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The API returns a 400 error - Claude Code

Quick fix

Upgrade CC Switch to v3.16.1 for the 400 caused by MiMo missing reasoning_content; on Github Copilot, check the API format and proxy config.

Symptom

Section titled “Symptom”
Error output
API 返回 400 错误
English translation

The API returns a 400 error

Cause

Section titled “Cause”

A 400 on a request proxied through CC Switch in Claude Code comes mainly from two mechanisms. First, the Xiaomi MiMo model requires the reasoning_content field to be sent back in a multi-turn conversation whose history contains tool calls; Claude Code does not support that field natively and drops it, and early CC Switch passed the request straight through, so the next request lacked the field and was rejected. Second, on the Github Copilot provider, the API format (OpenAI-Response versus Anthropic native) does not match the proxy settings, or a token refresh fails, and the request fails.

Fix

Section titled “Fix”

  1. Upgrade CC Switch to v3.16.1 or newer. That version added caching and completion of reasoning_content on the pass-through path, which resolves the MiMo 400.

  2. On the Github Copilot provider, try switching the API format in CC Switch settings (OpenAI-Response or Anthropic native) and make sure the local proxy is on.

  3. If the Copilot problem persists, try logging in to the Copilot account again, or uninstall CC Switch, delete the config directory (~/.cc-switch on macOS), and reinstall.

Affected Versions

Section titled “Affected Versions”
ToolClaude Code
Version< 3.16.1
PlatformsWindowsmacOSLinux

Source Issues

Section titled “Source Issues”

This page is distilled from 2 real issues

FAQ

Section titled “FAQ”
Why does configuring MiMo directly in Claude Code also error, when the Xiaomi announcement does not mention Claude Code?
Claude Code connecting to the official Anthropic API natively does not hit this, because the Anthropic API has no reasoning_content field. The constraint is only triggered when proxying through CC Switch to the MiMo Anthropic-compatible endpoint.
The Github Copilot test model passes but real use reports Error 400. How do I investigate?
Check the API format setting and try logging in to the Copilot account again. To investigate further, look at the CC Switch log (Windows: %APPDATA%\com.farion.ccswitch\logs\cc-switch.log, macOS: ~/Library/Application Support/com.farion.ccswitch/logs/cc-switch.log, Linux: ~/.config/com.farion.ccswitch/logs/cc-switch.log).
Section titled “Related problems”
Warmup request downgraded to modelTurn the Copilot warmupDowngrade option off in CC Switch so requests are not silently replaced with gpt-5-mini.5 issues mergedEGL_BAD_PARAMETER white screen error - cc-switchInstall the native .deb or AUR package instead, or run the AppImage with LD_PRELOAD pointing at the system library, to resolve the ABI conflict.6 issues mergedThe interface shows "query failed"Upgrade past v3.16.2 for the query routing fix, or use a custom balance query template. Put hooks in general config, and raise the failover timeout.4 issues merged

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