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claude-code - 💡(How to fix) Fix [BUG] API 400 "messages[1].role must be either 'user' or 'assistant', but got 'system'" (v2.1.156)

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Error Message

API Error: 400 messages[1].role must be either 'user' or 'assistant', but got 'system'

What Should Happen?

Claude Code should either:

place system prompts in the top-level system field or ensure messages[] only contains user / assistant roles

The request should be accepted by the Anthropic API.

Error Messages/Logs

Code Example

API Error: 400 messages[1].role must be either 'user' or 'assistant', but got 'system'

### What Should Happen?

Claude Code should either:

place system prompts in the top-level system field
or ensure messages[] only contains user / assistant roles

The request should be accepted by the Anthropic API.

### Error Messages/Logs
RAW_BUFFERClick to expand / collapse

Preflight Checklist

  • I have searched existing issues and this hasn't been reported yet
  • This is a single bug report (please file separate reports for different bugs)
  • I am using the latest version of Claude Code

What's Wrong?

Bug Description

Claude Code 2.1.156 appears to send an invalid system role inside the messages array when calling the Anthropic Messages API.

This results in the following API error:

API Error: 400 messages[1].role must be either 'user' or 'assistant', but got 'system'

### What Should Happen?

Claude Code should either:

place system prompts in the top-level system field
or ensure messages[] only contains user / assistant roles

The request should be accepted by the Anthropic API.

### Error Messages/Logs

```shell
API Error: 400 messages[1].role must be either 'user' or 'assistant', but got 'system'

Steps to Reproduce

  1. Start Claude Code normally

  2. Enter any prompt, for example: test

Claude Model

Other

Is this a regression?

Yes, this worked in a previous version

Last Working Version

No response

Claude Code Version

2.1.156

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

<img width="1178" height="324" alt="Image" src="https://github.com/user-attachments/assets/c886c2ed-e9d3-4b4a-ba6a-96dd0484fd2d" />

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