diff --git a/templates/sw.js b/templates/sw.js
index 0bf8170..394ee52 100644
--- a/templates/sw.js
+++ b/templates/sw.js
@@ -1,5 +1,7 @@
 // Served from / (not /static/) so its scope covers /admin/today/.
-const CACHE = "osiris-v1";
+// Bump on any change here or to the cached pages: activate() drops older caches,
+// which is what stops a stale Today page surviving a deploy.
+const CACHE = "osiris-v2";
 
 self.addEventListener("install", (event) => {
   self.skipWaiting();
diff --git a/templates/tracker/today.html b/templates/tracker/today.html
index 7a6188f..ea1e225 100644
--- a/templates/tracker/today.html
+++ b/templates/tracker/today.html
@@ -145,19 +145,34 @@
   {% endif %}
 </div>
 
-<div class="osiris-card" id="reminders" hidden>
+<div class="osiris-card" id="reminders">
   <h2 style="margin-top:0">Reminders</h2>
   <p class="dose-detail" id="reminder-status">Checking…</p>
   <div style="display:flex; gap:8px; flex-wrap:wrap">
-    <button type="button" id="reminder-toggle" class="osiris-save" style="flex:1; min-width:140px"></button>
+    <button type="button" id="reminder-toggle" class="osiris-save" style="flex:1; min-width:140px" hidden></button>
     <button type="button" id="reminder-test" class="osiris-save" style="flex:1; min-width:140px; background:transparent; color:inherit; border:1px solid var(--border-color)" hidden>
       Send test
     </button>
   </div>
-  <p class="dose-detail" style="margin-bottom:0">
+  <p class="dose-detail">
     A reminder is sent at each dose's reminder time, and only if it isn't already ticked off.
     Set those times on the medication in the admin.
   </p>
+  <details class="osiris-debug">
+    <summary class="dose-detail" style="cursor:pointer">Diagnostics</summary>
+    <ul id="reminder-checks"></ul>
+    <ul>
+      <li class="dose-detail">
+        {% if doses_with_reminders %}✅{% else %}❌{% endif %}
+        Doses with a reminder time: {{ doses_with_reminders }} of {{ day.doses|length }}
+        {% if not doses_with_reminders %}— set one on the medication in the admin, or nothing will ever fire{% endif %}
+      </li>
+      <li class="dose-detail">
+        {% if subscribed_devices %}✅{% else %}❌{% endif %}
+        Devices subscribed (this account): {{ subscribed_devices }}
+      </li>
+    </ul>
+  </details>
 </div>
 
 <p style="text-align:center"><a href="{% url 'admin:index' %}">Manage medication &amp; history &rarr;</a></p>
@@ -165,19 +180,80 @@
 <script>
 (function () {
   const VAPID_PUBLIC_KEY = "{{ vapid_public_key|escapejs }}";
-  const card = document.getElementById("reminders");
   const status = document.getElementById("reminder-status");
   const toggle = document.getElementById("reminder-toggle");
   const test = document.getElementById("reminder-test");
-
-  // No keys configured, or a browser without push: leave the card hidden entirely.
-  if (!VAPID_PUBLIC_KEY || !("serviceWorker" in navigator) || !("PushManager" in window)) {
-    return;
-  }
-  card.hidden = false;
+  const checks = document.getElementById("reminder-checks");
 
   const csrf = document.querySelector("[name=csrfmiddlewaretoken]").value;
 
+  // Every precondition for Web Push, each one reported by name. Push fails silently
+  // in a dozen ways; the point here is to say which one, rather than hide the card.
+  function preconditions() {
+    return [
+      {
+        name: "Server VAPID keys",
+        ok: Boolean(VAPID_PUBLIC_KEY),
+        detail: VAPID_PUBLIC_KEY
+          ? VAPID_PUBLIC_KEY.slice(0, 12) + "…"
+          : "not set — run: manage.py generate_vapid_keys, then set VAPID_PUBLIC_KEY and VAPID_PRIVATE_KEY in the server environment and restart",
+      },
+      {
+        name: "Secure context (HTTPS)",
+        ok: window.isSecureContext,
+        detail: window.isSecureContext
+          ? location.origin
+          : location.origin + " — push needs https:// (or localhost)",
+      },
+      {
+        name: "Service worker support",
+        ok: "serviceWorker" in navigator,
+        detail: "serviceWorker" in navigator ? "supported" : "not supported by this browser",
+      },
+      {
+        name: "Push support",
+        ok: "PushManager" in window,
+        detail:
+          "PushManager" in window
+            ? "supported"
+            : "not supported — on Android use Chrome; Firefox in a private window will not do",
+      },
+      {
+        name: "Notification permission",
+        ok: typeof Notification !== "undefined" && Notification.permission !== "denied",
+        detail:
+          typeof Notification === "undefined"
+            ? "Notification API missing"
+            : Notification.permission,
+      },
+    ];
+  }
+
+  function renderChecks(extra) {
+    const rows = preconditions().concat(extra || []);
+    checks.innerHTML = "";
+    for (const row of rows) {
+      const li = document.createElement("li");
+      li.className = "dose-detail";
+      li.textContent = `${row.ok ? "✅" : "❌"} ${row.name}: ${row.detail}`;
+      checks.appendChild(li);
+    }
+    return rows.every((row) => row.ok);
+  }
+
+  const blocking = preconditions().filter((row) => !row.ok);
+  if (blocking.length) {
+    // Notification.permission === "denied" is recoverable, so it is not fatal here;
+    // anything else means the button would do nothing at all.
+    const fatal = blocking.filter((row) => row.name !== "Notification permission");
+    if (fatal.length) {
+      status.textContent = `Reminders unavailable — ${fatal[0].name}: ${fatal[0].detail}`;
+      renderChecks();
+      document.querySelector(".osiris-debug").open = true;
+      return;
+    }
+  }
+
   function post(url, body) {
     return fetch(url, {
       method: "POST",
@@ -202,16 +278,37 @@
   }
 
   let subscription = null;
+  let worker = null;
 
   function render() {
-    const blocked = Notification.permission === "denied";
-    if (blocked) {
+    renderChecks([
+      {
+        name: "Service worker",
+        ok: Boolean(worker),
+        detail: worker ? "active at " + worker.scope : "not registered yet — reload the page",
+      },
+      {
+        name: "This device subscribed",
+        ok: Boolean(subscription),
+        detail: subscription ? subscription.endpoint.slice(0, 48) + "…" : "no",
+      },
+      {
+        name: "Installed as an app",
+        ok: window.matchMedia("(display-mode: standalone)").matches,
+        detail: window.matchMedia("(display-mode: standalone)").matches
+          ? "yes"
+          : "no — works in the browser too, but Android delivers reliably once installed",
+      },
+    ]);
+
+    if (Notification.permission === "denied") {
       status.textContent =
-        "Notifications are blocked for this site. Re-allow them in Chrome's site settings.";
+        "Notifications are blocked for this site. Re-allow them in Chrome → site settings → Notifications.";
       toggle.hidden = true;
       test.hidden = true;
       return;
     }
+
     toggle.hidden = false;
     toggle.disabled = false;
     toggle.textContent = subscription ? "Turn reminders off" : "Turn reminders on";
@@ -222,10 +319,18 @@
   }
 
   navigator.serviceWorker.ready
-    .then((registration) => registration.pushManager.getSubscription())
+    .then((registration) => {
+      worker = registration;
+      return registration.pushManager.getSubscription();
+    })
     .then((existing) => {
       subscription = existing;
       render();
+    })
+    .catch((error) => {
+      status.textContent = "Service worker failed: " + error.message;
+      renderChecks();
+      document.querySelector(".osiris-debug").open = true;
     });
 
   toggle.addEventListener("click", async () => {
diff --git a/tracker/tests.py b/tracker/tests.py
index 9db6d92..d796a7c 100644
--- a/tracker/tests.py
+++ b/tracker/tests.py
@@ -259,6 +259,34 @@ class PwaTests(TrackerTestCase):
         self.assertContains(response, 'rel="manifest"')
         self.assertContains(response, "serviceWorker.register")
 
+    @override_settings(VAPID_PUBLIC_KEY="", VAPID_PRIVATE_KEY="")
+    def test_the_reminder_card_explains_itself_when_keys_are_missing(self):
+        # It used to hide entirely, which is indistinguishable from a broken page.
+        self.client.force_login(self.user)
+        response = self.client.get(reverse("tracker:today"))
+
+        self.assertContains(response, "Reminders")
+        self.assertContains(response, "Diagnostics")
+        self.assertContains(response, "generate_vapid_keys")
+
+    @override_settings(VAPID_PUBLIC_KEY="BMn-test-public-key")
+    def test_the_diagnostics_report_reminder_times_and_devices(self):
+        self.client.force_login(self.user)
+
+        response = self.client.get(reverse("tracker:today"))
+        self.assertContains(response, "Doses with a reminder time: 0 of 3")
+        self.assertContains(response, "Devices subscribed (this account): 0")
+
+        self.morning.reminder_time = "08:00"
+        self.morning.save()
+        PushSubscription.objects.create(
+            user=self.user, endpoint="https://push.example/a", p256dh="k", auth="s"
+        )
+
+        response = self.client.get(reverse("tracker:today"))
+        self.assertContains(response, "Doses with a reminder time: 1 of 3")
+        self.assertContains(response, "Devices subscribed (this account): 1")
+
     def test_the_manifest_is_valid_json_pointing_at_real_icons(self):
         response = self.client.get(reverse("manifest"))
 
diff --git a/tracker/views.py b/tracker/views.py
index a5cc202..71717f3 100644
--- a/tracker/views.py
+++ b/tracker/views.py
@@ -104,7 +104,14 @@ def today(request):
             "next_date": day + timedelta(days=1),
             "chart": build_chart(readings),
             "reading_count": len(readings),
-            # Empty when push is not configured; the page then hides the card.
+            # Empty when push is not configured; the card then says so rather than
+            # vanishing, because a missing card looks like a bug in the page.
             "vapid_public_key": settings.VAPID_PUBLIC_KEY,
+            "doses_with_reminders": ScheduledDose.objects.filter(
+                is_active=True,
+                medication__is_active=True,
+                reminder_time__isnull=False,
+            ).count(),
+            "subscribed_devices": request.user.push_subscriptions.count(),
         },
     )