diff --git a/packages/server/src/routes/dm.spaceInvite.test.ts b/packages/server/src/routes/dm.spaceInvite.test.ts
new file mode 100644
index 00000000..142e31de
--- /dev/null
+++ b/packages/server/src/routes/dm.spaceInvite.test.ts
@@ -0,0 +1,300 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import Fastify, { type FastifyInstance } from 'fastify';
+import Database from 'better-sqlite3';
+import { drizzle } from 'drizzle-orm/better-sqlite3';
+import { eq } from 'drizzle-orm';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import * as schema from '../db/schema.js';
+import { setWorkerId } from '../utils/snowflake.js';
+
+setWorkerId(1);
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+// Module-level mutable state. The vi.mock factories below close over these
+// bindings via getter functions, so reassignment in beforeEach is observed.
+type TestDb = ReturnType<typeof drizzle<typeof schema>>;
+let sqlite: Database.Database;
+let testDb: TestDb;
+let currentUserId = 'alice';
+
+vi.mock('../db/index.js', () => ({
+  getDb: () => testDb,
+  getRawDb: () => sqlite,
+  schema,
+}));
+
+vi.mock('../utils/auth.js', () => ({
+  authenticate: async (req: { userId?: string }) => {
+    req.userId = currentUserId;
+  },
+}));
+
+vi.mock('../ws/handler.js', () => ({
+  connectionManager: {
+    sendToUser: vi.fn(),
+    sendToDmMembers: vi.fn(),
+    sendToAdmins: vi.fn(),
+    getAllOnlineUserIds: () => [],
+  },
+}));
+
+vi.mock('../utils/federationOutbox.js', async () => {
+  const actual = await vi.importActual<typeof import('../utils/federationOutbox.js')>('../utils/federationOutbox.js');
+  return {
+    ...actual,
+    isFederationRelayEnabled: () => false,
+    queueDmCloseRelay: vi.fn(),
+    sendTypingRelay: vi.fn(),
+    queueDmRelay: vi.fn(),
+    queueOutboxEvent: vi.fn(),
+    appendMutationLog: vi.fn(),
+  };
+});
+
+vi.mock('../utils/federationAuth.js', async (importActual) => {
+  const actual = await importActual<typeof import('../utils/federationAuth.js')>();
+  return { ...actual, getOurOrigin: () => 'https://local.test' };
+});
+
+// Mock the snapshot fetch so tests don't make real HTTP calls.
+vi.mock('../utils/spaceInviteSnapshot.js', () => ({
+  fetchSpaceInviteSnapshot: vi.fn(),
+}));
+import { fetchSpaceInviteSnapshot } from '../utils/spaceInviteSnapshot.js';
+
+function applyMigrations(db: Database.Database): void {
+  const migrationsDir = path.resolve(__dirname, '../../drizzle');
+  const files = fs.readdirSync(migrationsDir).filter(f => f.endsWith('.sql')).sort();
+  for (const f of files) {
+    const sqlText = fs.readFileSync(path.join(migrationsDir, f), 'utf8');
+    const statements = sqlText.split(/-->\s*statement-breakpoint/);
+    for (const stmt of statements) {
+      const clean = stmt.trim();
+      if (clean) db.exec(clean);
+    }
+  }
+}
+
+interface UserSeed {
+  id: string;
+  username: string;
+  displayName?: string | null;
+}
+
+function seedUser(u: UserSeed): void {
+  testDb.insert(schema.users).values({
+    id: u.id,
+    username: u.username,
+    displayName: u.displayName ?? null,
+    passwordHash: 'x',
+    status: 'offline',
+    isAdmin: 0,
+    isDeleted: 0,
+    discoverable: 1,
+    homeInstance: null,
+    homeUserId: null,
+    createdAt: Date.now(),
+  }).run();
+}
+
+function seedFriendship(a: string, b: string): void {
+  // The endpoint's friendship check looks at either ordering of (userId, friendId),
+  // so a single row is sufficient.
+  testDb.insert(schema.friends).values({
+    userId: a,
+    friendId: b,
+    createdAt: Date.now(),
+  }).run();
+}
+
+async function buildApp(): Promise<FastifyInstance> {
+  const app = Fastify({ logger: false });
+  const { dmRoutes } = await import('./dm.js');
+  await app.register(dmRoutes);
+  await app.ready();
+  return app;
+}
+
+describe('POST /api/dm/space-invite', () => {
+  let app: FastifyInstance;
+
+  beforeEach(async () => {
+    sqlite = new Database(':memory:');
+    testDb = drizzle(sqlite, { schema });
+    applyMigrations(sqlite);
+    seedUser({ id: 'alice', username: 'alice' });
+    seedUser({ id: 'bob', username: 'bob' });
+    seedFriendship('alice', 'bob');
+    currentUserId = 'alice';
+    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockReset();
+    app = await buildApp();
+  });
+
+  it('rejects 400 not_a_friend if target is not a friend', async () => {
+    seedUser({ id: 'charlie', username: 'charlie' });
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/dm/space-invite',
+      payload: {
+        target: { userId: 'charlie' },
+        spaceId: 'S1',
+        spaceInstanceOrigin: '',
+        inviteCode: 'abc123',
+      },
+    });
+    expect(res.statusCode).toBe(400);
+    expect(JSON.parse(res.body).error).toBe('not_a_friend');
+    // Snapshot should not be fetched if friendship gate fails first.
+    expect(fetchSpaceInviteSnapshot).not.toHaveBeenCalled();
+  });
+
+  it('rejects 400 invite_invalid when upstream preview 404s (snapshot null)', async () => {
+    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce(null);
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/dm/space-invite',
+      payload: {
+        target: { userId: 'bob' },
+        spaceId: 'S1',
+        spaceInstanceOrigin: '',
+        inviteCode: 'badcode',
+      },
+    });
+    expect(res.statusCode).toBe(400);
+    expect(JSON.parse(res.body).error).toBe('invite_invalid');
+    // No DM message should be inserted on failure.
+    const messageCount = testDb.select().from(schema.dmMessages).all().length;
+    expect(messageCount).toBe(0);
+  });
+
+  it('rejects 400 invite_invalid when snapshot.spaceId mismatches the requested spaceId', async () => {
+    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      spaceId: 'WRONG',
+      spaceName: 'X',
+      description: null,
+      icon: null,
+      avatarColor: null,
+      memberCount: 1,
+      instanceName: 'Backspace',
+    });
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/dm/space-invite',
+      payload: {
+        target: { userId: 'bob' },
+        spaceId: 'S1',
+        spaceInstanceOrigin: '',
+        inviteCode: 'abc',
+      },
+    });
+    expect(res.statusCode).toBe(400);
+    expect(JSON.parse(res.body).error).toBe('invite_invalid');
+    const messageCount = testDb.select().from(schema.dmMessages).all().length;
+    expect(messageCount).toBe(0);
+  });
+
+  it('inserts a type=system message with parseable space_invite content on success', async () => {
+    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      spaceId: 'S1',
+      spaceName: 'Aether',
+      description: 'desc',
+      icon: null,
+      avatarColor: 'mint',
+      memberCount: 12,
+      instanceName: 'Backspace',
+    });
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/dm/space-invite',
+      payload: {
+        target: { userId: 'bob' },
+        spaceId: 'S1',
+        spaceInstanceOrigin: '',
+        inviteCode: 'abc',
+      },
+    });
+    expect(res.statusCode).toBe(200);
+    const body = JSON.parse(res.body) as {
+      dmChannelId: string;
+      messageId: string;
+      message: { type: string };
+    };
+    expect(body.dmChannelId).toBeTruthy();
+    expect(body.messageId).toBeTruthy();
+    expect(body.message.type).toBe('system');
+
+    const stored = testDb.select()
+      .from(schema.dmMessages)
+      .where(eq(schema.dmMessages.id, body.messageId))
+      .get();
+    expect(stored).toBeTruthy();
+    expect(stored?.type).toBe('system');
+    expect(stored?.userId).toBe('alice');
+    expect(stored?.dmChannelId).toBe(body.dmChannelId);
+
+    const parsed = JSON.parse(stored!.content!);
+    expect(parsed.event).toBe('space_invite');
+    expect(parsed.spaceId).toBe('S1');
+    expect(parsed.inviteCode).toBe('abc');
+    expect(parsed.snapshot.spaceName).toBe('Aether');
+    expect(parsed.snapshot.memberCount).toBe(12);
+    expect(parsed.snapshot.avatarColor).toBe('mint');
+    expect(parsed.snapshot.description).toBe('desc');
+    expect(parsed.snapshot.instanceName).toBe('Backspace');
+  });
+
+  it('reuses an existing 1-on-1 DM rather than creating a new one', async () => {
+    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockResolvedValue({
+      spaceId: 'S1',
+      spaceName: 'Aether',
+      description: null,
+      icon: null,
+      avatarColor: null,
+      memberCount: 1,
+      instanceName: 'Backspace',
+    });
+
+    const r1 = await app.inject({
+      method: 'POST',
+      url: '/api/dm/space-invite',
+      payload: {
+        target: { userId: 'bob' },
+        spaceId: 'S1',
+        spaceInstanceOrigin: '',
+        inviteCode: 'abc',
+      },
+    });
+    expect(r1.statusCode).toBe(200);
+
+    const r2 = await app.inject({
+      method: 'POST',
+      url: '/api/dm/space-invite',
+      payload: {
+        target: { userId: 'bob' },
+        spaceId: 'S1',
+        spaceInstanceOrigin: '',
+        inviteCode: 'abc',
+      },
+    });
+    expect(r2.statusCode).toBe(200);
+
+    const body1 = JSON.parse(r1.body) as { dmChannelId: string; messageId: string };
+    const body2 = JSON.parse(r2.body) as { dmChannelId: string; messageId: string };
+    expect(body2.dmChannelId).toBe(body1.dmChannelId);
+    expect(body2.messageId).not.toBe(body1.messageId);
+
+    // Exactly one DM channel exists between alice and bob.
+    const channels = testDb.select().from(schema.dmChannels).all();
+    expect(channels.length).toBe(1);
+    // Two system messages were inserted into that single channel.
+    const messages = testDb.select()
+      .from(schema.dmMessages)
+      .where(eq(schema.dmMessages.dmChannelId, body1.dmChannelId))
+      .all();
+    expect(messages.length).toBe(2);
+    expect(messages.every(m => m.type === 'system')).toBe(true);
+  });
+});