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Using Compaction Causes Anthropic API Failure When Agent Is Invoked With Print_mode="Messages" Or Stream_mode="Messages"

When using Anthropic's compaction beta with LangChain agents, invoking with print_mode="messages" or streaming with stream_mode="messages" causes a BadRequestError because the message content includes an unsupported 'index' field in compaction blocks.

highConfidence 95%Langchain-Anthropic Affected Vlangchain-Anthropic==1.4.1 Affected Vlangchain-Core==1.3.0 Affected Vlanggraph==1.1.9 Affected Vanthropic==0.96.0

Origin Analysis

LangChain's langchain_anthropic/chat_models.py _format_messages method does not filter out the 'index' field added by LangGraph's streaming in 'messages' mode when formatting compaction blocks, leading Anthropic API to reject the request with 'Extra inputs are not permitted'.

1. Create a ChatAnthropic model with betas=['compact-2026-01-12'] and context_management configured as in the issue.\n2. Create an agent with create_agent(model=model).\n3. Build a conversation history exceeding 50,000 input tokens using repeated filler messages, plus a final user message.\n4. Invoke agent.invoke({'messages': history}, print_mode='messages').\n5. Take the returned result['messages'] and append a HumanMessage asking a follow-up question.\n6. Invoke agent.invoke again with print_mode='messages'.\n7. The second invoke raises anthropic.BadRequestError with message 'messages.63.content.0.compaction.index: Extra inputs are not permitted'.

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 change only affects formatting of compaction blocks and should not alter behavior in non-streaming or 'updates' modes. However, if Anthropic adds other valid fields for compaction blocks in future versions, this filter will discard them. It is recommended to review the Anthropic API spec when upgrading the langchain-anthropic or anthropic package and adjust the allowed keys accordingly. Rollback is simple: revert this branch to the original code that passed the entire block.