fix(mcp-shrink): shell:true on Windows so npx/.cmd upstream resolves

Folds in #387 — extract spawn options to spawn-options.js; shell:true only on win32 (PATHEXT resolution), POSIX unchanged. Args still come from installer-controlled config (trust boundary unchanged).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/mcp-servers/caveman-shrink/index.js

@@ -40,9 +40,9 @@ const debug = process.env.CAVEMAN_SHRINK_DEBUG === '1';
 const fields = (process.env.CAVEMAN_SHRINK_FIELDS || 'description')
   .split(',').map(s => s.trim()).filter(Boolean);
 
-const upstream = spawn(args[0], args.slice(1), {
-  stdio: ['pipe', 'pipe', 'inherit'],
-});
+const { getSpawnOptions } = require('./spawn-options');
+
+const upstream = spawn(args[0], args.slice(1), getSpawnOptions());
 
 upstream.on('error', err => {
   process.stderr.write(`caveman-shrink: failed to spawn upstream: ${err.message}\n`);

src/mcp-servers/caveman-shrink/spawn-options.js

@@ -0,0 +1,21 @@
+// Spawn options for the upstream MCP child process.
+//
+// Windows: spawn('npx', ...) (and any .cmd shim such as 'gemini') hits ENOENT
+// because PATHEXT resolution only happens when child_process spawns through
+// a shell. POSIX systems resolve fine without a shell. Keep shell:false on
+// POSIX to avoid argv quoting surprises.
+//
+// Exported standalone so the behavior is unit-testable without re-running
+// the CLI entry point (index.js exits immediately when args are empty).
+
+'use strict';
+
+function getSpawnOptions(platform = process.platform) {
+  return {
+    stdio: ['pipe', 'pipe', 'inherit'],
+    shell: platform === 'win32',
+    windowsHide: true,
+  };
+}
+
+module.exports = { getSpawnOptions };

tests/test_mcp_shrink.js

@@ -9,6 +9,9 @@ const ROOT = path.resolve(__dirname, '..');
 const { compress, compressDescriptionsInPlace } = require(
   path.join(ROOT, 'src', 'mcp-servers', 'caveman-shrink', 'compress.js')
 );
+const { getSpawnOptions } = require(
+  path.join(ROOT, 'src', 'mcp-servers', 'caveman-shrink', 'spawn-options.js')
+);
 
 let passed = 0;
 let failed = 0;
@@ -123,5 +126,34 @@ test('compressDescriptionsInPlace skips non-string description fields', () => {
   assert.deepStrictEqual(obj.description, { not: 'a string' });
 });
 
+// spawn-options: upstream MCP child process spawn flags.
+// Confirms shell:true on Windows (so npx and other .cmd shims resolve) and
+// shell:false on POSIX.
+
+test('win32 enables shell so npx and .cmd shims resolve', () => {
+  const opts = getSpawnOptions('win32');
+  assert.equal(opts.shell, true);
+  assert.equal(opts.windowsHide, true);
+  assert.deepEqual(opts.stdio, ['pipe', 'pipe', 'inherit']);
+});
+
+test('linux keeps shell off to avoid argv quoting surprises', () => {
+  const opts = getSpawnOptions('linux');
+  assert.equal(opts.shell, false);
+  assert.deepEqual(opts.stdio, ['pipe', 'pipe', 'inherit']);
+});
+
+test('darwin keeps shell off', () => {
+  const opts = getSpawnOptions('darwin');
+  assert.equal(opts.shell, false);
+});
+
+test('defaults to current platform when no arg passed', () => {
+  const opts = getSpawnOptions();
+  assert.equal(opts.shell, process.platform === 'win32');
+  assert.equal(opts.windowsHide, true);
+  assert.deepEqual(opts.stdio, ['pipe', 'pipe', 'inherit']);
+});
+
 console.log(`\n${passed} passed, ${failed} failed`);
 process.exit(failed ? 1 : 0);