AI & Agent Dev Bug Sandbox Back to Radar

ChatOpenAI Silently Drops Provider-Specific Tool-Call Fields, Breaking Multi-Turn Tool Calls On Compatible Endpoints

ChatOpenAI's message conversion discards unknown fields on tool calls (e.g., Gemini's thought_signature) in both directions, causing later provider-side 400 errors in multi-turn tool calling loops.

highConfidence 90%LangChain Affected V1.1.9 Affected V1.6.x

Origin Analysis

In langchain_openai.chat_models.base, _convert_dict_to_message calls parse_tool_call(...) which only keeps id/type/function, and _lc_tool_call_to_openai_tool_call reconstructs only those keys during serialization. The original raw tool_call dict is not preserved anywhere, so provider-specific extras are unrecoverable.

1. Use langchain-openai 1.1.9+ with an OpenAI-compatible endpoint that returns a tool call with an extra field such as 'thought_signature'. 2. Call _convert_dict_to_message on the raw assistant response containing that tool_call. 3. Convert the resulting AIMessage back with _convert_message_to_dict. 4. Observe that the extra field is absent from the outgoing tool_call dict, leading to provider 400 on the next turn.

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

This patch will forward unknown tool-call fields to the next endpoint. If a message created from one provider is replayed to a different provider (e.g., official OpenAI), the unknown fields may cause a 400 error. Users switching between providers should clear/rebuild messages or gate the forwarding based on the endpoint. Rollback: revert the changes; no data migration is needed. Additional care: ensure extra values are JSON-serializable and do not contain sensitive information that should not be echoed.