diff --git a/lib/mock-github.cjs b/lib/mock-github.cjs
new file mode 100644
index 0000000..1d707db
--- /dev/null
+++ b/lib/mock-github.cjs
@@ -0,0 +1,386 @@
+'use strict';
+
+/**
+ * lib/mock-github.cjs — GitHub API interceptor for tests
+ *
+ * Intercepts `child_process.execSync` calls that invoke the `gh` CLI, returning
+ * pre-baked fixture responses instead of making real network calls.
+ *
+ * Usage:
+ *
+ *   const mockGitHub = require('./lib/mock-github.cjs');
+ *
+ *   // Activate before test (optionally pass a scenario name)
+ *   mockGitHub.activate();             // base fixtures only
+ *   mockGitHub.activate('pr-error');   // load test/fixtures/github/pr-error/ overrides
+ *
+ *   // ... run code that calls gh CLI ...
+ *
+ *   const calls = mockGitHub.getCallLog();  // inspect what was called
+ *   mockGitHub.deactivate();                // restore real execSync
+ *
+ * Scenario support:
+ *   Scenarios live in test/fixtures/github/<scenario>/ and override base fixtures.
+ *   Any fixture key present in the scenario directory takes precedence over the
+ *   corresponding base fixture. This allows targeted per-test overrides.
+ *
+ * Inline overrides (highest precedence):
+ *   mockGitHub.setResponse('gh issue view', '{"number":999}');
+ *
+ * Call log format:
+ *   Each entry: { cmd, fixture, returnValue, timestamp }
+ *
+ * Safety:
+ *   - Re-activating without deactivating first is safe (auto-deactivates).
+ *   - Module never makes real gh CLI calls.
+ *   - Fixture load errors throw descriptive Error messages.
+ */
+
+const path = require('path');
+const fs = require('fs');
+const childProcess = require('child_process');
+
+// ---------------------------------------------------------------------------
+// Path resolution
+// ---------------------------------------------------------------------------
+
+/**
+ * Resolve the fixtures base directory relative to this file.
+ * Works whether installed as a package or used in-repo.
+ */
+function resolveFixturesDir() {
+  // Walk up from lib/ to find test/fixtures/github/
+  const libDir = __dirname;
+  const repoRoot = path.resolve(libDir, '..');
+  return path.join(repoRoot, 'test', 'fixtures', 'github');
+}
+
+// ---------------------------------------------------------------------------
+// Fixture loading
+// ---------------------------------------------------------------------------
+
+/**
+ * Load a fixture file and return its contents as a string.
+ * Returns the raw file content — callers receive exactly what execSync would.
+ *
+ * For fixtures whose JSON root is a string (e.g. `"https://..."`) we strip
+ * the outer quotes since execSync output never includes JSON string quoting.
+ * For fixtures whose JSON root is an object or array, we return the raw JSON.
+ *
+ * @param {string} fixtureKey - e.g. "issue-view", "pr-create"
+ * @param {string} baseDir - resolved fixtures base directory
+ * @param {string|null} scenarioDir - resolved scenario override directory (or null)
+ * @returns {string} fixture content as execSync would return it
+ * @throws {Error} if fixture file not found in either location
+ */
+function loadFixture(fixtureKey, baseDir, scenarioDir) {
+  const filename = `${fixtureKey}.json`;
+
+  // Scenario directory takes precedence
+  if (scenarioDir) {
+    const scenarioPath = path.join(scenarioDir, filename);
+    if (fs.existsSync(scenarioPath)) {
+      return parseFixtureFile(scenarioPath, fixtureKey);
+    }
+  }
+
+  // Base fixture
+  const basePath = path.join(baseDir, filename);
+  if (fs.existsSync(basePath)) {
+    return parseFixtureFile(basePath, fixtureKey);
+  }
+
+  throw new Error(
+    `mock-github: fixture not found: "${fixtureKey}" (looked for ${filename} in ${baseDir}${scenarioDir ? ` and ${scenarioDir}` : ''})`
+  );
+}
+
+/**
+ * Read a fixture file and convert its content to execSync-compatible output.
+ *
+ * JSON strings (root is a quoted string) are unwrapped: `"foo"` → `foo`
+ * JSON objects/arrays are returned as compact JSON strings.
+ * Empty strings (`""`) are returned as `""`.
+ *
+ * @param {string} filePath - absolute path to fixture .json file
+ * @param {string} fixtureKey - used in error messages
+ * @returns {string}
+ */
+function parseFixtureFile(filePath, fixtureKey) {
+  let raw;
+  try {
+    raw = fs.readFileSync(filePath, 'utf-8').trim();
+  } catch (err) {
+    throw new Error(`mock-github: failed to read fixture "${fixtureKey}" at ${filePath}: ${err.message}`);
+  }
+
+  let parsed;
+  try {
+    parsed = JSON.parse(raw);
+  } catch (err) {
+    throw new Error(`mock-github: fixture "${fixtureKey}" is not valid JSON (${filePath}): ${err.message}`);
+  }
+
+  // Unwrap JSON string values (execSync output is never JSON-encoded strings)
+  if (typeof parsed === 'string') {
+    return parsed;
+  }
+
+  // Objects and arrays: return compact JSON (callers parse with JSON.parse)
+  return JSON.stringify(parsed);
+}
+
+// ---------------------------------------------------------------------------
+// Command routing
+// ---------------------------------------------------------------------------
+
+/**
+ * Route table: ordered list of [pattern, fixtureKey] pairs.
+ * First match wins. Patterns are matched against the full command string.
+ *
+ * Built-in responses (not loaded from fixtures) are also handled in routeCommand().
+ */
+const ROUTE_TABLE = [
+  // Issue operations
+  [/\bgh issue view\b/, 'issue-view'],
+  [/\bgh issue list\b/, 'issue-list'],
+  [/\bgh issue comment\b/, 'issue-comment'],
+  [/\bgh issue edit\b/, 'issue-edit'],
+
+  // Milestone operations (order matters: PATCH before GET)
+  [/\bgh api\b.*\/milestones\/\d+.*--method PATCH/, 'milestone-close'],
+  [/\bgh api\b.*--method POST.*\/milestones/, 'milestone-create'],
+  [/\bgh api\b.*\/milestones\b.*--method POST/, 'milestone-create'],
+  [/\bgh api repos\/.*\/milestones\/\d+/, 'milestone-view'],
+
+  // Label operations
+  [/\bgh label create\b/, 'label-create'],
+  [/\bgh label list\b/, 'label-list'],
+
+  // PR operations
+  [/\bgh pr create\b/, 'pr-create'],
+  [/\bgh pr view\b/, 'pr-view'],
+
+  // Rate limit
+  [/\bgh api rate_limit\b/, 'rate-limit'],
+
+  // Board / GraphQL operations (order matters: specific mutations before generic graphql)
+  [/\bgh api graphql\b.*updateProjectV2ItemFieldValue/, 'graphql-board-mutation'],
+  [/\bgh api graphql\b.*discussionCategories/, 'repo-meta'],
+  [/\bgh api graphql\b.*createDiscussion/, 'discussion-create'],
+  [/\bgh project item-add\b/, 'board-item'],
+];
+
+/**
+ * Built-in responses — returned directly without loading a fixture file.
+ * These cover repo identity and user queries that are near-universal.
+ */
+const BUILTINS = [
+  [/\bgh repo view\b/, 'snipcodeit/mgw'],
+  [/\bgh api user\b/, '{"login":"snipcodeit"}'],
+  [/\bgh api\b.*\/user\b/, '{"login":"snipcodeit"}'],
+];
+
+/**
+ * Find the matching fixture key or builtin value for a command string.
+ *
+ * @param {string} cmd - the execSync command string
+ * @param {Map<string, string>} inlineOverrides - per-command inline overrides
+ * @returns {{ type: 'fixture'|'builtin'|'empty', key?: string, value?: string }}
+ */
+function routeCommand(cmd, inlineOverrides) {
+  // 1. Inline overrides (highest precedence) — match by prefix/substring
+  for (const [pattern, value] of inlineOverrides) {
+    if (cmd.includes(pattern)) {
+      return { type: 'builtin', key: pattern, value };
+    }
+  }
+
+  // 2. Builtin responses
+  for (const [pattern, value] of BUILTINS) {
+    if (pattern.test(cmd)) {
+      return { type: 'builtin', key: String(pattern), value };
+    }
+  }
+
+  // 3. Route table → fixture key
+  for (const [pattern, fixtureKey] of ROUTE_TABLE) {
+    if (pattern.test(cmd)) {
+      return { type: 'fixture', key: fixtureKey };
+    }
+  }
+
+  // 4. Default: empty string (unknown command)
+  return { type: 'empty', key: null };
+}
+
+// ---------------------------------------------------------------------------
+// State
+// ---------------------------------------------------------------------------
+
+/** The original child_process.execSync before any mock was installed */
+let _originalExecSync = null;
+
+/** Whether the mock is currently active */
+let _active = false;
+
+/** Ordered log of intercepted calls */
+let _callLog = [];
+
+/** Resolved path to base fixtures directory */
+let _baseDir = null;
+
+/** Resolved path to scenario override directory (or null) */
+let _scenarioDir = null;
+
+/** Per-command inline overrides: Map<string, string> (pattern string → return value) */
+let _inlineOverrides = new Map();
+
+// ---------------------------------------------------------------------------
+// Core API
+// ---------------------------------------------------------------------------
+
+/**
+ * Activate the mock. Replaces child_process.execSync with an interceptor.
+ *
+ * Safe to call when already active — deactivates first, then re-activates.
+ *
+ * @param {string} [scenario] - Optional scenario name. If provided, fixtures from
+ *   `test/fixtures/github/<scenario>/` override the base fixtures.
+ * @throws {Error} if the fixtures base directory does not exist
+ */
+function activate(scenario) {
+  if (_active) {
+    deactivate();
+  }
+
+  _baseDir = resolveFixturesDir();
+
+  if (!fs.existsSync(_baseDir)) {
+    throw new Error(
+      `mock-github: fixtures directory not found: ${_baseDir}\n` +
+      'Create test/fixtures/github/ with fixture JSON files before activating the mock.'
+    );
+  }
+
+  if (scenario) {
+    _scenarioDir = path.join(_baseDir, scenario);
+    if (!fs.existsSync(_scenarioDir)) {
+      throw new Error(
+        `mock-github: scenario directory not found: ${_scenarioDir}`
+      );
+    }
+  } else {
+    _scenarioDir = null;
+  }
+
+  _callLog = [];
+  _inlineOverrides = new Map();
+
+  // Store original and install interceptor
+  _originalExecSync = childProcess.execSync;
+
+  childProcess.execSync = function mockExecSync(cmd, _opts) {
+    const route = routeCommand(cmd, _inlineOverrides);
+
+    let returnValue;
+    let fixtureKey;
+
+    if (route.type === 'builtin') {
+      returnValue = route.value;
+      fixtureKey = route.key;
+    } else if (route.type === 'fixture') {
+      returnValue = loadFixture(route.key, _baseDir, _scenarioDir);
+      fixtureKey = route.key;
+    } else {
+      returnValue = '';
+      fixtureKey = null;
+    }
+
+    _callLog.push({
+      cmd,
+      fixture: fixtureKey,
+      returnValue,
+      timestamp: new Date().toISOString(),
+    });
+
+    return returnValue;
+  };
+
+  _active = true;
+}
+
+/**
+ * Deactivate the mock. Restores the original child_process.execSync.
+ * Safe to call when not active (no-op).
+ */
+function deactivate() {
+  if (!_active) return;
+
+  childProcess.execSync = _originalExecSync;
+  _originalExecSync = null;
+  _active = false;
+  _baseDir = null;
+  _scenarioDir = null;
+  _inlineOverrides = new Map();
+  // Note: call log is preserved after deactivation — callers inspect it after the test
+}
+
+/**
+ * Return the ordered array of intercepted call entries since the last activate().
+ * Each entry: { cmd, fixture, returnValue, timestamp }
+ *
+ * @returns {Array<{cmd: string, fixture: string|null, returnValue: string, timestamp: string}>}
+ */
+function getCallLog() {
+  return _callLog.slice(); // defensive copy
+}
+
+/**
+ * Clear the call log without deactivating the mock.
+ * Useful for resetting between sub-scenarios in a single test.
+ */
+function clearCallLog() {
+  _callLog = [];
+}
+
+/**
+ * Set an inline response override for commands matching the given pattern string.
+ * The pattern is matched with String.prototype.includes() against the full command.
+ * Inline overrides take precedence over all other routing (builtins and fixture table).
+ *
+ * Must be called after activate().
+ *
+ * @param {string} cmdPattern - Substring to match in the command string
+ * @param {string} returnValue - Value to return when the pattern matches
+ * @throws {Error} if called before activate()
+ */
+function setResponse(cmdPattern, returnValue) {
+  if (!_active) {
+    throw new Error('mock-github: setResponse() called before activate(). Call activate() first.');
+  }
+  _inlineOverrides.set(cmdPattern, returnValue);
+}
+
+/**
+ * Whether the mock is currently active.
+ * Useful for guard assertions in test setup/teardown.
+ *
+ * @returns {boolean}
+ */
+function isActive() {
+  return _active;
+}
+
+// ---------------------------------------------------------------------------
+// Exports
+// ---------------------------------------------------------------------------
+
+module.exports = {
+  activate,
+  deactivate,
+  getCallLog,
+  clearCallLog,
+  setResponse,
+  isActive,
+};
diff --git a/test/fixtures/github/board-item.json b/test/fixtures/github/board-item.json
new file mode 100644
index 0000000..cefad09
--- /dev/null
+++ b/test/fixtures/github/board-item.json
@@ -0,0 +1 @@
+"PVT_kwDOABC123"
diff --git a/test/fixtures/github/discussion-create.json b/test/fixtures/github/discussion-create.json
new file mode 100644
index 0000000..b20a069
--- /dev/null
+++ b/test/fixtures/github/discussion-create.json
@@ -0,0 +1,3 @@
+{
+  "url": "https://github.com/snipcodeit/mgw/discussions/99"
+}
diff --git a/test/fixtures/github/graphql-board-mutation.json b/test/fixtures/github/graphql-board-mutation.json
new file mode 100644
index 0000000..7616a6b
--- /dev/null
+++ b/test/fixtures/github/graphql-board-mutation.json
@@ -0,0 +1,9 @@
+{
+  "data": {
+    "updateProjectV2ItemFieldValue": {
+      "projectV2Item": {
+        "id": "PVTI_kwDOABC123"
+      }
+    }
+  }
+}
diff --git a/test/fixtures/github/issue-comment.json b/test/fixtures/github/issue-comment.json
new file mode 100644
index 0000000..e16c76d
--- /dev/null
+++ b/test/fixtures/github/issue-comment.json
@@ -0,0 +1 @@
+""
diff --git a/test/fixtures/github/issue-edit.json b/test/fixtures/github/issue-edit.json
new file mode 100644
index 0000000..fd710fa
--- /dev/null
+++ b/test/fixtures/github/issue-edit.json
@@ -0,0 +1 @@
+"https://github.com/snipcodeit/mgw/issues/42"
diff --git a/test/fixtures/github/issue-list.json b/test/fixtures/github/issue-list.json
new file mode 100644
index 0000000..a35c131
--- /dev/null
+++ b/test/fixtures/github/issue-list.json
@@ -0,0 +1,26 @@
+[
+  {
+    "number": 1,
+    "title": "First issue",
+    "state": "OPEN",
+    "labels": [{ "name": "bug" }],
+    "milestone": { "title": "v1.0", "number": 1 },
+    "assignees": [{ "login": "hat" }],
+    "createdAt": "2026-01-01T00:00:00Z",
+    "url": "https://github.com/snipcodeit/mgw/issues/1",
+    "body": "First issue body",
+    "comments": []
+  },
+  {
+    "number": 2,
+    "title": "Second issue",
+    "state": "OPEN",
+    "labels": [],
+    "milestone": null,
+    "assignees": [],
+    "createdAt": "2026-01-02T00:00:00Z",
+    "url": "https://github.com/snipcodeit/mgw/issues/2",
+    "body": "Second issue body",
+    "comments": []
+  }
+]
diff --git a/test/fixtures/github/issue-view.json b/test/fixtures/github/issue-view.json
new file mode 100644
index 0000000..8c395a7
--- /dev/null
+++ b/test/fixtures/github/issue-view.json
@@ -0,0 +1,13 @@
+{
+  "number": 42,
+  "title": "Fix everything",
+  "state": "OPEN",
+  "labels": [{ "name": "bug" }],
+  "milestone": { "title": "v1.0", "number": 1 },
+  "assignees": [{ "login": "hat" }],
+  "body": "Body text",
+  "url": "https://github.com/snipcodeit/mgw/issues/42",
+  "comments": [],
+  "createdAt": "2026-01-01T00:00:00Z",
+  "updatedAt": "2026-03-01T00:00:00Z"
+}
diff --git a/test/fixtures/github/label-create.json b/test/fixtures/github/label-create.json
new file mode 100644
index 0000000..e16c76d
--- /dev/null
+++ b/test/fixtures/github/label-create.json
@@ -0,0 +1 @@
+""
diff --git a/test/fixtures/github/label-list.json b/test/fixtures/github/label-list.json
new file mode 100644
index 0000000..29a42dc
--- /dev/null
+++ b/test/fixtures/github/label-list.json
@@ -0,0 +1,5 @@
+[
+  { "name": "bug", "color": "d73a4a", "description": "Something isn't working" },
+  { "name": "mgw:triaged", "color": "0e8a16", "description": "Issue triaged and ready for pipeline" },
+  { "name": "mgw:in-progress", "color": "1d76db", "description": "Pipeline actively executing" }
+]
diff --git a/test/fixtures/github/milestone-close.json b/test/fixtures/github/milestone-close.json
new file mode 100644
index 0000000..daeb03e
--- /dev/null
+++ b/test/fixtures/github/milestone-close.json
@@ -0,0 +1,11 @@
+{
+  "number": 3,
+  "title": "v1.0",
+  "state": "closed",
+  "open_issues": 0,
+  "closed_issues": 7,
+  "description": "First milestone",
+  "html_url": "https://github.com/snipcodeit/mgw/milestone/3",
+  "created_at": "2026-01-01T00:00:00Z",
+  "updated_at": "2026-03-06T00:00:00Z"
+}
diff --git a/test/fixtures/github/milestone-create.json b/test/fixtures/github/milestone-create.json
new file mode 100644
index 0000000..7b79653
--- /dev/null
+++ b/test/fixtures/github/milestone-create.json
@@ -0,0 +1,12 @@
+{
+  "number": 4,
+  "title": "v2.0",
+  "state": "open",
+  "open_issues": 0,
+  "closed_issues": 0,
+  "description": "Second milestone",
+  "html_url": "https://github.com/snipcodeit/mgw/milestone/4",
+  "id": 98765432,
+  "created_at": "2026-03-06T00:00:00Z",
+  "updated_at": "2026-03-06T00:00:00Z"
+}
diff --git a/test/fixtures/github/milestone-view.json b/test/fixtures/github/milestone-view.json
new file mode 100644
index 0000000..7e75491
--- /dev/null
+++ b/test/fixtures/github/milestone-view.json
@@ -0,0 +1,11 @@
+{
+  "number": 3,
+  "title": "v1.0",
+  "state": "open",
+  "open_issues": 2,
+  "closed_issues": 5,
+  "description": "First milestone",
+  "html_url": "https://github.com/snipcodeit/mgw/milestone/3",
+  "created_at": "2026-01-01T00:00:00Z",
+  "updated_at": "2026-03-01T00:00:00Z"
+}
diff --git a/test/fixtures/github/pr-create.json b/test/fixtures/github/pr-create.json
new file mode 100644
index 0000000..12199d8
--- /dev/null
+++ b/test/fixtures/github/pr-create.json
@@ -0,0 +1 @@
+"https://github.com/snipcodeit/mgw/pull/99"
diff --git a/test/fixtures/github/pr-view.json b/test/fixtures/github/pr-view.json
new file mode 100644
index 0000000..2526983
--- /dev/null
+++ b/test/fixtures/github/pr-view.json
@@ -0,0 +1,10 @@
+{
+  "number": 99,
+  "title": "feat: add mock github layer",
+  "state": "OPEN",
+  "mergedAt": null,
+  "url": "https://github.com/snipcodeit/mgw/pull/99",
+  "headRefName": "issue/247-mock-github",
+  "baseRefName": "main",
+  "body": "Implements mock GitHub API layer"
+}
diff --git a/test/fixtures/github/rate-limit.json b/test/fixtures/github/rate-limit.json
new file mode 100644
index 0000000..61b0521
--- /dev/null
+++ b/test/fixtures/github/rate-limit.json
@@ -0,0 +1,22 @@
+{
+  "resources": {
+    "core": {
+      "remaining": 4999,
+      "limit": 5000,
+      "reset": 1700000000,
+      "used": 1
+    },
+    "search": {
+      "remaining": 29,
+      "limit": 30,
+      "reset": 1700000060,
+      "used": 1
+    }
+  },
+  "rate": {
+    "remaining": 4999,
+    "limit": 5000,
+    "reset": 1700000000,
+    "used": 1
+  }
+}
diff --git a/test/fixtures/github/repo-meta.json b/test/fixtures/github/repo-meta.json
new file mode 100644
index 0000000..1d42641
--- /dev/null
+++ b/test/fixtures/github/repo-meta.json
@@ -0,0 +1,10 @@
+{
+  "id": "R_kgDOABC",
+  "discussionCategories": {
+    "nodes": [
+      { "id": "DIC_kwDOABC", "name": "Announcements" },
+      { "id": "DIC_kwDOXYZ", "name": "General" },
+      { "id": "DIC_kwDOQ&A", "name": "Q&A" }
+    ]
+  }
+}