LiteLLM / LiteLLM
LiteLLM Chat Completions to Responses API Bridge tool_choice 400 Error
Fix LiteLLM 400 error when using tool_choice named function format with GPT-5.4/5.5 models Includes evidence for LiteLLM troubleshooting demand.
- Category
- LiteLLM
- Error signature
400 Bad Request - tool_choice named function format- Quick fix
- Compare the failing environment with a known working setup, then change one configuration value at a time.
- Updated
What this error means
400 Bad Request - tool_choice named function format is a LiteLLM failure pattern reported for developers trying to fix litellm 400 error when using tool_choice named function format with gpt-5.4/5.5 models. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.
Why this happens
GitHub issue #27611 in BerriAI/litellm. Affects all GPT-5.4 and GPT-5.5 models through LiteLLM’s Chat Completions → Responses API bridge.
Common causes
- Calling GPT-5.4 or GPT-5.5 through LiteLLM’s Chat Completions endpoint with named tool_choice (type: function, function: {name: …}) causes a 400 error because LiteLLM’s bridge to the Responses API passes the parameter incorrectly.
- GitHub issue #27611 in BerriAI/litellm. Affects all GPT-5.4 and GPT-5.5 models through LiteLLM’s Chat Completions → Responses API bridge.
Quick fixes
- Confirm the exact error signature matches
400 Bad Request - tool_choice named function format. - 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.
Sources checked
Evidence note: GitHub issue #27611 in BerriAI/litellm. Affects all GPT-5.4 and GPT-5.5 models through LiteLLM’s Chat Completions → Responses API bridge.
Related errors
- LiteLLM tool use error OpenAI
- LiteLLM function calling 400
- OpenAI Responses API tool_choice error
FAQ
What should I check first?
Start with the exact 400 Bad Request - tool_choice named function format 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 400 Bad Request - tool_choice named function format.