fix(mcp_server): pass embedding_function= on collection reopen (#1299)

CHANGELOG.md

@@ -19,6 +19,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 
 ### Bug Fixes
 
+- **MCP server `tool_diary_write` SIGSEGV when default EF provider differs.** `mcp_server._get_collection` bypassed `ChromaBackend.get_collection` and called `client.get_collection` / `client.create_collection` without `embedding_function=`. ChromaDB 1.x persists the EF *identity* (its `name()`) with the collection but not the EF *instance/configuration*, so the MCP server's reopen silently bound chromadb's built-in `DefaultEmbeddingFunction` — its `name()` matches `mempalace.embedding`'s spoofed `"default"` so the identity check passes, but its provider list is chromadb's default rather than the user's resolved device. The miner / Stop hook ingest path routes through the backend helper and binds the configured EF instead. On bleeding-edge interpreters (python 3.14 + chromadb 1.5.x on Apple Silicon) the default provider selection could SIGSEGV the host process on first `col.add()`, killing the MCP stdio server and leaving every subsequent tool call returning `Connection closed` until Claude Code was relaunched. `_get_collection` now reuses `ChromaBackend._resolve_embedding_function()` on the reopen branches that actually open a collection (warm-cache reads stay zero-cost), matching the miner/backend path. (#1299, follow-up to #1262 / #1289)
 - **Cross-wing topic tunnels for hyphenated dir names.** `mempalace init` recorded the `topics_by_wing` registry key under the raw directory name (e.g. `mempalace-public`), while `mempalace.yaml`'s `wing` field used the lower-cased + separator-collapsed slug (`mempalace_public`). At mine time the miner read the slug from the yaml and missed the registry, so `_compute_topic_tunnels_for_wing` returned `0` silently. Real-world: any project whose folder contained a hyphen or space lost every topic tunnel. Now both call sites route through a shared `normalize_wing_name()` in `config.py`. (#1194, follow-up to #1180)
 - **CLI `mempalace search` retrieval quality.** The CLI was using pure ChromaDB cosine distance with no BM25 rerank, so drawers containing every query term but embedding as noise (directory listings, diff output, shell logs) scored `Match: 0.0` alongside genuinely irrelevant results with no way to tell them apart. Wired the CLI through the same `_hybrid_rank` the `mempalace_search` MCP tool already used, and surfaced both `cosine=` and `bm25=` scores in the output so users see which component of the match is firing. MCP search was unaffected; this fixes the human-facing CLI parity gap.
 - **Legacy-palace distance-metric warning.** CLI search now detects palaces created before `hnsw:space=cosine` was consistently set and prints a one-line notice pointing at `mempalace repair`. Without the warning such palaces silently used L2 distance, under which the similarity display floored every result to `Match: 0.0`. New palaces mined today already set cosine correctly and now have invariant tests pinning that behavior so future refactors can't silently regress it. (#1179)

mempalace/mcp_server.py

@@ -278,7 +278,25 @@ def _get_collection(create=False):
     global _collection_cache, _metadata_cache, _metadata_cache_time
     try:
         client = _get_client()
+        # ChromaDB 1.x persists the EF *identity* (its ``name()``) with the
+        # collection but not the EF *instance/configuration*. So a reader or
+        # writer that omits ``embedding_function=`` silently gets chromadb's
+        # built-in ``DefaultEmbeddingFunction`` — its ``name()`` matches the
+        # one we spoof in ``mempalace.embedding`` (both report ``"default"``,
+        # the identity check passes), but the *provider list* is chromadb's
+        # default rather than the user's resolved device. On bleeding-edge
+        # interpreters (#1299: python 3.14 + chromadb 1.5.x on Apple Silicon)
+        # that default provider selection can SIGSEGV the host process on
+        # first ``col.add()``. The miner / Stop hook ingest path avoids this
+        # because it routes through ``ChromaBackend.get_collection``, which
+        # resolves the EF via ``ChromaBackend._resolve_embedding_function``;
+        # the MCP server bypassed that abstraction. Resolve the EF inside the
+        # branches that actually open a collection so warm-cache reads stay
+        # zero-cost. Reuse the backend helper so the two call sites can't
+        # drift on logging or fallback semantics.
         if create:
+            ef = ChromaBackend._resolve_embedding_function()
+            ef_kwargs = {"embedding_function": ef} if ef is not None else {}
             # hnsw:num_threads=1 disables ChromaDB's multi-threaded ParallelFor
             # HNSW insert path, which has a race in repairConnectionsForUpdate /
             # addPoint (see issues #974, #965). Set via metadata on fresh
@@ -292,7 +310,7 @@ def _get_collection(create=False):
             # below skips the metadata-comparison codepath for existing
             # collections, mirroring the backend-layer fix from #1262.
             try:
-                raw = client.get_collection(_config.collection_name)
+                raw = client.get_collection(_config.collection_name, **ef_kwargs)
             except _ChromaNotFoundError:
                 raw = client.create_collection(
                     _config.collection_name,
@@ -301,13 +319,16 @@ def _get_collection(create=False):
                         "hnsw:num_threads": 1,
                         **_HNSW_BLOAT_GUARD,
                     },
+                    **ef_kwargs,
                 )
             _pin_hnsw_threads(raw)
             _collection_cache = ChromaCollection(raw)
             _metadata_cache = None
             _metadata_cache_time = 0
         elif _collection_cache is None:
-            raw = client.get_collection(_config.collection_name)
+            ef = ChromaBackend._resolve_embedding_function()
+            ef_kwargs = {"embedding_function": ef} if ef is not None else {}
+            raw = client.get_collection(_config.collection_name, **ef_kwargs)
             _pin_hnsw_threads(raw)
             _collection_cache = ChromaCollection(raw)
             _metadata_cache = None

tests/test_mcp_server.py

@@ -919,3 +919,59 @@ def _spy(self, *args, **kwargs):
         col2 = mcp_server._get_collection(create=True)
         assert col2 is not None
         assert calls == [], f"get_or_create_collection was called: {calls}"
+
+    def test_get_collection_passes_embedding_function(self, monkeypatch, config, palace_path, kg):
+        """Regression for #1299.
+
+        ``mcp_server._get_collection`` must pass ``embedding_function=`` into
+        both ``client.get_collection`` and ``client.create_collection``,
+        mirroring ``ChromaBackend.get_collection``. Without it, ChromaDB 1.x
+        falls back to its built-in ``DefaultEmbeddingFunction`` (whose lazy
+        ONNX provider selection has SIGSEGV'd on python 3.14 + Apple Silicon),
+        and writers/readers can disagree with the miner about which EF is
+        bound to the collection. The miner / Stop hook ingest path routes
+        through ``ChromaBackend.get_collection`` which does this correctly;
+        the MCP server must match.
+        """
+        _patch_mcp_server(monkeypatch, config, kg)
+        from mempalace import mcp_server
+
+        client = mcp_server._get_client()
+        client_cls = type(client)
+        captured: dict[str, list[dict]] = {"get": [], "create": []}
+        real_get = client_cls.get_collection
+        real_create = client_cls.create_collection
+
+        def _spy_get(self, name, **kwargs):
+            captured["get"].append(dict(kwargs))
+            return real_get(self, name, **kwargs)
+
+        def _spy_create(self, name, **kwargs):
+            captured["create"].append(dict(kwargs))
+            return real_create(self, name, **kwargs)
+
+        monkeypatch.setattr(client_cls, "get_collection", _spy_get)
+        monkeypatch.setattr(client_cls, "create_collection", _spy_create)
+        mcp_server._collection_cache = None
+
+        col = mcp_server._get_collection(create=True)
+        assert col is not None
+
+        all_calls = captured["get"] + captured["create"]
+        assert all_calls, "expected get_collection or create_collection to be called"
+        for kwargs in all_calls:
+            assert (
+                "embedding_function" in kwargs
+            ), f"missing embedding_function= in chromadb call: {kwargs}"
+            assert kwargs["embedding_function"] is not None
+
+        # Same expectation on the create=False (cache-miss) reopen path.
+        mcp_server._collection_cache = None
+        captured["get"].clear()
+        captured["create"].clear()
+        col2 = mcp_server._get_collection()
+        assert col2 is not None
+        assert captured["get"], "expected get_collection on cache-miss reopen"
+        for kwargs in captured["get"]:
+            assert "embedding_function" in kwargs
+            assert kwargs["embedding_function"] is not None