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Core: Finalize_tool_call_chunk Coerces Id=None To Empty String, Collapsing Parallel Id-Less Tool Calls

langchain-core's finalize_tool_call_chunk maps missing tool_call id (None) to "" in the tool_call and server_tool_call branches, causing multiple id-less parallel tool calls to share the same empty id and collide downstream (e.g., deepagents FilesystemMiddleware offloads all to /large_tool_results/unknown). The proposed fix replaces the empty-string fallback with uuid4().hex to guarantee distinct ids.

highConfidence 93%Langchain Affected V<=1.6.3

Origin Analysis

The compat bridge (libs/core/langchain_core/language_models/_compat_bridge.py) normalizes id_=None to id="" via `id=id_ or ""` in finalize_tool_call_chunk. This violates the ToolCall.id type contract (str | None), conflates 'absent id' with 'empty string', and makes every id-less tool call indistinguishable from others with the same name/args.

Run the provided MRE: create two ChatGenerationChunk with AIMessageChunk containing tool_call_chunks with id=None, indices 0 and 1. Pass them to chunks_to_events and dispatch to ChatModelStream. Print tool_calls ids; both show id='', proving collapse.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Potential secondary issue: if a single logical tool call is streamed in multiple chunks and some chunks carry the provider id while others do not, the fallback will generate a new UUID for each id-less chunk, causing the same logical call to fragment into multiple tool calls during aggregation. This is especially relevant for providers that intermittently omit the id field. Mitigation: ensure the streaming/provider layer assigns a stable id per call before the compat bridge, or track seen calls by index/name/args when id is missing. Rollback: reverting to `id_ or ""` restores prior behavior but reintroduces the collision; it may be acceptable if downstream handles empty ids explicitly. Also align the two dead fallback sites in chat_model_stream.py (`.get("id", "")`) to avoid confusion.