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Invalid parameter: This model does not support assistant message prefill - Codex

Quick fix

Upgrade to CC Switch 4.0 or the latest version and retry. If it still fails, submit complete reproduction details and logs.

Symptom

Section titled “Symptom”
Error output
Invalid parameter: ValidationException: This model does not support assistant message prefill.
The conversation must end with a user message.

Cause

Section titled “Cause”

In CC Switch v3.16.1, Codex takeover uses local routing to convert and forward `/responses` requests to a custom OpenAI-compatible endpoint that uses the Chat Completions protocol. If the forwarded `messages` end with an `assistant` message and the upstream model does not support assistant message prefill, the upstream service returns HTTP 400.

The reporter suggested adding a `strip_assistant_prefill` rectifier to remove the trailing `assistant` message before forwarding or merge it into the preceding `user` message. However, the issue was closed after timing out without new reproduction details, not as a confirmed fix. The maintainer only stated that 4.0 substantially refactored the relevant functionality and recommended updating and retrying.

Fix

Section titled “Fix”

  1. Update CC Switch to 4.0 or the latest version, then retest with the same Codex provider, Model Mapping, Local Routing, and Codex takeover configuration.

  2. If the problem persists after updating, resubmit the CC Switch version, macOS information, complete reproduction steps, and relevant logs or screenshots.

Affected Versions

Section titled “Affected Versions”
ToolCodex
Version受影响版本：3.16.1；4.0 是否修复未确认
PlatformsmacOS

Source Issues

Section titled “Source Issues”

This page is distilled from 1 real issues

FAQ

Section titled “FAQ”

(No FAQ yet)

Section titled “Related problems”
api failed deserialize json body intoDepending on the error: roll Claude Code back, upgrade cc-switch, turn the local proxy off, or stop sending unsupported fields.8 issues mergedthinking too short CodexStart a new Codex conversation to temporarily avoid it. The original post suggests filtering empty or overly short thinking; an official fix is unverified.1 issues mergedThe `content[].thinking` in thinking modeSome variants remain unresolved: upgrade to a version with the thinking history normalization fix; for Codex routed through DeepSeek, use a version with the…13 issues merged

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