OpenAI-Compatible API Fails: reasoning_content Must Be Passed Back for Thinking Mode

What this error means

Error from provider (DeepSeek): The reasoning_content in the thinking mode must be passed back to the API is a OpenAI API failure pattern reported for developers trying to fix openai-compatible api error requiring reasoning_content to be passed back when using thinking/reasoning mode. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

Zed IDE issue #56639: DeepSeek-v4-pro tool calling fails via Open Code API with error ‘The reasoning_content in the thinking mode must be passed back to the API.’ Affects AI coding workflows using reasoning-capable models.

Common causes

• Developers using DeepSeek models (like deepseek-v4-pro) through OpenAI-compatible API endpoints in IDEs like Zed get invalid_request_error when the thinking mode’s reasoning_content is not properly forwarded. This breaks AI coding tool integrations.
• Zed IDE issue #56639: DeepSeek-v4-pro tool calling fails via Open Code API with error ‘The reasoning_content in the thinking mode must be passed back to the API.’ Affects AI coding workflows using reasoning-capable models.

Quick fixes

1. Confirm the exact error signature matches Error from provider (DeepSeek): The reasoning_content in the thinking mode must be passed back to the API.
2. Check the OpenAI API 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/zed-industries/zed/issues/56639

Evidence note: Zed IDE issue #56639: DeepSeek-v4-pro tool calling fails via Open Code API with error ‘The reasoning_content in the thinking mode must be passed back to the API.’ Affects AI coding workflows using reasoning-capable models.

Related errors

• DeepSeek API extended thinking mode error
• OpenAI-compatible API tool calling with reasoning

FAQ

What should I check first?

Start with the exact Error from provider (DeepSeek): The reasoning_content in the thinking mode must be passed back to the API text and the smallest action that reproduces it.

Can I ignore this error?

No. Treat it as a failed OpenAI API 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 Error from provider (DeepSeek): The reasoning_content in the thinking mode must be passed back to the API.