[BUG] --chrome-native-host crashes with Bun assertion failure on Windows (all versions)

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?

The claude.exe --chrome-native-host command crashes immediately on startup with a Bun internal assertion
failure on Windows. This prevents the Claude in Chrome browser automation feature from working entirely.

The crash occurs after the socket listener path is created but before any messages are processed.

### Versions Tested

| Claude Code | Bun Version | Result |
|-------------|-------------|--------|
| 2.1.37 | Bun Canary v1.3.9-canary.51 | Crash |
| 2.1.36 | Bun Canary v1.3.9-canary.51 | Crash |
| 2.1.30 | Bun v1.3.5 | Crash |
| 2.1.0 | Bun v1.3.5 | Crash |
| 2.0.0 | N/A | --chrome-native-host not supported |

### Analysis

The crash appears to be in Bun's stdin handling (Bun.stdin(2)) when used as a Windows standalone executable.
The native messaging host protocol requires reading length-prefixed binary messages from stdin (piped from
Chrome), and this triggers the Bun assertion failure.

Key observations:

• Crash happens regardless of whether Chrome is the caller or stdin is /dev/null
• Same crash across multiple Bun versions (1.3.5 and 1.3.9-canary.51)
• The named pipe path \\.\pipe\claude-mcp-browser-bridge-<user> is created but never becomes functional
• All native messaging host configuration is correct (registry, JSON manifest, bat file, extension permissions)

### Verified Configuration

• Registry: HKCU\Software\Google\Chrome\NativeMessagingHosts\com.anthropic.claude_code_browser_extension →

points to correct JSON

• JSON manifest: correct allowed_origins, correct path to bat file
• Bat file: correctly calls claude.exe --chrome-native-host
• Extension: enabled, correct ID, correct permissions including "Communicate with cooperating native

applications"

• No Chrome enterprise policies blocking native messaging

What Should Happen?

claude.exe --chrome-native-host should start, create the named pipe, and bridge Chrome native messaging to the
MCP browser server without crashing. The Claude in Chrome browser automation tools should connect successfully.

Error Messages/Logs

[Claude Chrome Native Host] Initializing...
  [Claude Chrome Native Host] Creating socket listener: \\.\pipe\claude-mcp-browser-bridge-<user>
  ============================================================
  Bun Canary v1.3.9-canary.51 (d5628db2) Windows x64 (baseline)
  Windows v.win11_ge
  CPU: sse42 avx avx2
  Args: "C:\Users\<user>\.local\bin\claude.exe" "--chrome-native-host"
  Features: Bun.stdin(2) jsc standalone_executable
  Builtins: "bun:main" "node:buffer" "node:child_process" "node:crypto" "node:fs" ...
  Elapsed: 92ms | User: 0ms | Sys: 15ms
  RSS: 0.32GB | Peak: 0.32GB | Commit: 0.47GB | Faults: 78279 | Machine: 68.53GB

  panic(main thread): Internal assertion failure
  oh no: Bun has crashed. This indicates a bug in Bun, not your code.

  Bun crash report: https://bun.report/1.3.9/e_2d5628dbEkgggC+s1oU+vvkOu8t/uBysi5uB__qt24tCA0eNrzzCtJLcpLzFFILC5OLS
  rJzM9TSEvMzCktSgUAiSkKPg

Steps to Reproduce

Steps to Reproduce

1. Install Claude Code on Windows (any version with Chrome support: 2.1.0+)
2. Install the "Claude in Chrome" extension from Chrome Web Store (v1.0.47)
3. Run directly: "C:\Users\<user>\.local\bin\claude.exe" --chrome-native-host
4. Observe immediate crash with Bun assertion failure

Alternative reproduction:

1. Start Claude Code with claude --chrome
2. Attempt any mcp__claude-in-chrome__* tool call
3. Get "Browser extension is not connected" error

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.37 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

• Windows 11 build 22631.6199
• Chrome extension service worker shows no errors (the native host crashes before Chrome can communicate with it)
• The "browser is managed by your organization" flag is present but only due to a single

LocalNetworkAccessAllowedForUrls policy — no native messaging restrictions

• Tested with both stdin from Chrome and from /dev/null — same crash
• Bun crash report URL: https://bun.report/1.3.9/e_2d5628dbEkgggC+s1oU+vvkOu8t/uBysi5uB__qt24tCA0eNrzzCtJLcpLzFFI

LC5OLSrJzM9TSEvMzCktSgUAiSkKPg