LiteLLM 400 Error with Codex CLI and Domestic LLM Models

What this error means

400 Bad Request — unsupported parameter passed to model API is a LiteLLM failure pattern reported for developers trying to fix 400 error when using litellm with codex cli and chinese/domestic llm models. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

OpenAI Codex CLI uses Responses API protocol; domestic Chinese models only support Chat Completions API. LiteLLM’s protocol translation passes unsupported fields (client_metadata, reasoning_effort, reasoning, coding_plan, parallel_tool_calls, stream_options, modalities, prediction, audio, store, include, prompt_cache_key), causing 400 errors. Fix requires parameter filtering per model type.

Common causes

• Developers using OpenAI Codex CLI (v0.130.0) with LiteLLM proxy and Chinese/domestic LLM models get 400 errors because LiteLLM’s protocol translation layer passes Codex-specific parameters (client_metadata, reasoning_effort, coding_plan, parallel_tool_calls) that domestic models don’t support.
• OpenAI Codex CLI uses Responses API protocol; domestic Chinese models only support Chat Completions API. LiteLLM’s protocol translation passes unsupported fields (client_metadata, reasoning_effort, reasoning, coding_plan, parallel_tool_calls, stream_options, modalities, prediction, audio, store, include, prompt_cache_key), causing 400 errors. Fix requires parameter filtering per model type.

Quick fixes

1. Confirm the exact error signature matches 400 Bad Request — unsupported parameter passed to model API.
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/pull/27604

Evidence note: OpenAI Codex CLI uses Responses API protocol; domestic Chinese models only support Chat Completions API. LiteLLM’s protocol translation passes unsupported fields (client_metadata, reasoning_effort, reasoning, coding_plan, parallel_tool_calls, stream_options, modalities, prediction, audio, store, include, prompt_cache_key), causing 400 errors. Fix requires parameter filtering per model type.

Related errors

• OpenAI API invalid parameter error
• LiteLLM model routing error
• Codex CLI API compatibility issue

FAQ

What should I check first?

Start with the exact 400 Bad Request — unsupported parameter passed to model API 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 — unsupported parameter passed to model API.