diff --git a/openspec/changes/user-accounts/tasks.md b/openspec/changes/user-accounts/tasks.md
index 75d3617..27e5a59 100644
--- a/openspec/changes/user-accounts/tasks.md
+++ b/openspec/changes/user-accounts/tasks.md
@@ -22,7 +22,7 @@
 ## 4. Registration API
 
 - [x] 4.1 `[sonnet]` Create `POST /api/auth/register` endpoint: validate input (email required, password 8+ chars), check email uniqueness, hash password with bcryptjs, insert user, return 201
-- [ ] 4.2 `[sonnet]` Add default data seeding function: on new user creation, insert default annotation_types (break_up, break_down, line) and default span_label_types for the new user
+- [x] 4.2 `[sonnet]` Add default data seeding function: on new user creation, insert default annotation_types (break_up, break_down, line) and default span_label_types for the new user
 
 ## 5. Auth Middleware & Helpers
 
diff --git a/src/app/api/auth/register/route.ts b/src/app/api/auth/register/route.ts
index aa22ed5..1fcfec8 100644
--- a/src/app/api/auth/register/route.ts
+++ b/src/app/api/auth/register/route.ts
@@ -3,6 +3,7 @@ import bcryptjs from 'bcryptjs';
 import { eq } from 'drizzle-orm';
 import { db } from '@/lib/db';
 import { users } from '@/lib/db/schema';
+import { seedUserDefaults } from '@/lib/db/seed-user-defaults';
 
 export async function POST(request: NextRequest) {
   let body: unknown;
@@ -57,6 +58,9 @@ export async function POST(request: NextRequest) {
     })
     .returning({ id: users.id, email: users.email, name: users.name });
 
+  // Seed default annotation types and span label types for the new user
+  await seedUserDefaults(newUser.id);
+
   return NextResponse.json(
     { id: newUser.id, email: newUser.email, name: newUser.name },
     { status: 201 }
diff --git a/src/auth.ts b/src/auth.ts
index 55ac105..de06dda 100644
--- a/src/auth.ts
+++ b/src/auth.ts
@@ -5,6 +5,7 @@ import bcryptjs from "bcryptjs";
 import { eq } from "drizzle-orm";
 import { db } from "@/lib/db";
 import { users } from "@/lib/db/schema";
+import { seedUserDefaults } from "@/lib/db/seed-user-defaults";
 
 export const { handlers, auth, signIn, signOut } = NextAuth({
   secret: process.env.AUTH_SECRET,
@@ -72,14 +73,17 @@ export const { handlers, auth, signIn, signOut } = NextAuth({
 
         if (!existingUser) {
           // First Google sign-in: create a new user record
-          await db.insert(users).values({
+          const [newUser] = await db.insert(users).values({
             email,
             name: profile?.name ?? user.name ?? null,
             image: profile?.picture ?? (user.image ?? null),
             provider: "google",
             provider_account_id: account.providerAccountId,
             password_hash: null,
-          });
+          }).returning({ id: users.id });
+
+          // Seed default annotation types and span label types for the new user
+          await seedUserDefaults(newUser.id);
         }
         // Returning Google sign-in: existing user found, allow sign-in
         return true;
diff --git a/src/lib/db/seed-user-defaults.ts b/src/lib/db/seed-user-defaults.ts
new file mode 100644
index 0000000..b72a691
--- /dev/null
+++ b/src/lib/db/seed-user-defaults.ts
@@ -0,0 +1,50 @@
+import { db } from './index';
+import { annotationTypes, spanLabelTypes } from './schema';
+
+const DEFAULT_ANNOTATION_TYPES = [
+  {
+    name: 'break_up',
+    display_name: 'Break Up',
+    color: '#10b981',
+    category: 'marker',
+    icon: 'arrowUp',
+  },
+  {
+    name: 'break_down',
+    display_name: 'Break Down',
+    color: '#ef4444',
+    category: 'marker',
+    icon: 'arrowDown',
+  },
+  {
+    name: 'line',
+    display_name: 'Line',
+    color: '#3b82f6',
+    category: 'line',
+    icon: 'line',
+  },
+];
+
+const DEFAULT_SPAN_LABEL_TYPES = [
+  { name: 'bull_flag', display_name: 'Bull Flag', color: '#4CAF50', hotkey: '1', sort_order: 1 },
+  { name: 'bear_flag', display_name: 'Bear Flag', color: '#F44336', hotkey: '2', sort_order: 2 },
+  { name: 'head_and_shoulders', display_name: 'Head and Shoulders', color: '#9C27B0', hotkey: '3', sort_order: 3 },
+  { name: 'double_bottom', display_name: 'Double Bottom', color: '#2196F3', hotkey: '4', sort_order: 4 },
+  { name: 'wedge_up', display_name: 'Wedge Up', color: '#FF9800', hotkey: '5', sort_order: 5 },
+  { name: 'wedge_down', display_name: 'Wedge Down', color: '#FF5722', hotkey: '6', sort_order: 6 },
+  { name: 'custom', display_name: 'Custom', color: '#607D8B', hotkey: '7', sort_order: 7 },
+];
+
+/**
+ * Seeds default annotation_types and span_label_types for a newly created user.
+ * Safe to call on new user creation — inserts rows scoped to the given user_id.
+ */
+export async function seedUserDefaults(userId: string): Promise<void> {
+  await db.insert(annotationTypes).values(
+    DEFAULT_ANNOTATION_TYPES.map((t) => ({ ...t, user_id: userId }))
+  );
+
+  await db.insert(spanLabelTypes).values(
+    DEFAULT_SPAN_LABEL_TYPES.map((t) => ({ ...t, user_id: userId }))
+  );
+}