🪨 fix: Strip Assistant Prefill for Claude 4.6+ on Bedrock

[berry-13]

Summary

Claude 4.6 and every later model reject a trailing assistant message (prefill) with a 400. #621 widened the gate to every 4.6+ and 5.x id, but only CustomAnthropic applies it; CustomChatBedrockConverse sends the converted messages as they are, so a Bedrock run on anthropic.claude-sonnet-4-6, global.anthropic.claude-haiku-5-5 or any other 4.6+ id that ends on an assistant turn still fails.

The Bedrock stream and the local non-streaming converter now drop trailing assistant turns for those models after conversion, using the same modelDisallowsAssistantPrefill gate. A cache point that rode a stripped turn is re-anchored on the new tail when no surviving message carries one, as the Anthropic path does. Non-streaming calls for those models go through the local converter the stream already uses, since the inherited converter cannot strip and system turns are only lifted out during conversion. Claude 3.x through 4.5 and non-Anthropic Bedrock models (Nova, Llama, Mistral) are unchanged. The index.ts diff includes formatting the pre-commit Prettier hook applied to that file.

Tracked in berry-13/LibreChat#398.

Change Type

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

Testing

• npx jest src/llm/bedrock/prefill.test.ts: 25 passed, covering Bedrock, inference-profile and ARN ids for 4.6+, 5.x and Fable as stripped, 3.x through 4.5, dated 4.0, Nova, Llama, Mistral and missing ids as unchanged, cache point re-anchoring, and the invoke and stream requests for Haiku 5.5 (stripped) and Haiku 4.5 (kept), and a prefill followed by a system message. Without the index.ts wiring the two Haiku 5.5 request tests fail.
• npx jest src/llm/bedrock: 15 suites, 224 passed.
• npx tsc --noEmit and ESLint on the touched files: clean (only pre-existing warnings).

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes

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📝 Code Review	✅ Completed 2026-10-17T16:44:45.953156Z	f52beae	New commits

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