diff --git a/README.md b/README.md
index 2debe67..ac102d1 100644
--- a/README.md
+++ b/README.md
@@ -303,6 +303,19 @@ GET  /api/sdk/v1/resolve/:shortCode    # Resolve link to deep link data (no redi
 GET  /api/sdk/v1/health                # Health check
 ```
 
+An event whose `installId` this server no longer has is **recovered, not
+refused**: the install is recorded again (marked `recovered`, with no link,
+click or confidence score, so it is never mistaken for a fresh attributed
+install) and the event is stored against it.
+
+This matters because the id lives in the app's storage for the life of the
+install while the server's copy may not: a restore from an older backup, a
+manual cleanup, an app build pointed at a fresh database, or an analytics
+retention job all leave a live device holding an id nothing resolves. The
+bundled SDKs store the id once and never re-register, so without recovery
+that device is refused forever with no way back. The 404 that remains carries
+`code: "INSTALL_NOT_FOUND"` and `action: "reregister"`.
+
 ### Health
 
 ```bash
diff --git a/src/routes/sdk.event.test.ts b/src/routes/sdk.event.test.ts
index 125bd09..6dc2674 100644
--- a/src/routes/sdk.event.test.ts
+++ b/src/routes/sdk.event.test.ts
@@ -142,13 +142,124 @@ describe('POST /api/sdk/v1/event — last-click attribution stamp', () => {
     await app.close();
   });
 
-  it('returns 404 when the install does not exist', async () => {
+});
+
+/**
+ * An event whose install is gone.
+ *
+ * The install id lives in the app's storage for the life of the install, so a
+ * missing row is permanent from the device's point of view: refusing the event
+ * silences that app forever. A deployment that prunes analytics on a retention
+ * window reaches this the moment an install outlives the window.
+ */
+describe('POST /api/sdk/v1/event — an install the server no longer has', () => {
+  beforeEach(() => {
+    mockQuery.mockReset();
+  });
+
+  const orphanEvent = {
+    installId: INSTALL_ID,
+    eventName: 'purchase',
+    eventData: { value: 12 },
+    sdkName: 'android',
+    sdkVersion: '1.3.2',
+  };
+
+  it('records the install again and stores the event against it', async () => {
+    mockQuery.mockResolvedValueOnce({ rows: [] });                               // 1) install lookup: gone
+    mockQuery.mockResolvedValueOnce({ rows: [{ id: INSTALL_ID, link_id: null }] }); // 2) recovery insert
+    mockQuery.mockResolvedValueOnce({ rows: [{ id: EVENT_ID }] });               // 3) the event insert
+
+    const app = await buildApp();
+    const res = await app.inject({ method: 'POST', url: '/api/sdk/v1/event', payload: orphanEvent });
+
+    expect(res.statusCode).toBe(200);
+    expect(res.json()).toMatchObject({ eventId: EVENT_ID, acknowledged: true });
+
+    const recovery = mockQuery.mock.calls[1];
+    expect(recovery[0]).toMatch(/INSERT INTO install_events/);
+    // The id the client sent, so the device's next event resolves normally.
+    expect(recovery[1][0]).toBe(INSTALL_ID);
+    // A marker, never a real hash: it cannot collide with a fingerprint match.
+    expect(recovery[1][1]).toBe(`recovered:${INSTALL_ID}`);
+    expect(recovery[1].slice(2)).toEqual(['android', '1.3.2']);
+    // Concurrent events for the same install must not collide.
+    expect(recovery[0]).toMatch(/ON CONFLICT \(id\) DO NOTHING/);
+
+    // The event is stored against that install, unattributed.
+    const insert = mockQuery.mock.calls[2];
+    expect(insert[0]).toMatch(/INSERT INTO in_app_events/);
+    expect(insert[1][0]).toBe(INSTALL_ID);
+
+    await app.close();
+  });
+
+  it('claims no attribution for a recovered install', async () => {
     mockQuery.mockResolvedValueOnce({ rows: [] });
+    mockQuery.mockResolvedValueOnce({ rows: [{ id: INSTALL_ID, link_id: null }] });
+    mockQuery.mockResolvedValueOnce({ rows: [{ id: EVENT_ID }] });
 
     const app = await buildApp();
-    const res = await app.inject({ method: 'POST', url: '/api/sdk/v1/event', payload: stampedEvent });
+    await app.inject({ method: 'POST', url: '/api/sdk/v1/event', payload: orphanEvent });
+
+    const sql = mockQuery.mock.calls[1][0] as string;
+    expect(sql).toMatch(/'recovered'/);
+    // No link, click or confidence among the columns written: a device we have
+    // met before is not a new attributed install, and nothing downstream may
+    // read it as one. (The RETURNING clause reads link_id back; that is not a
+    // write.)
+    const columns = sql.slice(sql.indexOf('('), sql.indexOf('VALUES'));
+    expect(columns).not.toMatch(/link_id|click_id|confidence_score/);
+
+    await app.close();
+  });
+
+  it('uses the row a concurrent request created rather than failing', async () => {
+    mockQuery.mockResolvedValueOnce({ rows: [] });                                  // lookup: gone
+    mockQuery.mockResolvedValueOnce({ rows: [] });                                  // insert: lost the race
+    mockQuery.mockResolvedValueOnce({ rows: [{ id: INSTALL_ID, link_id: LINK_ID }] }); // re-read
+    mockQuery.mockResolvedValueOnce({ rows: [{ id: EVENT_ID }] });                  // event insert
+    mockQuery.mockResolvedValueOnce({ rows: [] });                                  // webhook lookup (link_id set)
+
+    const app = await buildApp();
+    const res = await app.inject({ method: 'POST', url: '/api/sdk/v1/event', payload: orphanEvent });
+
+    expect(res.statusCode).toBe(200);
+    expect(res.json()).toMatchObject({ eventId: EVENT_ID, acknowledged: true });
+
+    await app.close();
+  });
+
+  it('tells a client what to do when recovery cannot complete', async () => {
+    mockQuery.mockResolvedValueOnce({ rows: [] }); // lookup: gone
+    mockQuery.mockResolvedValueOnce({ rows: [] }); // insert: no row
+    mockQuery.mockResolvedValueOnce({ rows: [] }); // re-read: still nothing
+
+    const app = await buildApp();
+    const res = await app.inject({ method: 'POST', url: '/api/sdk/v1/event', payload: orphanEvent });
 
     expect(res.statusCode).toBe(404);
+    expect(res.json()).toEqual({
+      error: 'Install event not found',
+      code: 'INSTALL_NOT_FOUND',
+      action: 'reregister',
+    });
+
+    await app.close();
+  });
+
+  it('leaves a known install untouched', async () => {
+    mockQuery.mockResolvedValueOnce({ rows: [{ id: INSTALL_ID, link_id: null }] });
+    mockQuery.mockResolvedValueOnce({ rows: [{ id: EVENT_ID }] });
+
+    const app = await buildApp();
+    const res = await app.inject({ method: 'POST', url: '/api/sdk/v1/event', payload: orphanEvent });
+
+    expect(res.statusCode).toBe(200);
+    // Two queries only: the lookup and the event. No recovery insert.
+    expect(mockQuery.mock.calls).toHaveLength(2);
+    expect(mockQuery.mock.calls[1][0]).toMatch(/INSERT INTO in_app_events/);
+
     await app.close();
   });
 });
diff --git a/src/routes/sdk.ts b/src/routes/sdk.ts
index ec77e2b..44b3863 100644
--- a/src/routes/sdk.ts
+++ b/src/routes/sdk.ts
@@ -15,6 +15,59 @@ import { parseUserAgent, getLocationFromIP, detectDevice } from '../lib/utils.js
 import { emitClickEvent } from '../lib/event-emitter.js';
 import { classifyBot, edgeBotSignal } from '../lib/bot-detection.js';
 
+/**
+ * An event arrived for an install this server does not have. Put the install
+ * back rather than refusing the event.
+ *
+ * The id lives in the app's own storage for the life of the install while our
+ * copy may not, so a missing row is not a transient condition: every event
+ * that device will ever send refers to an id we cannot resolve, and the SDKs
+ * store the id once and never re-register, so refusing them means the app goes
+ * silent forever with no way back. A row goes missing in ordinary operation —
+ * a restore from an older backup, a manual cleanup, a build pointed at a fresh
+ * database, an analytics retention job — and none of those are the device's
+ * fault.
+ *
+ * The recovered row is deliberately thin. We know the device exists and which
+ * SDK it runs; we do not know what brought it here, and we must not invent
+ * that: `attribution_method` says `recovered` and there is no link, click or
+ * confidence score, so nothing downstream can mistake it for a fresh
+ * attributed install. The fingerprint is a marker rather than a hash — it can
+ * never equal a real one, so it cannot match a click by accident.
+ *
+ * Creating a row from a client-supplied id adds no exposure that
+ * `POST /api/sdk/v1/install` does not already have: that endpoint is
+ * unauthenticated and creates an install row for anyone who calls it. The only
+ * difference here is who chose the id, and a UUID the caller picked is still
+ * only ever their own row.
+ *
+ * Returns the row, or null when a concurrent request already created it and we
+ * lost the race but cannot read it back — the caller then answers 404 as
+ * before, and the SDK's next event succeeds.
+ */
+async function recoverInstall(
+  installId: string,
+  sdkName?: string,
+  sdkVersion?: string
+): Promise<{ id: string; link_id: string | null } | null> {
+  const inserted = await db.query(
+    `INSERT INTO install_events
+       (id, fingerprint_hash, attribution_method, installed_at, first_open_at, sdk_name, sdk_version)
+     VALUES ($1, $2, 'recovered', NOW(), NOW(), $3, $4)
+     ON CONFLICT (id) DO NOTHING
+     RETURNING id, link_id`,
+    [installId, `recovered:${installId}`, sdkName || null, sdkVersion || null]
+  );
+  if (inserted.rows[0]) return inserted.rows[0];
+
+  // Lost the race with a concurrent event for the same install.
+  const existing = await db.query(
+    `SELECT id, link_id FROM install_events WHERE id = $1`,
+    [installId]
+  );
+  return existing.rows[0] ?? null;
+}
+
 /**
  * SDK Routes - Mobile SDK endpoints for deferred deep linking
  * These endpoints are used by the mobile SDKs to report installs and retrieve attribution data
@@ -228,6 +281,13 @@ export async function sdkRoutes(fastify: FastifyInstance) {
    * Response:
    * - eventId: UUID of the tracked event
    * - acknowledged: Boolean confirmation
+   *
+   * An `installId` this server no longer has is recovered rather than
+   * refused: the install is recorded again, marked `recovered`, and the event
+   * is stored against it. See recoverInstall above. A 404 with
+   * `code: 'INSTALL_NOT_FOUND'` and `action: 'reregister'` is returned only in
+   * the rare case where recovery itself could not complete; an SDK seeing it
+   * should register a fresh install rather than retry the same id.
    */
   fastify.post('/api/sdk/v1/event', async (request, reply) => {
     const schema = z.object({
@@ -249,19 +309,23 @@ export async function sdkRoutes(fastify: FastifyInstance) {
     const body = schema.parse(request.body);
 
     try {
-      // Verify install exists and get link_id for webhook lookup
+      // The install this event belongs to, recovered if we no longer have it.
       const installCheck = await db.query(
         `SELECT id, link_id FROM install_events WHERE id = $1`,
         [body.installId]
       );
 
-      if (installCheck.rows.length === 0) {
+      const install =
+        installCheck.rows[0] ??
+        (await recoverInstall(body.installId, body.sdkName, body.sdkVersion));
+
+      if (!install) {
         return reply.status(404).send({
           error: 'Install event not found',
+          code: 'INSTALL_NOT_FOUND',
+          action: 'reregister',
         });
       }
-
-      const install = installCheck.rows[0];
       const eventTimestamp = body.timestamp || new Date().toISOString();
       const eventDataJson = JSON.stringify(body.eventData || {});