diff --git a/src/slack/sent-thread-cache.test.ts b/src/slack/sent-thread-cache.test.ts
index 57acea9fac0..05af1958895 100644
--- a/src/slack/sent-thread-cache.test.ts
+++ b/src/slack/sent-thread-cache.test.ts
@@ -55,4 +55,13 @@ describe("slack sent-thread-cache", () => {
     vi.spyOn(Date, "now").mockReturnValue(Date.now() + 25 * 60 * 60 * 1000);
     expect(hasSlackThreadParticipation("A1", "C123", "1700000000.000001")).toBe(false);
   });
+
+  it("enforces maximum entries by evicting oldest fresh entries", () => {
+    for (let i = 0; i < 5001; i += 1) {
+      recordSlackThreadParticipation("A1", "C123", `1700000000.${String(i).padStart(6, "0")}`);
+    }
+
+    expect(hasSlackThreadParticipation("A1", "C123", "1700000000.000000")).toBe(false);
+    expect(hasSlackThreadParticipation("A1", "C123", "1700000000.005000")).toBe(true);
+  });
 });
diff --git a/src/slack/sent-thread-cache.ts b/src/slack/sent-thread-cache.ts
index 9cce41b07bd..7fe8037c797 100644
--- a/src/slack/sent-thread-cache.ts
+++ b/src/slack/sent-thread-cache.ts
@@ -22,6 +22,13 @@ function evictExpired(): void {
   }
 }
 
+function evictOldest(): void {
+  const oldest = threadParticipation.keys().next().value;
+  if (oldest) {
+    threadParticipation.delete(oldest);
+  }
+}
+
 export function recordSlackThreadParticipation(
   accountId: string,
   channelId: string,
@@ -33,6 +40,9 @@ export function recordSlackThreadParticipation(
   if (threadParticipation.size >= MAX_ENTRIES) {
     evictExpired();
   }
+  if (threadParticipation.size >= MAX_ENTRIES) {
+    evictOldest();
+  }
   threadParticipation.set(makeKey(accountId, channelId, threadTs), Date.now());
 }
 
diff --git a/ui/src/ui/controllers/cron.test.ts b/ui/src/ui/controllers/cron.test.ts
index 5133994b6db..223eceb7c94 100644
--- a/ui/src/ui/controllers/cron.test.ts
+++ b/ui/src/ui/controllers/cron.test.ts
@@ -346,6 +346,55 @@ describe("cron controller", () => {
     });
   });
 
+  it("omits failureAlert.cooldownMs when custom cooldown is left blank", async () => {
+    const request = vi.fn(async (method: string, _payload?: unknown) => {
+      if (method === "cron.update") {
+        return { id: "job-alert-no-cooldown" };
+      }
+      if (method === "cron.list") {
+        return { jobs: [{ id: "job-alert-no-cooldown" }] };
+      }
+      if (method === "cron.status") {
+        return { enabled: true, jobs: 1, nextWakeAtMs: null };
+      }
+      return {};
+    });
+    const state = createState({
+      client: { request } as unknown as CronState["client"],
+      cronEditingJobId: "job-alert-no-cooldown",
+      cronForm: {
+        ...DEFAULT_CRON_FORM,
+        name: "alert job no cooldown",
+        payloadKind: "agentTurn",
+        payloadText: "run it",
+        failureAlertMode: "custom",
+        failureAlertAfter: "3",
+        failureAlertCooldownSeconds: "",
+        failureAlertChannel: "telegram",
+        failureAlertTo: "123456",
+      },
+    });
+
+    await addCronJob(state);
+
+    const updateCall = request.mock.calls.find(([method]) => method === "cron.update");
+    expect(updateCall).toBeDefined();
+    expect(updateCall?.[1]).toMatchObject({
+      id: "job-alert-no-cooldown",
+      patch: {
+        failureAlert: {
+          after: 3,
+          channel: "telegram",
+          to: "123456",
+        },
+      },
+    });
+    expect(
+      (updateCall?.[1] as { patch?: { failureAlert?: { cooldownMs?: number } } })?.patch
+        ?.failureAlert,
+    ).not.toHaveProperty("cooldownMs");
+  });
+
   it("includes failureAlert=false when disabled per job", async () => {
     const request = vi.fn(async (method: string, _payload?: unknown) => {
       if (method === "cron.update") {
diff --git a/ui/src/ui/controllers/cron.ts b/ui/src/ui/controllers/cron.ts
index 775b4cb650c..72d9ba8a91a 100644
--- a/ui/src/ui/controllers/cron.ts
+++ b/ui/src/ui/controllers/cron.ts
@@ -579,15 +579,17 @@ function buildFailureAlert(form: CronFormState) {
     return undefined;
   }
   const after = toNumber(form.failureAlertAfter.trim(), 0);
-  const cooldownSeconds = toNumber(form.failureAlertCooldownSeconds.trim(), 0);
+  const cooldownRaw = form.failureAlertCooldownSeconds.trim();
+  const cooldownSeconds = cooldownRaw.length > 0 ? toNumber(cooldownRaw, 0) : undefined;
+  const cooldownMs =
+    cooldownSeconds !== undefined && Number.isFinite(cooldownSeconds) && cooldownSeconds >= 0
+      ? Math.floor(cooldownSeconds * 1000)
+      : undefined;
   return {
     after: after > 0 ? Math.floor(after) : undefined,
     channel: form.failureAlertChannel.trim() || CRON_CHANNEL_LAST,
     to: form.failureAlertTo.trim() || undefined,
-    cooldownMs:
-      Number.isFinite(cooldownSeconds) && cooldownSeconds >= 0
-        ? Math.floor(cooldownSeconds * 1000)
-        : undefined,
+    ...(cooldownMs !== undefined ? { cooldownMs } : {}),
   };
 }