diff --git a/.gitignore b/.gitignore
index 30ed32ae..b10814b4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,7 @@ bower_components
 # Editors
 .idea
 *.iml
+.cursor
 
 # OS metadata
 .DS_Store
diff --git a/packages/core/src/core/subagent.ts b/packages/core/src/core/subagent.ts
index 3abe4816..cd9d9181 100644
--- a/packages/core/src/core/subagent.ts
+++ b/packages/core/src/core/subagent.ts
@@ -118,15 +118,15 @@ export interface ModelConfig {
    *
    * TODO: In the future, this needs to support 'auto' or some other string to support routing use cases.
    */
-  model: string;
+  model?: string;
   /**
    * The temperature for the model's sampling process.
    */
-  temp: number;
+  temp?: number;
   /**
    * The top-p value for nucleus sampling.
    */
-  top_p: number;
+  top_p?: number;
 }
 
 /**
@@ -138,7 +138,7 @@ export interface ModelConfig {
  */
 export interface RunConfig {
   /** The maximum execution time for the subagent in minutes. */
-  max_time_minutes: number;
+  max_time_minutes?: number;
   /**
    * The maximum number of conversational turns (a user message + model response)
    * before the execution is terminated. Helps prevent infinite loops.
@@ -387,7 +387,10 @@ export class SubAgentScope {
           break;
         }
         let durationMin = (Date.now() - startTime) / (1000 * 60);
-        if (durationMin >= this.runConfig.max_time_minutes) {
+        if (
+          this.runConfig.max_time_minutes &&
+          durationMin >= this.runConfig.max_time_minutes
+        ) {
           this.output.terminate_reason = SubagentTerminateMode.TIMEOUT;
           break;
         }
@@ -413,7 +416,10 @@ export class SubAgentScope {
         }
 
         durationMin = (Date.now() - startTime) / (1000 * 60);
-        if (durationMin >= this.runConfig.max_time_minutes) {
+        if (
+          this.runConfig.max_time_minutes &&
+          durationMin >= this.runConfig.max_time_minutes
+        ) {
           this.output.terminate_reason = SubagentTerminateMode.TIMEOUT;
           break;
         }
@@ -588,7 +594,9 @@ export class SubAgentScope {
         this.runtimeContext.getSessionId(),
       );
 
-      this.runtimeContext.setModel(this.modelConfig.model);
+      if (this.modelConfig.model) {
+        this.runtimeContext.setModel(this.modelConfig.model);
+      }
 
       return new GeminiChat(
         this.runtimeContext,
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index f7c89c64..53bb4602 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -67,6 +67,9 @@ export * from './tools/tools.js';
 export * from './tools/tool-error.js';
 export * from './tools/tool-registry.js';
 
+// Export subagents (Phase 1)
+export * from './subagents/index.js';
+
 // Export prompt logic
 export * from './prompts/mcp-prompts.js';
 
diff --git a/packages/core/src/subagents/index.ts b/packages/core/src/subagents/index.ts
new file mode 100644
index 00000000..3aa869cb
--- /dev/null
+++ b/packages/core/src/subagents/index.ts
@@ -0,0 +1,53 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Subagents Phase 1 implementation - File-based configuration layer
+ *
+ * This module provides the foundation for the subagents feature by implementing
+ * a file-based configuration system that builds on the existing SubAgentScope
+ * runtime system. It includes:
+ *
+ * - Type definitions for file-based subagent configurations
+ * - Validation system for configuration integrity
+ * - Runtime conversion functions integrated into the manager
+ * - Manager class for CRUD operations on subagent files
+ *
+ * The implementation follows the Markdown + YAML frontmatter format specified
+ * in the Claude Code product manual, with storage at both project and user levels.
+ */
+
+// Core types and interfaces
+export type {
+  SubagentConfig,
+  SubagentMetadata,
+  SubagentLevel,
+  SubagentRuntimeConfig,
+  ValidationResult,
+  ListSubagentsOptions,
+  CreateSubagentOptions,
+  SubagentErrorCode,
+} from './types.js';
+
+export { SubagentError } from './types.js';
+
+// Validation system
+export { SubagentValidator } from './validation.js';
+
+// Main management class
+export { SubagentManager } from './subagent-manager.js';
+
+// Re-export existing runtime types for convenience
+export type {
+  PromptConfig,
+  ModelConfig,
+  RunConfig,
+  ToolConfig,
+  SubagentTerminateMode,
+  OutputObject,
+} from '../core/subagent.js';
+
+export { SubAgentScope } from '../core/subagent.js';
diff --git a/packages/core/src/subagents/subagent-manager.test.ts b/packages/core/src/subagents/subagent-manager.test.ts
new file mode 100644
index 00000000..c40b3541
--- /dev/null
+++ b/packages/core/src/subagents/subagent-manager.test.ts
@@ -0,0 +1,800 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { vi, describe, it, expect, beforeEach, afterEach } from 'vitest';
+import * as fs from 'fs/promises';
+import * as path from 'path';
+import * as os from 'os';
+import { SubagentManager } from './subagent-manager.js';
+import { SubagentConfig, SubagentError } from './types.js';
+import { ToolRegistry } from '../tools/tool-registry.js';
+
+// Mock file system operations
+vi.mock('fs/promises');
+vi.mock('os');
+
+// Mock yaml parser - use vi.hoisted for proper hoisting
+const mockParseYaml = vi.hoisted(() => vi.fn());
+const mockStringifyYaml = vi.hoisted(() => vi.fn());
+
+vi.mock('../utils/yaml-parser.js', () => ({
+  parse: mockParseYaml,
+  stringify: mockStringifyYaml,
+}));
+
+// Mock dependencies - create mock functions at the top level
+const mockValidateConfig = vi.hoisted(() => vi.fn());
+const mockValidateOrThrow = vi.hoisted(() => vi.fn());
+
+vi.mock('./validation.js', () => ({
+  SubagentValidator: class MockSubagentValidator {
+    validateConfig = mockValidateConfig;
+    validateOrThrow = mockValidateOrThrow;
+  },
+}));
+
+vi.mock('../core/subagent.js');
+
+describe('SubagentManager', () => {
+  let manager: SubagentManager;
+  let mockToolRegistry: ToolRegistry;
+  const projectRoot = '/test/project';
+
+  beforeEach(() => {
+    mockToolRegistry = {} as ToolRegistry;
+
+    // Mock os.homedir
+    vi.mocked(os.homedir).mockReturnValue('/home/user');
+
+    // Reset and setup mocks
+    vi.clearAllMocks();
+    mockValidateConfig.mockReturnValue({
+      isValid: true,
+      errors: [],
+      warnings: [],
+    });
+    mockValidateOrThrow.mockImplementation(() => {});
+
+    // Setup yaml parser mocks with sophisticated behavior
+    mockParseYaml.mockImplementation((yamlString: string) => {
+      // Handle different test cases based on YAML content
+      if (yamlString.includes('tools:')) {
+        return {
+          name: 'test-agent',
+          description: 'A test subagent',
+          tools: ['read_file', 'write_file'],
+        };
+      }
+      if (yamlString.includes('modelConfig:')) {
+        return {
+          name: 'test-agent',
+          description: 'A test subagent',
+          modelConfig: { model: 'custom-model', temp: 0.5 },
+        };
+      }
+      if (yamlString.includes('runConfig:')) {
+        return {
+          name: 'test-agent',
+          description: 'A test subagent',
+          runConfig: { max_time_minutes: 5, max_turns: 10 },
+        };
+      }
+      if (yamlString.includes('name: agent1')) {
+        return { name: 'agent1', description: 'First agent' };
+      }
+      if (yamlString.includes('name: agent2')) {
+        return { name: 'agent2', description: 'Second agent' };
+      }
+      if (yamlString.includes('name: agent3')) {
+        return { name: 'agent3', description: 'Third agent' };
+      }
+      if (!yamlString.includes('name:')) {
+        return { description: 'A test subagent' }; // Missing name case
+      }
+      if (!yamlString.includes('description:')) {
+        return { name: 'test-agent' }; // Missing description case
+      }
+      // Default case
+      return {
+        name: 'test-agent',
+        description: 'A test subagent',
+      };
+    });
+
+    mockStringifyYaml.mockImplementation((obj: Record<string, unknown>) => {
+      let yaml = '';
+      for (const [key, value] of Object.entries(obj)) {
+        if (key === 'tools' && Array.isArray(value)) {
+          yaml += `tools:\n${value.map((tool) => `  - ${tool}`).join('\n')}\n`;
+        } else if (
+          key === 'modelConfig' &&
+          typeof value === 'object' &&
+          value
+        ) {
+          yaml += `modelConfig:\n`;
+          for (const [k, v] of Object.entries(
+            value as Record<string, unknown>,
+          )) {
+            yaml += `  ${k}: ${v}\n`;
+          }
+        } else if (key === 'runConfig' && typeof value === 'object' && value) {
+          yaml += `runConfig:\n`;
+          for (const [k, v] of Object.entries(
+            value as Record<string, unknown>,
+          )) {
+            yaml += `  ${k}: ${v}\n`;
+          }
+        } else {
+          yaml += `${key}: ${value}\n`;
+        }
+      }
+      return yaml.trim();
+    });
+
+    manager = new SubagentManager(projectRoot, mockToolRegistry);
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  const validConfig: SubagentConfig = {
+    name: 'test-agent',
+    description: 'A test subagent',
+    systemPrompt: 'You are a helpful assistant.',
+    level: 'project',
+    filePath: '/test/project/.qwen/agents/test-agent.md',
+  };
+
+  const validMarkdown = `---
+name: test-agent
+description: A test subagent
+---
+
+You are a helpful assistant.
+`;
+
+  describe('parseSubagentContent', () => {
+    it('should parse valid markdown content', () => {
+      const config = manager.parseSubagentContent(
+        validMarkdown,
+        validConfig.filePath,
+      );
+
+      expect(config.name).toBe('test-agent');
+      expect(config.description).toBe('A test subagent');
+      expect(config.systemPrompt).toBe('You are a helpful assistant.');
+      expect(config.level).toBe('project');
+      expect(config.filePath).toBe(validConfig.filePath);
+    });
+
+    it('should parse content with tools', () => {
+      const markdownWithTools = `---
+name: test-agent
+description: A test subagent
+tools:
+  - read_file
+  - write_file
+---
+
+You are a helpful assistant.
+`;
+
+      const config = manager.parseSubagentContent(
+        markdownWithTools,
+        validConfig.filePath,
+      );
+
+      expect(config.tools).toEqual(['read_file', 'write_file']);
+    });
+
+    it('should parse content with model config', () => {
+      const markdownWithModel = `---
+name: test-agent
+description: A test subagent
+modelConfig:
+  model: custom-model
+  temp: 0.5
+---
+
+You are a helpful assistant.
+`;
+
+      const config = manager.parseSubagentContent(
+        markdownWithModel,
+        validConfig.filePath,
+      );
+
+      expect(config.modelConfig).toEqual({ model: 'custom-model', temp: 0.5 });
+    });
+
+    it('should parse content with run config', () => {
+      const markdownWithRun = `---
+name: test-agent
+description: A test subagent
+runConfig:
+  max_time_minutes: 5
+  max_turns: 10
+---
+
+You are a helpful assistant.
+`;
+
+      const config = manager.parseSubagentContent(
+        markdownWithRun,
+        validConfig.filePath,
+      );
+
+      expect(config.runConfig).toEqual({ max_time_minutes: 5, max_turns: 10 });
+    });
+
+    it('should determine level from file path', () => {
+      const projectPath = '/test/project/.qwen/agents/test-agent.md';
+      const userPath = '/home/user/.qwen/agents/test-agent.md';
+
+      const projectConfig = manager.parseSubagentContent(
+        validMarkdown,
+        projectPath,
+      );
+      const userConfig = manager.parseSubagentContent(validMarkdown, userPath);
+
+      expect(projectConfig.level).toBe('project');
+      expect(userConfig.level).toBe('user');
+    });
+
+    it('should throw error for invalid frontmatter format', () => {
+      const invalidMarkdown = `No frontmatter here
+Just content`;
+
+      expect(() =>
+        manager.parseSubagentContent(invalidMarkdown, validConfig.filePath),
+      ).toThrow(SubagentError);
+    });
+
+    it('should throw error for missing name', () => {
+      const markdownWithoutName = `---
+description: A test subagent
+---
+
+You are a helpful assistant.
+`;
+
+      expect(() =>
+        manager.parseSubagentContent(markdownWithoutName, validConfig.filePath),
+      ).toThrow(SubagentError);
+    });
+
+    it('should throw error for missing description', () => {
+      const markdownWithoutDescription = `---
+name: test-agent
+---
+
+You are a helpful assistant.
+`;
+
+      expect(() =>
+        manager.parseSubagentContent(
+          markdownWithoutDescription,
+          validConfig.filePath,
+        ),
+      ).toThrow(SubagentError);
+    });
+  });
+
+  describe('serializeSubagent', () => {
+    it('should serialize basic configuration', () => {
+      const serialized = manager.serializeSubagent(validConfig);
+
+      expect(serialized).toContain('name: test-agent');
+      expect(serialized).toContain('description: A test subagent');
+      expect(serialized).toContain('You are a helpful assistant.');
+      expect(serialized).toMatch(/^---\n[\s\S]*\n---\n\n[\s\S]*\n$/);
+    });
+
+    it('should serialize configuration with tools', () => {
+      const configWithTools: SubagentConfig = {
+        ...validConfig,
+        tools: ['read_file', 'write_file'],
+      };
+
+      const serialized = manager.serializeSubagent(configWithTools);
+
+      expect(serialized).toContain('tools:');
+      expect(serialized).toContain('- read_file');
+      expect(serialized).toContain('- write_file');
+    });
+
+    it('should serialize configuration with model config', () => {
+      const configWithModel: SubagentConfig = {
+        ...validConfig,
+        modelConfig: { model: 'custom-model', temp: 0.5 },
+      };
+
+      const serialized = manager.serializeSubagent(configWithModel);
+
+      expect(serialized).toContain('modelConfig:');
+      expect(serialized).toContain('model: custom-model');
+      expect(serialized).toContain('temp: 0.5');
+    });
+
+    it('should not include empty optional fields', () => {
+      const serialized = manager.serializeSubagent(validConfig);
+
+      expect(serialized).not.toContain('tools:');
+      expect(serialized).not.toContain('modelConfig:');
+      expect(serialized).not.toContain('runConfig:');
+    });
+  });
+
+  describe('createSubagent', () => {
+    beforeEach(() => {
+      // Mock successful file operations
+      vi.mocked(fs.access).mockRejectedValue(new Error('File not found'));
+      vi.mocked(fs.mkdir).mockResolvedValue(undefined);
+      vi.mocked(fs.writeFile).mockResolvedValue(undefined);
+    });
+
+    it('should create subagent successfully', async () => {
+      await manager.createSubagent(validConfig, { level: 'project' });
+
+      expect(fs.mkdir).toHaveBeenCalledWith(
+        path.dirname(validConfig.filePath),
+        { recursive: true },
+      );
+      expect(fs.writeFile).toHaveBeenCalledWith(
+        expect.stringContaining('test-agent.md'),
+        expect.stringContaining('name: test-agent'),
+        'utf8',
+      );
+    });
+
+    it('should throw error if file already exists and overwrite is false', async () => {
+      vi.mocked(fs.access).mockResolvedValue(undefined); // File exists
+
+      await expect(
+        manager.createSubagent(validConfig, { level: 'project' }),
+      ).rejects.toThrow(SubagentError);
+
+      await expect(
+        manager.createSubagent(validConfig, { level: 'project' }),
+      ).rejects.toThrow(/already exists/);
+    });
+
+    it('should overwrite file when overwrite is true', async () => {
+      vi.mocked(fs.access).mockResolvedValue(undefined); // File exists
+
+      await manager.createSubagent(validConfig, {
+        level: 'project',
+        overwrite: true,
+      });
+
+      expect(fs.writeFile).toHaveBeenCalled();
+    });
+
+    it('should use custom path when provided', async () => {
+      const customPath = '/custom/path/agent.md';
+
+      await manager.createSubagent(validConfig, {
+        level: 'project',
+        customPath,
+      });
+
+      expect(fs.writeFile).toHaveBeenCalledWith(
+        customPath,
+        expect.any(String),
+        'utf8',
+      );
+    });
+
+    it('should throw error on file write failure', async () => {
+      vi.mocked(fs.writeFile).mockRejectedValue(new Error('Write failed'));
+
+      await expect(
+        manager.createSubagent(validConfig, { level: 'project' }),
+      ).rejects.toThrow(SubagentError);
+
+      await expect(
+        manager.createSubagent(validConfig, { level: 'project' }),
+      ).rejects.toThrow(/Failed to write subagent file/);
+    });
+  });
+
+  describe('loadSubagent', () => {
+    it('should load subagent from project level first', async () => {
+      vi.mocked(fs.readFile).mockResolvedValue(validMarkdown);
+
+      const config = await manager.loadSubagent('test-agent');
+
+      expect(config).toBeDefined();
+      expect(config!.name).toBe('test-agent');
+      expect(fs.readFile).toHaveBeenCalledWith(
+        '/test/project/.qwen/agents/test-agent.md',
+        'utf8',
+      );
+    });
+
+    it('should fall back to user level if project level fails', async () => {
+      vi.mocked(fs.readFile)
+        .mockRejectedValueOnce(new Error('Not found'))
+        .mockResolvedValueOnce(validMarkdown);
+
+      const config = await manager.loadSubagent('test-agent');
+
+      expect(config).toBeDefined();
+      expect(config!.name).toBe('test-agent');
+      expect(fs.readFile).toHaveBeenCalledWith(
+        '/home/user/.qwen/agents/test-agent.md',
+        'utf8',
+      );
+    });
+
+    it('should return null if not found at either level', async () => {
+      vi.mocked(fs.readFile).mockRejectedValue(new Error('Not found'));
+
+      const config = await manager.loadSubagent('nonexistent');
+
+      expect(config).toBeNull();
+    });
+  });
+
+  describe('updateSubagent', () => {
+    beforeEach(() => {
+      vi.mocked(fs.readFile).mockResolvedValue(validMarkdown);
+      vi.mocked(fs.writeFile).mockResolvedValue(undefined);
+    });
+
+    it('should update existing subagent', async () => {
+      const updates = { description: 'Updated description' };
+
+      await manager.updateSubagent('test-agent', updates);
+
+      expect(fs.writeFile).toHaveBeenCalledWith(
+        expect.stringContaining('test-agent.md'),
+        expect.stringContaining('Updated description'),
+        'utf8',
+      );
+    });
+
+    it('should throw error if subagent not found', async () => {
+      vi.mocked(fs.readFile).mockRejectedValue(new Error('Not found'));
+
+      await expect(manager.updateSubagent('nonexistent', {})).rejects.toThrow(
+        SubagentError,
+      );
+
+      await expect(manager.updateSubagent('nonexistent', {})).rejects.toThrow(
+        /not found/,
+      );
+    });
+
+    it('should throw error on write failure', async () => {
+      vi.mocked(fs.writeFile).mockRejectedValue(new Error('Write failed'));
+
+      await expect(manager.updateSubagent('test-agent', {})).rejects.toThrow(
+        SubagentError,
+      );
+
+      await expect(manager.updateSubagent('test-agent', {})).rejects.toThrow(
+        /Failed to update subagent file/,
+      );
+    });
+  });
+
+  describe('deleteSubagent', () => {
+    it('should delete subagent from specified level', async () => {
+      vi.mocked(fs.unlink).mockResolvedValue(undefined);
+
+      await manager.deleteSubagent('test-agent', 'project');
+
+      expect(fs.unlink).toHaveBeenCalledWith(
+        '/test/project/.qwen/agents/test-agent.md',
+      );
+    });
+
+    it('should delete from both levels if no level specified', async () => {
+      vi.mocked(fs.unlink).mockResolvedValue(undefined);
+
+      await manager.deleteSubagent('test-agent');
+
+      expect(fs.unlink).toHaveBeenCalledTimes(2);
+      expect(fs.unlink).toHaveBeenCalledWith(
+        '/test/project/.qwen/agents/test-agent.md',
+      );
+      expect(fs.unlink).toHaveBeenCalledWith(
+        '/home/user/.qwen/agents/test-agent.md',
+      );
+    });
+
+    it('should throw error if subagent not found', async () => {
+      vi.mocked(fs.unlink).mockRejectedValue(new Error('Not found'));
+
+      await expect(manager.deleteSubagent('nonexistent')).rejects.toThrow(
+        SubagentError,
+      );
+
+      await expect(manager.deleteSubagent('nonexistent')).rejects.toThrow(
+        /not found/,
+      );
+    });
+
+    it('should succeed if deleted from at least one level', async () => {
+      vi.mocked(fs.unlink)
+        .mockRejectedValueOnce(new Error('Not found'))
+        .mockResolvedValueOnce(undefined);
+
+      await expect(manager.deleteSubagent('test-agent')).resolves.not.toThrow();
+    });
+  });
+
+  describe('listSubagents', () => {
+    beforeEach(() => {
+      // Mock directory listing
+      vi.mocked(fs.readdir)
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        .mockResolvedValueOnce(['agent1.md', 'agent2.md', 'not-md.txt'] as any)
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        .mockResolvedValueOnce(['agent3.md', 'agent1.md'] as any); // user level
+
+      // Mock file reading for valid agents
+      vi.mocked(fs.readFile).mockImplementation((filePath) => {
+        const pathStr = String(filePath);
+        if (pathStr.includes('agent1.md')) {
+          return Promise.resolve(`---
+name: agent1
+description: First agent
+---
+System prompt 1`);
+        } else if (pathStr.includes('agent2.md')) {
+          return Promise.resolve(`---
+name: agent2
+description: Second agent
+---
+System prompt 2`);
+        } else if (pathStr.includes('agent3.md')) {
+          return Promise.resolve(`---
+name: agent3
+description: Third agent
+---
+System prompt 3`);
+        }
+        return Promise.reject(new Error('File not found'));
+      });
+    });
+
+    it('should list subagents from both levels', async () => {
+      const subagents = await manager.listSubagents();
+
+      expect(subagents).toHaveLength(3); // agent1 (project takes precedence), agent2, agent3
+      expect(subagents.map((s) => s.name)).toEqual([
+        'agent1',
+        'agent2',
+        'agent3',
+      ]);
+    });
+
+    it('should prioritize project level over user level', async () => {
+      const subagents = await manager.listSubagents();
+      const agent1 = subagents.find((s) => s.name === 'agent1');
+
+      expect(agent1!.level).toBe('project');
+    });
+
+    it('should filter by level', async () => {
+      const projectSubagents = await manager.listSubagents({
+        level: 'project',
+      });
+
+      expect(projectSubagents).toHaveLength(2); // agent1, agent2
+      expect(projectSubagents.every((s) => s.level === 'project')).toBe(true);
+    });
+
+    it('should sort by name', async () => {
+      const subagents = await manager.listSubagents({
+        sortBy: 'name',
+        sortOrder: 'asc',
+      });
+
+      const names = subagents.map((s) => s.name);
+      expect(names).toEqual(['agent1', 'agent2', 'agent3']);
+    });
+
+    it('should handle empty directories', async () => {
+      // Reset all mocks for this specific test
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      vi.mocked(fs.readdir).mockResolvedValue([] as any);
+      vi.mocked(fs.readFile).mockRejectedValue(new Error('No files'));
+
+      const subagents = await manager.listSubagents();
+
+      expect(subagents).toHaveLength(0);
+    });
+
+    it('should handle directory read errors', async () => {
+      // Reset all mocks for this specific test
+      vi.mocked(fs.readdir).mockRejectedValue(new Error('Directory not found'));
+      vi.mocked(fs.readFile).mockRejectedValue(new Error('No files'));
+
+      const subagents = await manager.listSubagents();
+
+      expect(subagents).toHaveLength(0);
+    });
+
+    it('should skip invalid subagent files', async () => {
+      // Reset all mocks for this specific test
+      vi.mocked(fs.readdir)
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        .mockResolvedValueOnce(['valid.md', 'invalid.md'] as any)
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        .mockResolvedValueOnce(['valid.md', 'invalid.md'] as any); // user level
+      vi.mocked(fs.readFile)
+        .mockResolvedValueOnce(validMarkdown) // valid.md project level
+        .mockRejectedValueOnce(new Error('Invalid YAML')) // invalid.md project level
+        .mockRejectedValueOnce(new Error('Not found')) // valid.md user level (already found)
+        .mockRejectedValueOnce(new Error('Invalid YAML')); // invalid.md user level
+
+      const consoleSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+
+      const subagents = await manager.listSubagents();
+
+      expect(subagents).toHaveLength(1);
+      expect(consoleSpy).toHaveBeenCalledWith(
+        expect.stringContaining('Skipping invalid subagent file'),
+      );
+
+      consoleSpy.mockRestore();
+    });
+  });
+
+  describe('findSubagentByName', () => {
+    it('should find existing subagent', async () => {
+      vi.mocked(fs.readFile).mockResolvedValue(validMarkdown);
+
+      const metadata = await manager.findSubagentByName('test-agent');
+
+      expect(metadata).toBeDefined();
+      expect(metadata!.name).toBe('test-agent');
+      expect(metadata!.description).toBe('A test subagent');
+    });
+
+    it('should return null for non-existent subagent', async () => {
+      vi.mocked(fs.readFile).mockRejectedValue(new Error('Not found'));
+
+      const metadata = await manager.findSubagentByName('nonexistent');
+
+      expect(metadata).toBeNull();
+    });
+  });
+
+  describe('isNameAvailable', () => {
+    it('should return true for available names', async () => {
+      vi.mocked(fs.readFile).mockRejectedValue(new Error('Not found'));
+
+      const available = await manager.isNameAvailable('new-agent');
+
+      expect(available).toBe(true);
+    });
+
+    it('should return false for existing names', async () => {
+      vi.mocked(fs.readFile).mockResolvedValue(validMarkdown);
+
+      const available = await manager.isNameAvailable('test-agent');
+
+      expect(available).toBe(false);
+    });
+
+    it('should check specific level when provided', async () => {
+      // The isNameAvailable method loads from both levels and checks if found subagent is at different level
+      // First call: loads subagent (found at user level), checks if it's at project level (different) -> available
+      vi.mocked(fs.readFile)
+        .mockRejectedValueOnce(new Error('Not found')) // project level
+        .mockResolvedValueOnce(validMarkdown); // user level - found here
+
+      const availableAtProject = await manager.isNameAvailable(
+        'test-agent',
+        'project',
+      );
+      expect(availableAtProject).toBe(true); // Available at project because found at user level
+
+      // Second call: loads subagent (found at user level), checks if it's at user level (same) -> not available
+      vi.mocked(fs.readFile)
+        .mockRejectedValueOnce(new Error('Not found')) // project level
+        .mockResolvedValueOnce(validMarkdown); // user level - found here
+
+      const availableAtUser = await manager.isNameAvailable(
+        'test-agent',
+        'user',
+      );
+      expect(availableAtUser).toBe(false); // Not available at user because found at user level
+    });
+  });
+
+  describe('Runtime Configuration Methods', () => {
+    describe('convertToRuntimeConfig', () => {
+      it('should convert basic configuration', () => {
+        const runtimeConfig = manager.convertToRuntimeConfig(validConfig);
+
+        expect(runtimeConfig.promptConfig.systemPrompt).toBe(
+          validConfig.systemPrompt,
+        );
+        expect(runtimeConfig.modelConfig).toEqual({});
+        expect(runtimeConfig.runConfig).toEqual({});
+        expect(runtimeConfig.toolConfig).toBeUndefined();
+      });
+
+      it('should include tool configuration when tools are specified', () => {
+        const configWithTools: SubagentConfig = {
+          ...validConfig,
+          tools: ['read_file', 'write_file'],
+        };
+
+        const runtimeConfig = manager.convertToRuntimeConfig(configWithTools);
+
+        expect(runtimeConfig.toolConfig).toBeDefined();
+        expect(runtimeConfig.toolConfig!.tools).toEqual([
+          'read_file',
+          'write_file',
+        ]);
+      });
+
+      it('should merge custom model and run configurations', () => {
+        const configWithCustom: SubagentConfig = {
+          ...validConfig,
+          modelConfig: { model: 'custom-model', temp: 0.5 },
+          runConfig: { max_time_minutes: 5 },
+        };
+
+        const runtimeConfig = manager.convertToRuntimeConfig(configWithCustom);
+
+        expect(runtimeConfig.modelConfig.model).toBe('custom-model');
+        expect(runtimeConfig.modelConfig.temp).toBe(0.5);
+        expect(runtimeConfig.runConfig.max_time_minutes).toBe(5);
+        // No default values are provided anymore
+        expect(Object.keys(runtimeConfig.modelConfig)).toEqual([
+          'model',
+          'temp',
+        ]);
+        expect(Object.keys(runtimeConfig.runConfig)).toEqual([
+          'max_time_minutes',
+        ]);
+      });
+    });
+
+    describe('mergeConfigurations', () => {
+      it('should merge basic properties', () => {
+        const updates = {
+          description: 'Updated description',
+          systemPrompt: 'Updated prompt',
+        };
+
+        const merged = manager.mergeConfigurations(validConfig, updates);
+
+        expect(merged.description).toBe('Updated description');
+        expect(merged.systemPrompt).toBe('Updated prompt');
+        expect(merged.name).toBe(validConfig.name); // Should keep original
+      });
+
+      it('should merge nested configurations', () => {
+        const configWithNested: SubagentConfig = {
+          ...validConfig,
+          modelConfig: { model: 'original-model', temp: 0.7 },
+          runConfig: { max_time_minutes: 10, max_turns: 20 },
+        };
+
+        const updates = {
+          modelConfig: { temp: 0.5 },
+          runConfig: { max_time_minutes: 5 },
+        };
+
+        const merged = manager.mergeConfigurations(configWithNested, updates);
+
+        expect(merged.modelConfig!.model).toBe('original-model'); // Should keep original
+        expect(merged.modelConfig!.temp).toBe(0.5); // Should update
+        expect(merged.runConfig!.max_time_minutes).toBe(5); // Should update
+        expect(merged.runConfig!.max_turns).toBe(20); // Should keep original
+      });
+    });
+  });
+});
diff --git a/packages/core/src/subagents/subagent-manager.ts b/packages/core/src/subagents/subagent-manager.ts
new file mode 100644
index 00000000..75470939
--- /dev/null
+++ b/packages/core/src/subagents/subagent-manager.ts
@@ -0,0 +1,647 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as fs from 'fs/promises';
+import * as path from 'path';
+import * as os from 'os';
+// Note: yaml package would need to be added as a dependency
+// For now, we'll use a simple YAML parser implementation
+import {
+  parse as parseYaml,
+  stringify as stringifyYaml,
+} from '../utils/yaml-parser.js';
+import {
+  SubagentConfig,
+  SubagentRuntimeConfig,
+  SubagentMetadata,
+  SubagentLevel,
+  ListSubagentsOptions,
+  CreateSubagentOptions,
+  SubagentError,
+  SubagentErrorCode,
+} from './types.js';
+import { SubagentValidator } from './validation.js';
+import {
+  SubAgentScope,
+  PromptConfig,
+  ModelConfig,
+  RunConfig,
+  ToolConfig,
+} from '../core/subagent.js';
+import { Config } from '../config/config.js';
+import { ToolRegistry } from '../tools/tool-registry.js';
+
+const QWEN_CONFIG_DIR = '.qwen';
+const AGENT_CONFIG_DIR = 'agents';
+
+/**
+ * Manages subagent configurations stored as Markdown files with YAML frontmatter.
+ * Provides CRUD operations, validation, and integration with the runtime system.
+ */
+export class SubagentManager {
+  private readonly validator: SubagentValidator;
+
+  constructor(
+    private readonly projectRoot: string,
+    private readonly toolRegistry?: ToolRegistry,
+  ) {
+    this.validator = new SubagentValidator(toolRegistry);
+  }
+
+  /**
+   * Creates a new subagent configuration.
+   *
+   * @param config - Subagent configuration to create
+   * @param options - Creation options
+   * @throws SubagentError if creation fails
+   */
+  async createSubagent(
+    config: SubagentConfig,
+    options: CreateSubagentOptions,
+  ): Promise<void> {
+    // Validate the configuration
+    this.validator.validateOrThrow(config);
+
+    // Determine file path
+    const filePath =
+      options.customPath || this.getSubagentPath(config.name, options.level);
+
+    // Check if file already exists
+    if (!options.overwrite) {
+      try {
+        await fs.access(filePath);
+        throw new SubagentError(
+          `Subagent "${config.name}" already exists at ${filePath}`,
+          SubagentErrorCode.ALREADY_EXISTS,
+          config.name,
+        );
+      } catch (error) {
+        if (error instanceof SubagentError) throw error;
+        // File doesn't exist, which is what we want
+      }
+    }
+
+    // Ensure directory exists
+    const dir = path.dirname(filePath);
+    await fs.mkdir(dir, { recursive: true });
+
+    // Update config with actual file path and level
+    const finalConfig: SubagentConfig = {
+      ...config,
+      level: options.level,
+      filePath,
+    };
+
+    // Serialize and write the file
+    const content = this.serializeSubagent(finalConfig);
+
+    try {
+      await fs.writeFile(filePath, content, 'utf8');
+    } catch (error) {
+      throw new SubagentError(
+        `Failed to write subagent file: ${error instanceof Error ? error.message : 'Unknown error'}`,
+        SubagentErrorCode.FILE_ERROR,
+        config.name,
+      );
+    }
+  }
+
+  /**
+   * Loads a subagent configuration by name.
+   * Searches project-level first, then user-level.
+   *
+   * @param name - Name of the subagent to load
+   * @returns SubagentConfig or null if not found
+   */
+  async loadSubagent(name: string): Promise<SubagentConfig | null> {
+    // Try project level first
+    const projectPath = this.getSubagentPath(name, 'project');
+    try {
+      const config = await this.parseSubagentFile(projectPath);
+      return config;
+    } catch (_error) {
+      // Continue to user level
+    }
+
+    // Try user level
+    const userPath = this.getSubagentPath(name, 'user');
+    try {
+      const config = await this.parseSubagentFile(userPath);
+      return config;
+    } catch (_error) {
+      // Not found at either level
+      return null;
+    }
+  }
+
+  /**
+   * Updates an existing subagent configuration.
+   *
+   * @param name - Name of the subagent to update
+   * @param updates - Partial configuration updates
+   * @throws SubagentError if subagent not found or update fails
+   */
+  async updateSubagent(
+    name: string,
+    updates: Partial<SubagentConfig>,
+  ): Promise<void> {
+    const existing = await this.loadSubagent(name);
+    if (!existing) {
+      throw new SubagentError(
+        `Subagent "${name}" not found`,
+        SubagentErrorCode.NOT_FOUND,
+        name,
+      );
+    }
+
+    // Merge updates with existing configuration
+    const updatedConfig = this.mergeConfigurations(existing, updates);
+
+    // Validate the updated configuration
+    this.validator.validateOrThrow(updatedConfig);
+
+    // Write the updated configuration
+    const content = this.serializeSubagent(updatedConfig);
+
+    try {
+      await fs.writeFile(existing.filePath, content, 'utf8');
+    } catch (error) {
+      throw new SubagentError(
+        `Failed to update subagent file: ${error instanceof Error ? error.message : 'Unknown error'}`,
+        SubagentErrorCode.FILE_ERROR,
+        name,
+      );
+    }
+  }
+
+  /**
+   * Deletes a subagent configuration.
+   *
+   * @param name - Name of the subagent to delete
+   * @param level - Specific level to delete from, or undefined to delete from both
+   * @throws SubagentError if deletion fails
+   */
+  async deleteSubagent(name: string, level?: SubagentLevel): Promise<void> {
+    const levelsToCheck: SubagentLevel[] = level
+      ? [level]
+      : ['project', 'user'];
+    let deleted = false;
+
+    for (const currentLevel of levelsToCheck) {
+      const filePath = this.getSubagentPath(name, currentLevel);
+
+      try {
+        await fs.unlink(filePath);
+        deleted = true;
+      } catch (_error) {
+        // File might not exist at this level, continue
+      }
+    }
+
+    if (!deleted) {
+      throw new SubagentError(
+        `Subagent "${name}" not found`,
+        SubagentErrorCode.NOT_FOUND,
+        name,
+      );
+    }
+  }
+
+  /**
+   * Lists all available subagents.
+   *
+   * @param options - Filtering and sorting options
+   * @returns Array of subagent metadata
+   */
+  async listSubagents(
+    options: ListSubagentsOptions = {},
+  ): Promise<SubagentMetadata[]> {
+    const subagents: SubagentMetadata[] = [];
+    const seenNames = new Set<string>();
+
+    const levelsToCheck: SubagentLevel[] = options.level
+      ? [options.level]
+      : ['project', 'user'];
+
+    // Collect subagents from each level (project takes precedence)
+    for (const level of levelsToCheck) {
+      const levelSubagents = await this.listSubagentsAtLevel(level);
+
+      for (const subagent of levelSubagents) {
+        // Skip if we've already seen this name (project takes precedence)
+        if (seenNames.has(subagent.name)) {
+          continue;
+        }
+
+        // Apply tool filter if specified
+        if (
+          options.hasTool &&
+          (!subagent.tools || !subagent.tools.includes(options.hasTool))
+        ) {
+          continue;
+        }
+
+        subagents.push(subagent);
+        seenNames.add(subagent.name);
+      }
+    }
+
+    // Sort results
+    if (options.sortBy) {
+      subagents.sort((a, b) => {
+        let comparison = 0;
+
+        switch (options.sortBy) {
+          case 'name':
+            comparison = a.name.localeCompare(b.name);
+            break;
+          case 'lastModified': {
+            const aTime = a.lastModified?.getTime() || 0;
+            const bTime = b.lastModified?.getTime() || 0;
+            comparison = aTime - bTime;
+            break;
+          }
+          case 'level':
+            // Project comes before user
+            comparison =
+              a.level === 'project' ? -1 : b.level === 'project' ? 1 : 0;
+            break;
+          default:
+            comparison = 0;
+            break;
+        }
+
+        return options.sortOrder === 'desc' ? -comparison : comparison;
+      });
+    }
+
+    return subagents;
+  }
+
+  /**
+   * Finds a subagent by name and returns its metadata.
+   *
+   * @param name - Name of the subagent to find
+   * @returns SubagentMetadata or null if not found
+   */
+  async findSubagentByName(name: string): Promise<SubagentMetadata | null> {
+    const config = await this.loadSubagent(name);
+    if (!config) {
+      return null;
+    }
+
+    return this.configToMetadata(config);
+  }
+
+  /**
+   * Parses a subagent file and returns the configuration.
+   *
+   * @param filePath - Path to the subagent file
+   * @returns SubagentConfig
+   * @throws SubagentError if parsing fails
+   */
+  async parseSubagentFile(filePath: string): Promise<SubagentConfig> {
+    let content: string;
+
+    try {
+      content = await fs.readFile(filePath, 'utf8');
+    } catch (error) {
+      throw new SubagentError(
+        `Failed to read subagent file: ${error instanceof Error ? error.message : 'Unknown error'}`,
+        SubagentErrorCode.FILE_ERROR,
+      );
+    }
+
+    return this.parseSubagentContent(content, filePath);
+  }
+
+  /**
+   * Parses subagent content from a string.
+   *
+   * @param content - File content
+   * @param filePath - File path for error reporting
+   * @returns SubagentConfig
+   * @throws SubagentError if parsing fails
+   */
+  parseSubagentContent(content: string, filePath: string): SubagentConfig {
+    try {
+      // Split frontmatter and content
+      const frontmatterRegex = /^---\n([\s\S]*?)\n---\n([\s\S]*)$/;
+      const match = content.match(frontmatterRegex);
+
+      if (!match) {
+        throw new Error('Invalid format: missing YAML frontmatter');
+      }
+
+      const [, frontmatterYaml, systemPrompt] = match;
+
+      // Parse YAML frontmatter
+      const frontmatter = parseYaml(frontmatterYaml) as Record<string, unknown>;
+
+      // Extract required fields
+      const name = frontmatter['name'] as string;
+      const description = frontmatter['description'] as string;
+
+      if (!name || typeof name !== 'string') {
+        throw new Error('Missing or invalid "name" in frontmatter');
+      }
+
+      if (!description || typeof description !== 'string') {
+        throw new Error('Missing or invalid "description" in frontmatter');
+      }
+
+      // Extract optional fields
+      const tools = frontmatter['tools'] as string[] | undefined;
+      const modelConfig = frontmatter['modelConfig'] as
+        | Record<string, unknown>
+        | undefined;
+      const runConfig = frontmatter['runConfig'] as
+        | Record<string, unknown>
+        | undefined;
+
+      // Determine level from file path
+      // Project level paths contain the project root, user level paths are in home directory
+      const isProjectLevel =
+        filePath.includes(this.projectRoot) &&
+        filePath.includes(`/${QWEN_CONFIG_DIR}/${AGENT_CONFIG_DIR}/`);
+      const level: SubagentLevel = isProjectLevel ? 'project' : 'user';
+
+      const config: SubagentConfig = {
+        name,
+        description,
+        tools,
+        systemPrompt: systemPrompt.trim(),
+        level,
+        filePath,
+        modelConfig: modelConfig as Partial<
+          import('../core/subagent.js').ModelConfig
+        >,
+        runConfig: runConfig as Partial<
+          import('../core/subagent.js').RunConfig
+        >,
+      };
+
+      // Validate the parsed configuration
+      const validation = this.validator.validateConfig(config);
+      if (!validation.isValid) {
+        throw new Error(`Validation failed: ${validation.errors.join(', ')}`);
+      }
+
+      return config;
+    } catch (error) {
+      throw new SubagentError(
+        `Failed to parse subagent file: ${error instanceof Error ? error.message : 'Unknown error'}`,
+        SubagentErrorCode.INVALID_CONFIG,
+      );
+    }
+  }
+
+  /**
+   * Serializes a subagent configuration to Markdown format.
+   *
+   * @param config - Configuration to serialize
+   * @returns Markdown content with YAML frontmatter
+   */
+  serializeSubagent(config: SubagentConfig): string {
+    // Build frontmatter object
+    const frontmatter: Record<string, unknown> = {
+      name: config.name,
+      description: config.description,
+    };
+
+    if (config.tools && config.tools.length > 0) {
+      frontmatter['tools'] = config.tools;
+    }
+
+    if (config.modelConfig) {
+      frontmatter['modelConfig'] = config.modelConfig;
+    }
+
+    if (config.runConfig) {
+      frontmatter['runConfig'] = config.runConfig;
+    }
+
+    // Serialize to YAML
+    const yamlContent = stringifyYaml(frontmatter, {
+      lineWidth: 0, // Disable line wrapping
+      minContentWidth: 0,
+    }).trim();
+
+    // Combine frontmatter and system prompt
+    return `---\n${yamlContent}\n---\n\n${config.systemPrompt}\n`;
+  }
+
+  /**
+   * Creates a SubAgentScope from a subagent configuration.
+   *
+   * @param config - Subagent configuration
+   * @param runtimeContext - Runtime context
+   * @returns Promise resolving to SubAgentScope
+   */
+  async createSubagentScope(
+    config: SubagentConfig,
+    runtimeContext: Config,
+  ): Promise<SubAgentScope> {
+    try {
+      const runtimeConfig = this.convertToRuntimeConfig(config);
+
+      return await SubAgentScope.create(
+        config.name,
+        runtimeContext,
+        runtimeConfig.promptConfig,
+        runtimeConfig.modelConfig,
+        runtimeConfig.runConfig,
+        runtimeConfig.toolConfig,
+      );
+    } catch (error) {
+      if (error instanceof Error) {
+        throw new SubagentError(
+          `Failed to create SubAgentScope: ${error.message}`,
+          SubagentErrorCode.INVALID_CONFIG,
+          config.name,
+        );
+      }
+      throw error;
+    }
+  }
+
+  /**
+   * Converts a file-based SubagentConfig to runtime configuration
+   * compatible with SubAgentScope.create().
+   *
+   * @param config - File-based subagent configuration
+   * @returns Runtime configuration for SubAgentScope
+   */
+  convertToRuntimeConfig(config: SubagentConfig): SubagentRuntimeConfig {
+    // Build prompt configuration
+    const promptConfig: PromptConfig = {
+      systemPrompt: config.systemPrompt,
+    };
+
+    // Build model configuration
+    const modelConfig: ModelConfig = {
+      ...config.modelConfig,
+    };
+
+    // Build run configuration
+    const runConfig: RunConfig = {
+      ...config.runConfig,
+    };
+
+    // Build tool configuration if tools are specified
+    let toolConfig: ToolConfig | undefined;
+    if (config.tools && config.tools.length > 0) {
+      toolConfig = {
+        tools: config.tools,
+      };
+    }
+
+    return {
+      promptConfig,
+      modelConfig,
+      runConfig,
+      toolConfig,
+    };
+  }
+
+  /**
+   * Merges partial configurations with defaults, useful for updating
+   * existing configurations.
+   *
+   * @param base - Base configuration
+   * @param updates - Partial updates to apply
+   * @returns New configuration with updates applied
+   */
+  mergeConfigurations(
+    base: SubagentConfig,
+    updates: Partial<SubagentConfig>,
+  ): SubagentConfig {
+    return {
+      ...base,
+      ...updates,
+      // Handle nested objects specially
+      modelConfig: updates.modelConfig
+        ? { ...base.modelConfig, ...updates.modelConfig }
+        : base.modelConfig,
+      runConfig: updates.runConfig
+        ? { ...base.runConfig, ...updates.runConfig }
+        : base.runConfig,
+    };
+  }
+
+  /**
+   * Gets the file path for a subagent at a specific level.
+   *
+   * @param name - Subagent name
+   * @param level - Storage level
+   * @returns Absolute file path
+   */
+  private getSubagentPath(name: string, level: SubagentLevel): string {
+    const baseDir =
+      level === 'project'
+        ? path.join(this.projectRoot, QWEN_CONFIG_DIR, AGENT_CONFIG_DIR)
+        : path.join(os.homedir(), QWEN_CONFIG_DIR, AGENT_CONFIG_DIR);
+
+    return path.join(baseDir, `${name}.md`);
+  }
+
+  /**
+   * Lists subagents at a specific level.
+   *
+   * @param level - Storage level to check
+   * @returns Array of subagent metadata
+   */
+  private async listSubagentsAtLevel(
+    level: SubagentLevel,
+  ): Promise<SubagentMetadata[]> {
+    const baseDir =
+      level === 'project'
+        ? path.join(this.projectRoot, QWEN_CONFIG_DIR, AGENT_CONFIG_DIR)
+        : path.join(os.homedir(), QWEN_CONFIG_DIR, AGENT_CONFIG_DIR);
+
+    try {
+      const files = await fs.readdir(baseDir);
+      const subagents: SubagentMetadata[] = [];
+
+      for (const file of files) {
+        if (!file.endsWith('.md')) continue;
+
+        const filePath = path.join(baseDir, file);
+
+        try {
+          const config = await this.parseSubagentFile(filePath);
+          const metadata = this.configToMetadata(config);
+          subagents.push(metadata);
+        } catch (error) {
+          // Skip invalid files but log the error
+          console.warn(
+            `Skipping invalid subagent file ${filePath}: ${error instanceof Error ? error.message : 'Unknown error'}`,
+          );
+        }
+      }
+
+      return subagents;
+    } catch (_error) {
+      // Directory doesn't exist or can't be read
+      return [];
+    }
+  }
+
+  /**
+   * Converts a SubagentConfig to SubagentMetadata.
+   *
+   * @param config - Full configuration
+   * @returns Metadata object
+   */
+  private configToMetadata(config: SubagentConfig): SubagentMetadata {
+    return {
+      name: config.name,
+      description: config.description,
+      tools: config.tools,
+      level: config.level,
+      filePath: config.filePath,
+      // Add file stats if available
+      lastModified: undefined, // Would need to stat the file
+    };
+  }
+
+  /**
+   * Validates that a subagent name is available (not already in use).
+   *
+   * @param name - Name to check
+   * @param level - Level to check, or undefined to check both
+   * @returns True if name is available
+   */
+  async isNameAvailable(name: string, level?: SubagentLevel): Promise<boolean> {
+    const existing = await this.loadSubagent(name);
+
+    if (!existing) {
+      return true; // Name is available
+    }
+
+    if (level && existing.level !== level) {
+      return true; // Name is available at the specified level
+    }
+
+    return false; // Name is already in use
+  }
+
+  /**
+   * Gets available tools from the tool registry.
+   * Useful for validation and UI purposes.
+   *
+   * @returns Array of available tool names
+   */
+  getAvailableTools(): string[] {
+    if (!this.toolRegistry) {
+      return [];
+    }
+
+    // This would need to be implemented in ToolRegistry
+    // For now, return empty array
+    return [];
+  }
+}
diff --git a/packages/core/src/subagents/types.test.ts b/packages/core/src/subagents/types.test.ts
new file mode 100644
index 00000000..ecf2d717
--- /dev/null
+++ b/packages/core/src/subagents/types.test.ts
@@ -0,0 +1,40 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import { SubagentError, SubagentErrorCode } from './types.js';
+
+describe('SubagentError', () => {
+  it('should create error with message and code', () => {
+    const error = new SubagentError('Test error', SubagentErrorCode.NOT_FOUND);
+
+    expect(error).toBeInstanceOf(Error);
+    expect(error.name).toBe('SubagentError');
+    expect(error.message).toBe('Test error');
+    expect(error.code).toBe(SubagentErrorCode.NOT_FOUND);
+    expect(error.subagentName).toBeUndefined();
+  });
+
+  it('should create error with subagent name', () => {
+    const error = new SubagentError(
+      'Test error',
+      SubagentErrorCode.INVALID_CONFIG,
+      'test-agent',
+    );
+
+    expect(error.subagentName).toBe('test-agent');
+  });
+
+  it('should have correct error codes', () => {
+    expect(SubagentErrorCode.NOT_FOUND).toBe('NOT_FOUND');
+    expect(SubagentErrorCode.ALREADY_EXISTS).toBe('ALREADY_EXISTS');
+    expect(SubagentErrorCode.INVALID_CONFIG).toBe('INVALID_CONFIG');
+    expect(SubagentErrorCode.INVALID_NAME).toBe('INVALID_NAME');
+    expect(SubagentErrorCode.FILE_ERROR).toBe('FILE_ERROR');
+    expect(SubagentErrorCode.VALIDATION_ERROR).toBe('VALIDATION_ERROR');
+    expect(SubagentErrorCode.TOOL_NOT_FOUND).toBe('TOOL_NOT_FOUND');
+  });
+});
diff --git a/packages/core/src/subagents/types.ts b/packages/core/src/subagents/types.ts
new file mode 100644
index 00000000..5e61fccf
--- /dev/null
+++ b/packages/core/src/subagents/types.ts
@@ -0,0 +1,182 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  PromptConfig,
+  ModelConfig,
+  RunConfig,
+  ToolConfig,
+} from '../core/subagent.js';
+
+/**
+ * Represents the storage level for a subagent configuration.
+ * - 'project': Stored in `.qwen/agents/` within the project directory
+ * - 'user': Stored in `~/.qwen/agents/` in the user's home directory
+ */
+export type SubagentLevel = 'project' | 'user';
+
+/**
+ * Core configuration for a subagent as stored in Markdown files.
+ * This interface represents the file-based configuration that gets
+ * converted to runtime configuration for SubAgentScope.
+ */
+export interface SubagentConfig {
+  /** Unique name identifier for the subagent */
+  name: string;
+
+  /** Human-readable description of when and how to use this subagent */
+  description: string;
+
+  /**
+   * Optional list of tool names that this subagent is allowed to use.
+   * If omitted, the subagent inherits all available tools.
+   */
+  tools?: string[];
+
+  /**
+   * System prompt content that defines the subagent's behavior.
+   * Supports ${variable} templating via ContextState.
+   */
+  systemPrompt: string;
+
+  /** Storage level - determines where the configuration file is stored */
+  level: SubagentLevel;
+
+  /** Absolute path to the configuration file */
+  filePath: string;
+
+  /**
+   * Optional model configuration. If not provided, uses defaults.
+   * Can specify model name, temperature, and top_p values.
+   */
+  modelConfig?: Partial<ModelConfig>;
+
+  /**
+   * Optional runtime configuration. If not provided, uses defaults.
+   * Can specify max_time_minutes and max_turns.
+   */
+  runConfig?: Partial<RunConfig>;
+}
+
+/**
+ * Metadata extracted from a subagent configuration file.
+ * Used for listing and discovery without loading full configuration.
+ */
+export interface SubagentMetadata {
+  /** Unique name identifier */
+  name: string;
+
+  /** Human-readable description */
+  description: string;
+
+  /** Optional list of allowed tools */
+  tools?: string[];
+
+  /** Storage level */
+  level: SubagentLevel;
+
+  /** File path */
+  filePath: string;
+
+  /** File modification time for sorting */
+  lastModified?: Date;
+
+  /** Additional metadata from YAML frontmatter */
+  [key: string]: unknown;
+}
+
+/**
+ * Runtime configuration that converts file-based config to existing SubAgentScope.
+ * This interface maps SubagentConfig to the existing runtime interfaces.
+ */
+export interface SubagentRuntimeConfig {
+  /** Prompt configuration for SubAgentScope */
+  promptConfig: PromptConfig;
+
+  /** Model configuration for SubAgentScope */
+  modelConfig: ModelConfig;
+
+  /** Runtime execution configuration for SubAgentScope */
+  runConfig: RunConfig;
+
+  /** Optional tool configuration for SubAgentScope */
+  toolConfig?: ToolConfig;
+}
+
+/**
+ * Result of a validation operation on a subagent configuration.
+ */
+export interface ValidationResult {
+  /** Whether the configuration is valid */
+  isValid: boolean;
+
+  /** Array of error messages if validation failed */
+  errors: string[];
+
+  /** Array of warning messages (non-blocking issues) */
+  warnings: string[];
+}
+
+/**
+ * Options for listing subagents.
+ */
+export interface ListSubagentsOptions {
+  /** Filter by storage level */
+  level?: SubagentLevel;
+
+  /** Filter by tool availability */
+  hasTool?: string;
+
+  /** Sort order for results */
+  sortBy?: 'name' | 'lastModified' | 'level';
+
+  /** Sort direction */
+  sortOrder?: 'asc' | 'desc';
+}
+
+/**
+ * Options for creating a new subagent.
+ */
+export interface CreateSubagentOptions {
+  /** Storage level for the new subagent */
+  level: SubagentLevel;
+
+  /** Whether to overwrite existing subagent with same name */
+  overwrite?: boolean;
+
+  /** Custom directory path (overrides default level-based path) */
+  customPath?: string;
+}
+
+/**
+ * Error thrown when a subagent operation fails.
+ */
+export class SubagentError extends Error {
+  constructor(
+    message: string,
+    readonly code: string,
+    readonly subagentName?: string,
+  ) {
+    super(message);
+    this.name = 'SubagentError';
+  }
+}
+
+/**
+ * Error codes for subagent operations.
+ */
+export const SubagentErrorCode = {
+  NOT_FOUND: 'NOT_FOUND',
+  ALREADY_EXISTS: 'ALREADY_EXISTS',
+  INVALID_CONFIG: 'INVALID_CONFIG',
+  INVALID_NAME: 'INVALID_NAME',
+  FILE_ERROR: 'FILE_ERROR',
+  VALIDATION_ERROR: 'VALIDATION_ERROR',
+  TOOL_NOT_FOUND: 'TOOL_NOT_FOUND',
+} as const;
+
+export type SubagentErrorCode =
+  (typeof SubagentErrorCode)[keyof typeof SubagentErrorCode];
diff --git a/packages/core/src/subagents/validation.test.ts b/packages/core/src/subagents/validation.test.ts
new file mode 100644
index 00000000..7d01564d
--- /dev/null
+++ b/packages/core/src/subagents/validation.test.ts
@@ -0,0 +1,448 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { vi, describe, it, expect, beforeEach } from 'vitest';
+import { SubagentValidator } from './validation.js';
+import { SubagentConfig, SubagentError } from './types.js';
+import { ToolRegistry } from '../tools/tool-registry.js';
+
+describe('SubagentValidator', () => {
+  let validator: SubagentValidator;
+  let mockToolRegistry: ToolRegistry;
+
+  beforeEach(() => {
+    mockToolRegistry = {
+      getTool: vi.fn(),
+    } as unknown as ToolRegistry;
+
+    validator = new SubagentValidator(mockToolRegistry);
+  });
+
+  describe('validateName', () => {
+    it('should accept valid names', () => {
+      const validNames = [
+        'test-agent',
+        'code_reviewer',
+        'agent123',
+        'my-helper',
+      ];
+
+      for (const name of validNames) {
+        const result = validator.validateName(name);
+        expect(result.isValid).toBe(true);
+        expect(result.errors).toHaveLength(0);
+      }
+    });
+
+    it('should reject empty or whitespace names', () => {
+      const invalidNames = ['', '   ', '\t', '\n'];
+
+      for (const name of invalidNames) {
+        const result = validator.validateName(name);
+        expect(result.isValid).toBe(false);
+        expect(result.errors).toContain('Name is required and cannot be empty');
+      }
+    });
+
+    it('should reject names that are too short', () => {
+      const result = validator.validateName('a');
+      expect(result.isValid).toBe(false);
+      expect(result.errors).toContain(
+        'Name must be at least 2 characters long',
+      );
+    });
+
+    it('should reject names that are too long', () => {
+      const longName = 'a'.repeat(51);
+      const result = validator.validateName(longName);
+      expect(result.isValid).toBe(false);
+      expect(result.errors).toContain('Name must be 50 characters or less');
+    });
+
+    it('should reject names with invalid characters', () => {
+      const invalidNames = ['test@agent', 'agent.name', 'test agent', 'agent!'];
+
+      for (const name of invalidNames) {
+        const result = validator.validateName(name);
+        expect(result.isValid).toBe(false);
+        expect(result.errors).toContain(
+          'Name can only contain letters, numbers, hyphens, and underscores',
+        );
+      }
+    });
+
+    it('should reject names starting with special characters', () => {
+      const invalidNames = ['-agent', '_agent'];
+
+      for (const name of invalidNames) {
+        const result = validator.validateName(name);
+        expect(result.isValid).toBe(false);
+        expect(result.errors).toContain(
+          'Name cannot start with a hyphen or underscore',
+        );
+      }
+    });
+
+    it('should reject names ending with special characters', () => {
+      const invalidNames = ['agent-', 'agent_'];
+
+      for (const name of invalidNames) {
+        const result = validator.validateName(name);
+        expect(result.isValid).toBe(false);
+        expect(result.errors).toContain(
+          'Name cannot end with a hyphen or underscore',
+        );
+      }
+    });
+
+    it('should reject reserved names', () => {
+      const reservedNames = [
+        'self',
+        'system',
+        'user',
+        'model',
+        'tool',
+        'config',
+        'default',
+      ];
+
+      for (const name of reservedNames) {
+        const result = validator.validateName(name);
+        expect(result.isValid).toBe(false);
+        expect(result.errors).toContain(
+          `"${name}" is a reserved name and cannot be used`,
+        );
+      }
+    });
+
+    it('should warn about naming conventions', () => {
+      const result = validator.validateName('TestAgent');
+      expect(result.isValid).toBe(true);
+      expect(result.warnings).toContain(
+        'Consider using lowercase names for consistency',
+      );
+    });
+
+    it('should warn about mixed separators', () => {
+      const result = validator.validateName('test-agent_helper');
+      expect(result.isValid).toBe(true);
+      expect(result.warnings).toContain(
+        'Consider using either hyphens or underscores consistently, not both',
+      );
+    });
+  });
+
+  describe('validateSystemPrompt', () => {
+    it('should accept valid system prompts', () => {
+      const validPrompts = [
+        'You are a helpful assistant.',
+        'You are a code reviewer. Analyze the provided code and suggest improvements.',
+        'Help the user with ${task} by using available tools.',
+      ];
+
+      for (const prompt of validPrompts) {
+        const result = validator.validateSystemPrompt(prompt);
+        expect(result.isValid).toBe(true);
+        expect(result.errors).toHaveLength(0);
+      }
+    });
+
+    it('should reject empty prompts', () => {
+      const invalidPrompts = ['', '   ', '\t\n'];
+
+      for (const prompt of invalidPrompts) {
+        const result = validator.validateSystemPrompt(prompt);
+        expect(result.isValid).toBe(false);
+        expect(result.errors).toContain(
+          'System prompt is required and cannot be empty',
+        );
+      }
+    });
+
+    it('should reject prompts that are too short', () => {
+      const result = validator.validateSystemPrompt('Short');
+      expect(result.isValid).toBe(false);
+      expect(result.errors).toContain(
+        'System prompt must be at least 10 characters long',
+      );
+    });
+
+    it('should reject prompts that are too long', () => {
+      const longPrompt = 'a'.repeat(10001);
+      const result = validator.validateSystemPrompt(longPrompt);
+      expect(result.isValid).toBe(false);
+      expect(result.errors).toContain(
+        'System prompt is too long (>10,000 characters)',
+      );
+    });
+
+    it('should warn about long prompts', () => {
+      const longPrompt = 'a'.repeat(5001);
+      const result = validator.validateSystemPrompt(longPrompt);
+      expect(result.isValid).toBe(true);
+      expect(result.warnings).toContain(
+        'System prompt is quite long (>5,000 characters), consider shortening',
+      );
+    });
+  });
+
+  describe('validateTools', () => {
+    it('should accept valid tool arrays', () => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      vi.mocked(mockToolRegistry.getTool).mockReturnValue({} as any);
+
+      const result = validator.validateTools(['read_file', 'write_file']);
+      expect(result.isValid).toBe(true);
+      expect(result.errors).toHaveLength(0);
+    });
+
+    it('should reject non-array inputs', () => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const result = validator.validateTools('not-an-array' as any);
+      expect(result.isValid).toBe(false);
+      expect(result.errors).toContain('Tools must be an array of strings');
+    });
+
+    it('should warn about empty arrays', () => {
+      const result = validator.validateTools([]);
+      expect(result.isValid).toBe(true);
+      expect(result.warnings).toContain(
+        'Empty tools array - subagent will inherit all available tools',
+      );
+    });
+
+    it('should warn about duplicate tools', () => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      vi.mocked(mockToolRegistry.getTool).mockReturnValue({} as any);
+
+      const result = validator.validateTools([
+        'read_file',
+        'read_file',
+        'write_file',
+      ]);
+      expect(result.isValid).toBe(true);
+      expect(result.warnings).toContain(
+        'Duplicate tool names found in tools array',
+      );
+    });
+
+    it('should reject non-string tool names', () => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const result = validator.validateTools([123, 'read_file'] as any);
+      expect(result.isValid).toBe(false);
+      expect(result.errors).toContain(
+        'Tool name must be a string, got: number',
+      );
+    });
+
+    it('should reject empty tool names', () => {
+      const result = validator.validateTools(['', 'read_file']);
+      expect(result.isValid).toBe(false);
+      expect(result.errors).toContain('Tool name cannot be empty');
+    });
+
+    it('should reject unknown tools when registry is available', () => {
+      vi.mocked(mockToolRegistry.getTool).mockReturnValue(undefined);
+
+      const result = validator.validateTools(['unknown_tool']);
+      expect(result.isValid).toBe(false);
+      expect(result.errors).toContain(
+        'Tool "unknown_tool" not found in tool registry',
+      );
+    });
+  });
+
+  describe('validateModelConfig', () => {
+    it('should accept valid model configurations', () => {
+      const validConfigs = [
+        { model: 'gemini-1.5-pro', temp: 0.7, top_p: 0.9 },
+        { temp: 0.5 },
+        { top_p: 1.0 },
+        {},
+      ];
+
+      for (const config of validConfigs) {
+        const result = validator.validateModelConfig(config);
+        expect(result.isValid).toBe(true);
+        expect(result.errors).toHaveLength(0);
+      }
+    });
+
+    it('should reject invalid model names', () => {
+      const result = validator.validateModelConfig({ model: '' });
+      expect(result.isValid).toBe(false);
+      expect(result.errors).toContain('Model name must be a non-empty string');
+    });
+
+    it('should reject invalid temperature values', () => {
+      const invalidTemps = [-0.1, 2.1, 'not-a-number'];
+
+      for (const temp of invalidTemps) {
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        const result = validator.validateModelConfig({ temp: temp as any });
+        expect(result.isValid).toBe(false);
+      }
+    });
+
+    it('should warn about high temperature', () => {
+      const result = validator.validateModelConfig({ temp: 1.5 });
+      expect(result.isValid).toBe(true);
+      expect(result.warnings).toContain(
+        'High temperature (>1) may produce very creative but unpredictable results',
+      );
+    });
+
+    it('should reject invalid top_p values', () => {
+      const invalidTopP = [-0.1, 1.1, 'not-a-number'];
+
+      for (const top_p of invalidTopP) {
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        const result = validator.validateModelConfig({ top_p: top_p as any });
+        expect(result.isValid).toBe(false);
+      }
+    });
+  });
+
+  describe('validateRunConfig', () => {
+    it('should accept valid run configurations', () => {
+      const validConfigs = [
+        { max_time_minutes: 10, max_turns: 20 },
+        { max_time_minutes: 5 },
+        { max_turns: 10 },
+        {},
+      ];
+
+      for (const config of validConfigs) {
+        const result = validator.validateRunConfig(config);
+        expect(result.isValid).toBe(true);
+        expect(result.errors).toHaveLength(0);
+      }
+    });
+
+    it('should reject invalid max_time_minutes', () => {
+      const invalidTimes = [0, -1, 'not-a-number'];
+
+      for (const time of invalidTimes) {
+        const result = validator.validateRunConfig({
+          // eslint-disable-next-line @typescript-eslint/no-explicit-any
+          max_time_minutes: time as any,
+        });
+        expect(result.isValid).toBe(false);
+      }
+    });
+
+    it('should warn about very long execution times', () => {
+      const result = validator.validateRunConfig({ max_time_minutes: 120 });
+      expect(result.isValid).toBe(true);
+      expect(result.warnings).toContain(
+        'Very long execution time (>60 minutes) may cause resource issues',
+      );
+    });
+
+    it('should reject invalid max_turns', () => {
+      const invalidTurns = [0, -1, 1.5, 'not-a-number'];
+
+      for (const turns of invalidTurns) {
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        const result = validator.validateRunConfig({ max_turns: turns as any });
+        expect(result.isValid).toBe(false);
+      }
+    });
+
+    it('should warn about high turn limits', () => {
+      const result = validator.validateRunConfig({ max_turns: 150 });
+      expect(result.isValid).toBe(true);
+      expect(result.warnings).toContain(
+        'Very high turn limit (>100) may cause long execution times',
+      );
+    });
+  });
+
+  describe('validateConfig', () => {
+    const validConfig: SubagentConfig = {
+      name: 'test-agent',
+      description: 'A test subagent',
+      systemPrompt: 'You are a helpful assistant.',
+      level: 'project',
+      filePath: '/path/to/test-agent.md',
+    };
+
+    it('should accept valid configurations', () => {
+      const result = validator.validateConfig(validConfig);
+      expect(result.isValid).toBe(true);
+      expect(result.errors).toHaveLength(0);
+    });
+
+    it('should collect errors from all validation steps', () => {
+      const invalidConfig: SubagentConfig = {
+        name: '',
+        description: '',
+        systemPrompt: '',
+        level: 'project',
+        filePath: '/path/to/invalid.md',
+      };
+
+      const result = validator.validateConfig(invalidConfig);
+      expect(result.isValid).toBe(false);
+      expect(result.errors.length).toBeGreaterThan(0);
+    });
+
+    it('should collect warnings from all validation steps', () => {
+      const configWithWarnings: SubagentConfig = {
+        ...validConfig,
+        name: 'TestAgent', // Will generate warning about case
+        description: 'A'.repeat(501), // Will generate warning about long description
+      };
+
+      const result = validator.validateConfig(configWithWarnings);
+      expect(result.isValid).toBe(true);
+      expect(result.warnings.length).toBeGreaterThan(0);
+    });
+  });
+
+  describe('validateOrThrow', () => {
+    const validConfig: SubagentConfig = {
+      name: 'test-agent',
+      description: 'A test subagent',
+      systemPrompt: 'You are a helpful assistant.',
+      level: 'project',
+      filePath: '/path/to/test-agent.md',
+    };
+
+    it('should not throw for valid configurations', () => {
+      expect(() => validator.validateOrThrow(validConfig)).not.toThrow();
+    });
+
+    it('should throw SubagentError for invalid configurations', () => {
+      const invalidConfig: SubagentConfig = {
+        ...validConfig,
+        name: '',
+      };
+
+      expect(() => validator.validateOrThrow(invalidConfig)).toThrow(
+        SubagentError,
+      );
+      expect(() => validator.validateOrThrow(invalidConfig)).toThrow(
+        /Validation failed/,
+      );
+    });
+
+    it('should include subagent name in error', () => {
+      const invalidConfig: SubagentConfig = {
+        ...validConfig,
+        name: '',
+      };
+
+      try {
+        validator.validateOrThrow(invalidConfig, 'custom-name');
+        expect.fail('Should have thrown');
+      } catch (error) {
+        expect(error).toBeInstanceOf(SubagentError);
+        expect((error as SubagentError).subagentName).toBe('custom-name');
+      }
+    });
+  });
+});
diff --git a/packages/core/src/subagents/validation.ts b/packages/core/src/subagents/validation.ts
new file mode 100644
index 00000000..b271d1d7
--- /dev/null
+++ b/packages/core/src/subagents/validation.ts
@@ -0,0 +1,368 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  SubagentConfig,
+  ValidationResult,
+  SubagentError,
+  SubagentErrorCode,
+} from './types.js';
+import { ToolRegistry } from '../tools/tool-registry.js';
+
+/**
+ * Validates subagent configurations to ensure they are well-formed
+ * and compatible with the runtime system.
+ */
+export class SubagentValidator {
+  constructor(private readonly toolRegistry?: ToolRegistry) {}
+
+  /**
+   * Validates a complete subagent configuration.
+   *
+   * @param config - The subagent configuration to validate
+   * @returns ValidationResult with errors and warnings
+   */
+  validateConfig(config: SubagentConfig): ValidationResult {
+    const errors: string[] = [];
+    const warnings: string[] = [];
+
+    // Validate name
+    const nameValidation = this.validateName(config.name);
+    if (!nameValidation.isValid) {
+      errors.push(...nameValidation.errors);
+    }
+
+    // Validate description
+    if (!config.description || config.description.trim().length === 0) {
+      errors.push('Description is required and cannot be empty');
+    } else if (config.description.length > 500) {
+      warnings.push(
+        'Description is quite long (>500 chars), consider shortening for better readability',
+      );
+    }
+
+    // Validate system prompt
+    const promptValidation = this.validateSystemPrompt(config.systemPrompt);
+    if (!promptValidation.isValid) {
+      errors.push(...promptValidation.errors);
+    }
+    warnings.push(...promptValidation.warnings);
+
+    // Validate tools if specified
+    if (config.tools) {
+      const toolsValidation = this.validateTools(config.tools);
+      if (!toolsValidation.isValid) {
+        errors.push(...toolsValidation.errors);
+      }
+      warnings.push(...toolsValidation.warnings);
+    }
+
+    // Validate model config if specified
+    if (config.modelConfig) {
+      const modelValidation = this.validateModelConfig(config.modelConfig);
+      if (!modelValidation.isValid) {
+        errors.push(...modelValidation.errors);
+      }
+      warnings.push(...modelValidation.warnings);
+    }
+
+    // Validate run config if specified
+    if (config.runConfig) {
+      const runValidation = this.validateRunConfig(config.runConfig);
+      if (!runValidation.isValid) {
+        errors.push(...runValidation.errors);
+      }
+      warnings.push(...runValidation.warnings);
+    }
+
+    return {
+      isValid: errors.length === 0,
+      errors,
+      warnings,
+    };
+  }
+
+  /**
+   * Validates a subagent name.
+   * Names must be valid identifiers that can be used in file paths and tool calls.
+   *
+   * @param name - The name to validate
+   * @returns ValidationResult
+   */
+  validateName(name: string): ValidationResult {
+    const errors: string[] = [];
+    const warnings: string[] = [];
+
+    if (!name || name.trim().length === 0) {
+      errors.push('Name is required and cannot be empty');
+      return { isValid: false, errors, warnings };
+    }
+
+    const trimmedName = name.trim();
+
+    // Check length constraints
+    if (trimmedName.length < 2) {
+      errors.push('Name must be at least 2 characters long');
+    }
+
+    if (trimmedName.length > 50) {
+      errors.push('Name must be 50 characters or less');
+    }
+
+    // Check valid characters (alphanumeric, hyphens, underscores)
+    const validNameRegex = /^[a-zA-Z0-9_-]+$/;
+    if (!validNameRegex.test(trimmedName)) {
+      errors.push(
+        'Name can only contain letters, numbers, hyphens, and underscores',
+      );
+    }
+
+    // Check that it doesn't start or end with special characters
+    if (trimmedName.startsWith('-') || trimmedName.startsWith('_')) {
+      errors.push('Name cannot start with a hyphen or underscore');
+    }
+
+    if (trimmedName.endsWith('-') || trimmedName.endsWith('_')) {
+      errors.push('Name cannot end with a hyphen or underscore');
+    }
+
+    // Check for reserved names
+    const reservedNames = [
+      'self',
+      'system',
+      'user',
+      'model',
+      'tool',
+      'config',
+      'default',
+    ];
+    if (reservedNames.includes(trimmedName.toLowerCase())) {
+      errors.push(`"${trimmedName}" is a reserved name and cannot be used`);
+    }
+
+    // Warnings for naming conventions
+    if (trimmedName !== trimmedName.toLowerCase()) {
+      warnings.push('Consider using lowercase names for consistency');
+    }
+
+    if (trimmedName.includes('_') && trimmedName.includes('-')) {
+      warnings.push(
+        'Consider using either hyphens or underscores consistently, not both',
+      );
+    }
+
+    return {
+      isValid: errors.length === 0,
+      errors,
+      warnings,
+    };
+  }
+
+  /**
+   * Validates a system prompt.
+   *
+   * @param prompt - The system prompt to validate
+   * @returns ValidationResult
+   */
+  validateSystemPrompt(prompt: string): ValidationResult {
+    const errors: string[] = [];
+    const warnings: string[] = [];
+
+    if (!prompt || prompt.trim().length === 0) {
+      errors.push('System prompt is required and cannot be empty');
+      return { isValid: false, errors, warnings };
+    }
+
+    const trimmedPrompt = prompt.trim();
+
+    // Check minimum length for meaningful prompts
+    if (trimmedPrompt.length < 10) {
+      errors.push('System prompt must be at least 10 characters long');
+    }
+
+    // Check maximum length to prevent token issues
+    if (trimmedPrompt.length > 10000) {
+      errors.push('System prompt is too long (>10,000 characters)');
+    } else if (trimmedPrompt.length > 5000) {
+      warnings.push(
+        'System prompt is quite long (>5,000 characters), consider shortening',
+      );
+    }
+
+    return {
+      isValid: errors.length === 0,
+      errors,
+      warnings,
+    };
+  }
+
+  /**
+   * Validates a list of tool names.
+   *
+   * @param tools - Array of tool names to validate
+   * @returns ValidationResult
+   */
+  validateTools(tools: string[]): ValidationResult {
+    const errors: string[] = [];
+    const warnings: string[] = [];
+
+    if (!Array.isArray(tools)) {
+      errors.push('Tools must be an array of strings');
+      return { isValid: false, errors, warnings };
+    }
+
+    if (tools.length === 0) {
+      warnings.push(
+        'Empty tools array - subagent will inherit all available tools',
+      );
+      return { isValid: true, errors, warnings };
+    }
+
+    // Check for duplicates
+    const uniqueTools = new Set(tools);
+    if (uniqueTools.size !== tools.length) {
+      warnings.push('Duplicate tool names found in tools array');
+    }
+
+    // Validate each tool name
+    for (const tool of tools) {
+      if (typeof tool !== 'string') {
+        errors.push(`Tool name must be a string, got: ${typeof tool}`);
+        continue;
+      }
+
+      if (tool.trim().length === 0) {
+        errors.push('Tool name cannot be empty');
+        continue;
+      }
+
+      // Check if tool exists in registry (if available)
+      if (this.toolRegistry) {
+        const toolInstance = this.toolRegistry.getTool(tool);
+        if (!toolInstance) {
+          errors.push(`Tool "${tool}" not found in tool registry`);
+        }
+      }
+    }
+
+    return {
+      isValid: errors.length === 0,
+      errors,
+      warnings,
+    };
+  }
+
+  /**
+   * Validates model configuration.
+   *
+   * @param modelConfig - Partial model configuration to validate
+   * @returns ValidationResult
+   */
+  validateModelConfig(
+    modelConfig: Partial<import('../core/subagent.js').ModelConfig>,
+  ): ValidationResult {
+    const errors: string[] = [];
+    const warnings: string[] = [];
+
+    if (modelConfig.model !== undefined) {
+      if (
+        typeof modelConfig.model !== 'string' ||
+        modelConfig.model.trim().length === 0
+      ) {
+        errors.push('Model name must be a non-empty string');
+      }
+    }
+
+    if (modelConfig.temp !== undefined) {
+      if (typeof modelConfig.temp !== 'number') {
+        errors.push('Temperature must be a number');
+      } else if (modelConfig.temp < 0 || modelConfig.temp > 2) {
+        errors.push('Temperature must be between 0 and 2');
+      } else if (modelConfig.temp > 1) {
+        warnings.push(
+          'High temperature (>1) may produce very creative but unpredictable results',
+        );
+      }
+    }
+
+    if (modelConfig.top_p !== undefined) {
+      if (typeof modelConfig.top_p !== 'number') {
+        errors.push('top_p must be a number');
+      } else if (modelConfig.top_p < 0 || modelConfig.top_p > 1) {
+        errors.push('top_p must be between 0 and 1');
+      }
+    }
+
+    return {
+      isValid: errors.length === 0,
+      errors,
+      warnings,
+    };
+  }
+
+  /**
+   * Validates runtime configuration.
+   *
+   * @param runConfig - Partial run configuration to validate
+   * @returns ValidationResult
+   */
+  validateRunConfig(
+    runConfig: Partial<import('../core/subagent.js').RunConfig>,
+  ): ValidationResult {
+    const errors: string[] = [];
+    const warnings: string[] = [];
+
+    if (runConfig.max_time_minutes !== undefined) {
+      if (typeof runConfig.max_time_minutes !== 'number') {
+        errors.push('max_time_minutes must be a number');
+      } else if (runConfig.max_time_minutes <= 0) {
+        errors.push('max_time_minutes must be greater than 0');
+      } else if (runConfig.max_time_minutes > 60) {
+        warnings.push(
+          'Very long execution time (>60 minutes) may cause resource issues',
+        );
+      }
+    }
+
+    if (runConfig.max_turns !== undefined) {
+      if (typeof runConfig.max_turns !== 'number') {
+        errors.push('max_turns must be a number');
+      } else if (runConfig.max_turns <= 0) {
+        errors.push('max_turns must be greater than 0');
+      } else if (!Number.isInteger(runConfig.max_turns)) {
+        errors.push('max_turns must be an integer');
+      } else if (runConfig.max_turns > 100) {
+        warnings.push(
+          'Very high turn limit (>100) may cause long execution times',
+        );
+      }
+    }
+
+    return {
+      isValid: errors.length === 0,
+      errors,
+      warnings,
+    };
+  }
+
+  /**
+   * Throws a SubagentError if validation fails.
+   *
+   * @param config - Configuration to validate
+   * @param subagentName - Name for error context
+   * @throws SubagentError if validation fails
+   */
+  validateOrThrow(config: SubagentConfig, subagentName?: string): void {
+    const result = this.validateConfig(config);
+    if (!result.isValid) {
+      throw new SubagentError(
+        `Validation failed: ${result.errors.join(', ')}`,
+        SubagentErrorCode.VALIDATION_ERROR,
+        subagentName || config.name,
+      );
+    }
+  }
+}
diff --git a/packages/core/src/utils/yaml-parser.ts b/packages/core/src/utils/yaml-parser.ts
new file mode 100644
index 00000000..eb561068
--- /dev/null
+++ b/packages/core/src/utils/yaml-parser.ts
@@ -0,0 +1,166 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Simple YAML parser for subagent frontmatter.
+ * This is a minimal implementation that handles the basic YAML structures
+ * needed for subagent configuration files.
+ */
+
+/**
+ * Parses a simple YAML string into a JavaScript object.
+ * Supports basic key-value pairs, arrays, and nested objects.
+ *
+ * @param yamlString - YAML string to parse
+ * @returns Parsed object
+ */
+export function parse(yamlString: string): Record<string, unknown> {
+  const lines = yamlString
+    .split('\n')
+    .map((line) => line.trim())
+    .filter((line) => line && !line.startsWith('#'));
+  const result: Record<string, unknown> = {};
+
+  let currentKey = '';
+  let currentArray: string[] = [];
+  let inArray = false;
+  let currentObject: Record<string, unknown> = {};
+  let inObject = false;
+  let objectKey = '';
+
+  for (const line of lines) {
+    // Handle array items
+    if (line.startsWith('- ')) {
+      if (!inArray) {
+        inArray = true;
+        currentArray = [];
+      }
+      currentArray.push(line.substring(2).trim());
+      continue;
+    }
+
+    // End of array
+    if (inArray && !line.startsWith('- ')) {
+      result[currentKey] = currentArray;
+      inArray = false;
+      currentArray = [];
+    }
+
+    // Handle nested object items (simple indentation)
+    if (line.startsWith('  ') && inObject) {
+      const [key, ...valueParts] = line.trim().split(':');
+      const value = valueParts.join(':').trim();
+      currentObject[key.trim()] = parseValue(value);
+      continue;
+    }
+
+    // End of object
+    if (inObject && !line.startsWith('  ')) {
+      result[objectKey] = currentObject;
+      inObject = false;
+      currentObject = {};
+    }
+
+    // Handle key-value pairs
+    if (line.includes(':')) {
+      const [key, ...valueParts] = line.split(':');
+      const value = valueParts.join(':').trim();
+
+      if (value === '') {
+        // This might be the start of an object or array
+        currentKey = key.trim();
+        // Check if next lines are indented (object) or start with - (array)
+        continue;
+      } else {
+        result[key.trim()] = parseValue(value);
+      }
+    } else if (currentKey && !inArray && !inObject) {
+      // This might be the start of an object
+      inObject = true;
+      objectKey = currentKey;
+      currentObject = {};
+      currentKey = '';
+    }
+  }
+
+  // Handle remaining array or object
+  if (inArray) {
+    result[currentKey] = currentArray;
+  }
+  if (inObject) {
+    result[objectKey] = currentObject;
+  }
+
+  return result;
+}
+
+/**
+ * Converts a JavaScript object to a simple YAML string.
+ *
+ * @param obj - Object to stringify
+ * @param options - Stringify options
+ * @returns YAML string
+ */
+export function stringify(
+  obj: Record<string, unknown>,
+  _options?: { lineWidth?: number; minContentWidth?: number },
+): string {
+  const lines: string[] = [];
+
+  for (const [key, value] of Object.entries(obj)) {
+    if (Array.isArray(value)) {
+      lines.push(`${key}:`);
+      for (const item of value) {
+        lines.push(`  - ${item}`);
+      }
+    } else if (typeof value === 'object' && value !== null) {
+      lines.push(`${key}:`);
+      for (const [subKey, subValue] of Object.entries(
+        value as Record<string, unknown>,
+      )) {
+        lines.push(`  ${subKey}: ${formatValue(subValue)}`);
+      }
+    } else {
+      lines.push(`${key}: ${formatValue(value)}`);
+    }
+  }
+
+  return lines.join('\n');
+}
+
+/**
+ * Parses a value string into appropriate JavaScript type.
+ */
+function parseValue(value: string): unknown {
+  if (value === 'true') return true;
+  if (value === 'false') return false;
+  if (value === 'null') return null;
+  if (value === '') return '';
+
+  // Try to parse as number
+  const num = Number(value);
+  if (!isNaN(num) && isFinite(num)) {
+    return num;
+  }
+
+  // Return as string
+  return value;
+}
+
+/**
+ * Formats a value for YAML output.
+ */
+function formatValue(value: unknown): string {
+  if (typeof value === 'string') {
+    // Quote strings that might be ambiguous
+    if (value.includes(':') || value.includes('#') || value.trim() !== value) {
+      return `"${value.replace(/"/g, '\\"')}"`;
+    }
+    return value;
+  }
+
+  return String(value);
+}