[BUG] `stop_reason` not parsed from `message_delta` when using `ANTHROPIC_BASE_URL` proxy

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?

When using ANTHROPIC_BASE_URL to route requests through a proxy, Claude Code fails to parse stop_reason from the message_delta streaming event. The stop_reason is logged as null even when the proxy correctly sends "stop_reason": "tool_use" in the message_delta event.

This causes tool_use responses to not trigger tool execution, making the conversation stop abruptly after the model attempts to use a tool.

Proxy chain:

Claude Code → LiteLLM (Anthropic format) → kiro-openai-gateway (OpenAI format) → Kiro

Evidence:

• Direct Anthropic API logs show stop_reason: "tool_use" correctly (43 occurrences in one session)
• Proxy logs show stop_reason: null with tool_use content present
• The proxy output is identical to Anthropic's documented streaming format

Proxy streaming output (correct format):

event: message_delta
data: {"type": "message_delta", "delta": {"stop_reason": "tool_use", "stop_sequence": null}, "usage": {"input_tokens": 608, "output_tokens": 46}}

What Should Happen?

Claude Code should parse stop_reason from the message_delta event when using ANTHROPIC_BASE_URL, the same way it does when connecting directly to the Anthropic API.

Steps to Reproduce

1. Set up a proxy that returns Anthropic-compatible streaming format (e.g., LiteLLM with anthropic_messages endpoint)
2. Run Claude Code with ANTHROPIC_BASE_URL=http://localhost:4000 claude
3. Send a prompt that triggers tool use (e.g., "search the web for X" with WebSearch tool enabled)
4. Observe that the conversation stops after the model returns tool_use, without executing the tool
5. Check conversation logs in ~/.claude/projects/ - stop_reason will be null

Claude Model

Opus (via Kiro)

Is this a regression?

I don't know

Last Working Version

No response

Claude Code Version

2.1.1 (Claude Code)

Platform

Anthropic API (via ANTHROPIC_BASE_URL proxy)

Operating System

macOS

Terminal/Shell

zsh

Additional Information

Tested with:

• LiteLLM proxy converting OpenAI format to Anthropic format
• Custom mock server sending exact Anthropic streaming format
• Both produce the same result: stop_reason: null in logs

The proxy streaming format was verified to match Anthropic's documented format exactly, including stop_sequence: null in the delta.