v2.1.154+ breaks third-party API compatibility - system role injected mid-conversation

Preflight Checklist

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

What's Wrong?

Last working version:2.1.153
OS: Ubuntu 24 (Linux)
API used: DeepSeek API (third-party OpenAI-compatible endpoint)

Error
API Error: 400 Failed to deserialize the JSON body into the target type:
messages[1].role: unknown variant system, expected user or assistant

What happened

After updating to v2.1.154+, Claude Code started injecting a message
with role: "system" into the middle of the messages[] array.
Third-party APIs that follow strict OpenAI spec only accept
"user" or "assistant" roles in message turns. This makes
Claude Code completely unusable with third-party API providers.

Workaround

Downgraded to v2.1.153 - works fine.

What Should Happen?

claude code should work correctly with third-party openAI-compatible api's (such as deepseek) the same way it did in v2.1.153 and earlier. the messages array sent to the api should only contain "user" and
"assistant" roles in the conversation turns, with any system level instructions placed at the beginning as a proper system prompt not injected as a mid conversation message with role: "system".
this was working correctly before v2.1.154.

Error Messages/Logs

Steps to Reproduce

1. Install Claude Code v2.1.154 or higher
2. Configure a third-party OpenAI-compatible API endpoint (e.g. DeepSeek)

in Claude Code settings

1. Launch Claude Code: $ claude
2. Give any simple prompt, e.g. "write a hello world python script"
3. Observe the API error immediately

Error received:
"400 Failed to deserialize the JSON body into the target type:
messages[1].role: unknown variant system, expected user or assistant"

Note: Downgrading to v2.1.153 resolves the issue immediately with
no other changes. The same API key and endpoint configuration
works perfectly on v2.1.153.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.154, 2.1.156

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_