diff --git a/src/app/api/invite/complete-join/route.ts b/src/app/api/invite/complete-join/route.ts
new file mode 100644
index 0000000..f71ce79
--- /dev/null
+++ b/src/app/api/invite/complete-join/route.ts
@@ -0,0 +1,116 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getServerSession } from 'next-auth';
+
+import { authOptions } from '@/lib/auth';
+import { db } from '@/lib/db';
+
+export async function POST(request: NextRequest) {
+  try {
+    const session = await getServerSession(authOptions);
+    console.log('[invite/complete-join] start', {
+      hasSessionUser: !!session?.user,
+      hasCookieInviteToken: !!request.cookies.get('invite_token')?.value,
+      inviteLibrary: request.cookies.get('invite_library')?.value,
+    });
+
+    if (!session?.user) {
+      console.log('[invite/complete-join] no session user');
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+    }
+
+    const inviteToken = request.cookies.get('invite_token')?.value;
+    const inviteLibrary = request.cookies.get('invite_library')?.value;
+    if (!inviteToken || !inviteLibrary) {
+      console.log('[invite/complete-join] missing cookies');
+      return NextResponse.json(
+        { success: false, error: 'No invite found' },
+        { status: 400 }
+      );
+    }
+
+    const userId = (session.user as { id?: string } | undefined)?.id;
+    if (!userId) {
+      console.log('[invite/complete-join] missing userId from session');
+      return NextResponse.json({ error: 'User ID not found' }, { status: 400 });
+    }
+
+    // Validate invite is still valid
+    const invitation = await db.invitation.findFirst({
+      where: {
+        token: inviteToken,
+        libraryId: inviteLibrary,
+        type: 'library',
+        status: { in: ['PENDING', 'SENT'] },
+        expiresAt: { gt: new Date() },
+      },
+      select: { id: true },
+    });
+
+    console.log('[invite/complete-join] invitation lookup', {
+      found: !!invitation,
+    });
+
+    if (!invitation) {
+      console.log('[invite/complete-join] invalid or expired invite');
+      return NextResponse.json(
+        { success: false, error: 'Invalid or expired invite' },
+        { status: 400 }
+      );
+    }
+
+    // Create or reactivate membership
+    const existing = await db.collectionMember.findUnique({
+      where: { userId_collectionId: { userId, collectionId: inviteLibrary } },
+      select: { id: true, isActive: true },
+    });
+
+    if (!existing) {
+      console.log('[invite/complete-join] creating membership', {
+        userId,
+        inviteLibrary,
+      });
+      await db.collectionMember.create({
+        data: {
+          userId,
+          collectionId: inviteLibrary,
+          role: 'member',
+          isActive: true,
+        },
+      });
+    } else if (!existing.isActive) {
+      console.log('[invite/complete-join] reactivating membership', {
+        id: existing.id,
+      });
+      await db.collectionMember.update({
+        where: { id: existing.id },
+        data: { isActive: true, joinedAt: new Date() },
+      });
+    } else {
+      console.log('[invite/complete-join] membership already active');
+    }
+
+    // Mark invite accepted
+    console.log('[invite/complete-join] marking invite accepted');
+    await db.invitation.updateMany({
+      where: { token: inviteToken, libraryId: inviteLibrary },
+      data: { status: 'ACCEPTED', acceptedAt: new Date(), receiverId: userId },
+    });
+
+    // Now clear the cookies since membership is created
+    const res = NextResponse.json({
+      success: true,
+      collectionId: inviteLibrary,
+    });
+    res.cookies.set('invite_token', '', { path: '/', maxAge: 0 });
+    res.cookies.set('invite_library', '', { path: '/', maxAge: 0 });
+
+    console.log('[invite/complete-join] membership created successfully');
+    return res;
+  } catch (e) {
+    console.error('[invite/complete-join] error', e);
+    return NextResponse.json(
+      { success: false, error: 'Failed to complete join' },
+      { status: 500 }
+    );
+  }
+}