diff --git a/openspec/changes/add-global-config-dir/design.md b/openspec/changes/archive/2025-12-20-add-global-config-dir/design.md
similarity index 100%
rename from openspec/changes/add-global-config-dir/design.md
rename to openspec/changes/archive/2025-12-20-add-global-config-dir/design.md
diff --git a/openspec/changes/add-global-config-dir/proposal.md b/openspec/changes/archive/2025-12-20-add-global-config-dir/proposal.md
similarity index 100%
rename from openspec/changes/add-global-config-dir/proposal.md
rename to openspec/changes/archive/2025-12-20-add-global-config-dir/proposal.md
diff --git a/openspec/changes/add-global-config-dir/specs/global-config/spec.md b/openspec/changes/archive/2025-12-20-add-global-config-dir/specs/global-config/spec.md
similarity index 100%
rename from openspec/changes/add-global-config-dir/specs/global-config/spec.md
rename to openspec/changes/archive/2025-12-20-add-global-config-dir/specs/global-config/spec.md
diff --git a/openspec/changes/add-global-config-dir/tasks.md b/openspec/changes/archive/2025-12-20-add-global-config-dir/tasks.md
similarity index 51%
rename from openspec/changes/add-global-config-dir/tasks.md
rename to openspec/changes/archive/2025-12-20-add-global-config-dir/tasks.md
index 5fecd731..240d5e88 100644
--- a/openspec/changes/add-global-config-dir/tasks.md
+++ b/openspec/changes/archive/2025-12-20-add-global-config-dir/tasks.md
@@ -1,25 +1,26 @@
 ## 1. Core Implementation
 
-- [ ] 1.1 Create `src/core/global-config.ts` with path resolution
+- [x] 1.1 Create `src/core/global-config.ts` with path resolution
   - Implement `getGlobalConfigDir()` following XDG spec
   - Support `$XDG_CONFIG_HOME` environment variable override
   - Platform-specific fallbacks (Unix: `~/.config/`, Windows: `%APPDATA%`)
-- [ ] 1.2 Define TypeScript interfaces for config shape
+- [x] 1.2 Define TypeScript interfaces for config shape
   - `GlobalConfig` interface with optional fields
   - Start minimal: just `featureFlags?: Record<string, boolean>`
-- [ ] 1.3 Implement config loading with defaults
+- [x] 1.3 Implement config loading with defaults
   - `getGlobalConfig()` - reads config.json if exists, merges with defaults
   - No directory/file creation on read (lazy initialization)
-- [ ] 1.4 Implement config saving
+- [x] 1.4 Implement config saving
   - `saveGlobalConfig(config)` - writes config.json, creates directory if needed
 
 ## 2. Integration
 
-- [ ] 2.1 Export new module from `src/core/index.ts`
-- [ ] 2.2 Add constants for config file name and directory name
+- [x] 2.1 Export new module from `src/core/index.ts`
+- [x] 2.2 Add constants for config file name and directory name
 
 ## 3. Testing
 
-- [ ] 3.1 Manual testing of path resolution on current platform
-- [ ] 3.2 Test with/without `$XDG_CONFIG_HOME` set
-- [ ] 3.3 Test config load when file doesn't exist (should return defaults)
+- [x] 3.1 Manual testing of path resolution on current platform
+- [x] 3.2 Test with/without `$XDG_CONFIG_HOME` set
+- [x] 3.3 Test config load when file doesn't exist (should return defaults)
+- [x] 3.4 Unit tests in `test/core/global-config.test.ts` (18 tests)
diff --git a/openspec/specs/global-config/spec.md b/openspec/specs/global-config/spec.md
new file mode 100644
index 00000000..b8538aad
--- /dev/null
+++ b/openspec/specs/global-config/spec.md
@@ -0,0 +1,81 @@
+# global-config Specification
+
+## Purpose
+
+This spec defines how OpenSpec resolves, reads, and writes user-level global configuration. It governs the `src/core/global-config.ts` module, which provides the foundation for storing user preferences, feature flags, and settings that persist across projects. The spec ensures cross-platform compatibility by following XDG Base Directory Specification with platform-specific fallbacks, and guarantees forward/backward compatibility through schema evolution rules.
+## Requirements
+### Requirement: Global Config Directory Path
+
+The system SHALL resolve the global configuration directory path following XDG Base Directory Specification with platform-specific fallbacks.
+
+#### Scenario: Unix/macOS with XDG_CONFIG_HOME set
+- **WHEN** `$XDG_CONFIG_HOME` environment variable is set to `/custom/config`
+- **THEN** `getGlobalConfigDir()` returns `/custom/config/openspec`
+
+#### Scenario: Unix/macOS without XDG_CONFIG_HOME
+- **WHEN** `$XDG_CONFIG_HOME` environment variable is not set
+- **AND** the platform is Unix or macOS
+- **THEN** `getGlobalConfigDir()` returns `~/.config/openspec` (expanded to absolute path)
+
+#### Scenario: Windows platform
+- **WHEN** the platform is Windows
+- **AND** `%APPDATA%` is set to `C:\Users\User\AppData\Roaming`
+- **THEN** `getGlobalConfigDir()` returns `C:\Users\User\AppData\Roaming\openspec`
+
+### Requirement: Global Config Loading
+
+The system SHALL load global configuration from the config directory with sensible defaults when the config file does not exist or cannot be parsed.
+
+#### Scenario: Config file exists and is valid
+- **WHEN** `config.json` exists in the global config directory
+- **AND** the file contains valid JSON matching the config schema
+- **THEN** `getGlobalConfig()` returns the parsed configuration
+
+#### Scenario: Config file does not exist
+- **WHEN** `config.json` does not exist in the global config directory
+- **THEN** `getGlobalConfig()` returns the default configuration
+- **AND** no directory or file is created
+
+#### Scenario: Config file is invalid JSON
+- **WHEN** `config.json` exists but contains invalid JSON
+- **THEN** `getGlobalConfig()` returns the default configuration
+- **AND** a warning is logged to stderr
+
+### Requirement: Global Config Saving
+
+The system SHALL save global configuration to the config directory, creating the directory if it does not exist.
+
+#### Scenario: Save config to new directory
+- **WHEN** `saveGlobalConfig(config)` is called
+- **AND** the global config directory does not exist
+- **THEN** the directory is created
+- **AND** `config.json` is written with the provided configuration
+
+#### Scenario: Save config to existing directory
+- **WHEN** `saveGlobalConfig(config)` is called
+- **AND** the global config directory already exists
+- **THEN** `config.json` is written (overwriting if exists)
+
+### Requirement: Default Configuration
+
+The system SHALL provide a default configuration that is used when no config file exists.
+
+#### Scenario: Default config structure
+- **WHEN** no config file exists
+- **THEN** the default configuration includes an empty `featureFlags` object
+
+### Requirement: Config Schema Evolution
+
+The system SHALL merge loaded configuration with default values to ensure new config fields are available even when loading older config files.
+
+#### Scenario: Config file missing new fields
+- **WHEN** `config.json` exists with `{ "featureFlags": {} }`
+- **AND** the current schema includes a new field `defaultAiTool`
+- **THEN** `getGlobalConfig()` returns `{ featureFlags: {}, defaultAiTool: <default> }`
+- **AND** the loaded values take precedence over defaults for fields that exist in both
+
+#### Scenario: Config file has extra unknown fields
+- **WHEN** `config.json` contains fields not in the current schema
+- **THEN** the unknown fields are preserved in the returned configuration
+- **AND** no error or warning is raised
+
diff --git a/src/core/global-config.ts b/src/core/global-config.ts
new file mode 100644
index 00000000..65c6a8d5
--- /dev/null
+++ b/src/core/global-config.ts
@@ -0,0 +1,102 @@
+import * as fs from 'node:fs';
+import * as path from 'node:path';
+import * as os from 'node:os';
+
+// Constants
+export const GLOBAL_CONFIG_DIR_NAME = 'openspec';
+export const GLOBAL_CONFIG_FILE_NAME = 'config.json';
+
+// TypeScript interfaces
+export interface GlobalConfig {
+  featureFlags?: Record<string, boolean>;
+}
+
+const DEFAULT_CONFIG: GlobalConfig = {
+  featureFlags: {}
+};
+
+/**
+ * Gets the global configuration directory path following XDG Base Directory Specification.
+ *
+ * - Unix/macOS: $XDG_CONFIG_HOME/openspec/ or ~/.config/openspec/
+ * - Windows: %APPDATA%/openspec/
+ */
+export function getGlobalConfigDir(): string {
+  const platform = os.platform();
+
+  if (platform === 'win32') {
+    // Windows: use %APPDATA%
+    const appData = process.env.APPDATA;
+    if (appData) {
+      return path.join(appData, GLOBAL_CONFIG_DIR_NAME);
+    }
+    // Fallback for Windows if APPDATA is not set
+    return path.join(os.homedir(), 'AppData', 'Roaming', GLOBAL_CONFIG_DIR_NAME);
+  }
+
+  // Unix/macOS: use XDG_CONFIG_HOME or fallback to ~/.config
+  const xdgConfigHome = process.env.XDG_CONFIG_HOME;
+  if (xdgConfigHome) {
+    return path.join(xdgConfigHome, GLOBAL_CONFIG_DIR_NAME);
+  }
+
+  return path.join(os.homedir(), '.config', GLOBAL_CONFIG_DIR_NAME);
+}
+
+/**
+ * Gets the path to the global config file.
+ */
+export function getGlobalConfigPath(): string {
+  return path.join(getGlobalConfigDir(), GLOBAL_CONFIG_FILE_NAME);
+}
+
+/**
+ * Loads the global configuration from disk.
+ * Returns default configuration if file doesn't exist or is invalid.
+ * Merges loaded config with defaults to ensure new fields are available.
+ */
+export function getGlobalConfig(): GlobalConfig {
+  const configPath = getGlobalConfigPath();
+
+  try {
+    if (!fs.existsSync(configPath)) {
+      return { ...DEFAULT_CONFIG };
+    }
+
+    const content = fs.readFileSync(configPath, 'utf-8');
+    const parsed = JSON.parse(content);
+
+    // Merge with defaults (loaded values take precedence)
+    return {
+      ...DEFAULT_CONFIG,
+      ...parsed,
+      // Deep merge featureFlags
+      featureFlags: {
+        ...DEFAULT_CONFIG.featureFlags,
+        ...(parsed.featureFlags || {})
+      }
+    };
+  } catch (error) {
+    // Log warning for parse errors, but not for missing files
+    if (error instanceof SyntaxError) {
+      console.error(`Warning: Invalid JSON in ${configPath}, using defaults`);
+    }
+    return { ...DEFAULT_CONFIG };
+  }
+}
+
+/**
+ * Saves the global configuration to disk.
+ * Creates the config directory if it doesn't exist.
+ */
+export function saveGlobalConfig(config: GlobalConfig): void {
+  const configDir = getGlobalConfigDir();
+  const configPath = getGlobalConfigPath();
+
+  // Create directory if it doesn't exist
+  if (!fs.existsSync(configDir)) {
+    fs.mkdirSync(configDir, { recursive: true });
+  }
+
+  fs.writeFileSync(configPath, JSON.stringify(config, null, 2) + '\n', 'utf-8');
+}
diff --git a/src/core/index.ts b/src/core/index.ts
index 0f52581e..22afd94d 100644
--- a/src/core/index.ts
+++ b/src/core/index.ts
@@ -1,2 +1,10 @@
 // Core OpenSpec logic will be implemented here
-export {};
\ No newline at end of file
+export {
+  GLOBAL_CONFIG_DIR_NAME,
+  GLOBAL_CONFIG_FILE_NAME,
+  type GlobalConfig,
+  getGlobalConfigDir,
+  getGlobalConfigPath,
+  getGlobalConfig,
+  saveGlobalConfig
+} from './global-config.js';
\ No newline at end of file
diff --git a/test/core/global-config.test.ts b/test/core/global-config.test.ts
new file mode 100644
index 00000000..cdf75f0a
--- /dev/null
+++ b/test/core/global-config.test.ts
@@ -0,0 +1,255 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import * as fs from 'node:fs';
+import * as path from 'node:path';
+import * as os from 'node:os';
+
+import {
+  getGlobalConfigDir,
+  getGlobalConfigPath,
+  getGlobalConfig,
+  saveGlobalConfig,
+  GLOBAL_CONFIG_DIR_NAME,
+  GLOBAL_CONFIG_FILE_NAME
+} from '../../src/core/global-config.js';
+
+describe('global-config', () => {
+  let tempDir: string;
+  let originalEnv: NodeJS.ProcessEnv;
+  let consoleErrorSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    // Create temp directory for tests
+    tempDir = path.join(os.tmpdir(), `openspec-global-config-test-${Date.now()}`);
+    fs.mkdirSync(tempDir, { recursive: true });
+
+    // Save original env
+    originalEnv = { ...process.env };
+
+    // Spy on console.error for warning tests
+    consoleErrorSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    // Restore original env
+    process.env = originalEnv;
+
+    // Clean up temp directory
+    fs.rmSync(tempDir, { recursive: true, force: true });
+
+    // Restore console.error
+    consoleErrorSpy.mockRestore();
+  });
+
+  describe('constants', () => {
+    it('should export correct directory name', () => {
+      expect(GLOBAL_CONFIG_DIR_NAME).toBe('openspec');
+    });
+
+    it('should export correct file name', () => {
+      expect(GLOBAL_CONFIG_FILE_NAME).toBe('config.json');
+    });
+  });
+
+  describe('getGlobalConfigDir', () => {
+    it('should use XDG_CONFIG_HOME when set', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+
+      const result = getGlobalConfigDir();
+
+      expect(result).toBe(path.join(tempDir, 'openspec'));
+    });
+
+    it('should fall back to ~/.config on Unix/macOS without XDG_CONFIG_HOME', () => {
+      delete process.env.XDG_CONFIG_HOME;
+
+      const result = getGlobalConfigDir();
+
+      // On non-Windows, should use ~/.config/openspec
+      if (os.platform() !== 'win32') {
+        expect(result).toBe(path.join(os.homedir(), '.config', 'openspec'));
+      }
+    });
+
+    it('should use APPDATA on Windows', () => {
+      // This test only makes sense conceptually - we can't change os.platform()
+      // But we can verify the APPDATA logic by checking the code path
+      if (os.platform() === 'win32') {
+        const appData = process.env.APPDATA;
+        if (appData) {
+          const result = getGlobalConfigDir();
+          expect(result).toBe(path.join(appData, 'openspec'));
+        }
+      }
+    });
+  });
+
+  describe('getGlobalConfigPath', () => {
+    it('should return path to config.json in config directory', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+
+      const result = getGlobalConfigPath();
+
+      expect(result).toBe(path.join(tempDir, 'openspec', 'config.json'));
+    });
+  });
+
+  describe('getGlobalConfig', () => {
+    it('should return defaults when config file does not exist', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+
+      const config = getGlobalConfig();
+
+      expect(config).toEqual({ featureFlags: {} });
+    });
+
+    it('should not create directory when reading non-existent config', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configDir = path.join(tempDir, 'openspec');
+
+      getGlobalConfig();
+
+      expect(fs.existsSync(configDir)).toBe(false);
+    });
+
+    it('should load valid config from file', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configDir = path.join(tempDir, 'openspec');
+      const configPath = path.join(configDir, 'config.json');
+
+      fs.mkdirSync(configDir, { recursive: true });
+      fs.writeFileSync(configPath, JSON.stringify({
+        featureFlags: { testFlag: true, anotherFlag: false }
+      }));
+
+      const config = getGlobalConfig();
+
+      expect(config.featureFlags).toEqual({ testFlag: true, anotherFlag: false });
+    });
+
+    it('should return defaults for invalid JSON', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configDir = path.join(tempDir, 'openspec');
+      const configPath = path.join(configDir, 'config.json');
+
+      fs.mkdirSync(configDir, { recursive: true });
+      fs.writeFileSync(configPath, '{ invalid json }');
+
+      const config = getGlobalConfig();
+
+      expect(config).toEqual({ featureFlags: {} });
+    });
+
+    it('should log warning for invalid JSON', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configDir = path.join(tempDir, 'openspec');
+      const configPath = path.join(configDir, 'config.json');
+
+      fs.mkdirSync(configDir, { recursive: true });
+      fs.writeFileSync(configPath, '{ invalid json }');
+
+      getGlobalConfig();
+
+      expect(consoleErrorSpy).toHaveBeenCalledWith(
+        expect.stringContaining('Invalid JSON')
+      );
+    });
+
+    it('should preserve unknown fields from config file', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configDir = path.join(tempDir, 'openspec');
+      const configPath = path.join(configDir, 'config.json');
+
+      fs.mkdirSync(configDir, { recursive: true });
+      fs.writeFileSync(configPath, JSON.stringify({
+        featureFlags: { x: true },
+        unknownField: 'preserved',
+        futureOption: 123
+      }));
+
+      const config = getGlobalConfig();
+
+      expect((config as any).unknownField).toBe('preserved');
+      expect((config as any).futureOption).toBe(123);
+    });
+
+    it('should merge loaded config with defaults', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configDir = path.join(tempDir, 'openspec');
+      const configPath = path.join(configDir, 'config.json');
+
+      // Config with only some fields
+      fs.mkdirSync(configDir, { recursive: true });
+      fs.writeFileSync(configPath, JSON.stringify({
+        featureFlags: { customFlag: true }
+      }));
+
+      const config = getGlobalConfig();
+
+      // Should have the custom flag
+      expect(config.featureFlags?.customFlag).toBe(true);
+    });
+  });
+
+  describe('saveGlobalConfig', () => {
+    it('should create directory if it does not exist', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configDir = path.join(tempDir, 'openspec');
+
+      saveGlobalConfig({ featureFlags: { test: true } });
+
+      expect(fs.existsSync(configDir)).toBe(true);
+    });
+
+    it('should write config to file', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configPath = path.join(tempDir, 'openspec', 'config.json');
+
+      saveGlobalConfig({ featureFlags: { myFlag: true } });
+
+      const content = fs.readFileSync(configPath, 'utf-8');
+      const parsed = JSON.parse(content);
+      expect(parsed.featureFlags.myFlag).toBe(true);
+    });
+
+    it('should overwrite existing config file', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configDir = path.join(tempDir, 'openspec');
+      const configPath = path.join(configDir, 'config.json');
+
+      // Create initial config
+      fs.mkdirSync(configDir, { recursive: true });
+      fs.writeFileSync(configPath, JSON.stringify({ featureFlags: { old: true } }));
+
+      // Overwrite
+      saveGlobalConfig({ featureFlags: { new: true } });
+
+      const content = fs.readFileSync(configPath, 'utf-8');
+      const parsed = JSON.parse(content);
+      expect(parsed.featureFlags.new).toBe(true);
+      expect(parsed.featureFlags.old).toBeUndefined();
+    });
+
+    it('should write formatted JSON with trailing newline', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const configPath = path.join(tempDir, 'openspec', 'config.json');
+
+      saveGlobalConfig({ featureFlags: {} });
+
+      const content = fs.readFileSync(configPath, 'utf-8');
+      expect(content).toContain('\n');
+      expect(content.endsWith('\n')).toBe(true);
+    });
+
+    it('should round-trip config correctly', () => {
+      process.env.XDG_CONFIG_HOME = tempDir;
+      const originalConfig = {
+        featureFlags: { flag1: true, flag2: false }
+      };
+
+      saveGlobalConfig(originalConfig);
+      const loadedConfig = getGlobalConfig();
+
+      expect(loadedConfig.featureFlags).toEqual(originalConfig.featureFlags);
+    });
+  });
+});