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diff --git a/packages/server/src/tools/grep.test.ts b/packages/server/src/tools/grep.test.ts
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+++ b/packages/server/src/tools/grep.test.ts
@@ -0,0 +1,257 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import { GrepTool, GrepToolParams } from './grep.js';
+import path from 'path';
+import fs from 'fs/promises';
+import os from 'os';
+
+// Mock the child_process module to control grep/git grep behavior
+vi.mock('child_process', () => ({
+  spawn: vi.fn(() => ({
+    on: (event: string, cb: (...args: unknown[]) => void) => {
+      if (event === 'error' || event === 'close') {
+        // Simulate command not found or error for git grep and system grep
+        // to force fallback to JS implementation.
+        setTimeout(() => cb(1), 0); // cb(1) for error/close
+      }
+    },
+    stdout: { on: vi.fn() },
+    stderr: { on: vi.fn() },
+  })),
+}));
+
+describe('GrepTool', () => {
+  let tempRootDir: string;
+  let grepTool: GrepTool;
+  const abortSignal = new AbortController().signal;
+
+  beforeEach(async () => {
+    tempRootDir = await fs.mkdtemp(path.join(os.tmpdir(), 'grep-tool-root-'));
+    grepTool = new GrepTool(tempRootDir);
+
+    // Create some test files and directories
+    await fs.writeFile(
+      path.join(tempRootDir, 'fileA.txt'),
+      'hello world\nsecond line with world',
+    );
+    await fs.writeFile(
+      path.join(tempRootDir, 'fileB.js'),
+      'const foo = "bar";\nfunction baz() { return "hello"; }',
+    );
+    await fs.mkdir(path.join(tempRootDir, 'sub'));
+    await fs.writeFile(
+      path.join(tempRootDir, 'sub', 'fileC.txt'),
+      'another world in sub dir',
+    );
+    await fs.writeFile(
+      path.join(tempRootDir, 'sub', 'fileD.md'),
+      '# Markdown file\nThis is a test.',
+    );
+  });
+
+  afterEach(async () => {
+    await fs.rm(tempRootDir, { recursive: true, force: true });
+  });
+
+  describe('validateToolParams', () => {
+    it('should return null for valid params (pattern only)', () => {
+      const params: GrepToolParams = { pattern: 'hello' };
+      expect(grepTool.validateToolParams(params)).toBeNull();
+    });
+
+    it('should return null for valid params (pattern and path)', () => {
+      const params: GrepToolParams = { pattern: 'hello', path: '.' };
+      expect(grepTool.validateToolParams(params)).toBeNull();
+    });
+
+    it('should return null for valid params (pattern, path, and include)', () => {
+      const params: GrepToolParams = {
+        pattern: 'hello',
+        path: '.',
+        include: '*.txt',
+      };
+      expect(grepTool.validateToolParams(params)).toBeNull();
+    });
+
+    it('should return error if pattern is missing', () => {
+      const params = { path: '.' } as unknown as GrepToolParams;
+      expect(grepTool.validateToolParams(params)).toContain(
+        'Parameters failed schema validation',
+      );
+    });
+
+    it('should return error for invalid regex pattern', () => {
+      const params: GrepToolParams = { pattern: '[[' };
+      expect(grepTool.validateToolParams(params)).toContain(
+        'Invalid regular expression pattern',
+      );
+    });
+
+    it('should return error if path does not exist', () => {
+      const params: GrepToolParams = { pattern: 'hello', path: 'nonexistent' };
+      // Check for the core error message, as the full path might vary
+      expect(grepTool.validateToolParams(params)).toContain(
+        'Failed to access path stats for',
+      );
+      expect(grepTool.validateToolParams(params)).toContain('nonexistent');
+    });
+
+    it('should return error if path is a file, not a directory', async () => {
+      const filePath = path.join(tempRootDir, 'fileA.txt');
+      const params: GrepToolParams = { pattern: 'hello', path: filePath };
+      expect(grepTool.validateToolParams(params)).toContain(
+        `Path is not a directory: ${filePath}`,
+      );
+    });
+  });
+
+  describe('execute', () => {
+    it('should find matches for a simple pattern in all files', async () => {
+      const params: GrepToolParams = { pattern: 'world' };
+      const result = await grepTool.execute(params, abortSignal);
+      expect(result.llmContent).toContain(
+        'Found 3 match(es) for pattern "world" in path "."',
+      );
+      expect(result.llmContent).toContain('File: fileA.txt');
+      expect(result.llmContent).toContain('L1: hello world');
+      expect(result.llmContent).toContain('L2: second line with world');
+      expect(result.llmContent).toContain('File: sub/fileC.txt');
+      expect(result.llmContent).toContain('L1: another world in sub dir');
+      expect(result.returnDisplay).toBe('Found 3 matche(s)');
+    });
+
+    it('should find matches in a specific path', async () => {
+      const params: GrepToolParams = { pattern: 'world', path: 'sub' };
+      const result = await grepTool.execute(params, abortSignal);
+      expect(result.llmContent).toContain(
+        'Found 1 match(es) for pattern "world" in path "sub"',
+      );
+      expect(result.llmContent).toContain('File: fileC.txt'); // Path relative to 'sub'
+      expect(result.llmContent).toContain('L1: another world in sub dir');
+      expect(result.returnDisplay).toBe('Found 1 matche(s)');
+    });
+
+    it('should find matches with an include glob', async () => {
+      const params: GrepToolParams = { pattern: 'hello', include: '*.js' };
+      const result = await grepTool.execute(params, abortSignal);
+      expect(result.llmContent).toContain(
+        'Found 1 match(es) for pattern "hello" in path "." (filter: "*.js")',
+      );
+      expect(result.llmContent).toContain('File: fileB.js');
+      expect(result.llmContent).toContain(
+        'L2: function baz() { return "hello"; }',
+      );
+      expect(result.returnDisplay).toBe('Found 1 matche(s)');
+    });
+
+    it('should find matches with an include glob and path', async () => {
+      await fs.writeFile(
+        path.join(tempRootDir, 'sub', 'another.js'),
+        'const greeting = "hello";',
+      );
+      const params: GrepToolParams = {
+        pattern: 'hello',
+        path: 'sub',
+        include: '*.js',
+      };
+      const result = await grepTool.execute(params, abortSignal);
+      expect(result.llmContent).toContain(
+        'Found 1 match(es) for pattern "hello" in path "sub" (filter: "*.js")',
+      );
+      expect(result.llmContent).toContain('File: another.js');
+      expect(result.llmContent).toContain('L1: const greeting = "hello";');
+      expect(result.returnDisplay).toBe('Found 1 matche(s)');
+    });
+
+    it('should return "No matches found" when pattern does not exist', async () => {
+      const params: GrepToolParams = { pattern: 'nonexistentpattern' };
+      const result = await grepTool.execute(params, abortSignal);
+      expect(result.llmContent).toContain(
+        'No matches found for pattern "nonexistentpattern" in path "."',
+      );
+      expect(result.returnDisplay).toBe('No matches found');
+    });
+
+    it('should handle regex special characters correctly', async () => {
+      const params: GrepToolParams = { pattern: 'foo.*bar' }; // Matches 'const foo = "bar";'
+      const result = await grepTool.execute(params, abortSignal);
+      expect(result.llmContent).toContain(
+        'Found 1 match(es) for pattern "foo.*bar" in path "."',
+      );
+      expect(result.llmContent).toContain('File: fileB.js');
+      expect(result.llmContent).toContain('L1: const foo = "bar";');
+    });
+
+    it('should be case-insensitive by default (JS fallback)', async () => {
+      const params: GrepToolParams = { pattern: 'HELLO' };
+      const result = await grepTool.execute(params, abortSignal);
+      expect(result.llmContent).toContain(
+        'Found 2 match(es) for pattern "HELLO" in path "."',
+      );
+      expect(result.llmContent).toContain('File: fileA.txt');
+      expect(result.llmContent).toContain('L1: hello world');
+      expect(result.llmContent).toContain('File: fileB.js');
+      expect(result.llmContent).toContain(
+        'L2: function baz() { return "hello"; }',
+      );
+    });
+
+    it('should return an error if params are invalid', async () => {
+      const params = { path: '.' } as unknown as GrepToolParams; // Invalid: pattern missing
+      const result = await grepTool.execute(params, abortSignal);
+      expect(result.llmContent).toContain(
+        'Error: Invalid parameters provided. Reason: Parameters failed schema validation',
+      );
+      expect(result.returnDisplay).toContain(
+        'Model provided invalid parameters. Error: Parameters failed schema validation',
+      );
+    });
+  });
+
+  describe('getDescription', () => {
+    it('should generate correct description with pattern only', () => {
+      const params: GrepToolParams = { pattern: 'testPattern' };
+      expect(grepTool.getDescription(params)).toBe("'testPattern'");
+    });
+
+    it('should generate correct description with pattern and include', () => {
+      const params: GrepToolParams = {
+        pattern: 'testPattern',
+        include: '*.ts',
+      };
+      expect(grepTool.getDescription(params)).toBe("'testPattern' in *.ts");
+    });
+
+    it('should generate correct description with pattern and path', () => {
+      const params: GrepToolParams = {
+        pattern: 'testPattern',
+        path: 'src/app',
+      };
+      // The path will be relative to the tempRootDir, so we check for containment.
+      expect(grepTool.getDescription(params)).toContain("'testPattern' within");
+      expect(grepTool.getDescription(params)).toContain('src/app');
+    });
+
+    it('should generate correct description with pattern, include, and path', () => {
+      const params: GrepToolParams = {
+        pattern: 'testPattern',
+        include: '*.ts',
+        path: 'src/app',
+      };
+      expect(grepTool.getDescription(params)).toContain(
+        "'testPattern' in *.ts within",
+      );
+      expect(grepTool.getDescription(params)).toContain('src/app');
+    });
+
+    it('should use ./ for root path in description', () => {
+      const params: GrepToolParams = { pattern: 'testPattern', path: '.' };
+      expect(grepTool.getDescription(params)).toBe("'testPattern' within ./");
+    });
+  });
+});