diff --git a/apps/solidcraft-cli/src/kernel_cases.rs b/apps/solidcraft-cli/src/kernel_cases.rs
index eeddf32e07..549ad11cd2 100644
--- a/apps/solidcraft-cli/src/kernel_cases.rs
+++ b/apps/solidcraft-cli/src/kernel_cases.rs
@@ -129,7 +129,7 @@ fn case_name(dir: &str) -> (String, String) {
     (name(p.parent()), name(Some(p)))
 }
 
-fn check_case(dir: &str) -> Value {
+fn check_case(dir: &str, dumps: Option<&std::path::Path>) -> Value {
     let (area, name) = case_name(dir);
     let mut out = json!({"case": name, "area": area, "kernel": kernel()});
     let want = match read_json(&format!("{dir}/expected.json")) {
@@ -167,6 +167,18 @@ fn check_case(dir: &str) -> Value {
     let outcome = want["outcome"].as_str().unwrap_or("solid");
     let refused = outcome == "refused";
     let may_refuse = refused || outcome == "solid_or_refused";
+    // The SMLib calls that fail in this case leave their inputs in its own folder.
+    if let Some(root) = dumps {
+        let folder = root.join(format!("{area}-{name}"));
+        if let Err(e) = std::fs::create_dir_all(&folder)
+            .map_err(|e| e.to_string())
+            .and_then(|()| solidcraft_engine::kernel::smlib_dump_dir(Some(&folder)).map_err(|e| e.to_string()))
+        {
+            out["status"] = json!("fail");
+            out["error"] = json!(format!("--dumps: {}: {e}", folder.display()));
+            return out;
+        }
+    }
     let mut s = solidcraft_engine::Session::default();
     let clock = std::time::Instant::now();
     let ran = s.run_script(&commands);
@@ -331,7 +343,15 @@ pub fn run(args: &[String]) -> Result<(), String> {
     if dirs.is_empty() {
         return Err("kernel-cases: give one or more case directories (each with recipe.json and expected.json)".into());
     }
-    let results: Vec<Value> = dirs.iter().map(|d| check_case(d)).collect();
+    // `--dumps=DIR`: each case's failing SMLib calls leave their inputs in DIR/<area>-<case>.
+    let dumps = args.iter().find_map(|a| a.strip_prefix("--dumps=")).map(std::path::PathBuf::from);
+    if dumps.as_ref().is_some_and(|d| d.as_os_str().is_empty()) {
+        return Err("kernel-cases: --dumps= needs a folder".into());
+    }
+    let results: Vec<Value> = dirs.iter().map(|d| check_case(d, dumps.as_deref())).collect();
+    if dumps.is_some() {
+        solidcraft_engine::kernel::smlib_dump_dir(None).map_err(|e| format!("kernel-cases: --dumps: {e}"))?;
+    }
     report(args, &results);
     let bad = results.iter().filter(|r| matches!(r["status"].as_str(), Some("fail" | "crash"))).count();
     if bad > 0 { Err(format!("{bad} kernel case(s) failed")) } else { Ok(()) }
diff --git a/apps/solidcraft-cli/src/main.rs b/apps/solidcraft-cli/src/main.rs
index 4fffaff44d..1bd2627a12 100644
--- a/apps/solidcraft-cli/src/main.rs
+++ b/apps/solidcraft-cli/src/main.rs
@@ -15,7 +15,7 @@
 //! solidcraft-cli commands                                     the command registry as JSON
 //! solidcraft-cli recipe <recipe.json>                         translate an oracle recipe to a script
 //! solidcraft-cli oracle <case-dir>...                         replay recipes and compare with expected.json
-//! solidcraft-cli kernel-cases [--json] <case-dir>...           the kernel regression rig (tests/kernel-cases)
+//! solidcraft-cli kernel-cases [--json] [--dumps=DIR] <case-dir>... the kernel regression rig (tests/kernel-cases)
 //! solidcraft-cli mcp [--headless] [--in design|script] [--connect HOST:PORT [--token T]] [--allow-path DIR]... [--allow-any-path]
 //!                    [--param-check off|warn|strict]
 //!                                                             MCP server on stdio (docs/mcp.md)
@@ -82,7 +82,7 @@ const USAGE_RECIPE: &str = "  solidcraft-cli recipe <recipe.json>\n";
 /// `oracle`'s lines in the usage.
 const USAGE_ORACLE: &str = "  solidcraft-cli oracle <case-dir>... [--json]\n";
 /// `kernel-cases`'s lines in the usage.
-const USAGE_KERNEL_CASES: &str = "  solidcraft-cli kernel-cases <case-dir>... [--json]\n      (the kernel regression rig, tests/kernel-cases; SOLIDCRAFT_KERNEL picks the kernel)\n";
+const USAGE_KERNEL_CASES: &str = "  solidcraft-cli kernel-cases <case-dir>... [--json] [--dumps=DIR]\n      (the kernel regression rig, tests/kernel-cases; SOLIDCRAFT_KERNEL picks the kernel; --dumps: each case's\n       failing SMLib calls leave their inputs for replay in DIR/<area>-<case>)\n";
 /// `fuzz`'s lines in the usage.
 const USAGE_FUZZ: &str =
     "  solidcraft-cli fuzz [--from N] [--count N] [--jobs J] [--timeout S] [--steps N] [--minimise] [--out DIR] [--report FILE]\n";
diff --git a/crates/kernel/src/lib.rs b/crates/kernel/src/lib.rs
index 0c53bbb1c3..b6c1e40fd7 100644
--- a/crates/kernel/src/lib.rs
+++ b/crates/kernel/src/lib.rs
@@ -217,7 +217,7 @@ pub use sweep_path::{RailFit, SweepControl, SweepOrientation, sweep_path, sweep_
 pub use topo::{CylinderFace, cylinder_end_at, cylinder_face, cylinder_face_at};
 pub use topo::{TopoCounts, merged_topology, seam_flags};
 pub use tspline::tspline_sheet;
-pub use validity::{SmlibCheck, Validity, smlib_built, validity};
+pub use validity::{SmlibCheck, Validity, smlib_built, smlib_dump_dir, validity};
 pub use whole_face::{WholeFace, cut_takes_only_the_face, cut_tool, face_prism, matching_face};
 
 #[derive(Debug, Clone, PartialEq, thiserror::Error)]
diff --git a/crates/kernel/src/validity.rs b/crates/kernel/src/validity.rs
index 52cb851295..e5dd336ee2 100644
--- a/crates/kernel/src/validity.rs
+++ b/crates/kernel/src/validity.rs
@@ -59,3 +59,17 @@ pub fn smlib_built() -> bool {
     cfg!(feature = "smlib")
 }
 
+/// Where every SMLib call that fails from now on leaves its inputs for replay (the C API's
+/// `smlib_set_dump_dir`: the call's arguments and B-reps, which `smlib_brep_read` reads back);
+/// `None` stops it. A build without SMLib makes no calls to dump.
+pub fn smlib_dump_dir(dir: Option<&std::path::Path>) -> crate::Result<()> {
+    #[cfg(feature = "smlib")]
+    {
+        solidcraft_smlib::set_dump_dir(dir).map_err(|e| crate::KernelError::Failed(format!("SMLib dump folder: {e}")))
+    }
+    #[cfg(not(feature = "smlib"))]
+    {
+        let _ = dir;
+        Ok(())
+    }
+}
diff --git a/tests/kernel-cases/README.md b/tests/kernel-cases/README.md
index 67733cf247..119e22bd40 100644
--- a/tests/kernel-cases/README.md
+++ b/tests/kernel-cases/README.md
@@ -8,6 +8,7 @@ them. `cargo xtask kernel-cases` runs them and writes [`docs/kernel-cases.md`](.
 cargo xtask kernel-cases                        # the default kernel
 cargo xtask kernel-cases --kernel both --cli target/release/solidcraft-cli   # and SMLib
 cargo run -p solidcraft-cli -- kernel-cases tests/kernel-cases/booleans/<id>     # one case
+cargo run -p solidcraft-cli --features smlib -- kernel-cases --dumps=/tmp/dumps <case-dir>  # failing SMLib calls leave replayable inputs
 ```
 
 For the `smlib` column, build `solidcraft-cli` with `--features smlib` (after