diff --git a/crates/smlib/src/ffi.rs b/crates/smlib/src/ffi.rs
index 17c1f66e84..1254d7c04c 100644
--- a/crates/smlib/src/ffi.rs
+++ b/crates/smlib/src/ffi.rs
@@ -191,6 +191,7 @@ pub mod flat;
 pub mod history;
 pub mod measure;
 pub mod ops;
+pub mod split;
 pub mod ssi;
 pub mod sweeps;
 pub mod trim;
diff --git a/crates/smlib/src/ffi/split.rs b/crates/smlib/src/ffi/split.rs
new file mode 100644
index 0000000000..6e07ae3aed
--- /dev/null
+++ b/crates/smlib/src/ffi/split.rs
@@ -0,0 +1,76 @@
+//! The split block of `capi/include/smlib.h` (`smlib_split_faces`, `smlib_split_body`; the
+//! fork's `capi/src/smlib_split.cpp`), declared by hand. Part of the FFI boundary, like its
+//! parent: called only through [`crate::split`].
+
+#![allow(unsafe_code)]
+
+use super::Handle;
+#[cfg(not(feature = "native"))]
+use super::NOT_BUILT;
+use super::ssi::SurfaceHandle;
+#[cfg(feature = "native")]
+use super::{ERR_ARG, ERR_EMPTY, OK, RawBrep, new_brep, ssi::RawSurface};
+
+/// `SMLIB_SPLIT_MAX_FACES`, `SMLIB_SPLIT_MAX_TOOLS`.
+pub const MAX_FACES: usize = 100_000;
+pub const MAX_TOOLS: usize = 4096;
+
+#[cfg(feature = "native")]
+unsafe extern "C" {
+    fn smlib_split_faces(
+        body: *const RawBrep,
+        faces: *const i64,
+        n_faces: i64,
+        tools: *const *const RawSurface,
+        n_tools: i64,
+        out: *mut *mut RawBrep,
+        made: *mut i64,
+    ) -> i32;
+    fn smlib_split_body(body: *const RawBrep, tool: *const RawSurface, back: *mut *mut RawBrep, front: *mut *mut RawBrep) -> i32;
+}
+
+/// `body` with `faces` cut by `tools` in turn, and per face how many faces it became besides
+/// itself.
+#[cfg(feature = "native")]
+pub fn split_faces(body: &Handle, faces: &[i64], tools: &[&SurfaceHandle]) -> Result<(Handle, Vec<i64>), i32> {
+    if faces.is_empty() || faces.len() > MAX_FACES || tools.is_empty() || tools.len() > MAX_TOOLS {
+        return Err(ERR_ARG);
+    }
+    let raw: Vec<*const RawSurface> = tools.iter().map(|t| t.raw()).collect();
+    let mut made = vec![0i64; faces.len()];
+    let n = i64::try_from(faces.len()).map_err(|_| ERR_ARG)?;
+    let m = i64::try_from(raw.len()).map_err(|_| ERR_ARG)?;
+    // SAFETY: the arrays outlive the call and hold `n` and `m` entries; the handles are live.
+    let h = new_brep(|out| unsafe { smlib_split_faces(body.raw(), faces.as_ptr(), n, raw.as_ptr(), m, out, made.as_mut_ptr()) })?;
+    Ok((h, made))
+}
+
+/// The parts of `body` behind and in front of `tool`.
+#[cfg(feature = "native")]
+pub fn split_body(body: &Handle, tool: &SurfaceHandle) -> Result<(Handle, Handle), i32> {
+    let mut back: *mut RawBrep = std::ptr::null_mut();
+    let mut front: *mut RawBrep = std::ptr::null_mut();
+    // SAFETY: the handles are live; the out-pointers are written only on success.
+    let s = unsafe { smlib_split_body(body.raw(), tool.raw(), &mut back, &mut front) };
+    let (b, f) = (std::ptr::NonNull::new(back), std::ptr::NonNull::new(front));
+    match (s, b, f) {
+        (OK, Some(b), Some(f)) => Ok((Handle(b), Handle(f))),
+        (OK, b, f) => {
+            // Free whichever came back alone.
+            drop(b.map(Handle));
+            drop(f.map(Handle));
+            Err(ERR_EMPTY)
+        }
+        (s, _, _) => Err(s),
+    }
+}
+
+#[cfg(not(feature = "native"))]
+pub fn split_faces(_: &Handle, _: &[i64], _: &[&SurfaceHandle]) -> Result<(Handle, Vec<i64>), i32> {
+    Err(NOT_BUILT)
+}
+
+#[cfg(not(feature = "native"))]
+pub fn split_body(_: &Handle, _: &SurfaceHandle) -> Result<(Handle, Handle), i32> {
+    Err(NOT_BUILT)
+}
diff --git a/crates/smlib/src/ffi/ssi.rs b/crates/smlib/src/ffi/ssi.rs
index f02533bcea..fb9b576297 100644
--- a/crates/smlib/src/ffi/ssi.rs
+++ b/crates/smlib/src/ffi/ssi.rs
@@ -110,6 +110,13 @@ unsafe impl Sync for SurfaceHandle {}
 unsafe impl Send for ResultHandle {}
 unsafe impl Sync for ResultHandle {}
 
+impl SurfaceHandle {
+    #[cfg_attr(not(feature = "native"), allow(dead_code))]
+    pub(crate) fn raw(&self) -> *const RawSurface {
+        self.0.as_ptr()
+    }
+}
+
 impl Drop for SurfaceHandle {
     fn drop(&mut self) {
         #[cfg(feature = "native")]
diff --git a/crates/smlib/src/lib.rs b/crates/smlib/src/lib.rs
index b2903e2ca1..91f2b98c13 100644
--- a/crates/smlib/src/lib.rs
+++ b/crates/smlib/src/lib.rs
@@ -23,6 +23,7 @@ pub mod history;
 pub mod import;
 mod measure;
 mod ops;
+mod split;
 pub mod ssi;
 pub mod sweeps;
 pub mod trim;
diff --git a/crates/smlib/src/split.rs b/crates/smlib/src/split.rs
new file mode 100644
index 0000000000..9a818f87d8
--- /dev/null
+++ b/crates/smlib/src/split.rs
@@ -0,0 +1,40 @@
+//! Split Face and Split Body (`capi/include/smlib.h`, "Split"): faces cut where tool surfaces
+//! meet them, and a solid cut in two by a surface. Tools are [`crate::ssi::Surface`]s, each
+//! taken over its whole domain.
+
+use crate::ssi::Surface;
+use crate::{Brep, Error, Result, error, ffi, init};
+
+impl Brep {
+    /// `self` with faces `faces` (indices in SMLib's face order, distinct) cut where each of
+    /// `tools` meets them, the tools in turn, each cutting the pieces the earlier ones left; and
+    /// for each face how many faces it became besides itself (0: no tool split it). `None` when
+    /// no face is split. Tagged operands ([`Brep::tagged`]) give the pieces' history.
+    pub fn split_faces(&self, faces: &[usize], tools: &[&Surface]) -> Result<Option<(Brep, Vec<usize>)>> {
+        init()?;
+        if faces.is_empty() || faces.len() > ffi::split::MAX_FACES || tools.is_empty() || tools.len() > ffi::split::MAX_TOOLS {
+            return Err(Error::Argument);
+        }
+        let faces: Vec<i64> = faces.iter().map(|f| i64::try_from(*f).map_err(|_| Error::Argument)).collect::<Result<_>>()?;
+        let handles: Vec<&ffi::ssi::SurfaceHandle> = tools.iter().map(|t| t.handle()).collect();
+        match ffi::split::split_faces(&self.0, &faces, &handles) {
+            Ok((h, made)) => Ok(Some((Brep(h), made.into_iter().map(|m| usize::try_from(m).unwrap_or(0)).collect()))),
+            Err(ffi::ops::ERR_NO_CUT) => Ok(None),
+            Err(s) => Err(error(s)),
+        }
+    }
+
+    /// The manifold solid `self` cut in two by `tool` (whose domain must reach past it): the
+    /// part behind the surface (its normal points away) and the part in front, each closed by
+    /// the tool's piece inside the body and each possibly several solids. A tool that misses
+    /// leaves both parts the whole body, with no face from the tool (tag `self` to tell); `None`
+    /// when a side is left empty.
+    pub fn split_body(&self, tool: &Surface) -> Result<Option<(Brep, Brep)>> {
+        init()?;
+        match ffi::split::split_body(&self.0, tool.handle()) {
+            Ok((b, f)) => Ok(Some((Brep(b), Brep(f)))),
+            Err(ffi::ops::ERR_NO_CUT) => Ok(None),
+            Err(s) => Err(error(s)),
+        }
+    }
+}
diff --git a/crates/smlib/src/ssi.rs b/crates/smlib/src/ssi.rs
index e05078b874..628b6dece7 100644
--- a/crates/smlib/src/ssi.rs
+++ b/crates/smlib/src/ssi.rs
@@ -25,6 +25,10 @@ fn int(n: usize) -> Result<i32> {
 }
 
 impl Surface {
+    pub(crate) fn handle(&self) -> &ffi::ssi::SurfaceHandle {
+        &self.0
+    }
+
     /// `origin + u*x + v*y` (x, y the axes made unit), for u in `u`, v in `v`.
     pub fn plane(origin: [f64; 3], xaxis: [f64; 3], yaxis: [f64; 3], u: [f64; 2], v: [f64; 2]) -> Result<Surface> {
         init()?;