Port: respect $SHELL on Windows when spawning child processes

[JohnnyVicious]

Port of openai/codex-plugin-cc#178 (merged)

Problem

On Windows, Node's child_process.spawn with shell: true uses COMSPEC (cmd.exe) by default. When the user runs under Git Bash or another POSIX-ish shell, commands that work in their interactive shell fail inside the plugin because the plugin spawned via cmd.exe instead.

plugins/opencode/scripts/lib/process.mjs::runCommand and resolveOpencodeBinary both use spawn("opencode"|"which", ...) without a shell: option at all today. This is different from the upstream issue shape — upstream has shell: process.platform === "win32" which picks cmd. Opencode has no shell, which means on Windows it can't find opencode.cmd, git.cmd, gh.cmd, etc. at all.

Both sides of the issue apply to us:

1. On Windows we need shell: true so .cmd shims resolve.
2. When we do pass shell: true, we should honor $SHELL for Git Bash users.

Fix (adapted from upstream)

lib/process.mjs::runCommand:

export function runCommand(cmd, args, opts = {}) {
  return new Promise((resolve) => {
    const shellOpt =
      process.platform === "win32"
        ? (process.env.SHELL || true)  // respect Git Bash if present, else cmd
        : false;

    const proc = spawn(cmd, args, {
      stdio: ["ignore", "pipe", "pipe"],
      cwd: opts.cwd,
      env: { ...process.env, ...opts.env },
      shell: shellOpt,
      windowsHide: true,
    });
    // ... rest unchanged ...
  });
}

lib/process.mjs::resolveOpencodeBinary:

which opencode doesn't work on Windows (no which binary). Use where on win32 and handle the .cmd shim:

export async function resolveOpencodeBinary() {
  return new Promise((resolve) => {
    const isWin = process.platform === "win32";
    const locator = isWin ? "where" : "which";
    const proc = spawn(locator, ["opencode"], {
      stdio: ["ignore", "pipe", "ignore"],
      shell: isWin ? (process.env.SHELL || true) : false,
      windowsHide: true,
    });
    let out = "";
    proc.stdout.on("data", (d) => (out += d));
    proc.on("close", (code) => {
      if (code !== 0) return resolve(null);
      // `where` returns all matches; take the first.
      resolve(out.trim().split(/\r?\n/)[0] ?? null);
    });
  });
}

lib/opencode-server.mjs::ensureServer already uses stdio: "ignore" + detached: true for opencode serve, but it still uses spawn("opencode", ...) without a shell option. Apply the same shellOpt fix there so .cmd shims resolve on Windows.

lib/process.mjs::spawnDetached — used by the background-task worker path — needs the same treatment:

export function spawnDetached(cmd, args, opts = {}) {
  const shellOpt =
    process.platform === "win32" ? (process.env.SHELL || true) : false;
  const child = spawn(cmd, args, {
    stdio: "ignore",
    detached: true,
    cwd: opts.cwd,
    env: { ...process.env, ...opts.env },
    shell: shellOpt,
    windowsHide: true,
  });
  child.unref();
  return child;
}

Test plan

1. On Windows with Git Bash installed, set SHELL=C:\Program Files\Git\bin\bash.exe, run /opencode:setup. Assert it locates opencode.cmd and reports the version.
2. On Windows without Git Bash, unset SHELL, run /opencode:setup. Assert cmd-shell fallback still works.
3. On POSIX (Linux/macOS), assert behavior is unchanged — the shell: false branch kicks in.
4. Confirm git and gh calls from lib/git.mjs (via runCommand) also find their .cmd shims on Windows.

Upstream reference

openai/codex-plugin-cc#178 (merged 2026-04-08). Related upstream context: #55 / #13 / #70 (Windows ENOENT cluster).

[JohnnyVicious]

Consolidated into #28. Windows SHELL/spawn behavior belongs with the same portable process spawning policy.