diff --git a/README.md b/README.md
index e1be709..f974940 100644
--- a/README.md
+++ b/README.md
@@ -15,8 +15,9 @@ phone each random ID belongs to.
    to deliver it.
 
 The relay doesn't know who you are, which servers you use, or what anybody wrote.
-The notification text is fixed here, so a server can't put anything in it.
-It says "Someone mentioned you", "New direct message" or "New message".
+The text the relay writes is fixed: "Someone mentioned you", "New direct message" or
+"New message". A server can add a preview, but it's sealed to the phone, so only the
+phone can read it and the relay just passes it along.
 Tapping it opens that server in the app, and the messages load from there.
 
 Capabilities are stored as SHA-256 hashes. A copy of the database has device tokens
@@ -27,12 +28,17 @@ in it but nothing that can make a phone ring.
 ```
 POST   /v1/devices   {"platform": "ios"|"android", "token": "...", "env": "production"|"sandbox"}
                      → 201 {"capability": "p_..."}
-POST   /v1/push      Authorization: Bearer p_...   {"kind": "mention"|"dm"|"message"}
+POST   /v1/push      Authorization: Bearer p_...   {"kind": "mention"|"dm"|"message", "preview"?: "..."}
                      → 202, or 404 unknown, 410 the phone is gone, 429, 502
 DELETE /v1/push      Authorization: Bearer p_...   → 204 or 404
 GET    /healthz
 ```
 
+A push can also carry `"preview"`: who wrote, where and the first line, sealed by the
+server to a key only the phone has (AES-256-GCM, base64url). The relay checks its shape,
+passes it on unread, and still sends its own fixed text. On iOS a notification extension
+in the app opens the preview and shows it instead. If it can't, the fixed text stays.
+
 A 410 means Apple or Google said the token is dead. The relay has already forgotten
 it, and the server should forget the capability too.
 
diff --git a/src/apns.ts b/src/apns.ts
index 4ebfd0e..82af9bf 100644
--- a/src/apns.ts
+++ b/src/apns.ts
@@ -30,8 +30,11 @@ export function apnsPayload(alert: Alert): string {
       alert: { title: alert.title, body: alert.body },
       sound: "default",
       "thread-id": alert.tag,
+      // Wakes the phone's notification extension, which swaps in the decrypted preview.
+      ...(alert.preview ? { "mutable-content": 1 } : {}),
     },
     c: alert.tag,
+    ...(alert.preview ? { p: alert.preview } : {}),
   });
 }
 
diff --git a/src/app.test.ts b/src/app.test.ts
index 7a0a01b..7b0d444 100644
--- a/src/app.test.ts
+++ b/src/app.test.ts
@@ -3,7 +3,9 @@ import { createServer, type Server } from "node:http";
 import type { AddressInfo } from "node:net";
 import { afterEach, beforeEach, describe, it } from "node:test";
 
+import { apnsPayload } from "./apns.ts";
 import { createHandler, type Senders } from "./app.ts";
+import { fcmMessage } from "./fcm.ts";
 import { capabilityTag, hashCapability } from "./caps.ts";
 import { loadConfig } from "./config.ts";
 import { Store } from "./db.ts";
@@ -70,6 +72,35 @@ describe("push relay", () => {
     ]);
   });
 
+  it("passes a sealed preview through unread, and asks iOS to wake the extension", async () => {
+    const cap = await register();
+    const preview = "AQ" + "x".repeat(60);
+    const res = await post("/v1/push", { kind: "message", preview }, cap);
+    assert.equal(res.status, 202);
+    assert.equal(sent[0].alert.preview, preview);
+    assert.equal(sent[0].alert.body, "New message", "the fixed text is still there for when the extension can't open it");
+    const apns = JSON.parse(apnsPayload(sent[0].alert));
+    assert.equal(apns.aps["mutable-content"], 1);
+    assert.equal(apns.p, preview);
+    assert.equal((fcmMessage("t", sent[0].alert) as { message: { data: { p: string } } }).message.data.p, preview);
+  });
+
+  it("leaves a push without a preview exactly as it was", async () => {
+    const cap = await register();
+    await post("/v1/push", { kind: "dm" }, cap);
+    assert.equal(sent[0].alert.preview, undefined);
+    assert.ok(!("mutable-content" in JSON.parse(apnsPayload(sent[0].alert)).aps));
+  });
+
+  it("refuses a preview that isn't the shape the server writes", async () => {
+    const cap = await register();
+    for (const preview of [42, "short", "has spaces in it and is long enough to pass the length check", "x".repeat(2049)]) {
+      const res = await post("/v1/push", { kind: "dm", preview }, cap);
+      assert.equal(res.status, 400, JSON.stringify(preview).slice(0, 30));
+    }
+    assert.equal(sent.length, 0);
+  });
+
   it("stores a hash of the capability, never the capability", async () => {
     const cap = await register();
     const rows = store.db.prepare("SELECT * FROM devices").all() as Record<string, unknown>[];
diff --git a/src/app.ts b/src/app.ts
index dfa96f5..4bb05ee 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -5,7 +5,7 @@ import type { Config } from "./config.ts";
 import type { ApnsEnv, Platform, Store } from "./db.ts";
 import { clientIp, header, HttpError, readJson, sendJson } from "./http.ts";
 import { RateLimiter } from "./limits.ts";
-import { alertFor, KINDS, type Alert, type Kind, type SendResult } from "./notification.ts";
+import { alertFor, KINDS, PREVIEW_SHAPE, type Alert, type Kind, type SendResult } from "./notification.ts";
 
 export interface Senders {
   ios?: (token: string, env: ApnsEnv, alert: Alert) => Promise<SendResult>;
@@ -58,6 +58,8 @@ export function createHandler(config: Config, store: Store, senders: Senders, lo
     const body = await readJson(req, MAX_BODY);
     const kind = body.kind as Kind;
     if (!KINDS.includes(kind)) throw new HttpError(400, "bad_kind");
+    const preview = body.preview;
+    if (preview !== undefined && (typeof preview !== "string" || !PREVIEW_SHAPE.test(preview))) throw new HttpError(400, "bad_preview");
 
     const now = clock();
     if (!perIp.take(ip, now)) throw new HttpError(429, "too_many_pushes");
@@ -66,7 +68,7 @@ export function createHandler(config: Config, store: Store, senders: Senders, lo
     if (!device) throw new HttpError(404, "unknown_capability");
     if (!perDevice.take(capHash, now)) throw new HttpError(429, "too_many_pushes");
 
-    const alert = alertFor(kind, capabilityTag(cap));
+    const alert = alertFor(kind, capabilityTag(cap), preview as string | undefined);
     const result = device.platform === "ios"
       ? await senders.ios?.(device.token, device.env, alert)
       : await senders.android?.(device.token, alert);
diff --git a/src/fcm.ts b/src/fcm.ts
index 35bc019..ddb80f7 100644
--- a/src/fcm.ts
+++ b/src/fcm.ts
@@ -21,7 +21,7 @@ export function fcmMessage(deviceToken: string, alert: Alert): object {
       token: deviceToken,
       notification: { title: alert.title, body: alert.body },
       android: { priority: "high", notification: { channel_id: "messages", sound: "default" } },
-      data: { c: alert.tag },
+      data: { c: alert.tag, ...(alert.preview ? { p: alert.preview } : {}) },
     },
   };
 }
diff --git a/src/notification.ts b/src/notification.ts
index 4b75691..acf2399 100644
--- a/src/notification.ts
+++ b/src/notification.ts
@@ -8,16 +8,21 @@ export interface Alert {
   body: string;
   /** Lets the phone tell which of its servers this came from. */
   tag: string;
+  /** Sealed by the server to the phone (GRYT-1688). Passed through unread; the text above is the fallback. */
+  preview?: string;
 }
 
+/** base64url of a version byte, a nonce, ciphertext and tag. Its shape only: the relay never opens it. */
+export const PREVIEW_SHAPE = /^[A-Za-z0-9_-]{40,2048}$/;
+
 const BODY: Record<Kind, string> = {
   mention: "Someone mentioned you",
   dm: "New direct message",
   message: "New message",
 };
 
-export function alertFor(kind: Kind, tag: string): Alert {
-  return { title: "Gryt", body: BODY[kind], tag };
+export function alertFor(kind: Kind, tag: string, preview?: string): Alert {
+  return { title: "Gryt", body: BODY[kind], tag, ...(preview ? { preview } : {}) };
 }
 
 export type SendResult = { ok: true } | { ok: false; gone: boolean; reason: string };