Session-start hook reports "The Vercel CLI is not installed" on Windows — execFileSync on vercel.cmd throws EINVAL

What happened

On Windows the session-start hook always injects this into the agent's context:

IMPORTANT: The Vercel CLI is not installed.
Strongly recommend the user install it with npm i -g vercel to unlock agentic features like vercel env pull, vercel deploy, and vercel logs.

The CLI is installed and authenticated:

$ vercel --version
Vercel CLI 56.5.0
$ vercel whoami
<my username>

The practical cost is not the wrong line, it's what the agent does with it: it takes the statement at face value and avoids vercel ls / vercel inspect for the rest of the session, so it can't check its own deployments.

Environment

• Plugin: vercel 0.48.0 (installed from anthropics/claude-plugins-official)
• OS: Windows 11
• Node: v24.13.0
• vercel installed globally with npm, so it resolves to %APPDATA%\npm\vercel.cmd

Cause

In hooks/src/session-start-profiler.mts (and identically in the compiled hooks/session-start-profiler.mjs that actually runs):

1. checkVercelCli() (~line 416) resolves the binary correctly. resolveBinaryFromPath does append the Windows executable extensions and finds vercel.cmd, so this part is not the bug.
2. It then calls execFileSync(vercelBinary, ["--version"]) at ~line 425 without shell: true.
3. Since Node 18.20 / 20.12 (the CVE-2024-27980 hardening), spawning a .cmd or .bat without a shell throws EINVAL.
4. The catch at ~line 433 swallows it and returns { installed: false } — which is what produces the message.

So the hook finds the CLI and then concludes it does not exist.

Minimal repro

node -e "const{execFileSync}=require('child_process');const p=require('path').join(process.env.APPDATA,'npm','vercel.cmd');try{execFileSync(p,['--version'],{stdio:['ignore','pipe','ignore']});console.log('ok')}catch(e){console.log(e.code)}"

Prints EINVAL. Adding shell: true (with the path quoted) makes it print ok.

Blast radius

• The npm view call at ~line 449 has exactly the same shape and would fail the same way, but it is unreachable: the catch on the first call returns before it.
• The "CLI is outdated (x → y)" branch lives in the else of installed, so on Windows it can never fire. Windows users don't get a false statement there — they get no upgrade notice at all.
• Any npm-installed CLI on Windows resolves to a .cmd shim, so the pattern isn't specific to vercel.

Suggested fix

Spawn through a shell on Windows when the resolved path ends in .cmd / .bat, or resolve to the JS entry point behind the shim instead of the shim itself. A bare shell: true needs care with argument escaping, so quoting the resolved path explicitly is probably the safer form.

Start in hooks/src/session-start-profiler.mts at checkVercelCli and compare it with the compiled hooks/session-start-profiler.mjs that runs. Run the Windows minimal reproduction and inspect the nearby npm view call for the same spawning pattern. Done means Windows npm-installed .cmd and .bat CLIs are detected and the session-start hook no longer reports an installed CLI as missing.