🧵 fix: Strip Unsupported Claude Assistant Prefill

[danny-avila]

Summary

I fixed Claude 4.6+ Anthropic dispatch so requests ending with assistant messages no longer fail on models that reject assistant prefill. This addresses the root cause behind danny-avila/LibreChat#13196.

• Detect Claude Opus, Sonnet, and Haiku 4.6+ model IDs across direct and provider-shaped names.
• Strip trailing assistant-role messages immediately before Anthropic streaming and non-streaming dispatch for affected models.
• Preserve existing assistant-prefill behavior for Claude 4.5 and older models.
• Add regression coverage for affected and unaffected Claude model IDs.

Change Type

• Bug fix (non-breaking change which fixes an issue)

Testing

I validated the fix with targeted unit coverage, package build, and live Vertex AI checks against claude-sonnet-4-6.

Test Configuration:

• Project: regulated-dev-441020
• Region: us-east5
• Model: claude-sonnet-4-6
• Auth: user ADC via gcloud auth application-default login

Commands and checks:

• npm test -- --runTestsByPath src/llm/anthropic/llm.spec.ts --runInBand --modulePathIgnorePatterns='<rootDir>/.claude' -t 'Claude assistant prefill compatibility'
• npm run build
• git diff --check
• Live Vertex raw SDK: user-final message succeeded, final assistant prefill failed with This model does not support assistant message prefill. The conversation must end with a user message.
• Live Vertex non-streaming: base ChatAnthropic failed with the same 400, patched CustomAnthropic succeeded with the same final AIMessage input.
• Live Vertex streaming: base ChatAnthropic failed with the same 400, patched CustomAnthropic succeeded with the same final AIMessage input.

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective
• Local unit tests pass with my changes

[danny-avila]

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[danny-avila]

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Codex Review: Didn't find any major issues. Keep them coming!

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