Bun.spawn -> CreateProcess only appends `.exe` to extensionless names; `.cmd`/`.bat` are never tried. Bare MCP commands like `npx` (which exists only as `npx.cmd` on Windows) crashed the subprocess ~140ms after spawn with ENOENT/EINVAL whenever our own PATH walk couldn't pin the file down (empty `Bun.env.PATH` under a restricted parent process, UNC mounts that reject `fs.access`, locked-down shells).
`resolveStdioSpawnCommand` now routes any unresolved bare command through `cmd.exe /d /s /c` so Windows's PATHEXT search runs Windows-native. Direct-spawn fast path is preserved for resolved `.exe`/`.com` files; the existing `.cmd`/`.bat` wrap is unchanged. The reporter's stated cause ("process.env not merged") was incorrect — the merge has been in place at `transports/stdio.ts:316-319` since well before 16.1.14 — but the symptom they hit is real.
Fixes#3250
roboomp
0b88ab32f8