diff --git a/python/packages/devui/agent_framework_devui/_server.py b/python/packages/devui/agent_framework_devui/_server.py
index 30d8ab4831..7f76aca6ba 100644
--- a/python/packages/devui/agent_framework_devui/_server.py
+++ b/python/packages/devui/agent_framework_devui/_server.py
@@ -55,6 +55,24 @@ def _extract_error_details(body: object) -> tuple[str | None, str | None, str |
     )
 
 
+def _validated_conversation_metadata(request_data: dict[str, Any]) -> dict[str, str]:
+    """Return conversation metadata that is safe for the store and every reader.
+
+    The Conversations API contract for metadata is a map of string keys to string
+    values. Values with any other shape are rejected here so the store can never
+    hold a value that makes the conversation list copy raise for every caller,
+    and an explicit null is treated as an empty map.
+    """
+    raw_metadata = request_data.get("metadata")
+    if raw_metadata is None:
+        return {}
+    if not isinstance(raw_metadata, dict) or not all(
+        isinstance(key, str) and isinstance(value, str) for key, value in raw_metadata.items()
+    ):
+        raise HTTPException(status_code=400, detail="metadata must be a map of string keys to string values")
+    return dict(raw_metadata)
+
+
 # Get package version
 try:
     __version__ = importlib.metadata.version("agent-framework-devui")
@@ -979,7 +997,7 @@ async def create_conversation(raw_request: Request) -> dict[str, Any] | JSONResp
                         )
 
                 # Local mode - use DevUI conversation store
-                metadata = request_data.get("metadata")
+                metadata = _validated_conversation_metadata(request_data)
                 executor = await self._ensure_executor()
                 conversation = executor.conversation_store.create_conversation(metadata=metadata)
                 return conversation.model_dump()
@@ -1051,7 +1069,7 @@ async def update_conversation(conversation_id: str, request_data: dict[str, Any]
             """Update conversation metadata - OpenAI standard."""
             try:
                 executor = await self._ensure_executor()
-                metadata = request_data.get("metadata", {})
+                metadata = _validated_conversation_metadata(request_data)
                 conversation = executor.conversation_store.update_conversation(conversation_id, metadata=metadata)
                 return conversation.model_dump()
             except ValueError as e:
diff --git a/python/packages/devui/tests/devui/test_server.py b/python/packages/devui/tests/devui/test_server.py
index 3c85085001..6d71806261 100644
--- a/python/packages/devui/tests/devui/test_server.py
+++ b/python/packages/devui/tests/devui/test_server.py
@@ -524,6 +524,32 @@ async def test_api_restrictions_in_user_mode():
     assert user_response.status_code in [200, 400, 404]
 
 
+def test_conversation_metadata_rejects_non_string_shapes_and_keeps_listing_healthy():
+    """Non-map metadata must be rejected at the boundary, not stored until listing crashes."""
+    from fastapi.testclient import TestClient
+
+    server = DevServer(mode="developer", auth_enabled=False)
+    client = TestClient(server.create_app(), base_url="http://127.0.0.1")
+
+    string_metadata = client.post("/v1/conversations", json={"metadata": "oops-not-a-dict"})
+    assert string_metadata.status_code == 400
+
+    created = client.post("/v1/conversations", json={"metadata": {"owner": "alice"}})
+    assert created.status_code == 200
+    conversation_id = created.json()["id"]
+
+    non_string_value = client.post(f"/v1/conversations/{conversation_id}", json={"metadata": {"count": 5}})
+    assert non_string_value.status_code == 400
+
+    cleared = client.post(f"/v1/conversations/{conversation_id}", json={"metadata": None})
+    assert cleared.status_code == 200
+    assert cleared.json()["metadata"] == {}
+
+    listing = client.get("/v1/conversations")
+    assert listing.status_code == 200
+    assert any(item["id"] == conversation_id for item in listing.json()["data"])
+
+
 if __name__ == "__main__":
     # Simple test runner
     async def run_tests():