[BUG] Opus 4.7 (1M context) stream stalls at ~660-730 bytes in every session — VS Code extension

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?

Streaming consistently stalls very early in every assistant turn when running
Opus 4.7 (1M context) via the Claude Code VS Code extension. The stream emits
one initial chunk, then goes idle indefinitely until the session ends with
had_error=true. This happens even in freshly started sessions with no prior
history and no MCP image calls — so it is not a session-size or transcript-bloat
issue.

Pattern in every failing turn:

1. Stream starts, first chunk received
2. After ~660-730 bytes of output, stream goes idle
3. [WARN] stream_idle_partial fires every 15s
4. sdk_stream_ended_no_result with had_error=true

What Should Happen?

The stream should continue sending chunks normally until the assistant turn
completes, as it does on previous model versions.

Error Messages/Logs

17:01:13 [WARN] [Stall] stream_idle_partial lastChunkAgeMs=15001 bytesTotal=661 idleDeadlineMs=300000
17:01:29 sdk_stream_ended_no_result had_error=true subagent_count=0 message_count=28
17:01:40 [WARN] [Stall] stream_idle_partial lastChunkAgeMs=15001 bytesTotal=697
17:01:55 [WARN] [Stall] stream_idle_partial lastChunkAgeMs=30004 bytesTotal=697
17:02:10 [WARN] [Stall] stream_idle_partial lastChunkAgeMs=15005 bytesTotal=733

Steps to Reproduce

1. Open Claude Code in the VS Code extension on macOS
2. Set model to Opus 4.7 (1M context) via /model
3. Start any new session, send any prompt that requires a non-trivial response
4. Observe: stream emits ~660-730 bytes then stalls
5. After ~30s of no chunks, session ends with had_error=true

Reproduces every time, across multiple fresh sessions and projects.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.141 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_