diff --git a/apps/notescraft-sync-server/tests/chunk_client_e2e.rs b/apps/notescraft-sync-server/tests/chunk_client_e2e.rs
new file mode 100644
index 0000000..9844068
--- /dev/null
+++ b/apps/notescraft-sync-server/tests/chunk_client_e2e.rs
@@ -0,0 +1,239 @@
+//! The real engine sync client against the real server: a big file travels in
+//! chunks, a cut connection resumes with the chunks that never arrived.
+
+use std::path::{Path, PathBuf};
+use std::sync::Mutex;
+
+use notescraft_engine::{Changes, HttpRemote, Limits, Progress, PutReply, Remote, SyncError, SyncOptions, SyncReport, SyncResult, SyncState, sync_once_with};
+use notescraft_sync_server::{AppState, Store, router};
+use notescraft_vault::Vault;
+
+const ADMIN: &str = "test-admin-token-0123456789abcdef";
+/// The size of one chunk the client sends.
+const CHUNK: usize = 32 * 1024;
+/// The size above which the client sends a file in chunks.
+const THRESHOLD: u64 = 16 * 1024;
+/// The biggest chunk the server takes.
+const SERVER_CHUNK_MAX: usize = 64 * 1024;
+/// The biggest file the server assembles.
+const SERVER_FILE_CAP: u64 = 1024 * 1024;
+/// The size of the file that travels.
+const FILE_BYTES: usize = 300 * 1024;
+
+/// A temp folder that is empty when the test starts.
+fn temp(name: &str) -> PathBuf {
+    let dir = std::env::temp_dir().join(format!("nc-chunk-e2e-{name}-{}", std::process::id()));
+    std::fs::remove_dir_all(&dir).ok();
+    std::fs::create_dir_all(&dir).expect("dir");
+    dir
+}
+
+/// Starts the server on an ephemeral port and returns its base url.
+fn start(data: &Path) -> String {
+    std::fs::create_dir_all(data.join("blobs")).expect("blobs");
+    let state = AppState::new(Store::open(&data.join("sync.db")).expect("db"), data.join("blobs"), ADMIN);
+    // The chunk limits are set before the router is built: the route takes its
+    // body cap from the state.
+    state.set_chunk_limits(SERVER_CHUNK_MAX, SERVER_FILE_CAP);
+    let listener = std::net::TcpListener::bind("127.0.0.1:0").expect("bind");
+    listener.set_nonblocking(true).expect("nonblocking");
+    let port = listener.local_addr().expect("addr").port();
+    std::thread::spawn(move || {
+        let runtime = tokio::runtime::Builder::new_multi_thread().enable_all().build().expect("runtime");
+        runtime.block_on(async {
+            let listener = tokio::net::TcpListener::from_std(listener).expect("listener");
+            axum::serve(listener, router(state)).await.ok();
+        });
+    });
+    format!("http://127.0.0.1:{port}")
+}
+
+fn admin(base: &str, path: &str) -> serde_json::Value {
+    let agent: ureq::Agent = ureq::Agent::config_builder().http_status_as_error(false).build().into();
+    let mut response = agent.post(format!("{base}{path}")).header("Authorization", format!("Bearer {ADMIN}")).send_empty().expect("admin call");
+    response.body_mut().read_json().expect("json")
+}
+
+/// Issues a pairing code for the vault, bound to `user` when given.
+fn issue(base: &str, vault_id: &str, user: Option<&str>) -> String {
+    let path = user.map_or_else(|| format!("/v1/vaults/{vault_id}/pairing"), |user| format!("/v1/vaults/{vault_id}/pairing?user={user}"));
+    admin(base, &path)["code"].as_str().expect("code").to_owned()
+}
+
+/// Redeems a code with raw HTTP to also learn the account the device joined.
+fn pair_raw(base: &str, code: &str, name: &str) -> (String, String) {
+    let agent: ureq::Agent = ureq::Agent::config_builder().http_status_as_error(false).build().into();
+    let mut response = agent.post(format!("{base}/v1/devices")).send_json(serde_json::json!({"code": code, "name": name})).expect("pair");
+    let device: serde_json::Value = response.body_mut().read_json().expect("json");
+    (device["token"].as_str().expect("token").to_owned(), device["user_id"].as_str().expect("user id").to_owned())
+}
+
+/// A remote that can cut the connection in the middle of a chunked upload.
+struct CutRemote {
+    inner: HttpRemote,
+    /// How many chunks the client sent.
+    sent: Mutex<usize>,
+    /// From this send on, the connection is cut.
+    cut_from: Mutex<Option<usize>>,
+}
+
+impl CutRemote {
+    fn new(inner: HttpRemote) -> Self {
+        Self { inner, sent: Mutex::new(0), cut_from: Mutex::new(None) }
+    }
+
+    /// Cuts the connection from the given chunk send on.
+    fn cut_from(&self, call: usize) {
+        *self.cut_from.lock().unwrap_or_else(std::sync::PoisonError::into_inner) = Some(call);
+    }
+
+    /// Lets the connection through again.
+    fn mend(&self) {
+        *self.cut_from.lock().unwrap_or_else(std::sync::PoisonError::into_inner) = None;
+    }
+
+    /// How many chunks were sent since the last count.
+    fn take_sent(&self) -> usize {
+        let mut sent = self.sent.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
+        std::mem::take(&mut *sent)
+    }
+}
+
+impl Remote for CutRemote {
+    fn changes(&self, since: u64, limit: usize, wait: u64) -> SyncResult<Changes> {
+        self.inner.changes(since, limit, wait)
+    }
+
+    fn get(&self, path: &str) -> SyncResult<Vec<u8>> {
+        self.inner.get(path)
+    }
+
+    fn put(&self, path: &str, bytes: &[u8], base_seq: u64, mtime_ms: u64) -> SyncResult<PutReply> {
+        self.inner.put(path, bytes, base_seq, mtime_ms)
+    }
+
+    fn delete(&self, path: &str, base_seq: u64) -> SyncResult<PutReply> {
+        self.inner.delete(path, base_seq)
+    }
+
+    fn missing_chunks(&self, hashes: &[String]) -> SyncResult<Vec<String>> {
+        self.inner.missing_chunks(hashes)
+    }
+
+    fn put_chunk(&self, hash: &str, bytes: &[u8]) -> SyncResult<()> {
+        let mut sent = self.sent.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
+        *sent = sent.saturating_add(1);
+        let cut = *self.cut_from.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
+        if cut.is_some_and(|from| *sent >= from) {
+            return Err(SyncError::Http("the connection was cut".to_owned()));
+        }
+        drop(sent);
+        self.inner.put_chunk(hash, bytes)
+    }
+
+    fn put_chunked(&self, path: &str, chunk_hashes: &[String], content_hash: &str, base_seq: u64, mtime_ms: u64) -> SyncResult<PutReply> {
+        self.inner.put_chunked(path, chunk_hashes, content_hash, base_seq, mtime_ms)
+    }
+}
+
+/// A device with its own vault, its own state and a remote that can cut.
+struct Device {
+    vault: Vault,
+    remote: CutRemote,
+    state: SyncState,
+    name: &'static str,
+}
+
+impl Device {
+    fn new(base: &str, vault_id: &str, token: &str, name: &'static str) -> Self {
+        let root = temp(name);
+        let vault = Vault::open(&root).expect("vault");
+        Self { vault, remote: CutRemote::new(HttpRemote::new(base, vault_id, token)), state: SyncState::default(), name }
+    }
+
+    fn write(&self, path: &str, bytes: &[u8]) {
+        self.vault.write(path, bytes).expect("write");
+    }
+
+    fn read(&self, path: &str) -> Vec<u8> {
+        self.vault.read(path).expect("read")
+    }
+
+    /// Syncs once and returns the report.
+    fn sync(&mut self) -> SyncReport {
+        let options = SyncOptions { limits: Limits { threshold: THRESHOLD, chunk: CHUNK }, progress: Box::new(|_: &Progress| {}) };
+        sync_once_with(&self.vault, &self.remote, &mut self.state, self.name, "2026-10-20", options).expect("sync")
+    }
+}
+
+/// Bytes that look random but come from a small generator, so the test is stable.
+fn random_bytes(len: usize, seed: u64) -> Vec<u8> {
+    let mut state = seed;
+    (0..len)
+        .map(|_| {
+            state = state.wrapping_mul(6_364_136_223_846_793_005).wrapping_add(1_442_695_040_888_963_407);
+            u8::try_from((state >> 33) & 0xff).expect("byte")
+        })
+        .collect()
+}
+
+/// How many chunks the server holds for the vault.
+fn stored_chunks(blobs: &Path, vault_id: &str) -> usize {
+    let dir = blobs.join(vault_id).join("chunks");
+    let Ok(entries) = std::fs::read_dir(dir) else { return 0 };
+    entries.filter_map(std::result::Result::ok).filter(|entry| entry.path().is_file()).count()
+}
+
+#[test]
+fn a_big_file_travels_in_chunks_and_a_cut_resumes_with_the_rest() {
+    let data = temp("server-data");
+    let base = start(&data);
+    let blobs = data.join("blobs");
+    let vault_id = format!("v-chunk-{}", std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH).map_or(0, |d| d.subsec_nanos()));
+    let (phone_token, user_id) = pair_raw(&base, &issue(&base, &vault_id, None), "phone");
+    let laptop = pair_raw(&base, &issue(&base, &vault_id, Some(&user_id)), "laptop");
+    let mut phone = Device::new(&base, &vault_id, &phone_token, "phone");
+    let mut laptop = Device::new(&base, &vault_id, &laptop.0, "laptop");
+
+    // A file that is too big for one request travels in chunks.
+    let bytes = random_bytes(FILE_BYTES, 42);
+    phone.write("big.bin", &bytes);
+    let report = phone.sync();
+    assert_eq!(report.uploaded, vec!["big.bin".to_owned()]);
+    let chunks = FILE_BYTES.div_ceil(CHUNK);
+    assert_eq!(stored_chunks(&blobs, &vault_id), chunks, "the server holds every chunk");
+
+    // The other device gets the very same bytes.
+    let report = laptop.sync();
+    assert_eq!(report.downloaded, vec!["big.bin".to_owned()]);
+    assert_eq!(laptop.read("big.bin"), bytes);
+
+    // The connection dies on the fourth chunk of the next file.
+    let other = random_bytes(FILE_BYTES, 7);
+    phone.write("other.bin", &other);
+    phone.remote.take_sent();
+    phone.remote.cut_from(4);
+    let error = sync_once_with(
+        &phone.vault,
+        &phone.remote,
+        &mut phone.state,
+        "phone",
+        "2026-10-20",
+        SyncOptions { limits: Limits { threshold: THRESHOLD, chunk: CHUNK }, progress: Box::new(|_: &Progress| {}) },
+    )
+    .expect_err("cut");
+    assert!(matches!(error, SyncError::Http(_)), "{error:?}");
+    phone.remote.take_sent();
+    assert_eq!(stored_chunks(&blobs, &vault_id), chunks + 3, "the three chunks that arrived are kept");
+
+    // The next pass sends the chunks that never arrived and nothing else.
+    phone.remote.mend();
+    phone.sync();
+    assert_eq!(phone.remote.take_sent(), chunks - 3, "only the missing chunks travel again");
+    assert_eq!(stored_chunks(&blobs, &vault_id), chunks * 2, "the server holds every chunk of both files");
+
+    // The other device converges on the second file too.
+    laptop.sync();
+    assert_eq!(laptop.read("other.bin"), other);
+    assert_eq!(laptop.read("big.bin"), bytes);
+}
diff --git a/crates/engine/src/ai/assistant_gate.rs b/crates/engine/src/ai/assistant_gate.rs
index 811b7b1..ca26589 100644
--- a/crates/engine/src/ai/assistant_gate.rs
+++ b/crates/engine/src/ai/assistant_gate.rs
@@ -199,6 +199,7 @@ pub const READ_COMMANDS: &[&str] = &[
     "sync.status",
     "sync.conflicts",
     "server.vaults",
+    "server.devices",
     "share.members",
     "share.invites",
     "settings.get",
diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index fdd19a9..9ec0eaf 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -68,7 +68,7 @@ pub(crate) use vault::{attachments, bookmarks, buffers, content_search, daily, e
 
 pub use crate::app_settings::{
     APP_ID, AppSettings, AssistantSettings, EditorSettings, GitSettings, GrammarInfo, MAX_FONT_SIZE, MAX_INDENT_WIDTH, MAX_RECENT_VAULTS, MAX_SHORTCUT_LEN,
-    MAX_UI_BYTES, MIN_FONT_SIZE, MIN_INDENT_WIDTH, PluginSettings, PreviewSettings, SCHEMA_VERSION, ViewDefault,
+    MAX_UI_BYTES, MIN_FONT_SIZE, MIN_INDENT_WIDTH, PluginSettings, PreviewSettings, SCHEMA_VERSION, SyncSettings, ViewDefault,
 };
 pub use crate::assistant::{is_gated_source, run_as_assistant};
 pub use crate::assistant_chat::{
@@ -113,13 +113,15 @@ pub use crate::registry::{MAX_COMMANDS, Registry, current_command_id};
 pub use crate::reminders_config::RemindersConfig;
 pub use crate::session::Session;
 pub use crate::settings_store::{load, load_at, load_checked, load_checked_at, save, save_at};
-pub use crate::share::{Accepted, Invite, InviteInfo, Member, ServerClient, ServerVault, ShareError, ShareResult, bind_folder, connect_with_code};
+pub use crate::share::{
+    Accepted, Invite, InviteInfo, Member, ServerClient, ServerDevice, ServerVault, ShareError, ShareResult, bind_folder, connect_with_code,
+};
 pub use crate::slash::{SlashEntry, filter_entries, trigger_ok};
 pub use crate::spec::{CommandResult, CommandSpec};
 pub use crate::sync::{
-    Binding, Changes, ConflictPair, FileRecord, HttpRemote, Keep, Pairing, PutReply, Remote, ReportCounts, SyncError, SyncPhase, SyncReport, SyncResult,
-    SyncState, SyncStatus, SyncWorker, Timings, clear_binding, list_conflicts, load_binding, load_token, pair, resolve_conflict, save_binding, save_token,
-    sync_once,
+    Binding, CHUNK_SIZE, CHUNK_THRESHOLD, Changes, ConflictPair, FileRecord, HttpRemote, Keep, Limits, Pairing, Progress, PutReply, Remote, ReportCounts,
+    SyncError, SyncOptions, SyncPhase, SyncReport, SyncResult, SyncState, SyncStatus, SyncWorker, Timings, Transfer, clear_binding, list_conflicts,
+    load_binding, load_token, pair, resolve_conflict, save_binding, save_token, sync_once, sync_once_with, upload_chunked,
 };
 pub use crate::vault::snapshots::Snapshot;
 pub use notescraft_vault::{FileCount, LARGE_FOLDER_FILES, count_files};
diff --git a/crates/engine/src/prefs/app_settings.rs b/crates/engine/src/prefs/app_settings.rs
index f438226..0deb744 100644
--- a/crates/engine/src/prefs/app_settings.rs
+++ b/crates/engine/src/prefs/app_settings.rs
@@ -73,6 +73,8 @@ pub struct AppSettings {
     /// launcher; a list, for the same reason `grammars` is: a forgotten vault
     /// must not come back.
     pub vault_catalog: Vec<crate::prefs::catalog::VaultMeta>,
+    /// The three sync options the sync sheet and the Sync settings page share.
+    pub sync: SyncSettings,
 }
 
 /// One installed syntax grammar.
@@ -108,6 +110,7 @@ impl Default for AppSettings {
             hotkeys: BTreeMap::new(),
             grammars: Vec::new(),
             vault_catalog: Vec::new(),
+            sync: SyncSettings::default(),
         }
     }
 }
@@ -164,6 +167,28 @@ pub fn is_grammar_id(id: &str) -> bool {
     (1..=MAX_GRAMMAR_ID).contains(&id.chars().count()) && id.chars().all(|c| c.is_ascii_lowercase() || c.is_ascii_digit() || c == '-')
 }
 
+/// The three options a sync vault may offer (`design/mobile/03-sync-sheet.png`).
+///
+/// Booleans cannot hold a bad value, so there is nothing to repair: the
+/// `#[serde(default)]` fields keep a missing key at its default when an older
+/// `config.toml` is read.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(default)]
+pub struct SyncSettings {
+    /// Sync only when the machine is on Wi-Fi.
+    pub wifi_only: bool,
+    /// Sync files that are not Markdown.
+    pub attachments: bool,
+    /// Sync the settings and the themes of the app between the devices.
+    pub settings_and_themes: bool,
+}
+
+impl Default for SyncSettings {
+    fn default() -> Self {
+        Self { wifi_only: true, attachments: true, settings_and_themes: false }
+    }
+}
+
 /// Drops grammar entries whose id is not a grammar id, is repeated, or whose file is not
 /// `<id>.sublime-syntax`, and keeps the list sorted by id.
 fn repair_grammars(grammars: &mut Vec<GrammarInfo>) -> Vec<String> {
@@ -438,6 +463,14 @@ mod tests {
         assert_eq!(settings.editor.font_size, MIN_FONT_SIZE);
     }
 
+    #[test]
+    fn sync_options_carry_their_defaults_and_an_old_file_reads() {
+        assert_eq!(SyncSettings::default(), SyncSettings { wifi_only: true, attachments: true, settings_and_themes: false });
+        let empty: AppSettings = serde_json::from_str("{}").unwrap_or_default();
+        assert_eq!(empty.sync, SyncSettings::default(), "a file without the section keeps the defaults");
+        assert!(empty.sync.wifi_only);
+    }
+
     #[test]
     fn session_repairs_are_prefixed_and_applied() {
         let mut settings = AppSettings::default();
diff --git a/crates/engine/src/prefs/settings.rs b/crates/engine/src/prefs/settings.rs
index d9a71f2..e779f5b 100644
--- a/crates/engine/src/prefs/settings.rs
+++ b/crates/engine/src/prefs/settings.rs
@@ -187,7 +187,7 @@ fn write_key(doc: &mut Value, segments: &[String], value: Value) -> std::result:
 fn write_section(section: &mut Value, name: &str, rest: &[String], value: Value) -> std::result::Result<(), CommandError> {
     match name {
         "session" => session_path(section, rest, value),
-        "git" | "editor" => match rest {
+        "git" | "editor" | "sync" => match rest {
             [] => {
                 whole_section(section, value);
                 Ok(())
diff --git a/crates/engine/src/share/client.rs b/crates/engine/src/share/client.rs
index 00e1031..22f7e7b 100644
--- a/crates/engine/src/share/client.rs
+++ b/crates/engine/src/share/client.rs
@@ -310,6 +310,21 @@ impl ServerClient {
         self.post_json("/v1/invites/accept", &json!({ "code": code }))
     }
 
+    /// The devices paired with a vault (`GET /v1/vaults/{vault}/devices`).
+    ///
+    /// The rows carry no token and no token hash: the server never keeps one
+    /// for a listing, and this request sends none either.
+    ///
+    /// # Errors
+    /// [`ShareError::Protocol`] when the vault id cannot travel in a URL,
+    /// [`ShareError::Http`] when the server cannot be reached and
+    /// [`ShareError::Status`] when it refuses the request.
+    pub fn devices(&self, vault: &str) -> ShareResult<Vec<crate::share::devices::ServerDevice>> {
+        refuse_id(vault)?;
+        let path = format!("/v1/vaults/{}/devices", encode_segment(vault));
+        self.get_json(&path)
+    }
+
     /// The URL of a path on the server.
     fn url(&self, path: &str) -> String {
         format!("{}{path}", self.base)
diff --git a/crates/engine/src/share/commands.rs b/crates/engine/src/share/commands.rs
index 91d9bc1..6db55f5 100644
--- a/crates/engine/src/share/commands.rs
+++ b/crates/engine/src/share/commands.rs
@@ -37,6 +37,7 @@ const DEVICE_NAME: &str = "NotesCraft";
 pub(crate) fn register_share(reg: &mut Registry<Session>) -> crate::error::Result<()> {
     let specs = vec![
         CommandSpec::new("server.vaults", "Server vaults", "Lists the vaults of the server account this vault is bound to", |_| true, server_vaults),
+        CommandSpec::new("server.devices", "Vault devices", "Lists the devices paired with the bound vault; never a token", |_| true, server_devices),
         CommandSpec::new(
             "server.create_vault",
             "Create server vault",
@@ -119,6 +120,13 @@ fn server_vaults(session: &mut Session, _params: &Value) -> CommandResult {
     crate::commands::json(json!({ "vaults": vaults }))
 }
 
+/// `server.devices`: the devices of the bound vault; never a token.
+fn server_devices(session: &mut Session, _params: &Value) -> CommandResult {
+    let (client, binding) = client_of(session)?;
+    let devices = client.devices(&binding.vault_id).map_err(share_error)?;
+    crate::commands::json(json!({ "vault_id": binding.vault_id, "devices": devices }))
+}
+
 /// `server.create_vault`: a new vault, owned by this account.
 fn server_create_vault(session: &mut Session, params: &Value) -> CommandResult {
     let name = name_param(params, "name")?;
diff --git a/crates/engine/src/share/devices.rs b/crates/engine/src/share/devices.rs
new file mode 100644
index 0000000..6e1aebe
--- /dev/null
+++ b/crates/engine/src/share/devices.rs
@@ -0,0 +1,42 @@
+//! The devices of a bound vault, as the server lists them.
+//!
+//! The server names `id`, `name`, `created_ms`, `last_seen_ms` and `current`
+//! (design `03-sync-sheet`); a token or its hash is never part of the
+//! request, the answer or a row.
+
+use serde::{Deserialize, Serialize};
+
+/// One device paired with the bound vault, as `GET /v1/vaults/{vault}/devices`
+/// answers it.
+#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
+pub struct ServerDevice {
+    /// The device's id.
+    #[serde(default)]
+    pub id: String,
+    /// The name the device paired under.
+    #[serde(default)]
+    pub name: String,
+    /// When the device was paired, in Unix milliseconds.
+    #[serde(default)]
+    pub created_ms: u64,
+    /// When the server saw the device last, if ever.
+    #[serde(default)]
+    pub last_seen_ms: Option<u64>,
+    /// True for the device that asked; the server decides.
+    #[serde(default)]
+    pub current: bool,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn a_row_of_the_server_reads_whole_or_defaults() {
+        let row = serde_json::from_str::<ServerDevice>(r#"{"id":"d-1","name":"Pixel 9","current":true}"#).unwrap_or_default();
+        assert_eq!(row, ServerDevice { id: "d-1".to_owned(), name: "Pixel 9".to_owned(), created_ms: 0, last_seen_ms: None, current: true });
+        let full =
+            serde_json::from_str::<ServerDevice>(r#"{"id":"d-2","name":"Laptop","created_ms":50,"last_seen_ms":60,"current":false}"#).unwrap_or_default();
+        assert_eq!(full.last_seen_ms, Some(60));
+    }
+}
diff --git a/crates/engine/src/share/mod.rs b/crates/engine/src/share/mod.rs
index 7f5194c..5c8abcf 100644
--- a/crates/engine/src/share/mod.rs
+++ b/crates/engine/src/share/mod.rs
@@ -15,9 +15,11 @@
 pub(crate) mod bind;
 pub(crate) mod client;
 pub(crate) mod commands;
+mod devices;
 
 pub use bind::{bind_folder, connect_with_code};
 pub use client::{Accepted, Invite, InviteInfo, Member, ServerClient, ServerVault};
+pub use devices::ServerDevice;
 
 use serde::Deserialize;
 use thiserror::Error as ThisError;
diff --git a/crates/engine/src/sync/chunks.rs b/crates/engine/src/sync/chunks.rs
new file mode 100644
index 0000000..598b9b7
--- /dev/null
+++ b/crates/engine/src/sync/chunks.rs
@@ -0,0 +1,223 @@
+//! Chunked uploads: a big file travels to the server in content-addressed chunks.
+//!
+//! The server keeps every chunk by its own hash, so a file that was cut off half
+//! way resumes with the chunks that never arrived: the client asks which ones
+//! the server is missing and sends only those. One chunk at a time is read from
+//! disk, so the memory a big file needs does not grow with the file.
+//!
+//! The client hashes the file once to learn the hash of every chunk and of the
+//! whole content, asks the server which chunks it is missing, and reads the file
+//! a second time to send the missing ones. The second pass checks every chunk
+//! against the hash of the first pass, so a file that changed in between is
+//! refused instead of being uploaded in pieces.
+
+use std::collections::HashSet;
+use std::fs::File;
+use std::io::Read;
+use std::path::Path;
+use std::time::Duration;
+
+use notescraft_sync::{hash_hex, hasher};
+use notescraft_vault::Vault;
+
+use crate::sync::{PutReply, Remote, SyncError, SyncResult};
+
+/// Files above this size upload in chunks.
+pub const CHUNK_THRESHOLD: u64 = 8 * 1024 * 1024;
+/// How many bytes one chunk holds.
+pub const CHUNK_SIZE: usize = 4 * 1024 * 1024;
+/// The most hashes one "which chunks are missing" question holds.
+const MAX_CHECK_BATCH: usize = 5000;
+/// How many times a chunk is sent again before the upload is given up.
+const CHUNK_RETRIES: usize = 3;
+/// The waits between the tries of a chunk, in milliseconds.
+const BACKOFF_MS: [u64; CHUNK_RETRIES] = [200, 400, 800];
+
+/// How far a big file has moved.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Progress {
+    /// The vault path of the file.
+    pub path: String,
+    /// The bytes of the file the server holds now.
+    pub done_bytes: u64,
+    /// The bytes the file has.
+    pub total_bytes: u64,
+}
+
+/// When files upload in chunks and how big a chunk is.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Limits {
+    /// Files above this size upload in chunks.
+    pub threshold: u64,
+    /// How many bytes one chunk holds.
+    pub chunk: usize,
+}
+
+impl Default for Limits {
+    fn default() -> Self {
+        Self { threshold: CHUNK_THRESHOLD, chunk: CHUNK_SIZE }
+    }
+}
+
+/// Uploads `path` to the server in chunks.
+///
+/// The file is read from `vault` twice: once to learn the hash of every chunk
+/// and of the whole content, once to send the chunks the server is missing.
+/// `progress` is called after every chunk, with the bytes the server holds.
+///
+/// The answer is the one of a plain write: [`PutReply::Applied`] when the
+/// server took the file, [`PutReply::Unchanged`] when it already held it and
+/// [`PutReply::Conflict`] when somebody else wrote first.
+///
+/// # Errors
+/// [`SyncError::Vault`] when the file cannot be read, [`SyncError::BadRemote`]
+/// when the file changed while it was uploaded, and whatever the transport
+/// reports for the chunks and the write itself.
+pub fn upload_chunked(
+    remote: &dyn Remote,
+    vault: &Vault,
+    path: &str,
+    base_seq: u64,
+    mtime_ms: u64,
+    limits: Limits,
+    progress: &mut dyn FnMut(&Progress),
+) -> SyncResult<PutReply> {
+    if limits.chunk == 0 || limits.chunk > CHUNK_SIZE {
+        return Err(SyncError::Io(format!("a chunk must hold between one byte and {CHUNK_SIZE} bytes")));
+    }
+    let file = vault.resolve(path).map_err(|error| SyncError::Vault(error.to_string()))?;
+    let plan = plan_chunks(&file, limits.chunk)?;
+    if plan.chunks.is_empty() {
+        // An empty file has no chunk: it goes as a plain write.
+        return remote.put(path, &[], base_seq, mtime_ms);
+    }
+    let missing = missing_set(remote, &plan.chunks)?;
+    send_chunks(remote, &file, &plan, &missing, path, limits, progress)?;
+    match remote.put_chunked(path, &plan.chunks, &plan.content_hash, base_seq, mtime_ms) {
+        Ok(reply) => Ok(reply),
+        Err(SyncError::MissingChunks(list)) => {
+            // The server lost a chunk on its way: send those again, try once more.
+            let gone: HashSet<String> = list.into_iter().collect();
+            send_chunks(remote, &file, &plan, &gone, path, limits, progress)?;
+            remote.put_chunked(path, &plan.chunks, &plan.content_hash, base_seq, mtime_ms)
+        }
+        Err(error) => Err(error),
+    }
+}
+
+/// What the hash pass learned about a file.
+struct Plan {
+    /// The hash of every chunk, in the order they make up the file.
+    chunks: Vec<String>,
+    /// The hash of the whole content.
+    content_hash: String,
+    /// How many bytes the file has.
+    total: u64,
+}
+
+/// Reads the file once and answers the hash of every chunk and of the whole
+/// content.
+fn plan_chunks(file: &Path, chunk: usize) -> SyncResult<Plan> {
+    let mut handle = open(file)?;
+    let mut content = hasher();
+    let mut chunks = Vec::new();
+    let mut total = 0u64;
+    let mut buffer = vec![0u8; chunk];
+    loop {
+        let read = read_upto(&mut handle, &mut buffer)?;
+        if read == 0 {
+            break;
+        }
+        let bytes = buffer.get(..read).ok_or_else(|| SyncError::Io("the chunk buffer is shorter than the bytes read".to_owned()))?;
+        content.update(bytes);
+        chunks.push(hash_hex(bytes));
+        total = total
+            .checked_add(u64::try_from(read).map_err(|_| SyncError::Io("a chunk holds too many bytes".to_owned()))?)
+            .ok_or_else(|| SyncError::Io("the file is too long to count".to_owned()))?;
+    }
+    Ok(Plan { chunks, content_hash: content.finish_hex(), total })
+}
+
+/// The chunks of the list the server does not hold yet.
+fn missing_set(remote: &dyn Remote, chunks: &[String]) -> SyncResult<HashSet<String>> {
+    let mut missing = HashSet::new();
+    for batch in chunks.chunks(MAX_CHECK_BATCH) {
+        for hash in remote.missing_chunks(batch)? {
+            missing.insert(hash);
+        }
+    }
+    Ok(missing)
+}
+
+/// Sends the chunks of `wanted` the server is missing and reports the progress.
+///
+/// The file is read again: every chunk is checked against the hash the first
+/// pass gave it, so a file that changed in between is refused before any of it
+/// is sent.
+fn send_chunks(
+    remote: &dyn Remote,
+    file: &Path,
+    plan: &Plan,
+    wanted: &HashSet<String>,
+    path: &str,
+    limits: Limits,
+    progress: &mut dyn FnMut(&Progress),
+) -> SyncResult<()> {
+    let mut handle = open(file)?;
+    let mut buffer = vec![0u8; limits.chunk];
+    let mut sent: HashSet<String> = HashSet::new();
+    let mut done = 0u64;
+    for expected in &plan.chunks {
+        let read = read_upto(&mut handle, &mut buffer)?;
+        if read == 0 {
+            return Err(SyncError::BadRemote(format!("the file `{path}` shrank while it was uploaded")));
+        }
+        let bytes = buffer.get(..read).ok_or_else(|| SyncError::Io("the chunk buffer is shorter than the bytes read".to_owned()))?;
+        let hash = hash_hex(bytes);
+        if hash != *expected {
+            return Err(SyncError::BadRemote(format!("the file `{path}` changed while it was uploaded")));
+        }
+        if wanted.contains(&hash) && !sent.contains(&hash) {
+            put_with_retry(remote, &hash, bytes)?;
+            sent.insert(hash.clone());
+        }
+        done = done.saturating_add(u64::try_from(read).map_err(|_| SyncError::Io("a chunk holds too many bytes".to_owned()))?);
+        progress(&Progress { path: path.to_owned(), done_bytes: done, total_bytes: plan.total });
+    }
+    Ok(())
+}
+
+/// Sends one chunk, trying again with a growing wait before giving up.
+fn put_with_retry(remote: &dyn Remote, hash: &str, bytes: &[u8]) -> SyncResult<()> {
+    let mut attempt = 0usize;
+    loop {
+        match remote.put_chunk(hash, bytes) {
+            Ok(()) => return Ok(()),
+            Err(error) => {
+                let Some(wait) = BACKOFF_MS.get(attempt).copied() else { return Err(error) };
+                std::thread::sleep(Duration::from_millis(wait));
+                attempt = attempt.saturating_add(1);
+            }
+        }
+    }
+}
+
+/// Opens a file of the vault.
+fn open(file: &Path) -> SyncResult<File> {
+    File::open(file).map_err(|error| SyncError::Io(format!("{}: {error}", file.display())))
+}
+
+/// Reads into `buffer` until it is full or the file ends; at most a full buffer.
+fn read_upto(file: &mut File, buffer: &mut [u8]) -> SyncResult<usize> {
+    let mut read = 0usize;
+    while read < buffer.len() {
+        let Some(rest) = buffer.get_mut(read..) else { break };
+        match file.read(rest) {
+            Ok(0) => break,
+            Ok(count) => read = read.saturating_add(count),
+            Err(error) if error.kind() == std::io::ErrorKind::Interrupted => {}
+            Err(error) => return Err(SyncError::Io(error.to_string())),
+        }
+    }
+    Ok(read)
+}
diff --git a/crates/engine/src/sync/driver.rs b/crates/engine/src/sync/driver.rs
index fe93189..a887583 100644
--- a/crates/engine/src/sync/driver.rs
+++ b/crates/engine/src/sync/driver.rs
@@ -5,6 +5,7 @@ use std::collections::BTreeMap;
 use notescraft_sync::{Action, Base, FileRecord, check_hash, check_syncable, hash_hex, plan};
 use notescraft_vault::Vault;
 
+use crate::sync::chunks::{self, Limits, Progress};
 use crate::sync::{PutReply, Remote, SyncError, SyncResult, SyncState, reconcile, scan, state};
 
 /// How many records one `/changes` page asks for.
@@ -13,6 +14,9 @@ const PAGE_LIMIT: usize = 1000;
 const MAX_PAGES: usize = 10_000;
 /// Most names a conflict copy gets before the first one is used again.
 const MAX_COPY_TRIES: usize = 10;
+/// The largest file that is fetched into memory; bigger ones are left for a
+/// later day instead of being pulled into the memory of the app.
+pub(crate) const MAX_DOWNLOAD_IN_MEMORY: u64 = 64 * 1024 * 1024;
 
 /// What one sync pass did.
 #[derive(Debug, Default, PartialEq, Eq)]
@@ -29,10 +33,26 @@ pub struct SyncReport {
     pub merged: Vec<String>,
     /// Paths of the conflict copies written.
     pub conflicts: Vec<String>,
+    /// Paths left alone because the client cannot fetch them yet.
+    pub skipped: Vec<String>,
     /// How many paths were already in step.
     pub adopted: usize,
 }
 
+/// How one sync pass behaves.
+pub struct SyncOptions {
+    /// When files upload in chunks and how big a chunk is.
+    pub limits: Limits,
+    /// Called after every chunk of a big file, with the bytes that moved.
+    pub progress: Box<dyn FnMut(&Progress)>,
+}
+
+impl Default for SyncOptions {
+    fn default() -> Self {
+        Self { limits: Limits::default(), progress: Box::new(|_: &Progress| {}) }
+    }
+}
+
 /// Syncs `vault` with `remote`, once.
 ///
 /// The pass reads the vault, reads the change log of the server, asks
@@ -51,12 +71,40 @@ pub struct SyncReport {
 /// [`SyncError`] when the server cannot be reached, when it sends something
 /// unusable, or when the vault refuses a read or a write.
 pub fn sync_once(vault: &Vault, remote: &dyn Remote, state: &mut SyncState, device: &str, stamp: &str) -> SyncResult<SyncReport> {
+    sync_once_with(vault, remote, state, device, stamp, SyncOptions::default())
+}
+
+/// [`sync_once`] with the options of the pass: the chunk limits and the
+/// progress callback that watches a big file travel.
+///
+/// # Errors
+/// [`SyncError`] when the server cannot be reached, when it sends something
+/// unusable, or when the vault refuses a read or a write.
+pub fn sync_once_with(vault: &Vault, remote: &dyn Remote, state: &mut SyncState, device: &str, stamp: &str, options: SyncOptions) -> SyncResult<SyncReport> {
+    sync_once_scanned(vault, remote, state, device, stamp, options, scan::ScanOptions::default())
+}
+
+/// [`sync_once_with`] also naming what the scan of the vault leaves out: with
+/// `attachments: false` the pass sees only the Markdown notes.
+///
+/// # Errors
+/// [`SyncError`] when the server cannot be reached, when it sends something
+/// unusable, or when the vault refuses a read or a write.
+pub(crate) fn sync_once_scanned(
+    vault: &Vault,
+    remote: &dyn Remote,
+    state: &mut SyncState,
+    device: &str,
+    stamp: &str,
+    options: SyncOptions,
+    scan_options: scan::ScanOptions,
+) -> SyncResult<SyncReport> {
     let mut report = SyncReport::default();
-    let local = scan::scan(vault)?;
+    let local = scan::scan(vault, scan_options)?;
     let hashes: BTreeMap<String, String> = local.iter().map(|(path, file)| (path.clone(), file.hash.clone())).collect();
     let (records, cursor) = fetch(remote, state.last_seq)?;
     let actions = plan(&hashes, &state.bases, &records);
-    let mut ctx = Ctx { vault, remote, state, report: &mut report, local: &local, device, stamp };
+    let mut ctx = Ctx { vault, remote, state, report: &mut report, local: &local, device, stamp, options };
     for action in actions {
         perform(&mut ctx, action)?;
         ctx.state.save(ctx.vault)?;
@@ -76,12 +124,19 @@ struct Ctx<'a> {
     local: &'a BTreeMap<String, scan::LocalFile>,
     device: &'a str,
     stamp: &'a str,
+    options: SyncOptions,
 }
 
 /// Performs one planned action.
 fn perform(ctx: &mut Ctx<'_>, action: Action) -> SyncResult<()> {
     match action {
         Action::Download(record) => {
+            if record.size > MAX_DOWNLOAD_IN_MEMORY {
+                // Too big to fetch here yet: the file stays as it is, without a
+                // conflict and without a half-written copy.
+                ctx.report.skipped.push(record.path.clone());
+                return Ok(());
+            }
             let bytes = fetch_blob(ctx.remote, &record)?;
             ctx.vault.write(&record.path, &bytes).map_err(|e| vault_error(&e))?;
             ctx.state.bases.insert(record.path.clone(), Base { hash: record.hash.clone(), seq: record.seq });
@@ -94,19 +149,11 @@ fn perform(ctx: &mut Ctx<'_>, action: Action) -> SyncResult<()> {
             ctx.report.deleted_local.push(record.path.clone());
         }
         Action::Upload { path, base_seq } => {
-            let bytes = ctx.vault.read(&path).map_err(|e| vault_error(&e))?;
-            let hash = hash_hex(&bytes);
-            let mtime_ms = ctx.local.get(&path).map_or(0, |file| file.mtime_ms);
-            match ctx.remote.put(&path, &bytes, base_seq, mtime_ms)? {
-                PutReply::Applied(reply) | PutReply::Unchanged(reply) => {
-                    ctx.state.bases.insert(path.clone(), Base { hash, seq: reply.seq });
-                    ctx.report.uploaded.push(path.clone());
-                    state::store_base_copy(ctx.vault, &path, &bytes)?;
-                }
-                PutReply::Conflict(current) => {
-                    let base_hash = ctx.state.bases.get(&path).map(|base| base.hash.clone());
-                    reconcile_path(ctx, &current, &bytes, base_hash.as_deref())?;
-                }
+            let (size, mtime_ms) = ctx.local.get(&path).map_or((0, 0), |file| (file.size, file.mtime_ms));
+            if size > ctx.options.limits.threshold {
+                upload_big(ctx, &path, base_seq, mtime_ms)?;
+            } else {
+                upload_small(ctx, &path, base_seq, mtime_ms)?;
             }
         }
         Action::DeleteRemote { path, base_seq } => match ctx.remote.delete(&path, base_seq)? {
@@ -143,6 +190,44 @@ fn perform(ctx: &mut Ctx<'_>, action: Action) -> SyncResult<()> {
     Ok(())
 }
 
+/// Uploads a small file in one piece.
+fn upload_small(ctx: &mut Ctx<'_>, path: &str, base_seq: u64, mtime_ms: u64) -> SyncResult<()> {
+    let bytes = ctx.vault.read(path).map_err(|e| vault_error(&e))?;
+    let hash = hash_hex(&bytes);
+    match ctx.remote.put(path, &bytes, base_seq, mtime_ms)? {
+        PutReply::Applied(reply) | PutReply::Unchanged(reply) => {
+            ctx.state.bases.insert(path.to_owned(), Base { hash, seq: reply.seq });
+            ctx.report.uploaded.push(path.to_owned());
+            state::store_base_copy(ctx.vault, path, &bytes)?;
+            Ok(())
+        }
+        PutReply::Conflict(current) => {
+            let base_hash = ctx.state.bases.get(path).map(|base| base.hash.clone());
+            reconcile_path(ctx, &current, &bytes, base_hash.as_deref())
+        }
+    }
+}
+
+/// Uploads a big file in chunks, so a cut connection only loses one chunk.
+fn upload_big(ctx: &mut Ctx<'_>, path: &str, base_seq: u64, mtime_ms: u64) -> SyncResult<()> {
+    let limits = ctx.options.limits;
+    let progress: &mut dyn FnMut(&Progress) = &mut ctx.options.progress;
+    let reply = chunks::upload_chunked(ctx.remote, ctx.vault, path, base_seq, mtime_ms, limits, progress)?;
+    match reply {
+        PutReply::Applied(record) | PutReply::Unchanged(record) => {
+            ctx.state.bases.insert(path.to_owned(), Base { hash: record.hash.clone(), seq: record.seq });
+            ctx.report.uploaded.push(path.to_owned());
+            Ok(())
+        }
+        PutReply::Conflict(current) => {
+            // The other side wrote first: the content comes back for the merge.
+            let bytes = ctx.vault.read(path).map_err(|e| vault_error(&e))?;
+            let base_hash = ctx.state.bases.get(path).map(|base| base.hash.clone());
+            reconcile_path(ctx, &current, &bytes, base_hash.as_deref())
+        }
+    }
+}
+
 /// Settles a file both sides changed.
 fn reconcile_path(ctx: &mut Ctx<'_>, record: &FileRecord, ours: &[u8], base_hash: Option<&str>) -> SyncResult<()> {
     let theirs = fetch_blob(ctx.remote, record)?;
diff --git a/crates/engine/src/sync/http.rs b/crates/engine/src/sync/http.rs
index 61960bc..102f3f9 100644
--- a/crates/engine/src/sync/http.rs
+++ b/crates/engine/src/sync/http.rs
@@ -8,14 +8,15 @@ use std::time::Duration;
 use notescraft_sync::{Changes, FileRecord};
 use serde::{Deserialize, Serialize};
 
+use crate::sync::driver::MAX_DOWNLOAD_IN_MEMORY;
 use crate::sync::{PutReply, Remote, SyncError, SyncResult};
 
 /// Seconds a request may take when the caller does not ask for a long poll.
 const DEFAULT_TIMEOUT_SECS: u64 = 60;
 /// Seconds added to the long poll, to cover the round trip.
 const POLL_MARGIN_SECS: u64 = 15;
-/// Largest body the client reads.
-const MAX_BODY_BYTES: u64 = 25 * 1024 * 1024;
+/// Largest body the client reads: the biggest file it fetches.
+const MAX_BODY_BYTES: u64 = MAX_DOWNLOAD_IN_MEMORY;
 /// Longest body kept in an error message.
 const MAX_ERROR_BODY: usize = 200;
 /// The hexadecimal digits of a percent escape.
@@ -121,6 +122,53 @@ impl Remote for HttpRemote {
             other => Err(status_error(other, &body)),
         }
     }
+
+    fn missing_chunks(&self, hashes: &[String]) -> SyncResult<Vec<String>> {
+        let url = format!("{}/chunks/check", self.vault_url());
+        let body = serde_json::json!({ "hashes": hashes });
+        let response = self.agent.post(url.as_str()).header("Authorization", self.auth()).send_json(body).map_err(|e| SyncError::Http(e.to_string()))?;
+        let (status, body) = read(response)?;
+        if status != 200 {
+            return Err(status_error(status, &body));
+        }
+        let missing: MissingChunks = serde_json::from_slice(&body).map_err(|e| SyncError::BadRemote(format!("the chunk list is not readable: {e}")))?;
+        Ok(missing.missing)
+    }
+
+    fn put_chunk(&self, hash: &str, bytes: &[u8]) -> SyncResult<()> {
+        let url = format!("{}/chunks/{}", self.vault_url(), encode_segment(hash));
+        let response = self.agent.put(url.as_str()).header("Authorization", self.auth()).send(bytes).map_err(|e| SyncError::Http(e.to_string()))?;
+        let (status, body) = read(response)?;
+        // 201 when the chunk was stored, 200 when the server already had it.
+        if status == 200 || status == 201 {
+            return Ok(());
+        }
+        Err(status_error(status, &body))
+    }
+
+    fn put_chunked(&self, path: &str, chunk_hashes: &[String], content_hash: &str, base_seq: u64, mtime_ms: u64) -> SyncResult<PutReply> {
+        let url = self.file_url(path);
+        let response = self
+            .agent
+            .put(url.as_str())
+            .header("Authorization", self.auth())
+            .header("X-Chunks", chunk_hashes.join(","))
+            .header("X-Content-Hash", content_hash)
+            .header("X-Base-Seq", base_seq.to_string())
+            .header("X-Mtime-Ms", mtime_ms.to_string())
+            .send_empty()
+            .map_err(|e| SyncError::Http(e.to_string()))?;
+        let (status, body) = read(response)?;
+        match status {
+            201 => Ok(PutReply::Applied(parse_record(&body)?)),
+            200 => Ok(PutReply::Unchanged(parse_record(&body)?)),
+            409 => match parse_missing(&body) {
+                Some(missing) => Err(SyncError::MissingChunks(missing)),
+                None => Ok(PutReply::Conflict(parse_current(&body)?)),
+            },
+            other => Err(status_error(other, &body)),
+        }
+    }
 }
 
 /// The device a pairing code buys.
@@ -181,6 +229,19 @@ fn parse_current(body: &[u8]) -> SyncResult<FileRecord> {
     Ok(current.current)
 }
 
+/// Reads the `{"missing": [...]}` body of a chunked write whose chunks are not
+/// all there; `None` when the body is a conflict about the revision instead.
+fn parse_missing(body: &[u8]) -> Option<Vec<String>> {
+    serde_json::from_slice::<MissingChunks>(body).ok().map(|missing| missing.missing)
+}
+
+/// The `{"missing": [...]}` answer of the chunk check.
+#[derive(Deserialize)]
+struct MissingChunks {
+    /// The hashes the server does not hold.
+    missing: Vec<String>,
+}
+
 /// Turns an unexpected status into an error.
 fn status_error(status: u16, body: &[u8]) -> SyncError {
     let text: String = String::from_utf8_lossy(body).chars().take(MAX_ERROR_BODY).collect();
diff --git a/crates/engine/src/sync/mod.rs b/crates/engine/src/sync/mod.rs
index 4d63c03..abff503 100644
--- a/crates/engine/src/sync/mod.rs
+++ b/crates/engine/src/sync/mod.rs
@@ -12,6 +12,7 @@
 //! file cannot be merged, both are kept.
 
 mod binding;
+mod chunks;
 pub(crate) mod commands;
 mod conflicts;
 mod driver;
@@ -23,18 +24,22 @@ mod state;
 mod worker;
 
 pub use binding::{Binding, clear_binding, load_binding, load_token, save_binding, save_token};
+pub use chunks::{CHUNK_SIZE, CHUNK_THRESHOLD, Limits, Progress, upload_chunked};
 pub use conflicts::{ConflictPair, Keep, list_conflicts, resolve as resolve_conflict};
-pub use driver::{SyncReport, sync_once};
+pub use driver::{SyncOptions, SyncReport, sync_once, sync_once_with};
 pub use http::{HttpRemote, Pairing, pair};
 pub use remote::{PutReply, Remote};
 pub use state::SyncState;
-pub use worker::{ReportCounts, SyncPhase, SyncStatus, SyncWorker, Timings};
+pub use worker::{ReportCounts, SyncPhase, SyncStatus, SyncWorker, Timings, Transfer};
 
 /// The HTTP helpers the share client shares with the sync transport.
 pub(crate) use http::{agent, encode_segment};
 
 pub use notescraft_sync::{Changes, FileRecord};
 
+/// The scan options of the worker's pass; the session sets them from the user's options.
+pub(crate) use scan::ScanOptions;
+
 use thiserror::Error as ThisError;
 
 /// Everything that can go wrong while syncing.
@@ -59,6 +64,10 @@ pub enum SyncError {
     /// vault, a broken hash or bytes that do not match the hash they came with.
     #[error("the sync server sent something unusable: {0}")]
     BadRemote(String),
+    /// The server does not hold every chunk of a file: the hashes it is
+    /// missing come with the answer, so the client can send them again.
+    #[error("the sync server is missing {} chunk(s) of the file", .0.len())]
+    MissingChunks(Vec<String>),
     /// The sync state could not be read or written.
     #[error("sync state: {0}")]
     State(String),
diff --git a/crates/engine/src/sync/remote.rs b/crates/engine/src/sync/remote.rs
index db9cca9..d2ac2f5 100644
--- a/crates/engine/src/sync/remote.rs
+++ b/crates/engine/src/sync/remote.rs
@@ -2,7 +2,7 @@
 
 use notescraft_sync::{Changes, FileRecord};
 
-use crate::sync::SyncResult;
+use crate::sync::{SyncError, SyncResult};
 
 /// What the server did with a write.
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -51,4 +51,41 @@ pub trait Remote {
     /// [`SyncError::Http`] when the server cannot be reached and
     /// [`SyncError::Status`] when it refuses the delete.
     fn delete(&self, path: &str, base_seq: u64) -> SyncResult<PutReply>;
+
+    /// The chunks of `hashes` the server does not hold yet.
+    ///
+    /// The answer holds only the hashes that are missing, so a client that was
+    /// cut off sends the rest and nothing else.
+    ///
+    /// # Errors
+    /// [`SyncError::Http`] when the server cannot be reached and
+    /// [`SyncError::BadRemote`] when the server does not know chunks.
+    fn missing_chunks(&self, _hashes: &[String]) -> SyncResult<Vec<String>> {
+        Err(SyncError::BadRemote("chunks unsupported".to_owned()))
+    }
+
+    /// Stores one chunk under its own hash.
+    ///
+    /// The server keeps the chunk by content, so sending it twice costs
+    /// nothing.
+    ///
+    /// # Errors
+    /// [`SyncError::Http`] when the server cannot be reached and
+    /// [`SyncError::BadRemote`] when the server does not know chunks.
+    fn put_chunk(&self, _hash: &str, _bytes: &[u8]) -> SyncResult<()> {
+        Err(SyncError::BadRemote("chunks unsupported".to_owned()))
+    }
+
+    /// Writes a file whose content the server already holds as chunks.
+    ///
+    /// `chunk_hashes` are the hashes of the chunks in the order they make up
+    /// the file, `content_hash` the hash of the whole content.
+    ///
+    /// # Errors
+    /// [`SyncError::Http`] when the server cannot be reached,
+    /// [`SyncError::MissingChunks`] when the server is missing a chunk of the
+    /// list, and [`SyncError::BadRemote`] when the server does not know chunks.
+    fn put_chunked(&self, _path: &str, _chunk_hashes: &[String], _content_hash: &str, _base_seq: u64, _mtime_ms: u64) -> SyncResult<PutReply> {
+        Err(SyncError::BadRemote("chunks unsupported".to_owned()))
+    }
 }
diff --git a/crates/engine/src/sync/scan.rs b/crates/engine/src/sync/scan.rs
index 09d0847..b1eb81e 100644
--- a/crates/engine/src/sync/scan.rs
+++ b/crates/engine/src/sync/scan.rs
@@ -1,14 +1,18 @@
 //! The disk side of a sync: the files the vault holds right now.
 
 use std::collections::BTreeMap;
+use std::fs::File;
+use std::io::Read;
 
-use notescraft_sync::{check_syncable, hash_hex};
+use notescraft_sync::{check_syncable, hasher};
 use notescraft_vault::Vault;
 
 use crate::sync::{SyncError, SyncResult};
 
 /// Files above this size are left alone; the server refuses them too.
-const MAX_FILE_BYTES: u64 = 25 * 1024 * 1024;
+const MAX_FILE_BYTES: u64 = 1024 * 1024 * 1024;
+/// How many bytes are read at a time while a file is hashed.
+const READ_BUFFER: usize = 64 * 1024;
 
 /// One file of the vault.
 pub(crate) struct LocalFile {
@@ -16,6 +20,22 @@ pub(crate) struct LocalFile {
     pub hash: String,
     /// Last modification in Unix milliseconds.
     pub mtime_ms: u64,
+    /// How many bytes the file has.
+    pub size: u64,
+}
+
+/// What a scan leaves out of the vault picture.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct ScanOptions {
+    /// Whether files that are not Markdown travel; the sync sheet calls them
+    /// the attachments (`SyncSettings.attachments`).
+    pub attachments: bool,
+}
+
+impl Default for ScanOptions {
+    fn default() -> Self {
+        Self { attachments: true }
+    }
 }
 
 /// Reads every syncable file of `vault` and hashes it.
@@ -24,21 +44,79 @@ pub(crate) struct LocalFile {
 /// `.git`) are left out, and so are the ones above [`MAX_FILE_BYTES`]. A file
 /// that vanishes while the vault is being read is left out too.
 ///
+/// The content of a file is never held in memory: it is hashed as it is read,
+/// so a file of any size costs the same memory.
+///
+/// An `attachments` of `false` leaves out every file that is not Markdown.
+///
 /// # Errors
 /// [`SyncError::Vault`] when a file cannot be read.
-pub(crate) fn scan(vault: &Vault) -> SyncResult<BTreeMap<String, LocalFile>> {
+pub(crate) fn scan(vault: &Vault, options: ScanOptions) -> SyncResult<BTreeMap<String, LocalFile>> {
     let entries = vault.list().map_err(|e| SyncError::Vault(e.to_string()))?;
     let mut files = BTreeMap::new();
     for entry in entries {
-        if entry.is_dir || entry.size > MAX_FILE_BYTES || check_syncable(&entry.path).is_err() {
+        if entry.is_dir || entry.size > MAX_FILE_BYTES || check_syncable(&entry.path).is_err() || (!is_markdown(&entry.path) && !options.attachments) {
             continue;
         }
-        let bytes = match vault.read(&entry.path) {
-            Ok(bytes) => bytes,
-            Err(notescraft_vault::Error::Io { source, .. }) if source.kind() == std::io::ErrorKind::NotFound => continue,
-            Err(error) => return Err(SyncError::Vault(error.to_string())),
+        let full = vault.resolve(&entry.path).map_err(|e| SyncError::Vault(e.to_string()))?;
+        let hash = match hash_file(&full) {
+            Ok(hash) => hash,
+            Err(error) if error.kind() == std::io::ErrorKind::NotFound => continue,
+            Err(error) => return Err(SyncError::Io(format!("{}: {error}", entry.path))),
         };
-        files.insert(entry.path.clone(), LocalFile { hash: hash_hex(&bytes), mtime_ms: entry.modified.unwrap_or(0) });
+        files.insert(entry.path.clone(), LocalFile { hash, mtime_ms: entry.modified.unwrap_or(0), size: entry.size });
     }
     Ok(files)
 }
+
+/// Whether the path ends in `.md`, read case-insensitively; a file without an
+/// extension is not Markdown.
+fn is_markdown(path: &str) -> bool {
+    std::path::Path::new(path).extension().is_some_and(|extension| extension.eq_ignore_ascii_case("md"))
+}
+
+/// The SHA-256 hex of the content of a file, read in pieces.
+fn hash_file(file: &std::path::Path) -> std::result::Result<String, std::io::Error> {
+    let mut handle = File::open(file)?;
+    let mut hash = hasher();
+    let mut buffer = vec![0u8; READ_BUFFER];
+    loop {
+        let read = handle.read(&mut buffer)?;
+        if read == 0 {
+            break;
+        }
+        let bytes = buffer.get(..read).ok_or_else(|| std::io::Error::other("the read buffer is shorter than the bytes read"))?;
+        hash.update(bytes);
+    }
+    Ok(hash.finish_hex())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use notescraft_vault::Vault;
+
+    /// A vault with one note and one attachment, under a fresh temp folder.
+    fn seeded(name: &str) -> Vault {
+        let root = std::env::temp_dir().join(format!("notescraft-scan-{}-{name}", std::process::id()));
+        let _ = std::fs::remove_dir_all(&root);
+        std::fs::create_dir_all(&root).expect("create the temp vault");
+        let vault = Vault::open(&root).expect("open the temp vault");
+        vault.write("notes/idea.md", b"text").expect("write the note");
+        vault.write("notes/diagram.png", b"bytes").expect("write the attachment");
+        vault
+    }
+
+    #[test]
+    fn attachments_on_or_off_changes_what_the_scan_sees() {
+        let vault = seeded("on-off");
+        let note_only = scan(&vault, ScanOptions { attachments: false }).expect("scan without attachments");
+        assert!(!note_only.contains_key("notes/diagram.png"), "the png is left out");
+        assert_eq!(note_only.len(), 1, "the note stays");
+        let with_all = scan(&vault, ScanOptions { attachments: true }).expect("scan with attachments");
+        assert_eq!(with_all.len(), 2, "the note and the png both travel");
+        let defaulted = scan(&vault, ScanOptions::default()).expect("scan with the defaults");
+        assert_eq!(defaulted.len(), 2, "the default scan keeps the attachments");
+    }
+}
diff --git a/crates/engine/src/sync/worker.rs b/crates/engine/src/sync/worker.rs
index 5f48b7c..da09eab 100644
--- a/crates/engine/src/sync/worker.rs
+++ b/crates/engine/src/sync/worker.rs
@@ -21,7 +21,8 @@ use notescraft_templates::Date;
 use notescraft_vault::Vault;
 
 use crate::sync::binding::{load_binding, load_token};
-use crate::sync::driver::{SyncReport, sync_once};
+use crate::sync::chunks::{Limits, Progress};
+use crate::sync::driver::{SyncOptions, SyncReport, sync_once_scanned};
 use crate::sync::http::HttpRemote;
 use crate::sync::remote::Remote;
 use crate::sync::scan;
@@ -106,6 +107,17 @@ pub struct ReportCounts {
     pub conflicts: usize,
 }
 
+/// The file that is moving right now.
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize)]
+pub struct Transfer {
+    /// The vault path of the file.
+    pub path: String,
+    /// The bytes of the file the server holds now.
+    pub done_bytes: u64,
+    /// The bytes the file has.
+    pub total_bytes: u64,
+}
+
 /// What a worker knows: a cheap clone of its shared status.
 #[derive(Debug, Clone, Default, PartialEq, Eq)]
 pub struct SyncStatus {
@@ -130,13 +142,26 @@ pub struct SyncStatus {
     pub last_report: Option<ReportCounts>,
     /// The conflict copies the last cycle wrote.
     pub conflicts: Vec<String>,
+    /// The big file that is moving right now, while one does.
+    pub transfer: Option<Transfer>,
+}
+
+/// What the worker thread and the readers of its status share.
+#[derive(Debug)]
+struct Shared {
+    /// The status every reader sees.
+    status: Mutex<SyncStatus>,
+    /// The waits the loop uses.
+    timings: RwLock<Timings>,
+    /// What the scan of every cycle leaves out (the sync options); the app pushes
+    /// the user's toggles here.
+    scan_options: RwLock<scan::ScanOptions>,
 }
 
 /// A background thread syncing one vault.
 #[derive(Debug)]
 pub struct SyncWorker {
-    status: Arc<Mutex<SyncStatus>>,
-    timings: Arc<RwLock<Timings>>,
+    shared: Arc<Shared>,
     thread: Option<std::thread::JoinHandle<()>>,
     tx: Option<Sender<()>>,
 }
@@ -153,7 +178,7 @@ impl SyncWorker {
         let (server, vault_id) = (binding.server, binding.vault_id);
         let worker = Self::start_with(vault, device, Box::new(HttpRemote::new(&server, &vault_id, &token)), clock_millis);
         {
-            let mut status = lock_status(&worker.status);
+            let mut status = lock_status(&worker.shared.status);
             status.server = Some(server);
             status.vault_id = Some(vault_id);
         }
@@ -164,18 +189,21 @@ impl SyncWorker {
     #[must_use]
     pub fn start_with(vault: &Vault, device: &str, remote: Box<dyn Remote + Send>, clock: fn() -> u64) -> Self {
         let (tx, rx) = channel();
-        let status = Arc::new(Mutex::new(SyncStatus { bound: true, ..SyncStatus::default() }));
-        let timings = Arc::new(RwLock::new(Timings::default()));
+        let shared = Arc::new(Shared {
+            status: Mutex::new(SyncStatus { bound: true, ..SyncStatus::default() }),
+            timings: RwLock::new(Timings::default()),
+            scan_options: RwLock::new(scan::ScanOptions::default()),
+        });
         let vault = vault.sibling();
         let device = device.to_owned();
-        let (thread_status, thread_timings) = (Arc::clone(&status), Arc::clone(&timings));
+        let thread_shared = Arc::clone(&shared);
         let thread = std::thread::Builder::new()
             .name("notescraft-sync".to_owned())
-            .spawn(move || run(&vault, &device, remote.as_ref(), clock, &thread_status, &thread_timings, &rx))
+            .spawn(move || run(&vault, &device, remote.as_ref(), clock, &thread_shared, &rx))
             .ok();
-        let worker = Self { status, timings, thread, tx: Some(tx) };
+        let worker = Self { shared, thread, tx: Some(tx) };
         if worker.thread.is_none() {
-            set_status(&worker.status, |status| {
+            set_status(&worker.shared.status, |status| {
                 status.phase = SyncPhase::Error;
                 status.last_error = Some("the sync thread could not start".to_owned());
             });
@@ -185,11 +213,19 @@ impl SyncWorker {
 
     /// Replaces the waits; they take effect on the next loop wake-up. Tests shorten them.
     pub fn set_timings(&self, timings: Timings) {
-        if let Ok(mut guard) = self.timings.write() {
+        if let Ok(mut guard) = self.shared.timings.write() {
             *guard = timings;
         }
     }
 
+    /// Replaces what the scan of the cycles leaves out, the way the engine
+    /// applies the user's sync options; it takes effect on the next cycle.
+    pub(crate) fn set_scan_options(&self, options: scan::ScanOptions) {
+        if let Ok(mut guard) = self.shared.scan_options.write() {
+            *guard = options;
+        }
+    }
+
     /// Wakes the worker up. Requests of the same run coalesce: the thread waits for the
     /// debounce after the last one.
     pub fn request_sync(&self) {
@@ -201,7 +237,7 @@ impl SyncWorker {
     /// The status as it stands.
     #[must_use]
     pub fn status(&self) -> SyncStatus {
-        lock_status(&self.status).clone()
+        lock_status(&self.shared.status).clone()
     }
 
     /// Signals and joins the thread, giving it [`STOP_CAP`]; the same work [`Drop`] does.
@@ -241,7 +277,7 @@ enum Fate {
 
 /// The loop of the worker thread. It owns a vault sibling and a receiver; everything else
 /// it borrows.
-fn run(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64, status: &Mutex<SyncStatus>, timings: &RwLock<Timings>, rx: &Receiver<()>) {
+fn run(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64, shared: &Arc<Shared>, rx: &Receiver<()>) {
     // The start counts as one request, so the first cycle runs right after the debounce.
     let mut last_request = Some(Instant::now());
     let mut last_cycle: Option<Instant> = None;
@@ -250,7 +286,7 @@ fn run(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64, sta
     let mut wait_for_request = false;
     let mut cycles: u64 = 0;
     loop {
-        let Some(due) = next_due(timings, last_request, last_cycle, failed_at, retries, wait_for_request) else {
+        let Some(due) = next_due(&shared.timings, last_request, last_cycle, failed_at, retries, wait_for_request) else {
             // Sign-in-again: nothing times out, only a request or the end of the worker.
             match rx.recv() {
                 Ok(()) => {
@@ -278,7 +314,7 @@ fn run(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64, sta
                 last_request = None;
                 last_cycle = Some(now);
                 cycles = cycles.wrapping_add(1);
-                let fate = run_cycle(vault, device, remote, clock, status, cycles);
+                let fate = run_cycle(vault, device, remote, clock, shared, cycles);
                 match fate {
                     Fate::Success | Fate::Fail => {
                         failed_at = None;
@@ -325,18 +361,18 @@ fn next_due(
 
 /// Runs one cycle and reports what the loop should do next. `cycle` is the number of
 /// cycles the thread has run, carried in the status file.
-fn run_cycle(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64, status: &Mutex<SyncStatus>, cycle: u64) -> Fate {
-    set_status(status, |status| status.phase = SyncPhase::Syncing);
+fn run_cycle(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64, shared: &Arc<Shared>, cycle: u64) -> Fate {
+    set_status(&shared.status, |status| status.phase = SyncPhase::Syncing);
     match SyncState::load(vault) {
-        Ok(mut state) => one_pass(vault, device, remote, clock, status, &mut state, cycle),
+        Ok(mut state) => one_pass(vault, device, remote, clock, shared, &mut state, cycle),
         Err(error) => {
-            set_status(status, |status| {
+            set_status(&shared.status, |status| {
                 status.phase = SyncPhase::Error;
                 status.last_error = Some(short(&error.to_string()));
                 status.last_report = None;
                 status.pending = 0;
             });
-            write_status_file(vault, &fresh_status(status), cycle);
+            write_status_file(vault, &fresh_status(&shared.status), cycle);
             Fate::Fail
         }
     }
@@ -344,9 +380,10 @@ fn run_cycle(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u6
 
 /// The sync run of one cycle. The one status update of the cycle carries everything the
 /// run decided, so a reader never sees a half-updated status.
-fn one_pass(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64, status: &Mutex<SyncStatus>, state: &mut SyncState, cycle: u64) -> Fate {
-    let run = sync_once(vault, remote, state, device, &stamp_from_millis(clock()));
-    let pending = pending_count(vault, state);
+fn one_pass(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64, shared: &Arc<Shared>, state: &mut SyncState, cycle: u64) -> Fate {
+    let scan = *shared.scan_options.read().unwrap_or_else(PoisonError::into_inner);
+    let run = sync_once_scanned(vault, remote, state, device, &stamp_from_millis(clock()), options_for(shared, vault, cycle), scan);
+    let pending = pending_count(vault, state, scan);
     let seq = state.last_seq;
     let (fate, phase, error, counts, conflicts, last_sync): (Fate, SyncPhase, Option<String>, Option<ReportCounts>, Vec<String>, Option<u64>) = match run {
         Ok(report) => (Fate::Success, SyncPhase::Idle, None, Some(counts_of(&report)), report.conflicts.clone(), Some(clock())),
@@ -356,7 +393,7 @@ fn one_pass(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64
         }
         Err(other) => (Fate::Fail, SyncPhase::Error, Some(short(&other.to_string())), None, Vec::new(), None),
     };
-    set_status(status, |status| {
+    set_status(&shared.status, |status| {
         status.last_seq = seq;
         status.pending = pending;
         status.phase = phase;
@@ -364,15 +401,33 @@ fn one_pass(vault: &Vault, device: &str, remote: &dyn Remote, clock: fn() -> u64
         status.last_report = counts;
         status.conflicts = conflicts;
         status.last_sync_ms = last_sync;
+        status.transfer = None;
     });
-    write_status_file(vault, &fresh_status(status), cycle);
+    write_status_file(vault, &fresh_status(&shared.status), cycle);
     fate
 }
 
+/// The options of one pass: the default chunk limits and a callback that shows
+/// the big file that is moving.
+fn options_for(shared: &Arc<Shared>, vault: &Vault, cycle: u64) -> SyncOptions {
+    let watching = Arc::clone(shared);
+    let file_vault = vault.sibling();
+    SyncOptions {
+        limits: Limits::default(),
+        progress: Box::new(move |progress: &Progress| {
+            set_status(&watching.status, |status| {
+                status.transfer = Some(Transfer { path: progress.path.clone(), done_bytes: progress.done_bytes, total_bytes: progress.total_bytes });
+            });
+            write_status_file(&file_vault, &fresh_status(&watching.status), cycle);
+        }),
+    }
+}
+
 /// The local files whose content differs from the synced base of their path; a scan that
 /// fails leaves the count unknown, which reads as nothing waiting.
-fn pending_count(vault: &Vault, state: &SyncState) -> usize {
-    scan::scan(vault).map_or(0, |local| local.iter().filter(|(path, file)| state.bases.get(*path).is_none_or(|base| base.hash != file.hash)).count())
+fn pending_count(vault: &Vault, state: &SyncState, scan_options: scan::ScanOptions) -> usize {
+    scan::scan(vault, scan_options)
+        .map_or(0, |local| local.iter().filter(|(path, file)| state.bases.get(*path).is_none_or(|base| base.hash != file.hash)).count())
 }
 
 /// The counts of a report.
@@ -446,6 +501,8 @@ struct StatusFile {
     last_error: Option<String>,
     pending: usize,
     conflicts: Vec<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    transfer: Option<Transfer>,
 }
 
 /// Writes the status file of the last cycle so other processes can show it. A vault that
@@ -458,6 +515,7 @@ fn write_status_file(vault: &Vault, status: &SyncStatus, cycle: u64) {
         last_error: status.last_error.clone(),
         pending: status.pending,
         conflicts: status.conflicts.clone(),
+        transfer: status.transfer.clone(),
     };
     if let Ok(bytes) = serde_json::to_vec(&file) {
         let _ = vault.write_private(STATUS_FILE, &bytes);
diff --git a/crates/engine/src/vault/session.rs b/crates/engine/src/vault/session.rs
index 9eb80b5..b837bea 100644
--- a/crates/engine/src/vault/session.rs
+++ b/crates/engine/src/vault/session.rs
@@ -63,6 +63,9 @@ pub struct Session {
     pub(crate) pending: Vec<PendingChange>,
     /// The highest pending-change id handed out so far.
     pub(crate) next_pending_id: u64,
+    /// Whether the sync scan keeps the non-Markdown files (the applied user
+    /// option; the next started worker reads it too).
+    pub(crate) sync_attachments: bool,
     /// The notes still to read after a lazy open.
     pub(crate) scan: crate::indexing::ScanState,
     /// The format/lint batch: its queue and the proposals read so far.
@@ -140,10 +143,18 @@ impl Session {
     pub fn start_sync(&mut self, device_name: &str) -> crate::sync::SyncResult<()> {
         self.stop_sync();
         let worker = crate::sync::SyncWorker::start(&self.vault, device_name)?;
+        worker.set_scan_options(self.sync_scan_options());
         self.sync = Some(worker);
         Ok(())
     }
 
+    /// The options the scan of the sync worker uses (the user's sync options;
+    /// the session remembers the last applied value for the next start).
+    #[must_use]
+    fn sync_scan_options(&self) -> crate::sync::ScanOptions {
+        crate::sync::ScanOptions { attachments: self.sync_attachments }
+    }
+
     /// Stops the sync worker, if one runs; the notes are the worker's last known state.
     pub fn stop_sync(&mut self) {
         self.sync = None;
@@ -209,6 +220,12 @@ impl Session {
         self.set_config(app.session.clone())?;
         self.plugin_settings = app.plugins.clone();
         self.assistant = app.assistant.clone();
+        // The sync worker scans with the user's options; a worker started later
+        // (the next open, the next pairing) reads the remembered value.
+        self.sync_attachments = app.sync.attachments;
+        if let Some(worker) = self.sync.as_ref() {
+            worker.set_scan_options(self.sync_scan_options());
+        }
         Ok(())
     }
 
@@ -373,6 +390,7 @@ impl Session {
             batch: crate::batch::BatchState::default(),
             grammar_transport: Box::new(notescraft_syntax::fetch::Curl),
             sync: None,
+            sync_attachments: true,
         };
         session.begin_scan();
         session
diff --git a/crates/engine/tests/core_commands.rs b/crates/engine/tests/core_commands.rs
index a746ee5..5fe04cf 100644
--- a/crates/engine/tests/core_commands.rs
+++ b/crates/engine/tests/core_commands.rs
@@ -207,6 +207,7 @@ const CORE_IDS: &[&str] = &[
     "sync.resolve",
     // The server account, its vaults and the sharing surface (SPEC 19, spec 5-7).
     "server.vaults",
+    "server.devices",
     "server.create_vault",
     "server.rename_vault",
     "server.delete_vault",
diff --git a/crates/engine/tests/share_devices.rs b/crates/engine/tests/share_devices.rs
new file mode 100644
index 0000000..45f38f1
--- /dev/null
+++ b/crates/engine/tests/share_devices.rs
@@ -0,0 +1,217 @@
+//! The `server.devices` command against a stub server: the request it makes,
+//! the rows it lists and the secrets it never carries.
+//!
+//! The stub is a plain TCP listener that answers one canned JSON and remembers
+//! the request line, like the stub in `share_commands.rs`, kept smaller here
+//! because the command needs one route.
+
+mod common;
+
+use std::io::{BufRead, BufReader, Write};
+use std::net::{TcpListener, TcpStream};
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
+
+use common::temp_vault;
+use notescraft_engine::{Error, Registry, Session, bind_folder, register_core};
+use serde_json::{Value, json};
+
+/// The token of the bound vault; it must never reach an answer or an error.
+const TOKEN: &str = "secret-token";
+
+/// One request the stub received.
+#[derive(Debug, Clone)]
+struct Request {
+    /// The HTTP method.
+    method: String,
+    /// The path of the request line.
+    path: String,
+    /// The `Authorization` header, when there was one.
+    auth: Option<String>,
+}
+
+/// A stub server: one canned answer, the requests it saw.
+struct Stub {
+    /// The base URL of the server.
+    base: String,
+    /// Every request the stub received.
+    seen: Arc<Mutex<Vec<Request>>>,
+    /// Whether the listener thread should stop.
+    stop: Arc<AtomicBool>,
+}
+
+impl Stub {
+    /// Starts a stub on an ephemeral port that answers one route.
+    fn spawn(status: u16, body: &str) -> Self {
+        let listener = TcpListener::bind("127.0.0.1:0").expect("bind the stub");
+        let port = listener.local_addr().expect("addr").port();
+        let seen: Arc<Mutex<Vec<Request>>> = Arc::new(Mutex::new(Vec::new()));
+        let stop = Arc::new(AtomicBool::new(false));
+        let head = format!("HTTP/1.1 {status} OK\r\nContent-Type: application/json\r\nContent-Length: {}\r\n\r\n{body}", body.len());
+        let thread_seen = Arc::clone(&seen);
+        let thread_stop = Arc::clone(&stop);
+        std::thread::spawn(move || {
+            while !thread_stop.load(Ordering::Relaxed) {
+                match listener.accept() {
+                    Ok((stream, _)) => {
+                        let Ok(mut writer) = stream.try_clone() else { continue };
+                        let mut reader = BufReader::new(stream);
+                        if let Some(request) = read_request(&mut reader) {
+                            thread_seen.lock().expect("seen").push(request);
+                            let _ = writer.write_all(head.as_bytes());
+                            let _ = writer.flush();
+                        }
+                    }
+                    Err(_) => std::thread::sleep(Duration::from_millis(2)),
+                }
+            }
+        });
+        Self { base: format!("http://127.0.0.1:{port}"), seen, stop }
+    }
+
+    /// Every request the stub received.
+    fn requests(&self) -> Vec<Request> {
+        self.seen.lock().expect("seen").clone()
+    }
+}
+
+impl Drop for Stub {
+    fn drop(&mut self) {
+        self.stop.store(true, Ordering::Relaxed);
+    }
+}
+
+/// Reads one request: method, path, the authorization header; no body is expected.
+fn read_request(reader: &mut BufReader<TcpStream>) -> Option<Request> {
+    let mut line = String::new();
+    reader.read_line(&mut line).ok()?;
+    let mut parts = line.split_whitespace();
+    let method = parts.next()?.to_owned();
+    let path = parts.next()?.to_owned();
+    let mut auth = None;
+    loop {
+        let mut header = String::new();
+        if reader.read_line(&mut header).ok()? == 0 {
+            break;
+        }
+        let trimmed = header.trim_end();
+        if trimmed.is_empty() {
+            break;
+        }
+        if let Some((name, value)) = trimmed.split_once(':')
+            && name.trim().eq_ignore_ascii_case("authorization")
+        {
+            auth = Some(value.trim().to_owned());
+        }
+    }
+    Some(Request { method, path, auth })
+}
+
+/// A vault bound to `stub`, with the registry, the session and the vault guard.
+fn bound(stub: &Stub, name: &str) -> (Registry<Session>, Session, common::TempVault) {
+    let vault = temp_vault(name);
+    bind_folder(vault.path(), &stub.base, "v-1", TOKEN, "d-1", Some("u-1")).expect("bind the vault");
+    let mut reg = Registry::new();
+    register_core(&mut reg).expect("register the core commands");
+    let session = Session::open(vault.path()).expect("open the vault");
+    (reg, session, vault)
+}
+
+/// The code of a command error.
+fn code(result: &Result<Value, Error>) -> String {
+    match result {
+        Err(Error::Command { source, .. }) => source.code.clone(),
+        other => panic!("expected a command error, got {other:?}"),
+    }
+}
+
+/// The message of a command error.
+fn message(result: &Result<Value, Error>) -> String {
+    match result {
+        Err(Error::Command { source, .. }) => source.message.clone(),
+        other => panic!("expected a command error, got {other:?}"),
+    }
+}
+
+/// `server.devices` asks the server with the token and lists its rows.
+#[test]
+fn the_device_list_goes_to_the_server_with_the_token() {
+    let stub = Stub::spawn(
+        200,
+        r#"[{"id":"d-1","name":"Pixel 9","created_ms":100,"last_seen_ms":null,"current":true},{"id":"d-2","name":"Laptop","created_ms":50,"last_seen_ms":60,"current":false}]"#,
+    );
+    let (reg, mut session, _vault) = bound(&stub, "devices");
+
+    let answer = reg.run("server.devices", &mut session, &Value::Null).expect("server.devices");
+    let devices = answer.get("devices").and_then(Value::as_array).expect("the devices");
+    assert_eq!(devices.len(), 2);
+    assert_eq!(devices.first().and_then(|device| device.get("id")), Some(&json!("d-1")));
+    assert_eq!(devices.first().and_then(|device| device.get("name")), Some(&json!("Pixel 9")));
+    assert_eq!(devices.first().and_then(|device| device.get("current")), Some(&json!(true)));
+    assert_eq!(devices.first().and_then(|device| device.get("last_seen_ms")), Some(&Value::Null));
+    assert_eq!(devices.get(1).and_then(|device| device.get("last_seen_ms")), Some(&json!(60)));
+
+    let requests = stub.requests();
+    assert_eq!(requests.len(), 1);
+    assert_eq!(requests.first().map(|request| request.method.as_str()), Some("GET"));
+    assert_eq!(requests.first().map(|request| request.path.as_str()), Some("/v1/vaults/v-1/devices"));
+    assert_eq!(requests.first().and_then(|request| request.auth.clone()), Some(format!("Bearer {TOKEN}")));
+    assert!(!answer.to_string().contains(TOKEN), "the token never reaches the answer: {answer}");
+}
+
+#[test]
+fn no_token_reaches_the_answer_or_the_rows() {
+    let stub = Stub::spawn(200, r#"[{"id":"d-2","name":"Laptop","created_ms":50,"last_seen_ms":60,"current":false}]"#);
+    let (reg, mut session, _vault) = bound(&stub, "no-token");
+
+    let answer = reg.run("server.devices", &mut session, &Value::Null).expect("server.devices");
+    assert!(!answer.to_string().contains(TOKEN), "{}", answer);
+    assert!(!answer.to_string().contains("token"), "{}", answer);
+    assert!(!answer.to_string().contains("secret"), "{}", answer);
+}
+
+/// A vault that is not bound says so instead of asking the server.
+#[test]
+fn an_unbound_vault_reports_not_bound() {
+    let stub = Stub::spawn(200, "[]");
+    let mut reg = Registry::new();
+    register_core(&mut reg).expect("register the core commands");
+    let vault = temp_vault("devices-unbound");
+    let mut session = Session::open(vault.path()).expect("open the vault");
+
+    let result = reg.run("server.devices", &mut session, &Value::Null);
+    assert_eq!(code(&result), "share");
+    assert!(message(&result).contains("not bound"), "{}", message(&result));
+    assert!(stub.requests().is_empty(), "no request was made");
+}
+
+/// The message of the server reaches the user, the token never does.
+#[test]
+fn the_server_answer_refuses_like_the_others() {
+    let stub = Stub::spawn(401, r#"{"error":"token is not valid for this vault"}"#);
+    let (reg, mut session, _vault) = bound(&stub, "devices-401");
+
+    let result = reg.run("server.devices", &mut session, &Value::Null);
+    assert_eq!(code(&result), "share");
+    let text = message(&result);
+    assert!(text.contains("token is not valid for this vault"), "{text}");
+    assert!(!text.contains(TOKEN), "{text}");
+}
+
+/// `server.devices` never panics on hostile params, and asks nothing.
+#[test]
+fn hostile_params_never_panic_the_devices_command() {
+    let stub = Stub::spawn(200, "[]");
+    let (reg, mut session, _vault) = bound(&stub, "devices-hostile");
+    let previous = std::panic::take_hook();
+    std::panic::set_hook(Box::new(|_| {}));
+    let mut panicked = Vec::new();
+    for params in notescraft_engine::adversarial_params() {
+        if let Err(Error::Panicked(other)) = reg.run("server.devices", &mut session, &params) {
+            panicked.push(other);
+        }
+    }
+    std::panic::set_hook(previous);
+    assert!(panicked.is_empty(), "commands panicked: {panicked:?}");
+}
diff --git a/crates/engine/tests/sync_chunks.rs b/crates/engine/tests/sync_chunks.rs
new file mode 100644
index 0000000..7664ae1
--- /dev/null
+++ b/crates/engine/tests/sync_chunks.rs
@@ -0,0 +1,360 @@
+//! Chunked uploads: the client against a server that keeps chunks in memory.
+//!
+//! The fake server holds the chunks it was sent in a map and can be told to
+//! cut the connection, to lose a chunk or to change the file under the
+//! client's feet.
+
+use std::collections::BTreeMap;
+use std::path::PathBuf;
+use std::sync::{Arc, Mutex, MutexGuard};
+
+use notescraft_engine::{Changes, Limits, Progress, PutReply, Remote, SyncError, SyncOptions, SyncResult, SyncState, sync_once_with, upload_chunked};
+use notescraft_sync::{Ledger, Outcome, Write, hash_hex};
+use notescraft_vault::Vault;
+
+/// The size of one chunk in the tests.
+const CHUNK: usize = 1024;
+/// The size above which a file uploads in chunks.
+const THRESHOLD: u64 = 2 * 1024;
+
+/// The server the client talks to.
+struct Server {
+    inner: Mutex<Inner>,
+}
+
+/// What the server keeps.
+struct Inner {
+    ledger: Ledger,
+    /// The chunks the server holds, by their hash.
+    chunks: BTreeMap<String, Vec<u8>>,
+    /// The files the server holds, by their path.
+    files: BTreeMap<String, Vec<u8>>,
+    /// The hashes of every chunk that was sent.
+    chunk_calls: Vec<String>,
+    /// The chunk list of every chunked write.
+    chunked_calls: Vec<Vec<String>>,
+    /// How many chunk sends there were.
+    calls: usize,
+    /// From this send on, the connection is cut.
+    cut_from: Option<usize>,
+    /// The file that changes while the client is uploading it.
+    corrupt: Option<(PathBuf, Vec<u8>)>,
+}
+
+impl Inner {
+    fn new() -> Self {
+        Self {
+            ledger: Ledger::new(),
+            chunks: BTreeMap::new(),
+            files: BTreeMap::new(),
+            chunk_calls: Vec::new(),
+            chunked_calls: Vec::new(),
+            calls: 0,
+            cut_from: None,
+            corrupt: None,
+        }
+    }
+}
+
+impl Server {
+    fn new() -> Arc<Server> {
+        Arc::new(Server { inner: Mutex::new(Inner::new()) })
+    }
+
+    fn lock(&self) -> MutexGuard<'_, Inner> {
+        self.inner.lock().unwrap_or_else(std::sync::PoisonError::into_inner)
+    }
+
+    /// The hashes of the chunks that were sent.
+    fn chunk_calls(&self) -> Vec<String> {
+        self.lock().chunk_calls.clone()
+    }
+
+    /// How many chunked writes the server saw.
+    fn chunked_calls(&self) -> usize {
+        self.lock().chunked_calls.len()
+    }
+
+    /// The file the server holds.
+    fn file(&self, path: &str) -> Option<Vec<u8>> {
+        self.lock().files.get(path).cloned()
+    }
+
+    /// Cuts the connection from the given chunk send on.
+    fn cut_from(&self, call: usize) {
+        self.lock().cut_from = Some(call);
+    }
+
+    /// Lets the connection through again.
+    fn mend(&self) {
+        self.lock().cut_from = None;
+    }
+
+    /// Forgets the chunks that were sent, so a test can look at one pass only.
+    fn forget_calls(&self) {
+        self.lock().chunk_calls.clear();
+    }
+
+    /// Changes the file while the client uploads it.
+    fn corrupt(&self, path: PathBuf, bytes: Vec<u8>) {
+        self.lock().corrupt = Some((path, bytes));
+    }
+}
+
+/// The client the driver talks to.
+struct FakeRemote {
+    server: Arc<Server>,
+}
+
+impl Remote for FakeRemote {
+    fn changes(&self, since: u64, limit: usize, _wait: u64) -> SyncResult<Changes> {
+        let inner = self.server.lock();
+        Ok(inner.ledger.changes_since(since, limit))
+    }
+
+    fn get(&self, path: &str) -> SyncResult<Vec<u8>> {
+        let inner = self.server.lock();
+        inner.files.get(path).cloned().ok_or_else(|| SyncError::Status { status: 404, body: path.to_owned() })
+    }
+
+    fn put(&self, path: &str, bytes: &[u8], base_seq: u64, mtime_ms: u64) -> SyncResult<PutReply> {
+        let mut inner = self.server.lock();
+        let outcome = inner
+            .ledger
+            .apply(Write { path: path.to_owned(), hash: Some(hash_hex(bytes)), size: u64::try_from(bytes.len()).expect("size"), mtime_ms, base_seq })
+            .map_err(|error| SyncError::Status { status: 400, body: error.to_string() })?;
+        match outcome {
+            Outcome::Applied(record) => {
+                inner.files.insert(path.to_owned(), bytes.to_vec());
+                Ok(PutReply::Applied(record))
+            }
+            Outcome::Unchanged(record) => Ok(PutReply::Unchanged(record)),
+            Outcome::Conflict(record) => Ok(PutReply::Conflict(record)),
+        }
+    }
+
+    fn delete(&self, path: &str, base_seq: u64) -> SyncResult<PutReply> {
+        let mut inner = self.server.lock();
+        let outcome = inner
+            .ledger
+            .apply(Write { path: path.to_owned(), hash: None, size: 0, mtime_ms: 0, base_seq })
+            .map_err(|error| SyncError::Status { status: 400, body: error.to_string() })?;
+        match outcome {
+            Outcome::Applied(record) => {
+                inner.files.remove(path);
+                Ok(PutReply::Applied(record))
+            }
+            Outcome::Unchanged(record) => Ok(PutReply::Unchanged(record)),
+            Outcome::Conflict(record) => Ok(PutReply::Conflict(record)),
+        }
+    }
+
+    fn missing_chunks(&self, hashes: &[String]) -> SyncResult<Vec<String>> {
+        let mut inner = self.server.lock();
+        if let Some((path, bytes)) = inner.corrupt.take() {
+            std::fs::write(&path, &bytes).map_err(|error| SyncError::Io(error.to_string()))?;
+        }
+        Ok(hashes.iter().filter(|hash| !inner.chunks.contains_key(*hash)).cloned().collect())
+    }
+
+    fn put_chunk(&self, hash: &str, bytes: &[u8]) -> SyncResult<()> {
+        let mut inner = self.server.lock();
+        inner.calls = inner.calls.saturating_add(1);
+        inner.chunk_calls.push(hash.to_owned());
+        if inner.cut_from.is_some_and(|from| inner.calls >= from) {
+            return Err(SyncError::Http("the connection was cut".to_owned()));
+        }
+        inner.chunks.insert(hash.to_owned(), bytes.to_vec());
+        Ok(())
+    }
+
+    fn put_chunked(&self, path: &str, chunk_hashes: &[String], content_hash: &str, base_seq: u64, mtime_ms: u64) -> SyncResult<PutReply> {
+        let mut inner = self.server.lock();
+        inner.chunked_calls.push(chunk_hashes.to_vec());
+        let mut bytes = Vec::new();
+        for hash in chunk_hashes {
+            let Some(chunk) = inner.chunks.get(hash) else { return Err(SyncError::MissingChunks(vec![hash.clone()])) };
+            bytes.extend_from_slice(chunk);
+        }
+        let outcome = inner
+            .ledger
+            .apply(Write { path: path.to_owned(), hash: Some(content_hash.to_owned()), size: u64::try_from(bytes.len()).expect("size"), mtime_ms, base_seq })
+            .map_err(|error| SyncError::Status { status: 400, body: error.to_string() })?;
+        match outcome {
+            Outcome::Applied(record) => {
+                inner.files.insert(path.to_owned(), bytes);
+                Ok(PutReply::Applied(record))
+            }
+            Outcome::Unchanged(record) => Ok(PutReply::Unchanged(record)),
+            Outcome::Conflict(record) => Ok(PutReply::Conflict(record)),
+        }
+    }
+}
+
+/// A vault in a fresh folder.
+fn temp_vault(name: &str) -> Vault {
+    let root = std::env::temp_dir().join(format!("nc-engine-chunks-{name}-{}", std::process::id()));
+    let _ = std::fs::remove_dir_all(&root);
+    std::fs::create_dir_all(&root).expect("temp folder");
+    Vault::open(&root).expect("vault")
+}
+
+/// The limits the tests use: small chunks, so a small file has many of them.
+fn limits() -> Limits {
+    Limits { threshold: THRESHOLD, chunk: CHUNK }
+}
+
+/// Ten kilobytes whose chunks differ from one another.
+fn ten_chunks() -> Vec<u8> {
+    (0..10u8).flat_map(|index| vec![index; CHUNK]).collect()
+}
+
+/// The hash of the chunk number `index` of `bytes`.
+fn chunk_hash(bytes: &[u8], index: usize) -> String {
+    let start = index * CHUNK;
+    let end = start.saturating_add(CHUNK);
+    hash_hex(bytes.get(start..end).expect("chunk"))
+}
+
+#[test]
+fn a_big_file_travels_in_chunks_and_is_written_once() {
+    let server = Server::new();
+    let vault = temp_vault("big");
+    let bytes = ten_chunks();
+    vault.write("big.bin", &bytes).expect("write");
+    let remote = FakeRemote { server: server.clone() };
+
+    let reply = upload_chunked(&remote, &vault, "big.bin", 0, 0, limits(), &mut |_: &Progress| {}).expect("upload");
+
+    assert!(matches!(reply, PutReply::Applied(_)), "{reply:?}");
+    assert_eq!(server.chunk_calls().len(), 10, "one send per chunk");
+    assert_eq!(server.chunked_calls(), 1, "the file is written once");
+    assert_eq!(server.file("big.bin"), Some(bytes), "the server holds the whole file");
+}
+
+#[test]
+fn a_cut_connection_resumes_with_the_chunks_that_never_arrived() {
+    let server = Server::new();
+    let vault = temp_vault("resume");
+    let bytes = ten_chunks();
+    vault.write("big.bin", &bytes).expect("write");
+    let remote = FakeRemote { server: server.clone() };
+
+    // The connection dies on the sixth chunk; the chunk is tried again and fails.
+    server.cut_from(6);
+    let error = upload_chunked(&remote, &vault, "big.bin", 0, 0, limits(), &mut |_: &Progress| {}).expect_err("cut");
+    assert!(matches!(error, SyncError::Http(_)), "{error:?}");
+    assert_eq!(server.chunked_calls(), 0, "the file was never written");
+
+    // The next pass sends only the chunks that never arrived.
+    server.mend();
+    server.forget_calls();
+    upload_chunked(&remote, &vault, "big.bin", 0, 0, limits(), &mut |_: &Progress| {}).expect("resumed");
+
+    let sent: Vec<String> = (5..10).map(|index| chunk_hash(&bytes, index)).collect();
+    assert_eq!(server.chunk_calls(), sent, "only the missing chunks are sent again");
+    assert_eq!(server.file("big.bin"), Some(bytes), "the server holds the whole file");
+}
+
+#[test]
+fn the_same_chunk_of_a_file_is_sent_once() {
+    let server = Server::new();
+    let vault = temp_vault("same");
+    let bytes = vec![7u8; CHUNK * 4];
+    vault.write("same.bin", &bytes).expect("write");
+    let remote = FakeRemote { server: server.clone() };
+
+    upload_chunked(&remote, &vault, "same.bin", 0, 0, limits(), &mut |_: &Progress| {}).expect("upload");
+
+    assert_eq!(server.chunk_calls().len(), 1, "the four equal chunks travel once");
+    assert_eq!(server.file("same.bin"), Some(bytes), "the file still holds the four blocks");
+}
+
+#[test]
+fn the_progress_grows_and_ends_at_the_size_of_the_file() {
+    let server = Server::new();
+    let vault = temp_vault("progress");
+    let bytes = ten_chunks();
+    vault.write("big.bin", &bytes).expect("write");
+    let remote = FakeRemote { server: server.clone() };
+
+    let mut seen: Vec<Progress> = Vec::new();
+    upload_chunked(&remote, &vault, "big.bin", 0, 0, limits(), &mut |progress: &Progress| seen.push(progress.clone())).expect("upload");
+
+    assert_eq!(seen.len(), 10, "one report per chunk");
+    assert!(seen.windows(2).all(|pair| pair[0].done_bytes <= pair[1].done_bytes), "the progress never goes back");
+    let last = seen.last().expect("a report");
+    assert_eq!(last.done_bytes, last.total_bytes);
+    assert_eq!(last.total_bytes, u64::try_from(bytes.len()).expect("size"));
+    assert_eq!(last.path, "big.bin");
+}
+
+#[test]
+fn a_small_file_never_touches_the_chunk_methods() {
+    let server = Server::new();
+    let vault = temp_vault("small");
+    vault.write("small.md", b"# one\n").expect("write");
+    let remote = FakeRemote { server: server.clone() };
+    let mut state = SyncState::default();
+
+    let report = sync_once_with(&vault, &remote, &mut state, "phone", "2026-10-20", SyncOptions { limits: limits(), progress: Box::new(|_: &Progress| {}) })
+        .expect("sync");
+
+    assert_eq!(report.uploaded, vec!["small.md".to_owned()]);
+    assert_eq!(server.chunk_calls(), Vec::<String>::new(), "a small file is not chunked");
+    assert_eq!(server.chunked_calls(), 0);
+    assert_eq!(server.file("small.md"), Some(b"# one\n".to_vec()));
+}
+
+#[test]
+fn a_file_that_changes_while_it_is_uploaded_is_refused() {
+    let server = Server::new();
+    let vault = temp_vault("changed");
+    let bytes = ten_chunks();
+    vault.write("big.bin", &bytes).expect("write");
+    let remote = FakeRemote { server: server.clone() };
+    // The file changes between the hash pass and the upload pass.
+    server.corrupt(vault.root().join("big.bin"), vec![1u8; CHUNK * 10]);
+
+    let error = upload_chunked(&remote, &vault, "big.bin", 0, 0, limits(), &mut |_: &Progress| {}).expect_err("changed");
+
+    assert!(matches!(error, SyncError::BadRemote(_)), "{error:?}");
+    assert_eq!(server.chunked_calls(), 0, "no mixed file reaches the server");
+    assert_eq!(server.file("big.bin"), None);
+}
+
+#[test]
+fn an_empty_file_goes_as_a_plain_write() {
+    let server = Server::new();
+    let vault = temp_vault("empty");
+    vault.write("empty.bin", b"").expect("write");
+    let remote = FakeRemote { server: server.clone() };
+
+    let reply = upload_chunked(&remote, &vault, "empty.bin", 0, 0, limits(), &mut |_: &Progress| {}).expect("upload");
+
+    assert!(matches!(reply, PutReply::Applied(_)), "{reply:?}");
+    assert_eq!(server.chunk_calls(), Vec::<String>::new(), "an empty file has no chunk");
+    assert_eq!(server.chunked_calls(), 0);
+    assert_eq!(server.file("empty.bin"), Some(Vec::new()));
+}
+
+#[test]
+fn the_threshold_decides_what_is_chunked() {
+    let server = Server::new();
+    let vault = temp_vault("threshold");
+    let at_threshold: Vec<u8> = (0..THRESHOLD).map(|index| u8::try_from(index % 251).expect("byte")).collect();
+    let over_threshold: Vec<u8> = (0..=THRESHOLD).map(|index| u8::try_from(index % 251).expect("byte")).collect();
+    vault.write("at.bin", &at_threshold).expect("write");
+    vault.write("over.bin", &over_threshold).expect("write");
+    let remote = FakeRemote { server: server.clone() };
+    let mut state = SyncState::default();
+
+    let report = sync_once_with(&vault, &remote, &mut state, "phone", "2026-10-20", SyncOptions { limits: limits(), progress: Box::new(|_: &Progress| {}) })
+        .expect("sync");
+
+    assert_eq!(report.uploaded.len(), 2);
+    assert_eq!(server.chunked_calls(), 1, "only the file over the threshold is chunked");
+    assert_eq!(server.chunk_calls().len(), 3, "the file over the threshold has three chunks");
+    assert_eq!(server.file("at.bin"), Some(at_threshold));
+    assert_eq!(server.file("over.bin"), Some(over_threshold));
+}
diff --git a/crates/i18n/locales/en.toml b/crates/i18n/locales/en.toml
index 5f9b73d..e4e43eb 100644
--- a/crates/i18n/locales/en.toml
+++ b/crates/i18n/locales/en.toml
@@ -1328,6 +1328,31 @@ ago-minutes = "{n} min ago"
 ago-hours = "{n} h ago"
 ago-days = "{n} d ago"
 
+[ui.sync-sheet]
+title = "Sync"
+status-up-to-date = "Everything is up to date"
+status-syncing = "Syncing..."
+status-offline = "Offline, will retry"
+status-sign-in = "Sign in again"
+status-error = "Sync stopped"
+last-sync = "Last sync {when} · {host}"
+never-synced = "Not yet synced"
+devices = "Devices"
+this-phone = "This phone"
+this-computer = "This computer"
+last-seen = "last seen {when}"
+never-seen = "last seen never"
+chip-online = "online"
+chip-synced = "synced"
+chip-stale = "stale"
+conflicts = "{n} conflicts to review"
+wifi-only = "Sync on Wi-Fi only"
+attachments = "Sync attachments"
+settings-and-themes = "Sync settings and themes"
+sync-now = "Sync now"
+loading = "Loading devices..."
+pairing-failed = "The pairing failed. Check the server address and the code."
+
 [ui.live]
 pill-live = "Live"
 go-live = "Go live"
diff --git a/crates/sync/src/hash.rs b/crates/sync/src/hash.rs
index 38f390f..4488b87 100644
--- a/crates/sync/src/hash.rs
+++ b/crates/sync/src/hash.rs
@@ -9,12 +9,33 @@ const HEX: &[u8; 16] = b"0123456789abcdef";
 /// SHA-256 of `bytes` as 64 lowercase hex digits.
 #[must_use]
 pub fn hash_hex(bytes: &[u8]) -> String {
-    let mut out = String::with_capacity(64);
-    for byte in &Sha256::digest(bytes) {
-        out.push(char::from(HEX.get(usize::from(byte >> 4)).copied().unwrap_or(b'0')));
-        out.push(char::from(HEX.get(usize::from(byte & 0x0f)).copied().unwrap_or(b'0')));
+    hex_digest(&Sha256::digest(bytes))
+}
+
+/// An incremental SHA-256 hash: the pieces are added one at a time, so content
+/// too large to hold in memory can still be hashed.
+#[derive(Debug, Clone, Default)]
+pub struct Hasher {
+    inner: Sha256,
+}
+
+impl Hasher {
+    /// Adds `bytes` to the hash.
+    pub fn update(&mut self, bytes: &[u8]) {
+        self.inner.update(bytes);
     }
-    out
+
+    /// The hash of everything added so far, as 64 lowercase hex digits.
+    #[must_use]
+    pub fn finish_hex(self) -> String {
+        hex_digest(&self.inner.finalize())
+    }
+}
+
+/// A fresh incremental hasher, for content that never sits in memory whole.
+#[must_use]
+pub fn hasher() -> Hasher {
+    Hasher::default()
 }
 
 /// Checks the shape of a hash received from outside.
@@ -28,3 +49,38 @@ pub fn check_hash(hash: &str) -> Result<()> {
         Err(Error::InvalidHash(hash.chars().take(80).collect()))
     }
 }
+
+/// A digest as 64 lowercase hex digits.
+fn hex_digest(digest: &[u8]) -> String {
+    let mut out = String::with_capacity(digest.len() * 2);
+    for byte in digest {
+        out.push(char::from(HEX.get(usize::from(byte >> 4)).copied().unwrap_or(b'0')));
+        out.push(char::from(HEX.get(usize::from(byte & 0x0f)).copied().unwrap_or(b'0')));
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn the_incremental_hash_is_the_hash_of_the_joined_bytes() {
+        let mut hasher = hasher();
+        hasher.update(b"hello ");
+        hasher.update(b"world");
+        assert_eq!(hasher.finish_hex(), hash_hex(b"hello world"));
+    }
+
+    #[test]
+    fn an_empty_hash_is_the_hash_of_no_bytes() {
+        assert_eq!(hasher().finish_hex(), hash_hex(b""));
+    }
+
+    #[test]
+    fn a_broken_hash_is_refused() {
+        assert!(check_hash(&hash_hex(b"x")).is_ok());
+        assert!(check_hash("not a hash").is_err());
+        assert!(check_hash(&hash_hex(b"x").to_uppercase()).is_err());
+    }
+}
diff --git a/crates/sync/src/lib.rs b/crates/sync/src/lib.rs
index 26ee097..8a3e37b 100644
--- a/crates/sync/src/lib.rs
+++ b/crates/sync/src/lib.rs
@@ -9,7 +9,7 @@
 //!   of each path and a `/changes` page to a list of [`Action`]s.
 //! * [`merge3`] — three-way line merge; anything doubtful is a conflict, never a guess.
 //! * [`conflict_path`] — the name of the copy a conflict leaves behind.
-//! * [`check_path`], [`check_syncable`], [`hash_hex`] — the gate every path and hash passes.
+//! * [`check_path`], [`check_syncable`], [`hash_hex`], [`hasher`] — the gate every path and hash passes.
 //!
 //! ```
 //! use notescraft_sync::{Ledger, Outcome, Write, hash_hex};
@@ -33,7 +33,7 @@ mod record;
 
 pub use conflict::conflict_path;
 pub use error::{Error, Result};
-pub use hash::{check_hash, hash_hex};
+pub use hash::{Hasher, check_hash, hash_hex, hasher};
 pub use ledger::{Ledger, Outcome, Write};
 pub use merge::{Merge, merge3};
 pub use path::{MAX_PATH_BYTES, check_path, check_syncable, is_excluded};
diff --git a/crates/ui-egui/src/panels/mod.rs b/crates/ui-egui/src/panels/mod.rs
index 421f7c2..031ed46 100644
--- a/crates/ui-egui/src/panels/mod.rs
+++ b/crates/ui-egui/src/panels/mod.rs
@@ -13,6 +13,7 @@ pub mod sidebar;
 pub mod sidebar_tabs;
 pub mod source_panel;
 pub mod sync_pill;
+pub mod sync_sheet;
 pub mod tags_panel;
 pub mod tree;
 #[cfg(test)]
diff --git a/crates/ui-egui/src/panels/sync_pill/mod.rs b/crates/ui-egui/src/panels/sync_pill/mod.rs
index b6dbcfe..1ec20d5 100644
--- a/crates/ui-egui/src/panels/sync_pill/mod.rs
+++ b/crates/ui-egui/src/panels/sync_pill/mod.rs
@@ -106,7 +106,13 @@ pub fn show(ui: &mut Ui, tokens: &Tokens, status: &SyncStatus, right: f32, middl
     ui.painter().galley(text, galley, tint);
     let response = ui.interact(rect, ui.id().with("sync-pill"), Sense::click());
     let _ = crate::motion::hover(ui.ctx(), "sync-pill", &response);
-    response.clicked()
+    let clicked = response.clicked();
+    // Phone width: the click asks the shell for the sheet, not for Settings >
+    // Sync; the flag is a same-frame egui temp the shell reads and clears.
+    if clicked && crate::style::compact::is_compact(ui.ctx()) {
+        ui.ctx().data_mut(|data| data.insert_temp(egui::Id::new("sync-sheet-request"), true));
+    }
+    clicked
 }
 
 #[cfg(test)]
diff --git a/crates/ui-egui/src/panels/sync_sheet/devices.rs b/crates/ui-egui/src/panels/sync_sheet/devices.rs
new file mode 100644
index 0000000..03fe993
--- /dev/null
+++ b/crates/ui-egui/src/panels/sync_sheet/devices.rs
@@ -0,0 +1,284 @@
+//! The state half of the sync sheet: the status card, the device rows and the
+//! conflict row — everything the vault's standing on the server decides.
+
+use egui::{Align2, CornerRadius, FontId, Rect, Sense, Ui, Vec2, vec2};
+use notescraft_engine::{ServerDevice, SyncPhase, SyncStatus};
+
+use super::{Facts, SheetAction, SyncSheet};
+use crate::text::sync_sheet as strings;
+use crate::theme::{Tokens, color};
+
+/// Radius of one card, points.
+const CARD_RADIUS: u8 = 12;
+/// Height of one device row, points.
+const DEVICE_ROW: f32 = 72.0;
+/// Height of the conflict row, points.
+const CONFLICT_ROW: f32 = 56.0;
+/// Height of the status card, points.
+const STATUS_ROW: f32 = 96.0;
+/// Side of the status icon, points.
+const STATUS_ICON: f32 = 24.0;
+/// Left pad inside a card, points.
+const CARD_PAD: f32 = 16.0;
+/// Gap between the icon and the text block, points.
+const TEXT_GAP: f32 = 16.0;
+/// Side of the chip's state dot, points.
+const CHIP_DOT: f32 = 8.0;
+/// Space between the chip's dot and its word, points.
+const CHIP_GAP: f32 = 6.0;
+/// Text size of a name, points.
+const NAME: f32 = 15.0;
+/// Text size of muted detail lines, points.
+const DETAIL: f32 = 13.0;
+/// Gap between the headline and the detail of the status card.
+const LINE_GAP: f32 = 16.0;
+/// Icons of the status card, by phase; a phase without one shows its error.
+const UP_TO_DATE_ICON: &str = "circle-check";
+const SYNCING_ICON: &str = "refresh-cw";
+const ATTENTION_ICON: &str = "circle-alert";
+const ERROR_ICON: &str = "triangle-alert";
+/// Milliseconds a device may be out of sight to still count as online.
+const ONLINE_WINDOW_MS: u64 = 2 * 60 * 1000;
+/// Milliseconds in one day.
+const MS_PER_DAY: u64 = 24 * 60 * 60 * 1000;
+
+/// The icon of the status card, from the phase.
+fn status_icon(status: Option<&SyncStatus>) -> &'static str {
+    match status.map(|status| status.phase) {
+        Some(SyncPhase::Idle) => UP_TO_DATE_ICON,
+        Some(SyncPhase::Syncing) => SYNCING_ICON,
+        Some(SyncPhase::Offline | SyncPhase::SignInAgain) => ATTENTION_ICON,
+        Some(SyncPhase::Error) | None => ERROR_ICON,
+    }
+}
+
+/// The tint of the status card's icon.
+fn status_tint(tokens: &Tokens, status: Option<&SyncStatus>) -> egui::Color32 {
+    let colors = &tokens.theme.colors;
+    match status.map(|status| status.phase) {
+        Some(SyncPhase::Idle) => color(colors.success),
+        Some(SyncPhase::Syncing | SyncPhase::SignInAgain) => color(colors.warning),
+        Some(SyncPhase::Offline) => color(colors.text_muted),
+        Some(SyncPhase::Error) | None => color(colors.danger),
+    }
+}
+
+/// The headline of the status card, from the phase; the error speaks for itself.
+#[must_use]
+pub fn headline(status: Option<&SyncStatus>) -> String {
+    match status.map(|status| status.phase) {
+        Some(SyncPhase::Idle) => strings::status_up_to_date().to_owned(),
+        Some(SyncPhase::Syncing) => strings::status_syncing().to_owned(),
+        Some(SyncPhase::Offline) => strings::status_offline().to_owned(),
+        Some(SyncPhase::SignInAgain) => strings::status_sign_in().to_owned(),
+        Some(SyncPhase::Error) | None => status.and_then(|status| status.last_error.clone()).unwrap_or_else(|| strings::status_error().to_owned()),
+    }
+}
+
+/// The second line of the status card: "Last sync 2 min ago · host".
+fn status_detail(status: Option<&SyncStatus>, now_ms: u64) -> String {
+    let host = status.and_then(|status| status.server.as_deref()).map(crate::views::settings_view::sync::server_host).unwrap_or_default();
+    let Some(last) = status.and_then(|status| status.last_sync_ms) else {
+        // Never synced: no "Last sync" prefix, and the host only when there is one.
+        let never = strings::never_synced();
+        return if host.is_empty() { never.to_owned() } else { format!("{never} · {host}") };
+    };
+    let text = strings::last_sync().replace("{when}", &crate::views::settings_view::sync::ago(last, now_ms)).replace("{host}", host);
+    text.trim_end_matches([' ', '·']).to_owned()
+}
+
+/// The status card (design: the check and the two lines).
+pub fn status_card(ui: &mut Ui, tokens: &Tokens, facts: &Facts<'_>) {
+    let colors = &tokens.theme.colors;
+    let (rect, _) = ui.allocate_exact_size(Vec2::new(ui.available_width(), STATUS_ROW), Sense::hover());
+    ui.painter().rect_filled(rect, CornerRadius::same(CARD_RADIUS), color(colors.surface_2));
+    let icon = Rect::from_center_size(rect.left_top() + vec2(CARD_PAD + STATUS_ICON / 2.0, rect.height() / 2.0), Vec2::splat(STATUS_ICON));
+    crate::icons::paint(ui, status_icon(facts.status.as_ref()), icon, status_tint(tokens, facts.status.as_ref()));
+    let text_left = rect.left() + CARD_PAD + STATUS_ICON + TEXT_GAP;
+    let width = rect.right() - text_left - CARD_PAD;
+    ui.painter().text(
+        egui::pos2(text_left, rect.center().y - LINE_GAP),
+        Align2::LEFT_CENTER,
+        headline(facts.status.as_ref()),
+        FontId::new(NAME, crate::fonts::semibold()),
+        color(colors.text),
+    );
+    let font = FontId::proportional(DETAIL);
+    let galley = ui.painter().layout(status_detail(facts.status.as_ref(), facts.now_ms), font, color(colors.text_muted), width);
+    ui.painter().galley(egui::pos2(text_left, rect.center().y + LINE_GAP - galley.size().y / 2.0), galley, color(colors.text_muted));
+}
+
+/// The devices: the heading, the rows (current first) and the line of a load.
+pub fn section(sheet: &mut SyncSheet, ui: &mut Ui, tokens: &Tokens, facts: &Facts<'_>, action: &mut Option<SheetAction>) {
+    let colors = &tokens.theme.colors;
+    heading(ui, tokens, strings::devices());
+    if sheet.loading() {
+        ui.horizontal(|ui| {
+            ui.add(egui::Spinner::new().size(14.0).color(color(colors.accent)));
+            ui.add_space(8.0);
+            ui.label(egui::RichText::new(strings::loading()).size(DETAIL).color(color(colors.text_muted)));
+        });
+    } else if let Some(error) = sheet.error.as_deref() {
+        ui.label(egui::RichText::new(error).size(DETAIL).color(color(colors.danger)));
+    } else {
+        for device in ordered(&sheet.devices) {
+            device_row(ui, tokens, &device, facts);
+        }
+    }
+    conflict_row(ui, tokens, facts, action);
+}
+
+/// The devices with the current one first, the others in the order of the server.
+fn ordered(devices: &[ServerDevice]) -> Vec<ServerDevice> {
+    let mut sorted = devices.to_vec();
+    sorted.sort_by_key(|device| !device.current);
+    sorted
+}
+
+/// The card of one device (design: name, "last seen ago", state chip).
+fn device_row(ui: &mut Ui, tokens: &Tokens, device: &ServerDevice, facts: &Facts<'_>) {
+    let colors = &tokens.theme.colors;
+    let (rect, _) = ui.allocate_exact_size(Vec2::new(ui.available_width(), DEVICE_ROW), Sense::hover());
+    ui.painter().rect_filled(rect, CornerRadius::same(CARD_RADIUS), color(colors.surface_2));
+    let name = match device.current {
+        true if facts.compact => strings::this_phone(),
+        true => strings::this_computer(),
+        false => device.name.as_str(),
+    };
+    ui.painter().text(
+        egui::pos2(rect.left() + CARD_PAD, rect.center().y - 12.0),
+        Align2::LEFT_CENTER,
+        name,
+        FontId::new(NAME, crate::fonts::semibold()),
+        color(colors.text),
+    );
+    let seen = match device.last_seen_ms {
+        Some(seen) => strings::last_seen().replace("{when}", &crate::views::settings_view::sync::ago(seen, facts.now_ms)),
+        None => strings::never_seen().to_owned(),
+    };
+    ui.painter().text(
+        egui::pos2(rect.left() + CARD_PAD, rect.center().y + 12.0),
+        Align2::LEFT_CENTER,
+        seen,
+        FontId::proportional(DETAIL),
+        color(colors.text_muted),
+    );
+    paint_chip(ui, tokens, rect, device, facts.now_ms);
+}
+
+/// The state chip at the right end of a device row.
+fn paint_chip(ui: &Ui, tokens: &Tokens, row: Rect, device: &ServerDevice, now_ms: u64) {
+    let (label, tint) = chip_state(device.last_seen_ms, now_ms, tokens);
+    let galley = ui.painter().layout_no_wrap(label.to_owned(), FontId::new(DETAIL, crate::fonts::semibold()), tint);
+    let chip_width = CHIP_DOT + CHIP_GAP + galley.size().x + CARD_PAD;
+    let left = row.right() - chip_width;
+    let centre_y = row.center().y;
+    ui.painter().circle_filled(egui::pos2(left + CHIP_DOT / 2.0, centre_y), CHIP_DOT / 2.0, tint);
+    ui.painter().galley(egui::pos2(left + CHIP_DOT + CHIP_GAP, centre_y - galley.size().y / 2.0), galley, tint);
+}
+
+/// The word and colour of the state chip: online, synced, stale.
+fn chip_state(last_seen_ms: Option<u64>, now_ms: u64, tokens: &Tokens) -> (&'static str, egui::Color32) {
+    let colors = &tokens.theme.colors;
+    let seen_ago = last_seen_ms.map(|seen| now_ms.saturating_sub(seen));
+    let label = match seen_ago {
+        Some(ago) if ago <= ONLINE_WINDOW_MS => strings::chip_online(),
+        Some(ago) if ago <= MS_PER_DAY => strings::chip_synced(),
+        _ => strings::chip_stale(),
+    };
+    (
+        label,
+        match seen_ago {
+            Some(ago) if ago <= MS_PER_DAY => color(colors.success),
+            Some(_) => color(colors.warning),
+            None => color(colors.text_muted),
+        },
+    )
+}
+
+/// The row "N conflicts to review"; hidden unless there is something to review.
+fn conflict_row(ui: &mut Ui, tokens: &Tokens, facts: &Facts<'_>, action: &mut Option<SheetAction>) {
+    if facts.conflicts == 0 {
+        return;
+    }
+    let colors = &tokens.theme.colors;
+    let label = strings::conflicts().replace("{n}", &facts.conflicts.to_string());
+    let (rect, hit) = ui.allocate_exact_size(Vec2::new(ui.available_width(), CONFLICT_ROW), Sense::click());
+    hit.widget_info(|| egui::WidgetInfo::selected(egui::WidgetType::Button, true, false, "conflicts"));
+    ui.painter().rect_filled(rect, CornerRadius::same(CARD_RADIUS), color(colors.surface_2));
+    ui.painter().text(
+        egui::pos2(rect.left() + CARD_PAD, rect.center().y),
+        Align2::LEFT_CENTER,
+        label,
+        FontId::new(NAME, crate::fonts::semibold()),
+        color(colors.text),
+    );
+    crate::icons::paint(
+        ui,
+        "chevron-right",
+        Rect::from_center_size(egui::pos2(rect.right() - CARD_PAD - 8.0, rect.center().y), vec2(16.0, 16.0)),
+        color(colors.text_muted),
+    );
+    if hit.clicked() {
+        *action = Some(SheetAction::ReviewConflicts);
+    }
+}
+
+/// The small caps heading over a group of cards.
+fn heading(ui: &mut Ui, tokens: &Tokens, text: &str) {
+    let colors = &tokens.theme.colors;
+    ui.add_space(4.0);
+    ui.label(egui::RichText::new(text).font(FontId::new(12.0, crate::fonts::semibold())).color(color(colors.text_muted)));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn the_headlines_follow_the_phases() {
+        let status = SyncStatus { phase: SyncPhase::Idle, ..SyncStatus::default() };
+        assert_eq!(headline(Some(&status)), "Everything is up to date");
+        let syncing = SyncStatus { phase: SyncPhase::Syncing, ..SyncStatus::default() };
+        assert_eq!(headline(Some(&syncing)), "Syncing...");
+        assert_eq!(headline(None), "Sync stopped", "a sheet without a status speaks its own state");
+    }
+
+    #[test]
+    fn the_error_speaks_as_the_headline() {
+        let status = SyncStatus { phase: SyncPhase::Error, last_error: Some("quota is full".to_owned()), ..SyncStatus::default() };
+        assert_eq!(headline(Some(&status)), "quota is full");
+    }
+
+    #[test]
+    fn the_detail_carries_the_last_sync_and_the_host() {
+        let status = SyncStatus { server: Some("https://sync.example.com".to_owned()), last_sync_ms: Some(998_000), ..SyncStatus::default() };
+        assert_eq!(status_detail(Some(&status), 1_000_000), "Last sync 2 s ago · sync.example.com");
+        assert_eq!(status_detail(None, 1_000_000), strings::never_synced());
+    }
+
+    #[test]
+    fn the_chips_age() {
+        let tokens = Tokens::new(notescraft_themes::builtin("catppuccin-mocha").expect("theme"));
+        let now = 1_000_000_000_u64;
+        let (online, _) = chip_state(Some(now - 1000), now, &tokens);
+        assert_eq!(online, strings::chip_online());
+        let (synced, _) = chip_state(Some(now - 3_600_000), now, &tokens);
+        assert_eq!(synced, strings::chip_synced());
+        let (stale, _) = chip_state(Some(now - MS_PER_DAY - 1), now, &tokens);
+        assert_eq!(stale, strings::chip_stale());
+        let (never, _) = chip_state(None, now, &tokens);
+        assert_eq!(never, strings::chip_stale());
+    }
+
+    #[test]
+    fn the_current_device_comes_first() {
+        let devices = [
+            ServerDevice { id: "d-2".to_owned(), name: "Laptop".to_owned(), ..ServerDevice::default() },
+            ServerDevice { id: "d-1".to_owned(), name: "Phone".to_owned(), current: true, ..ServerDevice::default() },
+        ];
+        let listed = ordered(&devices);
+        assert_eq!(listed.first().map(|device| device.id.as_str()), Some("d-1"), "the current device first");
+        assert_eq!(listed.get(1).map(|device| device.id.as_str()), Some("d-2"));
+    }
+}
diff --git a/crates/ui-egui/src/panels/sync_sheet/mod.rs b/crates/ui-egui/src/panels/sync_sheet/mod.rs
new file mode 100644
index 0000000..f9cca3d
--- /dev/null
+++ b/crates/ui-egui/src/panels/sync_sheet/mod.rs
@@ -0,0 +1,224 @@
+//! The sync sheet (design `03-sync-sheet`): everything about sync of the
+//! bound vault in one bottom sheet of the compact layout.
+//!
+//! The compact-layout sync pill opens it; on a wider window the pill keeps
+//! opening Settings > Sync (see [`crate::panels::sync_pill`]). The sheet holds
+//! the state (devices, the running load job) and gets the live facts — the
+//! status, the conflicts and the user's options — handed in every frame.
+//!
+//! No ask waits on the UI thread: the devices load on a worker thread that
+//! holds the one [`ServerClient`] (the token never crosses back) and answers
+//! through a channel the sheet polls, exactly like the Share dialog.
+
+mod devices;
+mod options;
+
+use std::sync::mpsc::{Receiver, TryRecvError};
+use std::sync::{Arc, Mutex};
+
+use egui::{Context, ScrollArea};
+use notescraft_engine::{ServerClient, ServerDevice, SyncSettings, SyncStatus, load_binding, load_token};
+
+use crate::text::sync_sheet as strings;
+use crate::theme::Tokens;
+
+/// The id of the sheet, also the salt of its widgets.
+const SHEET_ID: &str = "notescraft-sync-sheet";
+/// Most of the screen the open sheet may take, as a fraction, so the content
+/// scrolls instead of reaching the top edge of the window; the sheet of the
+/// design (status, three devices, the conflict row, three options and the
+/// button) is about 700 pt tall on a 390 x 844 phone and shows whole.
+const MAX_SHEET_HEIGHT: f32 = 0.9;
+/// How long a sheet without a job waits before it repaints for the spinner.
+const LOAD_MILLIS: u64 = 40;
+
+/// What the shell hands to the sheet every frame.
+pub struct Facts<'a> {
+    /// The status of the sync worker, `None` when the vault has no worker.
+    pub status: Option<SyncStatus>,
+    /// The conflict copies waiting for a decision.
+    pub conflicts: usize,
+    /// The user's three sync options.
+    pub options: &'a SyncSettings,
+    /// Now, in Unix milliseconds, for the ages.
+    pub now_ms: u64,
+    /// Whether the window is in the compact (phone) layout.
+    pub compact: bool,
+}
+
+/// What the sheet asks the app to do.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum SheetAction {
+    /// Close the sheet (the ×, a tap on the dimmed area or Escape).
+    Close,
+    /// Run `sync.now`.
+    SyncNow,
+    /// Open Settings > Sync's conflict list.
+    ReviewConflicts,
+    /// One of the three options flipped; the shell stores the new value.
+    Toggle(SheetOption, bool),
+}
+
+/// Which of the three options a [`SheetAction::Toggle`] names.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum SheetOption {
+    /// `sync.wifi_only`, stored but not yet honoured by the worker.
+    WifiOnly,
+    /// `sync.attachments`: `false` leaves the non-Markdown files out of the scan.
+    Attachments,
+    /// `sync.settings_and_themes`, stored but not yet honoured by the worker.
+    SettingsAndThemes,
+}
+
+impl SheetOption {
+    /// The settings key of the option.
+    #[must_use]
+    pub fn key(self) -> &'static str {
+        match self {
+            SheetOption::WifiOnly => "sync.wifi_only",
+            SheetOption::Attachments => "sync.attachments",
+            SheetOption::SettingsAndThemes => "sync.settings_and_themes",
+        }
+    }
+}
+
+/// The state of the open sheet between frames.
+pub struct SyncSheet {
+    /// The client of the account, `None` when the vault is not bound.
+    client: Option<Arc<Mutex<ServerClient>>>,
+    /// The vault id on the server.
+    vault_id: String,
+    /// The devices of the last load, current first.
+    devices: Vec<ServerDevice>,
+    /// What the last failed load said.
+    error: Option<String>,
+    /// The running load job, when there is one.
+    job: Option<Arc<Mutex<Receiver<DevicesOutcome>>>>,
+}
+
+/// The answer of a load job.
+enum DevicesOutcome {
+    /// The devices the server named.
+    Listed(Vec<ServerDevice>),
+    /// What the server or the connection said.
+    Failed(String),
+}
+
+impl SyncSheet {
+    /// Builds the sheet over the vault of `session` and asks for its devices.
+    #[must_use]
+    pub fn open(session: &notescraft_engine::Session) -> Self {
+        let mut sheet = Self { client: None, vault_id: String::new(), devices: Vec::new(), error: None, job: None };
+        let binding = load_binding(session.vault()).ok().flatten();
+        let token = load_token(session.vault()).ok().flatten();
+        if let Some(binding) = binding
+            && let Some(token) = token
+        {
+            sheet.client = Some(Arc::new(Mutex::new(ServerClient::new(&binding.server, &token))));
+            sheet.vault_id.clone_from(&binding.vault_id);
+            sheet.start_load();
+        }
+        sheet
+    }
+
+    /// Draws the sheet; returns what it asked the app to do.
+    pub fn show(&mut self, ctx: &Context, tokens: &Tokens, facts: &Facts<'_>) -> Option<SheetAction> {
+        self.poll();
+        let max_height = ctx.content_rect().height() * MAX_SHEET_HEIGHT;
+        let mut action = None;
+        let closed = crate::widgets::bottom_sheet::show(
+            ctx,
+            tokens,
+            &crate::widgets::bottom_sheet::Sheet {
+                id: egui::Id::new(SHEET_ID),
+                icon: crate::icons::image("refresh-cw", 16.0, egui::Color32::TRANSPARENT),
+                title: strings::title(),
+                chip: None,
+            },
+            |ui| {
+                ScrollArea::vertical()
+                    .id_salt("notescraft-sync-sheet-scroll")
+                    .auto_shrink([false, true])
+                    .min_scrolled_height(max_height)
+                    .max_height(max_height)
+                    .show(ui, |ui| {
+                        devices::status_card(ui, tokens, facts);
+                        let mut sheet_action = None;
+                        devices::section(self, ui, tokens, facts, &mut sheet_action);
+                        options::section(ui, tokens, facts, &mut sheet_action);
+                        if sheet_action.is_some() {
+                            action = sheet_action;
+                        }
+                    });
+            },
+        );
+        if self.job.is_some() {
+            ctx.request_repaint_after(std::time::Duration::from_millis(LOAD_MILLIS));
+        }
+        if closed {
+            return Some(action.unwrap_or(SheetAction::Close));
+        }
+        action
+    }
+
+    /// Whether a load job is running.
+    #[must_use]
+    pub fn loading(&self) -> bool {
+        self.job.is_some()
+    }
+
+    /// Starts a load of the devices on a fresh worker thread; the client moves
+    /// in, so the token never reaches the screen.
+    fn start_load(&mut self) {
+        if self.job.is_some() {
+            return;
+        }
+        let Some(client) = self.client.clone() else { return };
+        let vault_id = self.vault_id.clone();
+        let (sender, receiver) = std::sync::mpsc::channel();
+        match std::thread::Builder::new().name("notescraft-sync-sheet".to_owned()).spawn(move || {
+            let outcome = match client.lock().unwrap_or_else(std::sync::PoisonError::into_inner).devices(&vault_id) {
+                Ok(devices) => DevicesOutcome::Listed(devices),
+                Err(error) => DevicesOutcome::Failed(error.to_string()),
+            };
+            let _ = sender.send(outcome);
+        }) {
+            Ok(_) => {
+                self.job = Some(Arc::new(Mutex::new(receiver)));
+                self.error = None;
+            }
+            Err(error) => self.error = Some(error.to_string()),
+        }
+    }
+
+    /// Takes the answer of the load job, when it is there.
+    fn poll(&mut self) {
+        let Some(job) = self.job.clone() else { return };
+        let outcome = {
+            let guard = job.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
+            match guard.try_recv() {
+                Ok(outcome) => Some(outcome),
+                Err(TryRecvError::Empty) => None,
+                Err(TryRecvError::Disconnected) => Some(DevicesOutcome::Failed(strings::pairing_failed().to_owned())),
+            }
+        };
+        match outcome {
+            None => {}
+            Some(DevicesOutcome::Listed(devices)) => {
+                self.job = None;
+                self.devices = devices;
+                self.error = None;
+            }
+            Some(DevicesOutcome::Failed(error)) => {
+                self.job = None;
+                self.error = Some(error);
+            }
+        }
+    }
+}
+
+/// The current Unix time in milliseconds; the one clock of the sheet.
+#[must_use]
+pub fn now_millis() -> u64 {
+    std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH).map_or(0, |since| u64::try_from(since.as_millis()).unwrap_or(u64::MAX))
+}
diff --git a/crates/ui-egui/src/panels/sync_sheet/options.rs b/crates/ui-egui/src/panels/sync_sheet/options.rs
new file mode 100644
index 0000000..3594fb4
--- /dev/null
+++ b/crates/ui-egui/src/panels/sync_sheet/options.rs
@@ -0,0 +1,96 @@
+//! The options half of the sync sheet: the three toggles and Sync now.
+//!
+//! The toggles report a flip; the shell stores it through `settings.set` and
+//! the next frame's facts show it. The Sync now button is the primary action
+//! of the sheet; it asks for `sync.now` and stays open.
+
+use egui::{Align2, CornerRadius, FontId, Rect, Sense, Ui, Vec2, vec2};
+
+use super::{Facts, SHEET_ID, SheetAction, SheetOption};
+use crate::text::sync_sheet as strings;
+use crate::theme::{Tokens, color};
+
+/// Height of one toggle row, points.
+const TOGGLE_ROW: f32 = 48.0;
+/// Height of the Sync now button, points.
+const BUTTON_ROW: f32 = 52.0;
+/// Left pad inside a row, points.
+const ROW_PAD: f32 = 16.0;
+/// Radius of the Sync now card, points.
+const BUTTON_RADIUS: u8 = 12;
+/// Track of the switch: how wide and how high, points.
+const TRACK: Vec2 = vec2(40.0, 22.0);
+/// Radius of the switch's knob, points.
+const KNOB: f32 = 9.0;
+/// Space between a row and the button, points.
+const BUTTON_GAP: f32 = 8.0;
+
+/// The three rows and the primary button.
+pub fn section(ui: &mut Ui, tokens: &Tokens, facts: &Facts<'_>, action: &mut Option<SheetAction>) {
+    toggle(ui, tokens, SheetOption::WifiOnly, facts.options.wifi_only, action);
+    toggle(ui, tokens, SheetOption::Attachments, facts.options.attachments, action);
+    toggle(ui, tokens, SheetOption::SettingsAndThemes, facts.options.settings_and_themes, action);
+    ui.add_space(BUTTON_GAP);
+    button(ui, tokens, action);
+}
+
+/// The card of one option; `true` when this frame pressed it.
+fn toggle(ui: &mut Ui, tokens: &Tokens, option: SheetOption, on: bool, action: &mut Option<SheetAction>) {
+    let colors = &tokens.theme.colors;
+    let label = match option {
+        SheetOption::WifiOnly => strings::wifi_only(),
+        SheetOption::Attachments => strings::attachments(),
+        SheetOption::SettingsAndThemes => strings::settings_and_themes(),
+    };
+    let (rect, hit) = ui.allocate_exact_size(Vec2::new(ui.available_width(), TOGGLE_ROW), Sense::click());
+    hit.widget_info(|| egui::WidgetInfo::selected(egui::WidgetType::Checkbox, true, on, label));
+    let glow = crate::motion::hover(ui.ctx(), ui.id().with((SHEET_ID, "toggle", label)), &hit);
+    ui.painter().rect_filled(rect, CornerRadius::same(u8::MAX / 2), color(colors.selection).gamma_multiply(glow * 0.6));
+    ui.painter().text(egui::pos2(rect.left() + ROW_PAD, rect.center().y), Align2::LEFT_CENTER, label, FontId::proportional(15.0), color(colors.text));
+    paint_switch(ui, tokens, rect, on);
+    if hit.clicked() {
+        *action = Some(SheetAction::Toggle(option, !on));
+    }
+}
+
+/// The switch at the right end of a row (design: mauve track, dark knob).
+fn paint_switch(ui: &Ui, tokens: &Tokens, row: Rect, on: bool) {
+    let colors = &tokens.theme.colors;
+    let track = Rect::from_center_size(egui::pos2(row.right() - ROW_PAD - TRACK.x / 2.0, row.center().y), TRACK);
+    ui.painter().rect_filled(track, CornerRadius::same(u8::MAX / 2), if on { color(colors.accent) } else { color(colors.surface_2) });
+    let knob_x = if on { track.right() - KNOB } else { track.left() + KNOB };
+    ui.painter().circle_filled(egui::pos2(knob_x, track.center().y), KNOB, color(colors.text_muted));
+}
+
+/// The Sync now button, full width, the accent behind the primary word.
+fn button(ui: &mut Ui, tokens: &Tokens, action: &mut Option<SheetAction>) {
+    let colors = &tokens.theme.colors;
+    let (rect, hit) = ui.allocate_exact_size(Vec2::new(ui.available_width(), BUTTON_ROW), Sense::click());
+    hit.widget_info(|| egui::WidgetInfo::labeled(egui::WidgetType::Button, true, strings::sync_now()));
+    let glow = crate::motion::hover(ui.ctx(), ui.id().with((SHEET_ID, "sync-now")), &hit);
+    let base = crate::motion::mix(color(colors.accent), color(colors.text), glow * 0.12);
+    ui.painter().rect_filled(rect, CornerRadius::same(BUTTON_RADIUS), base);
+    ui.painter().text(rect.center(), Align2::CENTER_CENTER, strings::sync_now(), FontId::new(16.0, crate::fonts::semibold()), color(colors.text));
+    if hit.clicked() {
+        *action = Some(SheetAction::SyncNow);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use notescraft_engine::SyncSettings;
+
+    #[test]
+    fn the_toggles_target_the_settings_keys() {
+        assert_eq!(SheetOption::WifiOnly.key(), "sync.wifi_only");
+        assert_eq!(SheetOption::Attachments.key(), "sync.attachments");
+        assert_eq!(SheetOption::SettingsAndThemes.key(), "sync.settings_and_themes");
+    }
+
+    #[test]
+    fn the_sync_settings_carry_their_defaults() {
+        assert_eq!(SyncSettings::default(), SyncSettings { wifi_only: true, attachments: true, settings_and_themes: false });
+    }
+}
diff --git a/crates/ui-egui/src/shell/app.rs b/crates/ui-egui/src/shell/app.rs
index 52e1cbe..06f7882 100644
--- a/crates/ui-egui/src/shell/app.rs
+++ b/crates/ui-egui/src/shell/app.rs
@@ -106,6 +106,8 @@ pub struct NotesApp {
     pub quit_prompt: Option<SavePrompt>,
     /// The Share dialog and its worker result, while it is open.
     pub share_dialog: Option<crate::views::share_dialog::ShareDialog>,
+    /// The sync bottom sheet of the compact layout, while it is open.
+    pub sync_sheet: Option<crate::panels::sync_sheet::SyncSheet>,
     /// PDF export dialog and its worker result receiver.
     pub export_dialog: Option<crate::export_dialog::ExportDialog>,
     pub(crate) export_result: Option<Receiver<ExportResult>>,
@@ -263,6 +265,7 @@ impl NotesApp {
             quit_prompt: None,
             export_dialog: None,
             share_dialog: None,
+            sync_sheet: None,
             export_result: None,
             export_open_path: None,
             right_tab: RightTab::default(),
diff --git a/crates/ui-egui/src/shell/app/sync.rs b/crates/ui-egui/src/shell/app/sync.rs
index c8a06a7..454b809 100644
--- a/crates/ui-egui/src/shell/app/sync.rs
+++ b/crates/ui-egui/src/shell/app/sync.rs
@@ -24,7 +24,7 @@ impl NotesApp {
         if self.sync_next.is_none_or(|next| now >= next) {
             self.sync_next = Some(now + STATUS_PERIOD);
             self.ensure_sync_worker();
-            if self.tabs.active_path() == Some(crate::tabs::SETTINGS_PATH) {
+            if self.tabs.active_path() == Some(crate::tabs::SETTINGS_PATH) || self.sync_sheet.is_some() {
                 self.refresh_sync_status();
                 if self.sync_conflicts_next.is_none_or(|next| now >= next) {
                     self.sync_conflicts_next = Some(now + CONFLICTS_PERIOD);
diff --git a/crates/ui-egui/src/shell/app/window/frame.rs b/crates/ui-egui/src/shell/app/window/frame.rs
index 0756e6b..2bd5838 100644
--- a/crates/ui-egui/src/shell/app/window/frame.rs
+++ b/crates/ui-egui/src/shell/app/window/frame.rs
@@ -7,6 +7,7 @@
 use egui::Key;
 use serde_json::{Map, Value};
 
+use crate::panels::sync_sheet::Facts;
 use notescraft_engine::CommandInfo;
 
 use crate::app::NotesApp;
@@ -236,9 +237,17 @@ impl NotesApp {
         let edited = response.edited;
         let now = ctx.input(|i| i.time);
         self.handle_tree(tree_response, ctx);
+        // The sync pill asks for the sheet at phone width and for Settings > Sync
+        // otherwise; the request is a same-frame egui temp the pill wrote on click.
+        let sheet_requested = Self::take_sync_sheet_request(ctx);
         if let Some(category) = response.settings_navigated {
             self.cue(crate::sounds::Cue::Tick);
-            self.open_sync_page_if(category);
+            if !(sheet_requested && category == crate::views::settings_view::SettingsCategory::Sync) {
+                self.open_sync_page_if(category);
+            }
+        }
+        if sheet_requested {
+            self.sync_sheet = Some(crate::panels::sync_sheet::SyncSheet::open(&self.session));
         }
         self.apply_settings_changes(response.settings_changes.clone(), ctx);
         self.apply_settings_actions(std::mem::take(&mut response.settings_actions));
@@ -359,6 +368,37 @@ impl NotesApp {
                 self.open_sync_page_if(crate::views::settings_view::SettingsCategory::Sync);
             }
         }
+        // The sync sheet over everything; the shell stores a toggle and answers the rows.
+        let sheet_action = if self.sync_sheet.is_some() {
+            let facts = Facts {
+                status: self.session.sync_worker().map(notescraft_engine::SyncWorker::status),
+                conflicts: self.preferences.view.sync.data.conflicts.len(),
+                options: &self.preferences.settings.sync,
+                now_ms: crate::panels::sync_sheet::now_millis(),
+                compact: crate::style::compact::is_compact(ctx),
+            };
+            let action = self.sync_sheet.as_mut().and_then(|sheet| sheet.show(ctx, &self.tokens, &facts));
+            let click = ctx.input(|input| input.pointer.interact_pos());
+            let rect = ctx.read_response(egui::Id::new("notescraft-sync-sheet")).map(|response| response.rect);
+            println!("DIAG sheet action {action:?} pointer {click:?} rect {rect:?}");
+            action
+        } else {
+            None
+        };
+        if let Some(action) = sheet_action {
+            match action {
+                crate::panels::sync_sheet::SheetAction::Close => self.sync_sheet = None,
+                crate::panels::sync_sheet::SheetAction::SyncNow => self.run_command("sync.now", &Value::Null),
+                crate::panels::sync_sheet::SheetAction::ReviewConflicts => {
+                    self.sync_sheet = None;
+                    self.open_sync_page_if(crate::views::settings_view::SettingsCategory::Sync);
+                }
+                crate::panels::sync_sheet::SheetAction::Toggle(option, on) => {
+                    let params = serde_json::json!({ "key": option.key(), "value": on });
+                    self.run_command("settings.set", &params);
+                }
+            }
+        }
         crate::dialog::paint_closing(ctx, &self.tokens);
         if self.export_open_path.is_some() {
             egui::Area::new(egui::Id::new("export-open-action")).anchor(egui::Align2::RIGHT_BOTTOM, egui::vec2(-24.0, -24.0)).show(ctx, |ui| {
@@ -373,6 +413,17 @@ impl NotesApp {
         self.flush_cues(now);
     }
 
+    /// Whether the compact sync pill asked for the sheet this frame; the request
+    /// is read and cleared, so one click opens one sheet.
+    fn take_sync_sheet_request(ctx: &egui::Context) -> bool {
+        let id = egui::Id::new("sync-sheet-request");
+        ctx.data_mut(|data| {
+            let asked = data.get_temp::<bool>(id).unwrap_or(false);
+            data.remove::<bool>(id);
+            asked
+        })
+    }
+
     /// Runs a chosen engine/UI command; commands with required parameters toast instead.
     /// `ui.save` is intercepted: saving needs the session and the tab state.
     pub(in crate::shell::app) fn run_palette_choice(&mut self, id: &str, ctx: &egui::Context) {
diff --git a/crates/ui-egui/src/text.rs b/crates/ui-egui/src/text.rs
index 2966175..38ee392 100644
--- a/crates/ui-egui/src/text.rs
+++ b/crates/ui-egui/src/text.rs
@@ -32,7 +32,7 @@ fn lookup(key: &'static str) -> &'static str {
 
 pub mod app;
 mod features;
-pub use features::{compact, launcher, live};
+pub use features::{compact, launcher, live, sync_sheet};
 mod panels;
 mod settings;
 mod views;
diff --git a/crates/ui-egui/src/text/features/mod.rs b/crates/ui-egui/src/text/features/mod.rs
index 0397262..e74c08f 100644
--- a/crates/ui-egui/src/text/features/mod.rs
+++ b/crates/ui-egui/src/text/features/mod.rs
@@ -1,5 +1,6 @@
-//! Copy for the newer feature screens: compact layout, vault launcher, live editing.
+//! Copy for the newer feature screens: compact layout, vault launcher, live editing and the sync sheet.
 
 pub mod compact;
 pub mod launcher;
 pub mod live;
+pub mod sync_sheet;
diff --git a/crates/ui-egui/src/text/features/sync_sheet.rs b/crates/ui-egui/src/text/features/sync_sheet.rs
new file mode 100644
index 0000000..6305b30
--- /dev/null
+++ b/crates/ui-egui/src/text/features/sync_sheet.rs
@@ -0,0 +1,28 @@
+//! Copy for the sync sheet (design `03-sync-sheet`): status, devices, the
+//! conflict row, the three options and Sync now.
+
+table! {
+    title = "ui.sync-sheet.title";
+    status_up_to_date = "ui.sync-sheet.status-up-to-date";
+    status_syncing = "ui.sync-sheet.status-syncing";
+    status_offline = "ui.sync-sheet.status-offline";
+    status_sign_in = "ui.sync-sheet.status-sign-in";
+    status_error = "ui.sync-sheet.status-error";
+    last_sync = "ui.sync-sheet.last-sync";
+    never_synced = "ui.sync-sheet.never-synced";
+    devices = "ui.sync-sheet.devices";
+    this_phone = "ui.sync-sheet.this-phone";
+    this_computer = "ui.sync-sheet.this-computer";
+    last_seen = "ui.sync-sheet.last-seen";
+    never_seen = "ui.sync-sheet.never-seen";
+    chip_online = "ui.sync-sheet.chip-online";
+    chip_synced = "ui.sync-sheet.chip-synced";
+    chip_stale = "ui.sync-sheet.chip-stale";
+    conflicts = "ui.sync-sheet.conflicts";
+    wifi_only = "ui.sync-sheet.wifi-only";
+    attachments = "ui.sync-sheet.attachments";
+    settings_and_themes = "ui.sync-sheet.settings-and-themes";
+    sync_now = "ui.sync-sheet.sync-now";
+    loading = "ui.sync-sheet.loading";
+    pairing_failed = "ui.sync-sheet.pairing-failed";
+}
diff --git a/crates/ui-egui/tests/sync_sheet.rs b/crates/ui-egui/tests/sync_sheet.rs
new file mode 100644
index 0000000..fa37c9a
--- /dev/null
+++ b/crates/ui-egui/tests/sync_sheet.rs
@@ -0,0 +1,430 @@
+//! The sync sheet driven the way a user does: real frames at 390 x 844 over a
+//! vault bound to a stub server (design `03-sync-sheet`).
+
+use egui::{Context, Event, Key, PointerButton, Pos2, RawInput};
+use notescraft_engine::{Session, Timings, bind_folder};
+use notescraft_ui_egui::app::NotesApp;
+use std::io::{BufRead, BufReader, Read, Write};
+use std::net::{TcpListener, TcpStream};
+use std::path::Path;
+use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
+use std::sync::{Arc, Mutex};
+use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
+
+static NEXT: AtomicU64 = AtomicU64::new(0);
+/// The window of a phone.
+const PHONE: egui::Vec2 = egui::vec2(390.0, 844.0);
+/// The window of a desktop.
+const DESKTOP: egui::Vec2 = egui::vec2(1280.0, 800.0);
+/// The token of the bound vault; it must never reach the screen.
+const TOKEN: &str = "secret-token";
+/// Longest the tests wait for a stub request or an idle worker.
+const WAIT: Duration = Duration::from_secs(5);
+
+/// One request the stub received.
+#[derive(Debug, Clone)]
+struct Seen {
+    /// The HTTP method.
+    method: String,
+    /// The path of the request line, without the query.
+    path: String,
+}
+
+/// A stub server: canned answers per path and method, and every request it saw.
+struct Stub {
+    base: String,
+    seen: Arc<Mutex<Vec<Seen>>>,
+    stop: Arc<AtomicBool>,
+}
+
+impl Stub {
+    /// Starts a stub on an ephemeral port. `devices` is the body of the device
+    /// list; `/changes` answers an empty log and `/files/a.md` answers applied,
+    /// so the worker's cycle lands on "Synced".
+    fn spawn(devices: &str) -> Self {
+        let listener = TcpListener::bind("127.0.0.1:0").expect("bind the stub");
+        let port = listener.local_addr().expect("addr").port();
+        let record = r#"{"path":"a.md","hash":"x","size":2,"mtime_ms":0,"seq":1,"deleted":false}"#;
+        let empty = r#"{"head":0,"records":[],"more":false}"#;
+        let answer = |status: &str, body: &str| format!("HTTP/1.1 {status}\r\nContent-Type: application/json\r\nContent-Length: {}\r\n\r\n{body}", body.len());
+        let answers: Vec<(String, String, String)> = vec![
+            ("GET".to_owned(), "/changes".to_owned(), answer("200 OK", empty)),
+            ("PUT".to_owned(), "/files/a.md".to_owned(), answer("201 Created", record)),
+            ("GET".to_owned(), "/devices".to_owned(), answer("200 OK", devices)),
+        ];
+        let seen: Arc<Mutex<Vec<Seen>>> = Arc::new(Mutex::new(Vec::new()));
+        let stop = Arc::new(AtomicBool::new(false));
+        let (thread_seen, thread_stop) = (Arc::clone(&seen), Arc::clone(&stop));
+        std::thread::spawn(move || {
+            while !thread_stop.load(Ordering::Relaxed) {
+                match listener.accept() {
+                    Ok((stream, _)) => {
+                        // One thread per connection: the client may keep the connection
+                        // open, and the listener must go on accepting.
+                        let Ok(mut writer) = stream.try_clone() else { continue };
+                        let mut reader = BufReader::new(stream);
+                        let answers = Arc::new(answers.clone());
+                        let seen = Arc::clone(&thread_seen);
+                        std::thread::spawn(move || {
+                            while let Some(request) = read_request(&mut reader) {
+                                seen.lock().expect("seen").push(request.clone());
+                                let hit = answers.iter().find(|(method, path, _)| *method == request.method && request.path.contains(path.as_str()));
+                                let canned = hit.map_or("HTTP/1.1 404 Not Found\r\nContent-Length: 0\r\n\r\n", |(_, _, canned)| canned.as_str());
+                                let _ = writer.write_all(canned.as_bytes());
+                                let _ = writer.flush();
+                            }
+                        });
+                    }
+                    Err(_) => std::thread::sleep(Duration::from_millis(2)),
+                }
+            }
+        });
+        Self { base: format!("http://127.0.0.1:{port}"), seen, stop }
+    }
+
+    /// How many times the change log was polled.
+    fn change_polls(&self) -> u32 {
+        self.seen.lock().expect("seen").iter().filter(|seen| seen.method == "GET" && seen.path.ends_with("/changes")).count().try_into().unwrap_or(u32::MAX)
+    }
+
+    /// The requests the stub saw.
+    fn requests(&self) -> Vec<Seen> {
+        self.seen.lock().expect("seen").clone()
+    }
+
+    /// Whether the stub saw a write to `path`.
+    fn uploaded(&self, path: &str) -> bool {
+        self.requests().iter().any(|seen| seen.method == "PUT" && seen.path.ends_with(path))
+    }
+
+    /// Waits, bounded, until `wanted` holds.
+    fn wait_for(&self, wanted: impl Fn(&[Seen]) -> bool) {
+        let deadline = Instant::now() + WAIT;
+        while Instant::now() < deadline && !wanted(&self.requests()) {
+            std::thread::sleep(Duration::from_millis(5));
+        }
+    }
+}
+
+impl Drop for Stub {
+    fn drop(&mut self) {
+        self.stop.store(true, Ordering::Relaxed);
+    }
+}
+
+/// Reads one request: method, path, and its body skipped; the request ends when
+/// the client closes the connection.
+fn read_request(reader: &mut BufReader<TcpStream>) -> Option<Seen> {
+    let mut line = String::new();
+    reader.read_line(&mut line).ok()?;
+    let mut parts = line.split_whitespace();
+    let method = parts.next()?.to_owned();
+    let path = parts.next()?.to_owned();
+    let mut length = 0_usize;
+    loop {
+        let mut header = String::new();
+        if reader.read_line(&mut header).ok()? == 0 {
+            break;
+        }
+        if header.trim_end().is_empty() {
+            break;
+        }
+        if let Some((name, value)) = header.trim_end().split_once(':')
+            && name.trim().eq_ignore_ascii_case("content-length")
+        {
+            length = value.trim().parse().unwrap_or(0);
+        }
+    }
+    let mut body = vec![0_u8; length];
+    if length > 0 {
+        reader.read_exact(&mut body).ok()?;
+    }
+    Some(Seen { method, path: path.split('?').next().unwrap_or_default().to_owned() })
+}
+
+struct Driver {
+    app: NotesApp,
+    ctx: Context,
+    time: f64,
+    size: egui::Vec2,
+}
+
+/// A painted text: what it says, where it starts and its galley.
+#[derive(Debug)]
+struct Painted {
+    text: String,
+    pos: Pos2,
+    galley: Arc<egui::Galley>,
+}
+
+/// An app over a fresh vault bound to `stub`, with a note open; the settings
+/// dir lives beside the vault, never inside it (the vault is the sync scope).
+fn driven(stub: &Stub, files: &[(&str, &str)], size: egui::Vec2) -> Driver {
+    let root = std::env::temp_dir().join(format!("notescraft-sheet-{}-{}", std::process::id(), NEXT.fetch_add(1, Ordering::Relaxed)));
+    let _ = std::fs::remove_dir_all(&root);
+    std::fs::create_dir_all(&root).expect("root");
+    seed(&root, files);
+    bind_folder(&root, &stub.base, "v-1", TOKEN, "d-1", Some("u-1")).expect("bind the vault");
+    app_for(&root, &settings_dir(&root), size)
+}
+
+/// Writes `(path, content)` into `root`, creating the folders.
+fn seed(root: &Path, files: &[(&str, &str)]) {
+    for (path, text) in files {
+        if let Some((folder, _)) = path.rsplit_once('/') {
+            std::fs::create_dir_all(root.join(folder)).expect("folder");
+        }
+        std::fs::write(root.join(path), text).expect("seed");
+    }
+}
+
+/// The settings dir beside `root`; the vault never holds the settings (SPEC 4.1).
+fn settings_dir(root: &Path) -> std::path::PathBuf {
+    root.parent().unwrap_or(root).join("notescraft-sheet-config")
+}
+
+/// An app over an already seeded and bound vault at `root`.
+fn app_for(root: &Path, config: &Path, size: egui::Vec2) -> Driver {
+    let mut session = Session::open(root).expect("session");
+    session.use_settings_dir(config);
+    let mut app = NotesApp::new(session, notescraft_themes::builtin("catppuccin-mocha").expect("theme"));
+    app.state.animations = false;
+    app.open_note("a.md");
+    Driver { app, ctx: Context::default(), time: 1.0, size }
+}
+
+impl Driver {
+    /// One frame with `events`; the texts it painted.
+    fn frame(&mut self, events: Vec<Event>) -> Vec<Painted> {
+        self.time += 0.05;
+        let input = RawInput { screen_rect: Some(egui::Rect::from_min_size(Pos2::ZERO, self.size)), time: Some(self.time), events, ..Default::default() };
+        let app = &mut self.app;
+        let mut out = self.ctx.run_ui(input, |ui| app.ui(ui.ctx()));
+        out.textures_delta.clear();
+        out.shapes
+            .iter()
+            .filter_map(|shape| match &shape.shape {
+                egui::Shape::Text(text) => Some(Painted { text: text.galley.text().to_owned(), pos: text.pos, galley: text.galley.clone() }),
+                _ => None,
+            })
+            .collect()
+    }
+
+    fn settle(&mut self) -> Vec<Painted> {
+        let mut last = Vec::new();
+        for _ in 0..24 {
+            last = self.frame(Vec::new());
+        }
+        last
+    }
+
+    fn click(&mut self, at: Pos2) -> Vec<Painted> {
+        self.frame(vec![Event::PointerMoved(at)]);
+        self.frame(vec![Event::PointerButton { pos: at, button: PointerButton::Primary, pressed: true, modifiers: egui::Modifiers::NONE }]);
+        self.frame(vec![Event::PointerButton { pos: at, button: PointerButton::Primary, pressed: false, modifiers: egui::Modifiers::NONE }]);
+        self.settle()
+    }
+
+    /// The centre of the pill's painted `Synced` label; the hit area holds it.
+    fn pill_centre(&mut self) -> Pos2 {
+        let texts = self.settle();
+        let status = self.app.session.sync_worker().map(|worker| format!("{:?}", worker.status()));
+        let pill = texts
+            .iter()
+            .find(|painted| painted.text == "Synced")
+            .unwrap_or_else(|| panic!("no pill among {:?} -- worker {status:?}", texts.iter().map(|painted| painted.text.as_str()).collect::<Vec<_>>()));
+        pill.pos + pill.galley.size() / 2.0
+    }
+}
+
+fn painted<'a>(texts: &'a [Painted], wanted: &str) -> Option<&'a Painted> {
+    texts.iter().find(|painted| painted.text == wanted)
+}
+
+fn expect<'a>(texts: &'a [Painted], wanted: &str) -> &'a Painted {
+    painted(texts, wanted).unwrap_or_else(|| panic!("no {wanted:?} among {:?}", texts.iter().map(|painted| painted.text.as_str()).collect::<Vec<_>>()))
+}
+
+/// The device list the stub answers: another device last seen an hour ago,
+/// this device now.
+fn devices_json(now_ms: u64) -> String {
+    format!(
+        r#"[{{"id":"d-2","name":"Laptop","created_ms":0,"last_seen_ms":{},"current":false}},{{"id":"d-1","name":"Pixel 9","created_ms":0,"last_seen_ms":{},"current":true}}]"#,
+        now_ms - 3_600_000,
+        now_ms
+    )
+}
+
+/// The Unix time now, in milliseconds.
+fn now_ms() -> u64 {
+    SystemTime::now().duration_since(UNIX_EPOCH).map_or(0, |since| u64::try_from(since.as_millis()).unwrap_or(u64::MAX))
+}
+
+/// The worker runs fast: the tests never wait for the real debounce.
+fn fast_worker(d: &mut Driver) {
+    if let Some(worker) = d.app.session.sync_worker() {
+        worker.set_timings(Timings {
+            debounce: Duration::from_millis(10),
+            poll: Duration::from_mins(15),
+            backoff: Duration::from_millis(10),
+            backoff_max: Duration::from_secs(1),
+        });
+    }
+}
+
+/// Waits, bounded, until the worker is idle; the cycles land before the frames read it.
+fn worker_idle(d: &mut Driver) {
+    let deadline = Instant::now() + WAIT;
+    while Instant::now() < deadline && !d.app.session.sync_worker().is_some_and(|worker| worker.status().phase == notescraft_engine::SyncPhase::Idle) {
+        std::thread::sleep(Duration::from_millis(5));
+    }
+}
+
+#[test]
+fn the_pill_opens_the_sheet_and_lists_the_current_device_first() {
+    let stub = Stub::spawn(&devices_json(now_ms()));
+    let mut d = driven(&stub, &[("a.md", "A")], PHONE);
+    d.settle();
+    fast_worker(&mut d);
+    stub.wait_for(|requests| requests.iter().any(|seen| seen.method == "PUT" && seen.path.ends_with("a.md")));
+    worker_idle(&mut d);
+
+    let from = d.pill_centre();
+    let texts = d.click(from);
+    assert!(d.app.sync_sheet.is_some(), "the pill opened the sheet");
+    let current = expect(&texts, "This phone");
+    let laptop = expect(&texts, "Laptop");
+    assert!(current.pos.y < laptop.pos.y, "the current device first: {current:?} vs {laptop:?}");
+    assert!(
+        texts.iter().any(|painted| painted.text.contains("last seen")),
+        "the ages show: {:?}",
+        texts.iter().map(|painted| painted.text.as_str()).collect::<Vec<_>>()
+    );
+    assert!(!texts.iter().any(|painted| painted.text.contains(TOKEN)), "the token never reaches the screen");
+    assert!(painted(&texts, "Everything is up to date").is_some(), "the status headline shows");
+    assert!(painted(&texts, "Sync now").is_some(), "the primary button shows");
+}
+
+#[test]
+fn the_conflicts_row_appears_only_over_a_conflict_file_on_disk() {
+    let stub = Stub::spawn(&devices_json(now_ms()));
+    let mut with = driven(&stub, &[("a.md", "A"), ("a (conflict Laptop 2026-10-20).md", "A")], PHONE);
+    with.settle();
+    let from = with.pill_centre();
+    with.click(from);
+    let over = with.settle();
+    assert!(painted(&over, "1 conflicts to review").is_some(), "the row shows");
+    drop(with);
+
+    let mut none_driver = driven(&stub, &[("a.md", "A")], PHONE);
+    none_driver.settle();
+    let from = none_driver.pill_centre();
+    none_driver.click(from);
+    let none = none_driver.settle();
+    assert!(painted(&none, "conflicts to review").is_none(), "nothing to review: {:?}", none.iter().map(|painted| painted.text.as_str()).collect::<Vec<_>>());
+    let _ = none_driver.app.sync_sheet.take();
+}
+
+#[test]
+fn toggling_attachments_persists_and_the_first_cycle_skips_the_png() {
+    let stub = Stub::spawn(&devices_json(now_ms()));
+    let mut d = driven(&stub, &[("a.md", "A"), ("img/scan.png", "bytes")], PHONE);
+    d.settle();
+    worker_idle(&mut d);
+    let from = d.pill_centre();
+    d.click(from);
+    let texts = d.settle();
+    let label = expect(&texts, "Sync attachments");
+    let hit = label.pos + label.galley.size() / 2.0;
+    d.click(hit);
+    // The stored option goes into the settings file the next load reads; the
+    // vault root and its settings dir outlive the app they were opened over.
+    let root = d.app.session.vault().root().to_path_buf();
+    let config = settings_dir(&root);
+    let read = |text: &str| text.lines().filter(|line| line.contains("attachments") || line.contains("[sync]")).collect::<Vec<_>>().join(", ");
+    let stored = std::fs::read_to_string(config.join("config.toml")).unwrap_or_default();
+    println!("DIAG toml: {:?}", read(&stored));
+    assert!(stored.contains("attachments = false"), "the toggle stored: {:?}", read(&stored));
+    drop(d);
+
+    let mut again = app_for(&root, &config, PHONE);
+    again.settle();
+    fast_worker(&mut again);
+    worker_idle(&mut again);
+    assert!(!stub.uploaded("scan.png"), "the png never left: {}", stub_uploaded_list(&stub));
+    assert!(again.app.session.sync_worker().is_some_and(|worker| worker.status().phase == notescraft_engine::SyncPhase::Idle), "the cycle ran");
+    drop(again);
+    let _ = std::fs::remove_dir_all(&root);
+    let _ = std::fs::remove_dir_all(&config);
+}
+
+/// The paths the stub saw written to, for the failure messages.
+fn stub_uploaded_list(stub: &Stub) -> String {
+    stub.requests().iter().map(|seen| format!("{}/{}", seen.method, seen.path)).collect::<Vec<_>>().join(", ")
+}
+
+#[test]
+fn sync_now_wakes_the_worker() {
+    let stub = Stub::spawn(&devices_json(now_ms()));
+    let mut d = driven(&stub, &[("a.md", "A")], PHONE);
+    d.settle();
+    fast_worker(&mut d);
+    stub.wait_for(|requests| requests.iter().any(|seen| seen.method == "PUT" && seen.path.ends_with("a.md")));
+    worker_idle(&mut d);
+    let before = stub.change_polls();
+    let from = d.pill_centre();
+    d.click(from);
+    let texts = d.settle();
+    let label = expect(&texts, "Sync now");
+    let hit = label.pos + label.galley.size() / 2.0;
+    d.click(hit);
+    stub.wait_for(|requests| requests.iter().filter(|seen| seen.method == "GET" && seen.path.ends_with("/changes")).count() > before as usize);
+    assert!(stub.change_polls() > before, "the change log was polled again");
+    assert!(d.app.sync_sheet.is_some(), "Sync now keeps the sheet open");
+}
+
+#[test]
+fn the_sheet_closes_with_x_and_escape_and_a_tap_outside() {
+    let stub = Stub::spawn(&devices_json(now_ms()));
+    let mut d = driven(&stub, &[("a.md", "A")], PHONE);
+    d.settle();
+    let from = d.pill_centre();
+    d.click(from);
+    assert!(d.app.sync_sheet.is_some(), "open");
+    d.settle();
+    // The × sits in the title row, in the top-right of the sheet.
+    let sheet = d.ctx.read_response(egui::Id::new("notescraft-sync-sheet")).expect("the sheet area");
+    let rect = sheet.rect;
+    d.click(egui::pos2(rect.right() - 16.0 - 22.0, rect.top() + 16.0 + 4.0 + 12.0 + 22.0));
+    assert!(d.app.sync_sheet.is_none(), "the × closed the sheet");
+
+    let again_from = d.pill_centre();
+    d.click(again_from);
+    assert!(d.app.sync_sheet.is_some(), "open again");
+    let escape = Event::Key { key: Key::Escape, physical_key: None, pressed: true, repeat: false, modifiers: egui::Modifiers::NONE };
+    d.frame(vec![escape]);
+    d.settle();
+    assert!(d.app.sync_sheet.is_none(), "Escape closed the sheet");
+
+    let third_from = d.pill_centre();
+    d.click(third_from);
+    assert!(d.app.sync_sheet.is_some(), "open again for the tap outside");
+    d.click(egui::pos2(20.0, 20.0));
+    assert!(d.app.sync_sheet.is_none(), "a tap on the dimmed area closed the sheet");
+}
+
+#[test]
+fn at_desktop_width_the_pill_opens_the_settings_page() {
+    let stub = Stub::spawn(&devices_json(now_ms()));
+    let mut d = driven(&stub, &[("a.md", "A")], DESKTOP);
+    d.settle();
+    let from = d.pill_centre();
+    let texts = d.click(from);
+    assert!(d.app.sync_sheet.is_none(), "no sheet at desktop width");
+    assert_eq!(d.app.tabs.active_path(), Some(notescraft_ui_egui::tabs::SETTINGS_PATH), "the settings page opened");
+    assert!(
+        texts.iter().any(|painted| painted.text == "Server") && texts.iter().any(|painted| painted.text == "Sync now"),
+        "the Sync page shows: {:?}",
+        texts.iter().map(|painted| painted.text.as_str()).collect::<Vec<_>>()
+    );
+}
diff --git a/specs/attachments-and-history/spec.md b/specs/attachments-and-history/spec.md
index 31d7a2b..a7a3185 100644
--- a/specs/attachments-and-history/spec.md
+++ b/specs/attachments-and-history/spec.md
@@ -30,7 +30,7 @@ Real-time delta sync of Markdown (files are small), cross-vault dedup in encrypt
 2. Content-defined chunking (rolling hash) instead of fixed 4 MiB? Recommended: fixed first; revisit if edits to big files are common.
 
 ## Tasks
-1. Server: `chunks` table and endpoints, assembled-file reads, quota accounting.
+1. [x] Server: `chunks` table and endpoints, assembled-file reads, quota accounting. Done 2026-10-20 by GLM 5.3 Flash via Pi.
 2. [x] Server: `file_revisions`, history, restore, retention GC. Done 2026-10-20 by GLM 5.3 Flash via Pi (MiniMax was out of quota): `src/history/`, `store/revisions.rs`, endpoints `files-history`, `files/{path}?seq=`, `files-restore`, per-vault `retention_days` (default 30, newest 20 always kept), quota counts retained revisions; verified by the orchestrator (all server suites green, clippy clean). Engine and UI still open (task 4).
-3. Engine: chunked upload/download in the driver with resume and progress events.
+3. [x] Engine: chunked upload in the driver with resume and progress events (`sync/chunks.rs`, `Remote::{missing_chunks, put_chunk, put_chunked}`, `SyncStatus.transfer`). Done 2026-10-20 by MiniMax M3.1 Flash through the Claude Code harness; verified against the real server. Big-file download still goes through the existing `get` (capped by what fits in memory).
 4. UI: transfer progress in the status pill and Settings > Sync; Versions tab shows server revisions.
diff --git a/specs/mobile-and-sync/tasks.md b/specs/mobile-and-sync/tasks.md
index 5defecb..59b7b14 100644
--- a/specs/mobile-and-sync/tasks.md
+++ b/specs/mobile-and-sync/tasks.md
@@ -53,7 +53,7 @@ Rules: each task is independently testable; order is execution order. `[x]` done
 - [ ] 25. Quick capture sheet and share-target receive (`note.new_from_share` command).
 - [ ] 26. Quick switcher and command palette on phone, over the engine registry.
 - [ ] 27. Background sync via WorkManager and `BGTaskScheduler` (the worker from task 5 runs headless).
-- [ ] 28. Conflict resolution UI plus an offline queue view on phone (builds on task 6).
+- [~] 28. Conflict resolution UI plus an offline queue view on phone (builds on task 6). The phone sync sheet with devices, conflicts row, options and Sync now is done (2026-10-20, GLM 5.3 Flash, 6 driven tests); the offline queue view is open.
 - [ ] 29. Mobile toolbar above the keyboard and long-press context menus (design 02).
 
 ## Related spec sets