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

`mcp_server._get_collection` bypassed `ChromaBackend.get_collection`
and called `client.get_collection` / `client.create_collection` without
`embedding_function=`. ChromaDB 1.x does not persist the EF identity
with the collection, so the MCP server's reopen silently bound
chromadb's built-in `DefaultEmbeddingFunction` while the miner / Stop
hook ingest path bound `mempalace.embedding.get_embedding_function()`.

On bleeding-edge interpreters (python 3.14 + chromadb 1.5.x on Apple
Silicon, per #1299) the default EF's lazy ONNX 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. Reads worked
because `col.get(ids=...)` and metadata fetches don't invoke the EF;
the auto-ingest path worked because mining routes through the backend
abstraction. Diary writes were the consistent failure surface.

Resolve the EF up front (matching `ChromaBackend._resolve_embedding_function`)
and pass it into both reopen branches. Falls back to the chromadb default
only if `mempalace.embedding.get_embedding_function` itself raises.

Regression test patches the chromadb client class to capture
`embedding_function=` on every `get_collection` / `create_collection`
call from `_get_collection(create=True)` and `_get_collection()`, and
fails if any call omits it.

Follow-up to #1262 / #1289 (which fixed the metadata-mismatch SIGSEGV
path); this addresses the EF-mismatch SIGSEGV path on the same surface.

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 EF default differs.** `mcp_server._get_collection` bypassed `ChromaBackend.get_collection` and called `client.get_collection` / `client.create_collection` without `embedding_function=`. ChromaDB 1.x does not persist the EF identity with the collection, so the MCP server's reopen silently bound chromadb's built-in `DefaultEmbeddingFunction` to the collection while the miner / Stop hook ingest path bound `mempalace.embedding.get_embedding_function()`. On bleeding-edge interpreters (python 3.14 + chromadb 1.5.x on Apple Silicon) the default's lazy ONNX 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 resolves the EF via `mempalace.embedding.get_embedding_function` and passes it into both reopen branches, 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

@@ -66,6 +66,7 @@ from .backends.chroma import (  # noqa: E402
     _pin_hnsw_threads,
     hnsw_capacity_status,
 )
+from .embedding import get_embedding_function  # noqa: E402
 from .query_sanitizer import sanitize_query  # noqa: E402
 from .searcher import search_memories  # noqa: E402
 from .palace_graph import (  # noqa: E402
@@ -278,6 +279,22 @@ def _get_collection(create=False):
     global _collection_cache, _metadata_cache, _metadata_cache_time
     try:
         client = _get_client()
+        # ChromaDB 1.x does not persist the embedding function with the
+        # collection, so a reader/writer that omits ``embedding_function=``
+        # silently gets the chromadb-built-in default. On bleeding-edge
+        # interpreters (#1299: python 3.14 + chromadb 1.5.x on Apple Silicon)
+        # the default's lazy ONNX 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 ``mempalace.embedding.get_embedding_function``.
+        # The MCP server bypassed that abstraction; mirror its behaviour so
+        # ``tool_diary_write`` / ``tool_add_drawer`` get the same EF as mining.
+        try:
+            ef = get_embedding_function()
+        except Exception:
+            logger.exception("Failed to build embedding function; using chromadb default")
+            ef = None
+        ef_kwargs = {"embedding_function": ef} if ef is not None else {}
         if create:
             # hnsw:num_threads=1 disables ChromaDB's multi-threaded ParallelFor
             # HNSW insert path, which has a race in repairConnectionsForUpdate /
@@ -292,7 +309,7 @@ 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 +318,14 @@ 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)
+            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 @@ class TestCacheInvalidation:
         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