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The `reasoning_content` in the thinking mode must be passed back to the API. Codex

Quick fix

Configure DeepSeek as native mode instead of the default; if it still fails, set model_reasoning_effort to none in Codex.

Symptom

Section titled “Symptom”
Error output
CC Switch local proxy failed while handling Codex endpoint /responses. Provider: DeepSeek; model: deepseek-v4-flash; upstream_status: HTTP 400; cause: The `reasoning_content` in the thinking mode must be passed back to the API.

Cause

Section titled “Cause”

In thinking mode, Codex carries reasoning items in subsequent requests. When CC Switch's local proxy converts them to the DeepSeek chat endpoint, the upstream requires `reasoning_content` to be passed back unchanged. If this requirement is not met, it returns HTTP 400, after which Codex may hit stream broken.

Users have confirmed that switching DeepSeek's advanced option from the default to native mode fixes this. Other reports reproduced on CC Switch 3.16.5 show that upgrading alone may not help; after disabling Codex's reasoning effort, multi-turn tool calls no longer trigger this error.

Fix

Section titled “Fix”

  1. In the advanced options of the DeepSeek configuration in CC Switch, if the provider offers a native mode, select native mode instead of the default mode.

  2. If multi-turn tool calls still fail after enabling native mode, disable Codex's reasoning effort.

    ~/.codex/config.toml
    model_reasoning_effort = "none"

Affected Versions

Section titled “Affected Versions”
ToolCodex
VersionUnknown
PlatformsWindows

Source Issues

Section titled “Source Issues”

This page is distilled from 2 real issues

FAQ

Section titled “FAQ”
Is there a fix?
First select native mode in the advanced options of the DeepSeek configuration; if it still fails, set `model_reasoning_effort = "none"` in `~/.codex/config.toml`.
Where exactly do I change this? I can't find it.
The confirmed location is the advanced options of that provider's configuration in CC Switch; the issue does not provide a more specific UI path.
Section titled “Related problems”
invalid thinkingKimi's deepseek-v4-flash rejects the current thinking parameter; the original post provides no verified solution.1 issues mergedCC Switch is currently a black box; users cannot customize processing logicThis is a feature request for programmable routing transformation scripts in CC Switch; the original post has no verifiable official solution.1 issues merged<thinking> tags output as body text in Claude CodeChanging the provider's apiFormat from openai_chat to anthropic, if supported upstream, can bypass inline <thinking> leakage.1 issues merged

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