diff --git a/CLAUDE.md b/CLAUDE.md
index ee19dc9..18d1731 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -9,7 +9,14 @@ OpenSCAD AST and produces Manifold CSG geometry. This port builds on two depende
 
 - `openscad_cpp_parser` (git submodule, `external/openscad_cpp_parser`) for lexing/parsing/AST/scope
 - Manifold (https://github.com/elalish/manifold) v3.5.2, pulled in via CMake `FetchContent` (pinned
-  tag, not vendored) — wired in as of Phase 2
+  tag, not vendored) — wired in as of Phase 2. **Patched at fetch time** by
+  `cmake/patch_manifold.cmake`: it deletes the `combined.RemoveDegenerates()` call in
+  `CsgLeafNode::Compose`, which on some inputs leaves the face data inconsistent so the sort after
+  it reads past an array (heap-buffer-overflow under ASan, reached by
+  `MinkowskiDifference.ShrinksANonConvexBodyOnEverySide`). Bisected with a Manifold-only repro to
+  upstream #1789 (969b1417), which fixes it by deleting the same call; unreleased as of 3.5.4.
+  The script fails the configure if the source around the call has changed, so **drop the patch
+  when bumping past #1789** rather than letting it rot
 - Boost.Polygon (`boostorg/polygon` + `boostorg/config`, fetched individually — not the full Boost
   superproject) for `roof()`'s Voronoi-diagram construction — wired in as of Phase 6, see below
 - `nothings/stb` (`stb_image.h`; header-only, no build step) for `surface()`'s PNG loader — wired
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 36b7661..39509eb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -137,6 +137,9 @@ FetchContent_Declare(
   GIT_REPOSITORY https://github.com/elalish/manifold.git
   GIT_TAG v3.5.2
   GIT_SHALLOW TRUE
+  # Backports the one line of upstream #1789 that fixes a heap overflow in
+  # CsgLeafNode::Compose -- see cmake/patch_manifold.cmake.
+  PATCH_COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_SOURCE_DIR}/cmake/patch_manifold.cmake
 )
 FetchContent_MakeAvailable(manifold)
 
diff --git a/cmake/patch_manifold.cmake b/cmake/patch_manifold.cmake
new file mode 100644
index 0000000..6c8c036
--- /dev/null
+++ b/cmake/patch_manifold.cmake
@@ -0,0 +1,31 @@
+# Patches the fetched Manifold v3.5.2 source; run as its FetchContent
+# PATCH_COMMAND, in the source directory.
+#
+# CsgLeafNode::Compose calls RemoveDegenerates() on the combined mesh and
+# then sorts it, and on some inputs the cleanup leaves the face data
+# inconsistent: SortGeometry -> GatherFaces then reads past the end of an
+# array (a heap-buffer-overflow under AddressSanitizer). Our
+# MinkowskiDifference.ShrinksANonConvexBodyOnEverySide test reaches it;
+# reproduced with Manifold alone, and bisected upstream to elalish/manifold
+# #1789 ("Improved decimation", 969b1417), which fixes it by deleting this
+# same call -- the only part of that commit the fix needs. Not in any
+# release yet; drop this patch when Manifold is bumped past it.
+#
+# Idempotent: a re-run on already-patched source changes nothing. Anything
+# else -- the call or its surroundings changed -- is an error, so a version
+# bump can never silently skip the patch.
+set(file src/csg_tree.cpp)
+file(READ ${file} src)
+set(call "  // required to remove parts that are smaller than the tolerance\n  combined.RemoveDegenerates();\n")
+string(FIND "${src}" "${call}" at)
+if(at EQUAL -1)
+  string(FIND "${src}" "combined.RemoveDegenerates();" any)
+  if(NOT any EQUAL -1)
+    message(FATAL_ERROR "patch_manifold.cmake: ${file} changed around RemoveDegenerates(); update the patch")
+  endif()
+  message(STATUS "patch_manifold.cmake: already applied")
+  return()
+endif()
+string(REPLACE "${call}" "" src "${src}")
+file(WRITE ${file} "${src}")
+message(STATUS "patch_manifold.cmake: removed RemoveDegenerates() from CsgLeafNode::Compose")