diff --git a/lib/mock-gsd-agent.cjs b/lib/mock-gsd-agent.cjs
new file mode 100644
index 0000000..e537a0a
--- /dev/null
+++ b/lib/mock-gsd-agent.cjs
@@ -0,0 +1,367 @@
+'use strict';
+
+/**
+ * lib/mock-gsd-agent.cjs — Fake GSD agent runner for tests
+ *
+ * Intercepts Task() spawns in MGW command tests, returning configurable
+ * fixture outputs and recording spawn calls for assertion.
+ *
+ * Usage:
+ *
+ *   const mockAgent = require('./lib/mock-gsd-agent.cjs');
+ *
+ *   // Activate before test (optionally pass a scenario name)
+ *   mockAgent.activate();                   // base fixtures only
+ *   mockAgent.activate('planner-error');    // load test/fixtures/agents/planner-error/ overrides
+ *
+ *   // ... call spawnStub() where code would call Task() ...
+ *   const output = mockAgent.spawnStub({
+ *     subagent_type: 'gsd-planner',
+ *     prompt: 'Plan phase 47...',
+ *     model: 'inherit',
+ *     description: 'Plan Phase 47'
+ *   });
+ *
+ *   const calls = mockAgent.getCallLog();   // inspect what was spawned
+ *   mockAgent.assertSpawned('gsd-planner'); // assert a specific agent type was used
+ *   mockAgent.deactivate();                 // clean up
+ *
+ * Scenario support:
+ *   Scenarios live in test/fixtures/agents/<scenario>/ and override base fixtures.
+ *   Any fixture file present in the scenario directory takes precedence over the
+ *   corresponding base fixture. This allows targeted per-test overrides.
+ *
+ * Inline overrides (highest precedence):
+ *   mockAgent.setResponse('gsd-planner', '## PLANNING COMPLETE\n...');
+ *
+ * Call log format:
+ *   Each entry: { subagent_type, prompt, model, description, output, timestamp }
+ *
+ * Safety:
+ *   - Re-activating without deactivating first is safe (auto-deactivates).
+ *   - Fixture load errors throw descriptive Error messages.
+ *   - spawnStub() throws if called before activate().
+ *   - All state is module-local — safe to require multiple times.
+ */
+
+const path = require('path');
+const fs = require('fs');
+const assert = require('assert');
+
+// ---------------------------------------------------------------------------
+// Path resolution
+// ---------------------------------------------------------------------------
+
+/**
+ * Resolve the fixtures base directory relative to this file.
+ * Works whether installed as a package or used in-repo.
+ *
+ * @returns {string} Absolute path to test/fixtures/agents/
+ */
+function resolveFixturesDir() {
+  // Walk up from lib/ to find test/fixtures/agents/
+  const libDir = __dirname;
+  const repoRoot = path.resolve(libDir, '..');
+  return path.join(repoRoot, 'test', 'fixtures', 'agents');
+}
+
+// ---------------------------------------------------------------------------
+// Fixture loading
+// ---------------------------------------------------------------------------
+
+/**
+ * Load a fixture file and return its contents as a string.
+ * Returns the raw fixture content — callers receive exactly what the agent "returned".
+ *
+ * For fixtures whose JSON root is a string (e.g. `"## PLANNING COMPLETE\n..."`) we
+ * strip the outer quotes since agent outputs are plain text, not JSON-encoded strings.
+ * For fixtures whose JSON root is an object or array, we return compact JSON.
+ *
+ * @param {string} agentType - e.g. "gsd-planner", "general-purpose"
+ * @param {string} baseDir - resolved fixtures base directory
+ * @param {string|null} scenarioDir - resolved scenario override directory (or null)
+ * @returns {string} fixture content as the agent would return it
+ * @throws {Error} if fixture file not found in either location
+ */
+function loadFixture(agentType, baseDir, scenarioDir) {
+  const filename = `${agentType}.json`;
+
+  // Scenario directory takes precedence
+  if (scenarioDir) {
+    const scenarioPath = path.join(scenarioDir, filename);
+    if (fs.existsSync(scenarioPath)) {
+      return parseFixtureFile(scenarioPath, agentType);
+    }
+  }
+
+  // Base fixture
+  const basePath = path.join(baseDir, filename);
+  if (fs.existsSync(basePath)) {
+    return parseFixtureFile(basePath, agentType);
+  }
+
+  // No fixture found — return empty string (unknown agent type)
+  return '';
+}
+
+/**
+ * Read a fixture file and convert its content to agent-output-compatible string.
+ *
+ * JSON strings (root is a quoted string) are unwrapped: `"foo"` → `foo`
+ * JSON objects/arrays are returned as compact JSON strings.
+ * Empty strings (`""`) are returned as `""`.
+ *
+ * @param {string} filePath - absolute path to fixture .json file
+ * @param {string} agentType - used in error messages
+ * @returns {string}
+ */
+function parseFixtureFile(filePath, agentType) {
+  let raw;
+  try {
+    raw = fs.readFileSync(filePath, 'utf-8').trim();
+  } catch (err) {
+    throw new Error(`mock-gsd-agent: failed to read fixture "${agentType}" at ${filePath}: ${err.message}`);
+  }
+
+  let parsed;
+  try {
+    parsed = JSON.parse(raw);
+  } catch (err) {
+    throw new Error(`mock-gsd-agent: fixture "${agentType}" is not valid JSON (${filePath}): ${err.message}`);
+  }
+
+  // Unwrap JSON string values (agent outputs are plain text, not JSON-encoded strings)
+  if (typeof parsed === 'string') {
+    return parsed;
+  }
+
+  // Objects and arrays: return compact JSON (callers parse with JSON.parse)
+  return JSON.stringify(parsed);
+}
+
+// ---------------------------------------------------------------------------
+// State
+// ---------------------------------------------------------------------------
+
+/** Whether the mock is currently active */
+let _active = false;
+
+/** Ordered log of recorded spawn calls */
+let _callLog = [];
+
+/** Resolved path to base fixtures directory */
+let _baseDir = null;
+
+/** Resolved path to scenario override directory (or null) */
+let _scenarioDir = null;
+
+/** Per-agent-type inline overrides: Map<string, string> (agentType → output string) */
+let _inlineOverrides = new Map();
+
+// ---------------------------------------------------------------------------
+// Core API
+// ---------------------------------------------------------------------------
+
+/**
+ * Activate the mock. Sets up fixture directory resolution and resets state.
+ *
+ * Safe to call when already active — deactivates first, then re-activates.
+ *
+ * @param {string} [scenario] - Optional scenario name. If provided, fixtures from
+ *   `test/fixtures/agents/<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-gsd-agent: fixtures directory not found: ${_baseDir}\n` +
+      'Create test/fixtures/agents/ with fixture JSON files before activating the mock.'
+    );
+  }
+
+  if (scenario) {
+    _scenarioDir = path.join(_baseDir, scenario);
+    if (!fs.existsSync(_scenarioDir)) {
+      throw new Error(
+        `mock-gsd-agent: scenario directory not found: ${_scenarioDir}`
+      );
+    }
+  } else {
+    _scenarioDir = null;
+  }
+
+  _callLog = [];
+  _inlineOverrides = new Map();
+  _active = true;
+}
+
+/**
+ * Deactivate the mock. Clears scenario dir and inline overrides.
+ * Preserves the call log — callers inspect it after the test.
+ * Safe to call when not active (no-op).
+ */
+function deactivate() {
+  if (!_active) return;
+
+  _active = false;
+  _baseDir = null;
+  _scenarioDir = null;
+  _inlineOverrides = new Map();
+  // Note: _callLog is preserved after deactivation
+}
+
+/**
+ * Simulate a Task() spawn. Records the call and returns fixture output.
+ *
+ * This is the test-side replacement for the Task() orchestrator primitive.
+ * Call it in test code wherever the production path would call Task().
+ *
+ * Output resolution order (highest to lowest precedence):
+ *   1. Inline override set via setResponse(agentType, output)
+ *   2. Scenario fixture: test/fixtures/agents/<scenario>/<agentType>.json
+ *   3. Base fixture: test/fixtures/agents/<agentType>.json
+ *   4. Empty string (if no fixture found)
+ *
+ * @param {object} config - Spawn configuration
+ * @param {string} config.subagent_type - Agent type (e.g. "gsd-planner", "gsd-executor")
+ * @param {string} [config.prompt] - The prompt that would be sent to the agent
+ * @param {string} [config.model] - Model identifier (e.g. "inherit", "sonnet")
+ * @param {string} [config.description] - Human-readable spawn description
+ * @returns {string} The fixture output representing what the agent "returned"
+ * @throws {Error} if called before activate()
+ */
+function spawnStub(config) {
+  if (!_active) {
+    throw new Error('mock-gsd-agent: spawnStub() called before activate(). Call activate() first.');
+  }
+
+  const {
+    subagent_type,
+    prompt = '',
+    model = '',
+    description = '',
+  } = config || {};
+
+  if (!subagent_type) {
+    throw new Error('mock-gsd-agent: spawnStub() requires config.subagent_type');
+  }
+
+  // Output resolution: inline override > scenario fixture > base fixture > empty
+  let output;
+  if (_inlineOverrides.has(subagent_type)) {
+    output = _inlineOverrides.get(subagent_type);
+  } else {
+    output = loadFixture(subagent_type, _baseDir, _scenarioDir);
+  }
+
+  _callLog.push({
+    subagent_type,
+    prompt,
+    model,
+    description,
+    output,
+    timestamp: new Date().toISOString(),
+  });
+
+  return output;
+}
+
+/**
+ * Return the ordered array of recorded spawn entries since the last activate().
+ * Each entry: { subagent_type, prompt, model, description, output, timestamp }
+ *
+ * @returns {Array<{subagent_type: string, prompt: string, model: string, description: string, output: 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 a given agent type.
+ * Inline overrides take precedence over all fixture loading.
+ *
+ * Must be called after activate().
+ *
+ * @param {string} agentType - Agent type to override (e.g. "gsd-planner")
+ * @param {string} output - Output string to return when this agent type is spawned
+ * @throws {Error} if called before activate()
+ */
+function setResponse(agentType, output) {
+  if (!_active) {
+    throw new Error('mock-gsd-agent: setResponse() called before activate(). Call activate() first.');
+  }
+  _inlineOverrides.set(agentType, output);
+}
+
+/**
+ * Whether the mock is currently active.
+ * Useful for guard assertions in test setup/teardown.
+ *
+ * @returns {boolean}
+ */
+function isActive() {
+  return _active;
+}
+
+/**
+ * Return the number of spawn calls recorded.
+ *
+ * @param {string} [agentType] - If provided, count only calls with this subagent_type.
+ *   If omitted, return total spawn count.
+ * @returns {number}
+ */
+function getSpawnCount(agentType) {
+  if (agentType === undefined) {
+    return _callLog.length;
+  }
+  return _callLog.filter(entry => entry.subagent_type === agentType).length;
+}
+
+/**
+ * Assert that a specific agent type was spawned at least once.
+ * Throws an AssertionError with a descriptive message if not.
+ *
+ * Useful as a single-line assertion in tests:
+ *   mockAgent.assertSpawned('gsd-planner');
+ *
+ * @param {string} agentType - Agent type that should have been spawned
+ * @throws {AssertionError} if the agent type was not spawned
+ */
+function assertSpawned(agentType) {
+  const count = getSpawnCount(agentType);
+  assert.ok(
+    count > 0,
+    `mock-gsd-agent: assertSpawned('${agentType}') failed — no calls recorded for this agent type.\n` +
+    `Recorded spawns: [${_callLog.map(e => e.subagent_type).join(', ')}]`
+  );
+}
+
+// ---------------------------------------------------------------------------
+// Exports
+// ---------------------------------------------------------------------------
+
+module.exports = {
+  activate,
+  deactivate,
+  spawnStub,
+  getCallLog,
+  clearCallLog,
+  setResponse,
+  isActive,
+  getSpawnCount,
+  assertSpawned,
+};
diff --git a/test/fixtures/agents/general-purpose.json b/test/fixtures/agents/general-purpose.json
new file mode 100644
index 0000000..81086b4
--- /dev/null
+++ b/test/fixtures/agents/general-purpose.json
@@ -0,0 +1 @@
+{"classification":"informational","reasoning":"Standard status update","new_requirements":[],"blocking_reason":null}
diff --git a/test/fixtures/agents/gsd-executor.json b/test/fixtures/agents/gsd-executor.json
new file mode 100644
index 0000000..e086646
--- /dev/null
+++ b/test/fixtures/agents/gsd-executor.json
@@ -0,0 +1 @@
+"## EXECUTION COMPLETE\n\nAll tasks completed successfully.\n\n- Files modified as specified\n- No errors encountered\n- Commits staged"
diff --git a/test/fixtures/agents/gsd-plan-checker.json b/test/fixtures/agents/gsd-plan-checker.json
new file mode 100644
index 0000000..236715a
--- /dev/null
+++ b/test/fixtures/agents/gsd-plan-checker.json
@@ -0,0 +1 @@
+"## VERIFICATION PASSED\n\nPlan quality checks passed:\n- Frontmatter valid\n- Tasks specific and actionable\n- Dependencies correctly identified\n- must_haves present"
diff --git a/test/fixtures/agents/gsd-planner.json b/test/fixtures/agents/gsd-planner.json
new file mode 100644
index 0000000..5611620
--- /dev/null
+++ b/test/fixtures/agents/gsd-planner.json
@@ -0,0 +1 @@
+"## PLANNING COMPLETE\n\nPlan created successfully.\n\n- 1 plan in 1 wave\n- Tasks: actionable and specific\n- Frontmatter: valid\n- Verification criteria: included"
diff --git a/test/fixtures/agents/gsd-verifier.json b/test/fixtures/agents/gsd-verifier.json
new file mode 100644
index 0000000..d403b6e
--- /dev/null
+++ b/test/fixtures/agents/gsd-verifier.json
@@ -0,0 +1 @@
+"## VERIFICATION PASSED\n\nAll must_haves confirmed:\n- Implementation matches plan\n- Files exist at specified paths\n- No regressions detected"