[format_command] - fixed fallback logic within formatter command class

script/tool/lib/src/format_command.dart

@@ -272,16 +272,47 @@ class FormatCommand extends PackageLoopingCommand {
   }
 
   Future<String> _findValidSwiftFormat() async {
-    final String swiftFormat = getStringArg(_swiftFormatPathArg);
-    if (await _hasDependency(swiftFormat)) {
-      return swiftFormat;
+  final String swiftFormatArg = getStringArg(_swiftFormatPathArg);
+
+  // 1) Explicit override.
+  if (await _hasDependency(swiftFormatArg)) {
+    return swiftFormatArg;
+  }
+
+  // 2) Scan for any `swift-format` on PATH.
+  for (final String candidate in await _whichAll('swift-format')) {
+    if (await _hasDependency(candidate)) {
+      return candidate;
     }
+  }
 
-    printError('Unable to run "swift-format". Make sure that it is in your '
-        'path, or provide a full path with --$_swiftFormatPathArg.');
-    throw ToolExit(_exitDependencyMissing);
+  // 3) On macOS, fall back to Xcode’s bundled tool via xcrun.
+  if (platform.isMacOS) {
+    try {
+      final io.ProcessResult result = await processRunner.run(
+        'xcrun',
+        <String>['--find', 'swift-format'],
+      );
+      if (result.exitCode == 0) {
+        final String found = (result.stdout as String).trim();
+        if (found.isNotEmpty && await _hasDependency(found)) {
+          return found;
+        }
+      }
+    } on io.ProcessException {
+      // Ignore and continue to error.
+    }
   }
 
+  printError(
+    'Unable to run "swift-format". Make sure it is in your '
+    'path, provide a full path with --$_swiftFormatPathArg, '
+    'or install Xcode 16+.',
+  );
+  throw ToolExit(_exitDependencyMissing);
+}
+
+
   Future<void> _formatJava(Iterable<String> files, String formatterPath) async {
     final Iterable<String> javaFiles =
         _getPathsWithExtensions(files, <String>{'.java'});

script/tool/test/format_command_test.dart

@@ -745,6 +745,72 @@ void main() {
             contains('Failed to format Swift files: exit code 1.'),
           ]));
     });
+
+      // ─── BEGIN xcrun fallback test ─────────────────────────────────────────────
+    test('uses xcrun fallback when swift-format not on PATH or --swift-format-path',
+        () async {
+      // 1) Force macOS behavior
+      mockPlatform.isMacOS = true;
+
+      // 2) Make a fake plugin with one Swift file
+      const files = <String>['macos/foo.swift'];
+      final plugin = createFakePlugin('a_plugin', packagesDir, extraFiles: files);
+      fakePubGet(plugin);
+
+      // 3) Stub out the default `swift-format --version` to fail
+      processRunner.mockProcessesForExecutable['swift-format'] = <FakeProcessInfo>[
+        FakeProcessInfo(MockProcess(exitCode: 1), <String>['--version']),
+      ];
+
+      // 4) Stub `which -a swift-format` to return nothing
+      processRunner.mockProcessesForExecutable['which'] = <FakeProcessInfo>[
+        FakeProcessInfo(MockProcess(stdout: ''), <String>['-a', 'swift-format']),
+      ];
+
+      // 5) Stub `xcrun --find swift-format` to return a real path
+      processRunner.mockProcessesForExecutable['xcrun'] = <FakeProcessInfo>[
+        FakeProcessInfo(
+          MockProcess(stdout: '/usr/bin/swift-format\n'),
+          <String>['--find', 'swift-format'],
+        ),
+      ];
+
+      // 6) Stub the fallback binary so that --version, -i and lint all succeed
+      processRunner.mockProcessesForExecutable['/usr/bin/swift-format'] = <FakeProcessInfo>[
+        FakeProcessInfo(MockProcess(), <String>['--version']),
+        FakeProcessInfo(MockProcess(), <String>['-i']),
+        FakeProcessInfo(MockProcess(), <String>['lint', '--parallel', '--strict']),
+      ];
+
+      // 7) Run the command
+      await runCapturingPrint(runner, <String>['format', '--swift']);
+
+      // 8) Verify we called the fallback binary (not `xcrun` in the final recording)
+      expect(
+        processRunner.recordedCalls,
+        orderedEquals(<ProcessCall>[
+          // First invocation of the fallback swift-format
+          const ProcessCall(
+            '/usr/bin/swift-format',
+            <String>['--version'],
+            null,
+          ),
+          // Then formatting
+          ProcessCall(
+            '/usr/bin/swift-format',
+            <String>['-i', ...getPackagesDirRelativePaths(plugin, files)],
+            packagesDir.path,
+          ),
+          // Finally linting
+          ProcessCall(
+            '/usr/bin/swift-format',
+            <String>['lint', '--parallel', '--strict', ...getPackagesDirRelativePaths(plugin, files)],
+            packagesDir.path,
+          ),
+        ]),
+      );
+    });
+    // ─── END xcrun fallback test ────────────────────────────────────────────────
   });
 
   test('skips known non-repo files', () async {