diff --git a/src/resources/extensions/gsd/gsd-db.ts b/src/resources/extensions/gsd/gsd-db.ts
index f6d379048..2ece198ed 100644
--- a/src/resources/extensions/gsd/gsd-db.ts
+++ b/src/resources/extensions/gsd/gsd-db.ts
@@ -2200,6 +2200,39 @@ export function deleteSlice(milestoneId: string, sliceId: string): void {
   });
 }
 
+export function deleteMilestone(milestoneId: string): void {
+  if (!currentDb) throw new GSDError(GSD_STALE_STATE, "gsd-db: No database open");
+  transaction(() => {
+    currentDb!.prepare(
+      `DELETE FROM verification_evidence WHERE milestone_id = :mid`,
+    ).run({ ":mid": milestoneId });
+    currentDb!.prepare(
+      `DELETE FROM quality_gates WHERE milestone_id = :mid`,
+    ).run({ ":mid": milestoneId });
+    currentDb!.prepare(
+      `DELETE FROM tasks WHERE milestone_id = :mid`,
+    ).run({ ":mid": milestoneId });
+    currentDb!.prepare(
+      `DELETE FROM slice_dependencies WHERE milestone_id = :mid`,
+    ).run({ ":mid": milestoneId });
+    currentDb!.prepare(
+      `DELETE FROM slices WHERE milestone_id = :mid`,
+    ).run({ ":mid": milestoneId });
+    currentDb!.prepare(
+      `DELETE FROM replan_history WHERE milestone_id = :mid`,
+    ).run({ ":mid": milestoneId });
+    currentDb!.prepare(
+      `DELETE FROM assessments WHERE milestone_id = :mid`,
+    ).run({ ":mid": milestoneId });
+    currentDb!.prepare(
+      `DELETE FROM artifacts WHERE milestone_id = :mid`,
+    ).run({ ":mid": milestoneId });
+    currentDb!.prepare(
+      `DELETE FROM milestones WHERE id = :mid`,
+    ).run({ ":mid": milestoneId });
+  });
+}
+
 export function updateSliceFields(milestoneId: string, sliceId: string, fields: {
   title?: string;
   risk?: string;
diff --git a/src/resources/extensions/gsd/milestone-actions.ts b/src/resources/extensions/gsd/milestone-actions.ts
index 06562a893..51f1814ad 100644
--- a/src/resources/extensions/gsd/milestone-actions.ts
+++ b/src/resources/extensions/gsd/milestone-actions.ts
@@ -20,7 +20,8 @@ import {
 } from "./paths.js";
 import { invalidateAllCaches } from "./cache.js";
 import { loadQueueOrder, saveQueueOrder } from "./queue-order.js";
-import { getMilestone, isDbAvailable, updateMilestoneStatus } from "./gsd-db.js";
+import { deleteMilestone, getMilestone, isDbAvailable, updateMilestoneStatus } from "./gsd-db.js";
+import { removeWorktree } from "./worktree-manager.js";
 import { logWarning } from "./workflow-logger.js";
 
 // ─── Park ──────────────────────────────────────────────────────────────────
@@ -110,6 +111,15 @@ export function discardMilestone(basePath: string, milestoneId: string): boolean
   const mDir = resolveMilestonePath(basePath, milestoneId);
   if (!mDir || !existsSync(mDir)) return false;
 
+  try {
+    removeWorktree(basePath, milestoneId, {
+      branch: `milestone/${milestoneId}`,
+      deleteBranch: true,
+    });
+  } catch (err) {
+    logWarning("engine", `discardMilestone worktree cleanup failed for ${milestoneId}: ${(err as Error).message}`);
+  }
+
   rmSync(mDir, { recursive: true, force: true });
 
   // Prune from queue order if present
@@ -118,6 +128,14 @@ export function discardMilestone(basePath: string, milestoneId: string): boolean
     saveQueueOrder(basePath, order.filter(id => id !== milestoneId));
   }
 
+  if (isDbAvailable()) {
+    try {
+      deleteMilestone(milestoneId);
+    } catch (err) {
+      logWarning("engine", `discardMilestone DB cleanup failed for ${milestoneId}: ${(err as Error).message}`);
+    }
+  }
+
   invalidateAllCaches();
   return true;
 }
diff --git a/src/resources/extensions/gsd/tests/park-milestone.test.ts b/src/resources/extensions/gsd/tests/park-milestone.test.ts
index 5d9cd4efd..442a1cb74 100644
--- a/src/resources/extensions/gsd/tests/park-milestone.test.ts
+++ b/src/resources/extensions/gsd/tests/park-milestone.test.ts
@@ -3,10 +3,22 @@ import assert from 'node:assert/strict';
 import { mkdtempSync, mkdirSync, rmSync, writeFileSync, existsSync, readFileSync } from 'node:fs';
 import { join } from 'node:path';
 import { tmpdir } from 'node:os';
+import { execSync } from 'node:child_process';
 
 import { deriveState, invalidateStateCache, getActiveMilestoneId } from '../state.ts';
 import { clearPathCache } from '../paths.ts';
 import { parkMilestone, unparkMilestone, discardMilestone, isParked, getParkedReason } from '../milestone-actions.ts';
+import {
+  closeDatabase,
+  getMilestone,
+  getMilestoneSlices,
+  getSliceTasks,
+  insertMilestone,
+  insertSlice,
+  insertTask,
+  openDatabase,
+} from "../gsd-db.ts";
+import { createWorktree } from "../worktree-manager.ts";
 
 
 
@@ -60,9 +72,29 @@ function createMilestone(base: string, mid: string, opts?: { withRoadmap?: boole
 }
 
 function cleanup(base: string): void {
+  try {
+    closeDatabase();
+  } catch {
+    // ignore
+  }
   rmSync(base, { recursive: true, force: true });
 }
 
+function run(cmd: string, cwd: string): string {
+  return execSync(cmd, { cwd, stdio: ["ignore", "pipe", "pipe"], encoding: "utf-8" }).trim();
+}
+
+function initGitRepo(base: string): void {
+  writeFileSync(join(base, "README.md"), "# test\n", "utf-8");
+  writeFileSync(join(base, ".gsd", "STATE.md"), "# State\n", "utf-8");
+  run("git init", base);
+  run("git config user.email test@test.com", base);
+  run("git config user.name Test", base);
+  run("git add .", base);
+  run('git commit -m "init"', base);
+  run("git branch -M main", base);
+}
+
 function clearCaches(): void {
   clearPathCache();
   invalidateStateCache();
@@ -294,6 +326,38 @@ test('discardMilestone updates queue order', () => {
     }
 });
 
+test('discardMilestone removes DB rows, worktree, and milestone branch', () => {
+    const base = createFixtureBase();
+    try {
+      createMilestone(base, 'M001', { withRoadmap: true });
+      initGitRepo(base);
+      clearCaches();
+
+      assert.ok(openDatabase(join(base, '.gsd', 'gsd.db')), 'database opens');
+      insertMilestone({ id: 'M001', title: 'Discard me', status: 'active' });
+      insertSlice({ milestoneId: 'M001', id: 'S01', title: 'Only slice', status: 'pending' });
+      insertTask({ milestoneId: 'M001', sliceId: 'S01', id: 'T01', title: 'Only task', status: 'pending' });
+
+      const wt = createWorktree(base, 'M001', { branch: 'milestone/M001' });
+      assert.ok(existsSync(wt.path), 'worktree exists before discard');
+      assert.ok(run('git branch', base).includes('milestone/M001'), 'milestone branch exists before discard');
+      assert.ok(getMilestone('M001'), 'milestone exists in DB before discard');
+      assert.equal(getMilestoneSlices('M001').length, 1, 'slice exists in DB before discard');
+      assert.equal(getSliceTasks('M001', 'S01').length, 1, 'task exists in DB before discard');
+
+      const success = discardMilestone(base, 'M001');
+      assert.ok(success, 'discardMilestone returns true');
+
+      assert.equal(getMilestone('M001'), null, 'milestone row removed from DB');
+      assert.equal(getMilestoneSlices('M001').length, 0, 'slice rows removed from DB');
+      assert.equal(getSliceTasks('M001', 'S01').length, 0, 'task rows removed from DB');
+      assert.ok(!existsSync(wt.path), 'worktree removed after discard');
+      assert.ok(!run('git branch', base).includes('milestone/M001'), 'milestone branch removed after discard');
+    } finally {
+      cleanup(base);
+    }
+});
+
   // ─── Test 12: All milestones parked → no active milestone ─────────────
 test('All milestones parked → no active', async () => {
     const base = createFixtureBase();