Three related Windows-only fixes that together make the browser toolset
actually usable on Windows. Symptom chain: user invokes browser_navigate
-> tool returns {"success": false, "error": "Daemon process exited
during startup with no error output"} and the CLI exits mid-turn with
the session summary.
Root cause (3 layers):
1. tools/browser_tool.py::_find_agent_browser() resolved
node_modules/.bin/agent-browser to the extensionless POSIX shell
shim via Path.exists(). On Windows, CreateProcessW cannot execute
that script (WinError 193 "not a valid Win32 application"). Fix:
delegate to shutil.which with path=node_modules/.bin so PATHEXT
picks up agent-browser.CMD on Windows and the extensionless shim
stays correct on POSIX.
2. Windows Terminal / Win32 delivers a spurious CTRL_C_EVENT to the
parent hermes.exe whenever a background thread spawns a .cmd
subprocess. Python 3.11's default SIGINT handler raises
KeyboardInterrupt in MainThread, which unwinds prompt_toolkit's
app.run() -> cli.py::run()'s finally block calls _run_cleanup()
-> _emergency_cleanup_all_sessions -> spawns a concurrent
_run_browser_command("close", ...) on the same session the agent
thread just opened. Two agent-browser processes race on the same
--session name, the daemon startup loses, and the tool returns
the "Daemon process exited during startup" error. Fix: install a
Windows-only SIGINT handler that absorbs the signal silently.
Real user Ctrl+C still routes through prompt_toolkit's own c-c
keybinding at the TUI layer, which is how Claude Code handles the
same quirk (driving cancellation via the TUI key handler, not
signals).
3. In tools/browser_tool.py, both Popen sites now pass
creationflags=CREATE_NO_WINDOW | STARTF_USESTDHANDLES with
close_fds=True on Windows. CREATE_NO_WINDOW suppresses the .cmd
console flash; STARTF_USESTDHANDLES + close_fds ensures the child
inherits only our three chosen handles (DEVNULL stdin, temp-file
stdout/stderr) and no leaked parent console handles that could
confuse agent-browser's native daemon spawn. Notably we do NOT
add CREATE_NEW_PROCESS_GROUP - on Python 3.11 Windows the flag
interacts badly with asyncio's ProactorEventLoop and makes things
worse.
Verified end-to-end on Windows 10 / Windows Terminal / PowerShell:
browser_navigate to https://example.com returns
{"success": true, "title": "Example Domain"} and the CLI stays alive
for follow-up tool calls and assistant turns.
Refs: earlier Windows quirks commits 1cebb3bad (Ctrl+Enter newline),
26f5af52a (environment hints), aefd1a37f (Playwright Chromium).
0ba1e12abc