LiteLLM Mid-Stream Fallback Broken for Claude Models Without Assistant Prefill

What this error means

HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message. is a LiteLLM failure pattern reported for developers trying to fix litellm streaming fallback sending unsupported assistant prefill to claude models causing 400 error. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

Filed 2026-05-14. Detailed bug report with repro code. Affects LiteLLM Router with streaming fallbacks to Claude Sonnet 4.6/Opus 4.7. The escape hatches (disable_fallbacks, CustomLogger hooks) don’t work. Users resort to monkey-patching.

Common causes

• When a streaming request fails mid-stream, LiteLLM’s fallback mechanism unconditionally appends an assistant prefill block. Claude Sonnet 4.6/Opus 4.7 don’t support this, so fallback silently fails with 400. Users see the wrong error instead of the real upstream failure.
• Filed 2026-05-14. Detailed bug report with repro code. Affects LiteLLM Router with streaming fallbacks to Claude Sonnet 4.6/Opus 4.7. The escape hatches (disable_fallbacks, CustomLogger hooks) don’t work. Users resort to monkey-patching.

Quick fixes

1. Confirm the exact error signature matches HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message..
2. Check the LiteLLM account, local tool state, and provider configuration involved in the failing workflow.
3. Compare the failing environment with a known working setup, then change one configuration value at a time.

Platform/tool-specific checks

• Verify the command, editor, extension, or API client that produced the error.
• Compare local settings with CI, deployment, or editor-level settings when the error appears in only one environment.
• Avoid deleting credentials, local model data, or project settings until the failing scope is clear.

Step-by-step troubleshooting

1. Capture the exact error message and the command, editor action, or request that triggered it.
2. Check whether the failure is account/auth, quota/rate, model/provider, local runtime, or deployment configuration.
3. Review the source evidence below and compare it with your environment.
4. Apply one change at a time and rerun the smallest failing action.
5. Keep the working fix documented for the team or deployment environment.

How to prevent it

• Keep provider/tool configuration documented.
• Record non-secret diagnostics such as tool version, provider name, model name, and command path.
• Add a lightweight check before CI or production workflows depend on the tool.

Sources checked

• https://github.com/BerriAI/litellm/issues/27967

Evidence note: Filed 2026-05-14. Detailed bug report with repro code. Affects LiteLLM Router with streaming fallbacks to Claude Sonnet 4.6/Opus 4.7. The escape hatches (disable_fallbacks, CustomLogger hooks) don’t work. Users resort to monkey-patching.

Related errors

• LiteLLM streaming fallback assistant prefill
• LiteLLM MidStreamFallbackError Claude
• LiteLLM disable_fallbacks ignored mid-stream

FAQ

What should I check first?

Start with the exact HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message. text and the smallest action that reproduces it.

Can I ignore this error?

No. Treat it as a failed LiteLLM workflow until the root cause is understood.

Is this guaranteed to have one fix?

No. The imported evidence supports the troubleshooting path above, but tool behavior can vary by account, plan, version, provider, and local configuration.

How do I know the fix worked?

Rerun the same command, editor action, or request. The fix is working when that action completes without HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message..