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Langchain-Openrouter Sends ToolMessage Content Blocks (E.G. Images) Unconverted, Causing Client-Side ValidationError

ToolMessage content that contains standard LangChain content blocks (like images) is forwarded to the OpenRouter API without conversion, leading to a client-side ValidationError. This makes agent threads unrecoverable until history is trimmed.

highConfidence 95%Langchain Affected V<=0.2.8

Origin Analysis

In langchain_openrouter's _convert_message_to_dict, _format_message_content is applied only for HumanMessage. The ToolMessage branch forwards message.content verbatim, so standard 'image' blocks are not mapped to OpenRouter's expected 'image_url' tag, causing the OpenRouter SDK to reject the payload during validation.

1. Install langchain-openrouter (tested on 0.2.8) and langchain-core. 2. Create a ChatOpenRouter instance with a dummy API key. 3. Build a conversation with a HumanMessage, an AIMessage containing a tool_call, and a ToolMessage whose content is a list containing an image block: [{"type": "image", "base64": <valid base64>, "mime_type": "image/png"}]. 4. Call llm.invoke(messages). 5. Observe ValidationError before any network request. A text-only ToolMessage passes validation and fails later with 401 (dummy key).

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

Applying _format_message_content to ToolMessage may introduce edge cases if _format_message_content assumes user-message semantics (e.g. it might reject certain block types or alter formatting). Test with various tool result content types (text, image_url, file, audio) and across multiple OpenRouter models. The change is backward-compatible for string content. Rollback: revert to original branch that passes content verbatim. If the function raises on unsupported blocks, consider adding a fallback that converts only known safe types or logs a warning and sends a placeholder.