fix(rag): detect ChromaDB dim mismatch and recreate collection on provider switch

src/rag_engine.py

@@ -122,12 +122,49 @@ class RAGEngine:
    chromadb, Settings = chroma_module
    self.client     = chromadb.PersistentClient(path=db_path)
    self.collection = self.client.get_or_create_collection(name=vs_config.collection_name)
+   self._validate_collection_dim()
   elif vs_config.provider == 'mock':
    self.client     = "mock"
    self.collection = "mock"
   else:
    raise ValueError(f"Unknown vector store provider: {vs_config.provider}")
 
+ def _validate_collection_dim(self) -> None:
+  """
+  Detect dimension mismatch between an existing collection's vectors and
+  the current embedding provider's output. When mismatched (e.g. the user
+  switched from Gemini 3072-dim to local 384-dim, or vice versa), the
+  collection is deleted and recreated empty so the next index pass
+  populates it with the correct dim. Prevents silent corruption that
+  would later surface as a search error ("Collection expecting
+  embedding with dimension of X, got Y") and hang live_gui tests.
+  [C: tests/test_rag_engine.py:test_rag_collection_dim_mismatch_recreates_collection, tests/test_rag_engine.py:test_rag_collection_dim_match_preserves_collection]
+  """
+  if self.collection is None or self.collection == "mock" or self.embedding_provider is None:
+   return
+  try:
+   res = self.collection.get(limit=1, include=["embeddings"])
+   if not res:
+    return
+   embeddings = res.get("embeddings") if isinstance(res, dict) else None
+   if not embeddings or len(embeddings) == 0:
+    return
+   existing_dim = len(embeddings[0])
+   expected_dim = len(self.embedding_provider.embed(["__rag_dim_check__"])[0])
+   if existing_dim == expected_dim:
+    return
+   sys.stderr.write(
+    f"RAG: Collection '{self.collection.name}' dim mismatch "
+    f"(existing={existing_dim}, expected={expected_dim}). "
+    f"Recreating collection to prevent silent corruption.\n"
+   )
+   sys.stderr.flush()
+   self.client.delete_collection(self.collection.name)
+   self.collection = self.client.get_or_create_collection(name=self.collection.name)
+  except Exception as e:
+   sys.stderr.write(f"RAG: Failed to validate collection dim: {e}\n")
+   sys.stderr.flush()
+
  def is_empty(self) -> bool:
   if not self.config.enabled:
    return True