fix: normalize tool results to strings for OpenAI API compatibility

[Ayaan Ahmad (czarflix)]

🐛 Bug Fix

Issue

Addresses langchain-ai/langchain#34669
Users reported receiving 400 BadRequestError from OpenAI/Azure Chat Models when using MCP tools. The error Input should be a valid dictionary or instance of Content occurs because the adapter was returning tool content as a list[dict] (e.g., [{"type": "text", "text": "..."}]).
Root Cause: OpenAI's Chat Completion API strictly requires tool_message content to be a string (not an array of content blocks).

The Fix

This PR normalizes MCP tool results into a string format compatible with langchain-openai:

1. Single Text Block → Returns plain str (e.g., "Result")
2. Multiple Text Blocks → Returns newline-joined str (e.g., "Line 1\nLine 2")
3. Mixed/Complex Content (e.g., images) → Returns JSON-serialized str (e.g., '[{"type":"image",...}]')

Compatibility Matrix

Provider	Status	Notes
OpenAI / Azure	✅ Fixed	Resolves the 400 Crash.
Anthropic	✅ Compatible	Model parses the JSON-serialized string content.
Google Vertex	✅ Compatible	Uses string fallback.

Verification

• ✅ Reproduction: Confirmed crash locally using the list[dict] format.
• ✅ Fix Validation: Verified string output passes to OpenAI without error.
• ✅ Unit Tests: Updated tests/test_tools.py. Note: Previous tests asserted list output (the buggy behavior). I rewrote these assertions to expect the new normalized str output.

Change Impact

This PR changes the return type of _convert_call_tool_result from list[dict] to str.
Why this is NOT a breaking change:

1. The previous list[dict] format caused crashes with OpenAI (the primary consumer).
2. No working code could depend on the crashing format.
3. Legacy Support: The raw MCPToolArtifact is still preserved and returned if downstream users need the original objects.
   Proposed Release: Patch (Bug Fix)

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Maintainer Note: This issue is tracked in the main langchain repo (#34669). Since cross-repo auto-close is unreliable, I can manually update that issue once this is merged.

[Ayaan Ahmad (czarflix)]

Furkan Zeki ÖZYURT (@fzozyurt) thanks for the review. Could a maintainer approve the workflows? Tagging Bagatur (@baskaryan) for visibility.

[blanketspy99]

What if MCP returns an image which is expected to be understood by LLM for feature extraction? It cannot read bytes/base64 data. I guess openai responses can interpret list[dict] supporting multimodal capabilities.

[Ayaan Ahmad (czarflix)]

What if MCP returns an image which is expected to be understood by LLM for feature extraction? It cannot read bytes/base64 data. I guess openai responses can interpret list[dict] supporting multimodal capabilities.

Yeah so Chat Completions API doesn't actually support images in tool messages - it throws a 400 if you try. This fix just makes sure we don't crash.

For image understanding you'd need the Responses API which langchain-openai doesn't use by default yet. Could be a follow-up.