Checked other resources

• This is a bug, not a usage question.
• I added a clear and descriptive title that summarizes this issue.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain rather than my code.
• The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).
• This is not related to the langchain-community package.
• I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Package (Required)

• langchain
• langchain-openai
• langchain-anthropic
• langchain-classic
• langchain-core
• langchain-model-profiles
• langchain-tests
• langchain-text-splitters
• langchain-chroma
• langchain-deepseek
• langchain-exa
• langchain-fireworks
• langchain-groq
• langchain-huggingface
• langchain-mistralai
• langchain-nomic
• langchain-ollama
• langchain-openrouter
• langchain-perplexity
• langchain-qdrant
• langchain-xai
• Other / not sure / general

Related Issues / PRs

No response

Reproduction Steps / Example Code (Python)

from langchain_openrouter import ChatOpenRouter
from langchain_core.messages import AIMessage, HumanMessage

model = ChatOpenRouter(
    model="anthropic/claude-sonnet-4-6",
    api_key="your-key-here",
    reasoning={"effort": "high", "summary": "auto"},
)

# Turn 1 — streaming
msgs = [HumanMessage(content="What is 2+2? Think step by step.")]
chunks = list(model.stream(msgs))
merged = chunks[0]
for c in chunks[1:]:
    merged = merged + c

# Inspect: reasoning_details is fragmented (e.g. 6 entries instead of 1)
details = merged.additional_kwargs.get("reasoning_details", [])
print(f"reasoning_details count: {len(details)}")  # Expected: 1, Actual: 6

# Turn 2 — fails
ai_msg = AIMessage(
    content=merged.content,
    additional_kwargs=merged.additional_kwargs,
    response_metadata=merged.response_metadata,
)
msgs.append(ai_msg)
msgs.append(HumanMessage(content="Now what is 3+3?"))

# This raises BadRequestResponseError
chunks2 = list(model.stream(msgs))

Error Message and Stack Trace (if applicable)

BadRequestResponseError: Provider returned error

Description

During streaming, AIMessageChunk.__add__ list-concatenates reasoning_details in additional_kwargs, fragmenting a single entry into many. When _convert_message_to_dict() serializes conversation history back to the OpenRouter API for the next turn, these fragmented entries are passed through as-is. The API rejects the malformed payload with BadRequestResponseError.

Non-streaming (invoke()) is unaffected — it returns complete entries in a single response.

The fix should merge fragmented reasoning_details entries (same type + same index) back into single entries before serialization. A PR with the fix and tests is ready.

System Info

System Information

OS: Darwin
OS Version: Darwin Kernel Version 25.3.0
Python Version: 3.11.14

Package Information

langchain_core: 1.2.23
langchain: 1.2.13
langchain_openrouter: 0.2.1
langchain_openai: 1.1.11

Other Dependencies

openrouter: 0.8.0
pydantic: 2.12.5