diff --git a/src/mcp/tools/device/test_device.ts b/src/mcp/tools/device/test_device.ts
index 1e78e62c..81634454 100644
--- a/src/mcp/tools/device/test_device.ts
+++ b/src/mcp/tools/device/test_device.ts
@@ -26,6 +26,7 @@ import {
   getSessionAwareToolSchemaShape,
 } from '../../../utils/typed-tool-factory.ts';
 import { nullifyEmptyStrings } from '../../../utils/schema-helpers.ts';
+import { validateXcodebuildExtraArgs } from '../../../utils/xcodebuild-validation.ts';
 
 // Unified schema: XOR between projectPath and workspacePath
 const baseSchemaObject = z.object({
@@ -183,6 +184,13 @@ export async function testDeviceLogic(
   executor: CommandExecutor = getDefaultCommandExecutor(),
   fileSystemExecutor: FileSystemExecutor = getDefaultFileSystemExecutor(),
 ): Promise<ToolResponse> {
+  // Validate extraArgs for invalid xcodebuild options
+  const validation = validateXcodebuildExtraArgs(params.extraArgs);
+  if (!validation.isValid) {
+    log('error', `Invalid extraArgs: ${validation.error}`);
+    return createTextResponse(validation.error!, true);
+  }
+
   log(
     'info',
     `Starting test run for scheme ${params.scheme} on platform ${params.platform ?? 'iOS'} (internal)`,
diff --git a/src/mcp/tools/macos/test_macos.ts b/src/mcp/tools/macos/test_macos.ts
index 238aacea..5cbe27d7 100644
--- a/src/mcp/tools/macos/test_macos.ts
+++ b/src/mcp/tools/macos/test_macos.ts
@@ -26,6 +26,7 @@ import {
   getSessionAwareToolSchemaShape,
 } from '../../../utils/typed-tool-factory.ts';
 import { nullifyEmptyStrings } from '../../../utils/schema-helpers.ts';
+import { validateXcodebuildExtraArgs } from '../../../utils/xcodebuild-validation.ts';
 
 // Unified schema: XOR between projectPath and workspacePath
 const baseSchemaObject = z.object({
@@ -239,6 +240,13 @@ export async function testMacosLogic(
   executor: CommandExecutor = getDefaultCommandExecutor(),
   fileSystemExecutor: FileSystemExecutor = getDefaultFileSystemExecutor(),
 ): Promise<ToolResponse> {
+  // Validate extraArgs for invalid xcodebuild options
+  const validation = validateXcodebuildExtraArgs(params.extraArgs);
+  if (!validation.isValid) {
+    log('error', `Invalid extraArgs: ${validation.error}`);
+    return createTextResponse(validation.error!, true);
+  }
+
   log('info', `Starting test run for scheme ${params.scheme} on platform macOS (internal)`);
 
   try {
diff --git a/src/mcp/tools/simulator/__tests__/test_sim.test.ts b/src/mcp/tools/simulator/__tests__/test_sim.test.ts
index 69bf3ab8..7880fad0 100644
--- a/src/mcp/tools/simulator/__tests__/test_sim.test.ts
+++ b/src/mcp/tools/simulator/__tests__/test_sim.test.ts
@@ -6,7 +6,8 @@
 import { describe, it, expect, beforeEach } from 'vitest';
 import * as z from 'zod';
 import { sessionStore } from '../../../../utils/session-store.ts';
-import testSim from '../test_sim.ts';
+import testSim, { test_simLogic } from '../test_sim.ts';
+import { createMockExecutor } from '../../../../test-utils/mock-executors.ts';
 
 describe('test_sim tool', () => {
   beforeEach(() => {
@@ -97,4 +98,47 @@ describe('test_sim tool', () => {
       expect(result.content[0].text).toContain('simulatorName');
     });
   });
+
+  describe('Validation', () => {
+    it('should reject invalid xcodebuild options in extraArgs', async () => {
+      const mockExecutor = createMockExecutor({ success: true, output: 'mock output' });
+
+      const result = await test_simLogic(
+        {
+          projectPath: '/path/to/project.xcodeproj',
+          scheme: 'MyScheme',
+          simulatorName: 'iPhone 16',
+          configuration: 'Debug',
+          extraArgs: ['-test-arg', '--snapshot-record'],
+        },
+        mockExecutor,
+      );
+
+      expect(result.isError).toBe(true);
+      expect(result.content[0].text).toContain('-test-arg');
+      expect(result.content[0].text).toContain('not a recognized xcodebuild argument');
+      expect(result.content[0].text).toContain('testRunnerEnv');
+    });
+
+    it('should accept valid extraArgs', async () => {
+      const mockExecutor = createMockExecutor({ success: true, output: 'mock output' });
+
+      const result = await test_simLogic(
+        {
+          projectPath: '/path/to/project.xcodeproj',
+          scheme: 'MyScheme',
+          simulatorName: 'iPhone 16',
+          configuration: 'Debug',
+          extraArgs: ['-only-testing:MyTests/MyTestClass'],
+        },
+        mockExecutor,
+      );
+
+      // Should not fail due to validation (but might fail for other reasons in mock)
+      // The key is that it doesn't fail with the validation error message
+      if (result.isError) {
+        expect(result.content[0].text).not.toContain('not a recognized xcodebuild argument');
+      }
+    });
+  });
 });
diff --git a/src/mcp/tools/simulator/test_sim.ts b/src/mcp/tools/simulator/test_sim.ts
index 66d40a3d..003f0557 100644
--- a/src/mcp/tools/simulator/test_sim.ts
+++ b/src/mcp/tools/simulator/test_sim.ts
@@ -18,6 +18,8 @@ import {
   createSessionAwareTool,
   getSessionAwareToolSchemaShape,
 } from '../../../utils/typed-tool-factory.ts';
+import { validateXcodebuildExtraArgs } from '../../../utils/xcodebuild-validation.ts';
+import { createTextResponse } from '../../../utils/validation.ts';
 
 // Define base schema object with all fields
 const baseSchemaObject = z.object({
@@ -91,6 +93,13 @@ export async function test_simLogic(
   params: TestSimulatorParams,
   executor: CommandExecutor,
 ): Promise<ToolResponse> {
+  // Validate extraArgs for invalid xcodebuild options
+  const validation = validateXcodebuildExtraArgs(params.extraArgs);
+  if (!validation.isValid) {
+    log('error', `Invalid extraArgs: ${validation.error}`);
+    return createTextResponse(validation.error!, true);
+  }
+
   // Log warning if useLatestOS is provided with simulatorId
   if (params.simulatorId && params.useLatestOS !== undefined) {
     log(
diff --git a/src/utils/__tests__/xcodebuild-validation.test.ts b/src/utils/__tests__/xcodebuild-validation.test.ts
new file mode 100644
index 00000000..977b6d65
--- /dev/null
+++ b/src/utils/__tests__/xcodebuild-validation.test.ts
@@ -0,0 +1,119 @@
+/**
+ * Tests for xcodebuild validation utilities
+ */
+
+import { describe, it, expect } from 'vitest';
+import { validateXcodebuildExtraArgs } from '../xcodebuild-validation.ts';
+
+describe('validateXcodebuildExtraArgs', () => {
+  describe('valid extraArgs', () => {
+    it('should return valid for undefined extraArgs', () => {
+      const result = validateXcodebuildExtraArgs(undefined);
+      expect(result.isValid).toBe(true);
+      expect(result.error).toBeUndefined();
+    });
+
+    it('should return valid for empty extraArgs array', () => {
+      const result = validateXcodebuildExtraArgs([]);
+      expect(result.isValid).toBe(true);
+      expect(result.error).toBeUndefined();
+    });
+
+    it('should return valid for legitimate xcodebuild options', () => {
+      const result = validateXcodebuildExtraArgs([
+        '-only-testing:MyTests/MyTestClass',
+        '-verbose',
+        '-dry-run',
+      ]);
+      expect(result.isValid).toBe(true);
+      expect(result.error).toBeUndefined();
+    });
+
+    it('should return valid for result bundle path option', () => {
+      const result = validateXcodebuildExtraArgs([
+        '-resultBundlePath',
+        '/path/to/results.xcresult',
+      ]);
+      expect(result.isValid).toBe(true);
+      expect(result.error).toBeUndefined();
+    });
+
+    it('should return valid for multiple legitimate options', () => {
+      const result = validateXcodebuildExtraArgs([
+        '-only-testing:MyTests/MyTestClass',
+        '-configuration',
+        'Debug',
+        '-sdk',
+        'iphonesimulator',
+      ]);
+      expect(result.isValid).toBe(true);
+      expect(result.error).toBeUndefined();
+    });
+  });
+
+  describe('invalid extraArgs', () => {
+    it('should reject -test-arg option', () => {
+      const result = validateXcodebuildExtraArgs(['-test-arg', '--snapshot-record']);
+      expect(result.isValid).toBe(false);
+      expect(result.error).toContain('-test-arg');
+      expect(result.error).toContain('not a recognized xcodebuild argument');
+      expect(result.error).toContain('testRunnerEnv');
+    });
+
+    it('should reject --test-arg option (double dash)', () => {
+      const result = validateXcodebuildExtraArgs(['--test-arg', 'value']);
+      expect(result.isValid).toBe(false);
+      expect(result.error).toContain('--test-arg');
+    });
+
+    it('should reject -testArg option (camelCase)', () => {
+      const result = validateXcodebuildExtraArgs(['-testArg', 'value']);
+      expect(result.isValid).toBe(false);
+      expect(result.error).toContain('-testArg');
+    });
+
+    it('should reject --testArg option (camelCase with double dash)', () => {
+      const result = validateXcodebuildExtraArgs(['--testArg', 'value']);
+      expect(result.isValid).toBe(false);
+      expect(result.error).toContain('--testArg');
+    });
+
+    it('should reject when invalid option is mixed with valid options', () => {
+      const result = validateXcodebuildExtraArgs([
+        '-only-testing:MyTests/MyTestClass',
+        '-test-arg',
+        '--snapshot-record',
+        '-verbose',
+      ]);
+      expect(result.isValid).toBe(false);
+      expect(result.error).toContain('-test-arg');
+    });
+
+    it('should provide helpful error message with testRunnerEnv suggestion', () => {
+      const result = validateXcodebuildExtraArgs(['-test-arg', 'value']);
+      expect(result.error).toContain('testRunnerEnv');
+      expect(result.error).toContain('{ "testRunnerEnv": { "MY_VAR": "value" } }');
+    });
+  });
+
+  describe('edge cases', () => {
+    it('should handle array with only invalid option', () => {
+      const result = validateXcodebuildExtraArgs(['-test-arg']);
+      expect(result.isValid).toBe(false);
+      expect(result.error).toContain('-test-arg');
+    });
+
+    it('should detect first invalid option in sequence', () => {
+      const result = validateXcodebuildExtraArgs(['-test-arg', '--test-arg', '-testArg']);
+      expect(result.isValid).toBe(false);
+      // Should return error for first invalid option encountered
+      expect(result.error).toBeDefined();
+    });
+
+    it('should handle options that look similar but are valid', () => {
+      // -test is a valid xcodebuild action, not in our invalid list
+      const result = validateXcodebuildExtraArgs(['-test', '-verbose']);
+      expect(result.isValid).toBe(true);
+    });
+  });
+});
diff --git a/src/utils/xcodebuild-validation.ts b/src/utils/xcodebuild-validation.ts
new file mode 100644
index 00000000..6d6169ac
--- /dev/null
+++ b/src/utils/xcodebuild-validation.ts
@@ -0,0 +1,49 @@
+/**
+ * XcodeBuild Validation Utilities
+ *
+ * This module provides validation for xcodebuild command parameters,
+ * particularly for detecting invalid or unsupported options.
+ */
+
+/**
+ * List of invalid or commonly misused xcodebuild options.
+ * These options are not recognized by xcodebuild and will cause it to fail.
+ */
+const INVALID_XCODEBUILD_OPTIONS = new Set([
+  '-test-arg', // Not a valid xcodebuild option; use testRunnerEnv instead
+  '--test-arg',
+  '-testArg',
+  '--testArg',
+]);
+
+/**
+ * Validates extraArgs for invalid xcodebuild options.
+ *
+ * @param extraArgs - Array of extra arguments to validate
+ * @returns Object with isValid flag and error message if invalid
+ */
+export function validateXcodebuildExtraArgs(extraArgs?: string[]): {
+  isValid: boolean;
+  error?: string;
+} {
+  if (!extraArgs || extraArgs.length === 0) {
+    return { isValid: true };
+  }
+
+  // Check for invalid options
+  for (const arg of extraArgs) {
+    if (INVALID_XCODEBUILD_OPTIONS.has(arg)) {
+      return {
+        isValid: false,
+        error: `Invalid xcodebuild option: '${arg}'. This is not a recognized xcodebuild argument. 
+
+If you're trying to pass arguments to the test runner, use the 'testRunnerEnv' parameter instead.
+Example: { "testRunnerEnv": { "MY_VAR": "value" } }
+
+For more information, see the xcodebuild man page or documentation.`,
+      };
+    }
+  }
+
+  return { isValid: true };
+}