LiteLLM Mid-Stream Fallback Breaks Claude Models — Includes Unsupported Assistant Prefill

What this error means

LiteLLM mid-stream fallback request includes assistant prefill block, breaks Claude Sonnet 4.6 / Opus 4.7 is a LiteLLM failure pattern reported for developers trying to fix litellm mid-stream fallback including assistant prefill block that breaks claude model compatibility. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

Router.stream_with_fallbacks adds assistant prefill when constructing fallback request. Claude models reject requests with assistant prefill + prefix=True. Fallback mechanism itself fails, leaving users with no recovery path.

Common causes

• When a streaming request fails mid-stream (timeout, 5xx), Router.stream_with_fallbacks constructs fallback with an extra assistant prefill block. Claude Sonnet 4.6 and Opus 4.7 don’t support prefix=True, causing the fallback to fail too.
• Router.stream_with_fallbacks adds assistant prefill when constructing fallback request. Claude models reject requests with assistant prefill + prefix=True. Fallback mechanism itself fails, leaving users with no recovery path.

Quick fixes

1. Confirm the exact error signature matches LiteLLM mid-stream fallback request includes assistant prefill block, breaks Claude Sonnet 4.6 / Opus 4.7.
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: Router.stream_with_fallbacks adds assistant prefill when constructing fallback request. Claude models reject requests with assistant prefill + prefix=True. Fallback mechanism itself fails, leaving users with no recovery path.

Related errors

• LiteLLM fallback Claude error
• LiteLLM streaming fallback broken
• LiteLLM assistant prefill unsupported

FAQ

What should I check first?

Start with the exact LiteLLM mid-stream fallback request includes assistant prefill block, breaks Claude Sonnet 4.6 / Opus 4.7 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 LiteLLM mid-stream fallback request includes assistant prefill block, breaks Claude Sonnet 4.6 / Opus 4.7.