diff --git a/packages/server/src/db/backfillDmMembership.test.ts b/packages/server/src/db/backfillDmMembership.test.ts
new file mode 100644
index 00000000..b269ca04
--- /dev/null
+++ b/packages/server/src/db/backfillDmMembership.test.ts
@@ -0,0 +1,47 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import Database from 'better-sqlite3';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { backfillOneOnOneDmMembership } from './migrate.js';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+let db: Database.Database;
+
+function applyMigrations(d: Database.Database): void {
+  const dir = path.resolve(__dirname, '../../drizzle');
+  for (const f of fs.readdirSync(dir).filter(x => x.endsWith('.sql')).sort())
+    for (const stmt of fs.readFileSync(path.join(dir, f), 'utf8').split(/-->\s*statement-breakpoint/)) {
+      const c = stmt.trim(); if (c) d.exec(c);
+    }
+}
+const now = Date.now();
+const insUser = (id: string, isDeleted = 0) => db.prepare('INSERT INTO users (id, username, password_hash, created_at, is_deleted) VALUES (?,?,?,?,?)').run(id, id, 'x', now, isDeleted);
+const insDm = (id: string) => db.prepare('INSERT INTO dm_channels (id, owner_id, created_at) VALUES (?,NULL,?)').run(id, now);
+const insMember = (dm: string, u: string) => db.prepare('INSERT INTO dm_members (dm_channel_id, user_id, closed) VALUES (?,?,0)').run(dm, u);
+const insMsg = (id: string, dm: string, u: string) => db.prepare('INSERT INTO dm_messages (id, dm_channel_id, user_id, content, created_at) VALUES (?,?,?,?,?)').run(id, dm, u, 'hi', now);
+const memberCount = (dm: string) => (db.prepare('SELECT COUNT(*) AS c FROM dm_members WHERE dm_channel_id = ?').get(dm) as { c: number }).c;
+
+describe('backfillOneOnOneDmMembership', () => {
+  beforeEach(() => { db = new Database(':memory:'); applyMigrations(db); });
+  afterEach(() => db.close());
+
+  it('re-inserts the missing (deleted) partner membership from message authorship', () => {
+    insUser('dead', 1); insUser('survivor');
+    insDm('dm1'); insMember('dm1', 'survivor');       // partner membership missing (old bug)
+    insMsg('m1', 'dm1', 'dead'); insMsg('m2', 'dm1', 'survivor');
+    backfillOneOnOneDmMembership(db);
+    expect(memberCount('dm1')).toBe(2);
+    expect(db.prepare('SELECT 1 FROM dm_members WHERE dm_channel_id=? AND user_id=?').get('dm1', 'dead')).toBeTruthy();
+  });
+
+  it('is idempotent and leaves healthy / unrecoverable channels untouched', () => {
+    insUser('a'); insUser('b');
+    insDm('healthy'); insMember('healthy', 'a'); insMember('healthy', 'b'); // 2 members → skip
+    insDm('unrecoverable'); insMember('unrecoverable', 'a'); // partner never messaged / gone
+    backfillOneOnOneDmMembership(db);
+    backfillOneOnOneDmMembership(db); // second run must be a no-op
+    expect(memberCount('healthy')).toBe(2);
+    expect(memberCount('unrecoverable')).toBe(1);
+  });
+});
diff --git a/packages/server/src/db/index.ts b/packages/server/src/db/index.ts
index 4ebcbdb7..1e0dc932 100644
--- a/packages/server/src/db/index.ts
+++ b/packages/server/src/db/index.ts
@@ -3,7 +3,7 @@ import { drizzle } from 'drizzle-orm/better-sqlite3';
 import { migrate } from 'drizzle-orm/better-sqlite3/migrator';
 import { config } from '../config.js';
 import * as schema from './schema.js';
-import { ensureDefaults } from './migrate.js';
+import { ensureDefaults, backfillOneOnOneDmMembership } from './migrate.js';
 import { setWorkerId } from '../utils/snowflake.js';
 import { createSnapshot } from '../utils/backup.js';
 import { hasPendingMigrations } from './pendingMigrations.js';
@@ -50,6 +50,9 @@ export function initDatabase() {
 
   // Ensure data invariants (settings row, worker ID, first admin)
   ensureDefaults(sqlite);
+  // Recover pre-fix broken 1-on-1 DM threads (deleted partner's membership row
+  // lost before the tombstone fix). Idempotent — safe no-op on every later boot.
+  backfillOneOnOneDmMembership(sqlite);
 
   // Initialize Snowflake worker ID from persisted value
   const settings = sqlite.prepare('SELECT worker_id FROM instance_settings WHERE id = 1').get() as { worker_id: number } | undefined;
diff --git a/packages/server/src/db/migrate.ts b/packages/server/src/db/migrate.ts
index 7375d409..2b122b33 100644
--- a/packages/server/src/db/migrate.ts
+++ b/packages/server/src/db/migrate.ts
@@ -51,3 +51,39 @@ export function ensureDefaults(db: Database.Database): void {
     }
   }
 }
+
+/**
+ * One-time, idempotent recovery of 1-on-1 DM threads broken before the DM
+ * tombstone fix: those had the deleted partner's dm_members row removed, making
+ * the thread UI-unreachable. For each ownerId-NULL channel with exactly one
+ * member, re-insert membership for any distinct dm_messages author that is
+ * missing from dm_members and still exists in users. Safe to run every boot.
+ */
+export function backfillOneOnOneDmMembership(db: Database.Database): void {
+  const broken = db.prepare(`
+    SELECT dc.id AS channelId
+    FROM dm_channels dc
+    WHERE dc.owner_id IS NULL
+      AND (SELECT COUNT(*) FROM dm_members dm WHERE dm.dm_channel_id = dc.id) = 1
+  `).all() as { channelId: string }[];
+  if (broken.length === 0) return;
+
+  const missingAuthors = db.prepare(`
+    SELECT DISTINCT msg.user_id AS userId
+    FROM dm_messages msg
+    JOIN users u ON u.id = msg.user_id
+    WHERE msg.dm_channel_id = ?
+      AND msg.user_id NOT IN (SELECT user_id FROM dm_members WHERE dm_channel_id = ?)
+  `);
+  const insertMember = db.prepare('INSERT INTO dm_members (dm_channel_id, user_id, closed) VALUES (?, ?, 0)');
+
+  let restored = 0;
+  const run = db.transaction(() => {
+    for (const { channelId } of broken) {
+      const authors = missingAuthors.all(channelId, channelId) as { userId: string }[];
+      for (const { userId } of authors) { insertMember.run(channelId, userId); restored++; }
+    }
+  });
+  run();
+  if (restored > 0) console.log(`[backfill] restored ${restored} deleted-partner DM membership row(s)`);
+}