[BUG] Opus 4.8 leaks raw tool-call syntax (<invoke>/call) as text, killing the turn

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?

• Tool calls intermittently render as literal text (e.g. "call <invoke ...>")

in the reply body instead of being executed, which corrupts/stalls the turn.

• Happens even on the FIRST tool call of a fresh session.
• Reproducible on claude-opus-4-8. Switching to claude-sonnet-4-6 in the

SAME session/workspace makes it disappear entirely.

• Environment: Claude Code, VS Code extension, Windows 10.

What Should Happen?

• Tool calls intermittently render as literal text (e.g. "call <invoke ...>")

in the reply body instead of being executed, which corrupts/stalls the turn.

• Happens even on the FIRST tool call of a fresh session.
• Reproducible on claude-opus-4-8. Switching to claude-sonnet-4-6 in the

SAME session/workspace makes it disappear entirely.

• Environment: Claude Code, VS Code extension, Windows 10.

Error Messages/Logs

Steps to Reproduce

1. Open Claude Code via the VS Code extension on Windows 10.
2. Start a fresh session and set the model to Opus 4.8 (claude-opus-4-8).
3. Give any task that requires the assistant to call a tool

(e.g. "read this file and check X", "run git status").

1. Observe: instead of actually executing the tool, the assistant emits the

raw tool-call syntax as plain text in the reply body, e.g.:

call
<invoke name="Bash">
<parameter name="command">...</parameter>

The turn then stalls / dies — the tool never runs.

Additional notes:

• This happens even on the FIRST tool call of a brand-new session.
• It is NOT fixed by instructions/system prompt rules telling the model not to

do it — the model intends to call the tool but the call leaks as text at the
decoding level.

• Switching the model to Sonnet 4.6 (claude-sonnet-4-6) in the SAME session and

workspace makes the problem disappear completely; tools execute normally.

• So the issue appears specific to Opus 4.8 decoding/tool-call emission, not the

workspace or task.

Environment:

• Claude Code, VS Code extension
• Windows 10 Pro (10.0.19045)
• Model where it breaks: claude-opus-4-8
• Model where it works: claude-sonnet-4-6

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

2.1.181

Claude Code Version

2.1.181

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_