LiteLLM / LiteLLM

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

Fix LiteLLM streaming fallback sending unsupported assistant prefill to Claude models causing 400 error Includes evidence for LiteLLM troubleshooting demand.

Source-backed Last updated May 15, 2026 1 source Needs local verification

Category: LiteLLM
Error signature: HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message.
Quick fix: Compare the failing environment with a known working setup, then change one configuration value at a time.
Updated: May 15, 2026
Verification status: Source-backed
Evidence: 1 public source URL

Before you change production

This page includes public source URLs in the imported troubleshooting record. Compare those references with your version and environment before applying changes.

Reproduce the smallest failing action and save non-secret logs before changing configuration.
Check versions for LiteLLM, related SDKs, package managers, CI runners, and hosting providers.
Change one setting or dependency at a time, then rerun the same failing command or request.
Avoid destructive commands, credential rotation, billing changes, or security relaxations without a rollback plan.

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

Confirm the exact error signature matches HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message..
Check the LiteLLM account, local tool state, and provider configuration involved in the failing workflow.
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

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

Diagnostic flow for this page

Match HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message. exactly before applying the quick fix.
Compare the failing environment with LiteLLM versions, account scope, provider settings, and deployment context.
Check the listed common causes in order, starting with the cause that best matches your logs.
Use the evidence status below to decide whether to confirm against public sources or official documentation.
Apply one reversible change, rerun the smallest failing action, and keep rollback notes.

FAQ

What does HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message. mean in LiteLLM?

HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message. indicates the workflow described by "LiteLLM Mid-Stream Fallback Broken for Claude Models Without Assistant Prefill" failed in a LiteLLM context. Start by confirming the exact signature and the component that emitted it.

What should I check first for HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message.?

Check whether this cause matches your environment: 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.

Is the HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message. guidance source-backed?

The "LiteLLM Mid-Stream Fallback Broken for Claude Models Without Assistant Prefill" page is labeled source-backed. It has 1 public source URL from github.com, so use that evidence status when deciding how much additional verification you need.

When should I avoid the quick fix for HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message.?

For "LiteLLM Mid-Stream Fallback Broken for Claude Models Without Assistant Prefill", avoid applying "Compare the failing environment with a known working setup, then change one configuration value at a time." when the error text differs, the affected version or provider is different, or the change would touch credentials, billing, DNS, production deploy settings, or irreversible data.

What should I collect before debugging HTTP 400: This model does not support assistant message prefill. The conversation must end with a user message.?

For "LiteLLM Mid-Stream Fallback Broken for Claude Models Without Assistant Prefill", collect the failing LiteLLM command or request, non-secret configuration, versions, timestamps, request IDs when available, and the smallest reproduction that still triggers the signature.