diff --git a/crates/smlib/src/ffi/ops.rs b/crates/smlib/src/ffi/ops.rs
index 248086e4b5..47c055c6ba 100644
--- a/crates/smlib/src/ffi/ops.rs
+++ b/crates/smlib/src/ffi/ops.rs
@@ -1,4 +1,4 @@
-//! The operations block of `capi/include/smlib.h` (`smlib_boolean`, `smlib_boolean2d`, `smlib_fillet`,
+//! The operations block of `capi/include/smlib.h` (`smlib_boolean`, `smlib_boolean_union_all`, `smlib_boolean2d`, `smlib_fillet`,
 //! `smlib_chamfer`, `smlib_blend`, `smlib_fillet_variable`, `smlib_chamfer_sides`, `smlib_shell`, `smlib_offset_faces`,
 //! `smlib_thicken`, `smlib_draft`, `smlib_merge_faces`, `smlib_brep_solid_count`, `smlib_imprint`; the fork's `capi/src/smlib_ops.cpp`), declared by
 //! hand. Part of the FFI boundary, like its parent: called only through
@@ -48,6 +48,7 @@ type PickFn = unsafe extern "C" fn(*const RawBrep, *const f64, i64, f64, f64, *m
 #[cfg(feature = "native")]
 unsafe extern "C" {
     fn smlib_boolean(a: *const RawBrep, b: *const RawBrep, op: i32, out: *mut *mut RawBrep) -> i32;
+    fn smlib_boolean_union_all(breps: *const *const RawBrep, n: i64, out: *mut *mut RawBrep) -> i32;
     fn smlib_boolean2d(a: *const RawBrep, b: *const RawBrep, op: i32, out: *mut *mut RawBrep) -> i32;
     fn smlib_brep_solid_count(brep: *const RawBrep, out: *mut i64) -> i32;
     fn smlib_imprint(body: *const RawBrep, tool: *const RawBrep, out: *mut *mut RawBrep) -> i32;
@@ -149,6 +150,13 @@ pub fn boolean(a: &Handle, b: &Handle, op: i32) -> Result<Handle, i32> {
     new_brep(|out| unsafe { smlib_boolean(a.raw(), b.raw(), op, out) })
 }
 
+#[cfg(feature = "native")]
+pub fn boolean_union_all(breps: &[&Handle]) -> Result<Handle, i32> {
+    let raw: Vec<*const RawBrep> = breps.iter().map(|h| h.raw()).collect();
+    let n = i64::try_from(raw.len()).map_err(|_| ERR_ARG)?;
+    new_brep(|out| unsafe { smlib_boolean_union_all(raw.as_ptr(), n, out) })
+}
+
 #[cfg(feature = "native")]
 pub fn boolean2d(a: &Handle, b: &Handle, op: i32) -> Result<Handle, i32> {
     new_brep(|out| unsafe { smlib_boolean2d(a.raw(), b.raw(), op, out) })
@@ -315,6 +323,11 @@ pub fn boolean(_: &Handle, _: &Handle, _: i32) -> Result<Handle, i32> {
     Err(NOT_BUILT)
 }
 
+#[cfg(not(feature = "native"))]
+pub fn boolean_union_all(_: &[&Handle]) -> Result<Handle, i32> {
+    Err(NOT_BUILT)
+}
+
 #[cfg(not(feature = "native"))]
 pub fn boolean2d(_: &Handle, _: &Handle, _: i32) -> Result<Handle, i32> {
     Err(NOT_BUILT)
diff --git a/crates/smlib/src/lib.rs b/crates/smlib/src/lib.rs
index e832683ae6..8b683d17f4 100644
--- a/crates/smlib/src/lib.rs
+++ b/crates/smlib/src/lib.rs
@@ -31,7 +31,7 @@ pub mod trim;
 
 pub use check::{CheckLevel, CheckReport};
 pub use measure::{MEASURE_ACCURACY, MassProperties};
-pub use ops::{Blend, BooleanOp, ChamferSecond, MAX_POINTS};
+pub use ops::{Blend, BooleanOp, ChamferSecond, MAX_POINTS, MAX_UNION_BREPS};
 
 /// Why an SMLib call failed.
 #[derive(Clone, Debug, PartialEq)]
diff --git a/crates/smlib/src/ops.rs b/crates/smlib/src/ops.rs
index 866d65b891..0e18c4d719 100644
--- a/crates/smlib/src/ops.rs
+++ b/crates/smlib/src/ops.rs
@@ -41,6 +41,9 @@ pub enum ChamferSecond {
     Angle(f64),
 }
 
+/// Most bodies [`Brep::union_all`] takes (the C API's limit).
+pub const MAX_UNION_BREPS: usize = 100_000;
+
 /// Most points one selection may hold (the C API's limit).
 pub const MAX_POINTS: usize = 4096;
 
@@ -71,6 +74,19 @@ impl Brep {
         }
     }
 
+    /// The union of `breps` in one call: they are unioned pairwise, level by level, so none takes
+    /// part in more than about log2(n) unions. The result is as [`Brep::boolean`]'s; operands
+    /// tagged each with its own number ([`Brep::tagged`]) give a result whose [`Brep::history`]
+    /// names them. At least one body and at most [`MAX_UNION_BREPS`].
+    pub fn union_all(breps: &[&Brep]) -> Result<Brep> {
+        init()?;
+        if breps.is_empty() || breps.len() > MAX_UNION_BREPS {
+            return Err(error(ffi::ERR_ARG));
+        }
+        let handles: Vec<&ffi::Handle> = breps.iter().map(|b| &b.0).collect();
+        ffi::ops::boolean_union_all(&handles).map(Brep).map_err(error)
+    }
+
     /// `self` and `other`, sheets of planar faces on one plane ([`Brep::planar_faces`]), combined
     /// as regions of that plane by `op`: `None` when the result is empty. The result's edges are
     /// the inputs' own curves, split where they cross.