diff --git a/core/src/host/tasks.js b/core/src/host/tasks.js
index 9407659e..edf3cd43 100644
--- a/core/src/host/tasks.js
+++ b/core/src/host/tasks.js
@@ -569,7 +569,7 @@ export function install(rt) {
       if (task.status === 'accepted') {
         if (countsAsDelivery(task)) row.deliveries += 1
         const lease = leaseOf(task)
-        if (lease !== undefined && Number.isFinite(lease.takenAt)) row.workerSeconds += Math.max(0, Math.round((task.updatedAt - lease.takenAt) / 1000))
+        if (lease !== undefined && Number.isFinite(lease.takenAt) && Number.isFinite(task.updatedAt)) row.workerSeconds += Math.max(0, Math.round((task.updatedAt - lease.takenAt) / 1000))
       }
       if (run !== '') {
         const runRow = bucket(runs, run)
diff --git a/core/test/task-counters-updated-at.test.js b/core/test/task-counters-updated-at.test.js
new file mode 100644
index 00000000..2817f5b5
--- /dev/null
+++ b/core/test/task-counters-updated-at.test.js
@@ -0,0 +1,90 @@
+// `rt.taskCounters` adds a card's own `updatedAt` into `workerSeconds`. A card may be stored by
+// an older version or edited by hand, so the field can be missing or non-numeric while the lease
+// is valid. The arithmetic must guard that operand too: `NaN` would become `null` through JSON
+// for every reader of the counter. The numeric case still measures the lease window.
+import assert from 'node:assert/strict'
+import { mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { test } from 'node:test'
+import { createRuntime } from '../index.js'
+import { fakeContext } from './fake-ctx.js'
+
+const TAKEN_AT = 1_700_000_000_000
+
+const acceptedCard = (updatedAt) => ({
+  id: 'task-1',
+  title: 'Count the delivery',
+  status: 'accepted',
+  taskClass: 'work',
+  ownerBotId: 'd1',
+  createdAt: TAKEN_AT,
+  ...(updatedAt === undefined ? {} : { updatedAt }),
+  lease: { holder: 'd1', takenAt: TAKEN_AT, expiresAt: TAKEN_AT + 1_800_000 },
+})
+
+const seed = (task) => ({
+  version: 2,
+  revision: 1,
+  mainBotIds: ['a'],
+  members: { 'owner-1': { id: 'owner-1', createdAt: 1, designatedChiefId: 'a' } },
+  bots: {
+    a: { id: 'a', name: 'Chief', role: 'Lead', sessionId: 'a', createdAt: 1, ownerId: 'owner-1' },
+    d1: { id: 'd1', name: 'Dev1', role: 'Dev', sessionId: 'd1', createdAt: 2, ownerId: 'owner-1', createdBy: 'a' },
+  },
+  rooms: {},
+  grants: {},
+  agents: {},
+  exchanges: [],
+  exchangeTurns: {},
+  questions: {},
+  prefs: {},
+  tasks: { [task.id]: task },
+})
+
+async function boot(t, task) {
+  const home = mkdtempSync(join(tmpdir(), 'ds-bot-counters-'))
+  writeFileSync(join(home, 'state.json'), JSON.stringify(seed(task)))
+  const ctx = fakeContext()
+  const agents = new Map()
+  const agentOf = (sessionId) => {
+    if (!agents.has(sessionId)) {
+      const inbox = { nextStep: [], nextTurn: [], remove() { return true } }
+      agents.set(sessionId, {
+        session: { id: sessionId }, id: sessionId, calls: [], status: 'idle', inbox,
+        followup() {}, steer() {}, inject() {}, cancel() {}, async whenIdle() {},
+      })
+    }
+    return agents.get(sessionId)
+  }
+  Object.assign(ctx, {
+    llm: { listProviders: () => [], listModels: async () => [], resolveModelInfo: async () => ({}) },
+    workspaceRegistry: { get: () => undefined, resolveByPath: async () => undefined, create: async path => ({ id: 'w1', path }), archiveSession: async () => {} },
+    sessionController: { create: async () => ({ sessionId: 'made-1' }), resolveAgent: async id => ({ agent: agentOf(id) }), selectModel: async () => {}, page: async () => ({ records: [], hasMore: false }) },
+    sessionTitle: { rename: async () => {} },
+    sessions: { flush: async () => {} },
+  })
+  const rt = createRuntime(ctx, { home })
+  await rt.load()
+  t.after(async () => {
+    try { await ctx.dispose() } catch {}
+    rmSync(home, { recursive: true, force: true, maxRetries: 5 })
+  })
+  return rt
+}
+
+test('a non-numeric or missing updatedAt keeps workerSeconds finite (#2588)', async (t) => {
+  for (const updatedAt of ['not-a-number', undefined, null]) {
+    const rt = await boot(t, acceptedCard(updatedAt))
+    const counts = rt.taskCounters()
+    const workerSeconds = counts.roles.Dev.workerSeconds
+    assert.equal(Number.isFinite(workerSeconds), true, `updatedAt=${String(updatedAt)} gave workerSeconds=${workerSeconds}`)
+    const throughJson = JSON.parse(JSON.stringify(counts)).roles.Dev.workerSeconds
+    assert.notEqual(throughJson, null, `updatedAt=${String(updatedAt)} serialised to null`)
+  }
+})
+
+test('a numeric updatedAt still measures the lease window (#2588)', async (t) => {
+  const rt = await boot(t, acceptedCard(TAKEN_AT + 90_000))
+  assert.equal(rt.taskCounters().roles.Dev.workerSeconds, 90)
+})
diff --git a/patches/ds-bot/files/src/host/tasks.js b/patches/ds-bot/files/src/host/tasks.js
index 9407659e..edf3cd43 100644
--- a/patches/ds-bot/files/src/host/tasks.js
+++ b/patches/ds-bot/files/src/host/tasks.js
@@ -569,7 +569,7 @@ export function install(rt) {
       if (task.status === 'accepted') {
         if (countsAsDelivery(task)) row.deliveries += 1
         const lease = leaseOf(task)
-        if (lease !== undefined && Number.isFinite(lease.takenAt)) row.workerSeconds += Math.max(0, Math.round((task.updatedAt - lease.takenAt) / 1000))
+        if (lease !== undefined && Number.isFinite(lease.takenAt) && Number.isFinite(task.updatedAt)) row.workerSeconds += Math.max(0, Math.round((task.updatedAt - lease.takenAt) / 1000))
       }
       if (run !== '') {
         const runRow = bucket(runs, run)
diff --git a/patches/ds-bot/files/test/task-counters-updated-at.test.js b/patches/ds-bot/files/test/task-counters-updated-at.test.js
new file mode 100644
index 00000000..2817f5b5
--- /dev/null
+++ b/patches/ds-bot/files/test/task-counters-updated-at.test.js
@@ -0,0 +1,90 @@
+// `rt.taskCounters` adds a card's own `updatedAt` into `workerSeconds`. A card may be stored by
+// an older version or edited by hand, so the field can be missing or non-numeric while the lease
+// is valid. The arithmetic must guard that operand too: `NaN` would become `null` through JSON
+// for every reader of the counter. The numeric case still measures the lease window.
+import assert from 'node:assert/strict'
+import { mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { test } from 'node:test'
+import { createRuntime } from '../index.js'
+import { fakeContext } from './fake-ctx.js'
+
+const TAKEN_AT = 1_700_000_000_000
+
+const acceptedCard = (updatedAt) => ({
+  id: 'task-1',
+  title: 'Count the delivery',
+  status: 'accepted',
+  taskClass: 'work',
+  ownerBotId: 'd1',
+  createdAt: TAKEN_AT,
+  ...(updatedAt === undefined ? {} : { updatedAt }),
+  lease: { holder: 'd1', takenAt: TAKEN_AT, expiresAt: TAKEN_AT + 1_800_000 },
+})
+
+const seed = (task) => ({
+  version: 2,
+  revision: 1,
+  mainBotIds: ['a'],
+  members: { 'owner-1': { id: 'owner-1', createdAt: 1, designatedChiefId: 'a' } },
+  bots: {
+    a: { id: 'a', name: 'Chief', role: 'Lead', sessionId: 'a', createdAt: 1, ownerId: 'owner-1' },
+    d1: { id: 'd1', name: 'Dev1', role: 'Dev', sessionId: 'd1', createdAt: 2, ownerId: 'owner-1', createdBy: 'a' },
+  },
+  rooms: {},
+  grants: {},
+  agents: {},
+  exchanges: [],
+  exchangeTurns: {},
+  questions: {},
+  prefs: {},
+  tasks: { [task.id]: task },
+})
+
+async function boot(t, task) {
+  const home = mkdtempSync(join(tmpdir(), 'ds-bot-counters-'))
+  writeFileSync(join(home, 'state.json'), JSON.stringify(seed(task)))
+  const ctx = fakeContext()
+  const agents = new Map()
+  const agentOf = (sessionId) => {
+    if (!agents.has(sessionId)) {
+      const inbox = { nextStep: [], nextTurn: [], remove() { return true } }
+      agents.set(sessionId, {
+        session: { id: sessionId }, id: sessionId, calls: [], status: 'idle', inbox,
+        followup() {}, steer() {}, inject() {}, cancel() {}, async whenIdle() {},
+      })
+    }
+    return agents.get(sessionId)
+  }
+  Object.assign(ctx, {
+    llm: { listProviders: () => [], listModels: async () => [], resolveModelInfo: async () => ({}) },
+    workspaceRegistry: { get: () => undefined, resolveByPath: async () => undefined, create: async path => ({ id: 'w1', path }), archiveSession: async () => {} },
+    sessionController: { create: async () => ({ sessionId: 'made-1' }), resolveAgent: async id => ({ agent: agentOf(id) }), selectModel: async () => {}, page: async () => ({ records: [], hasMore: false }) },
+    sessionTitle: { rename: async () => {} },
+    sessions: { flush: async () => {} },
+  })
+  const rt = createRuntime(ctx, { home })
+  await rt.load()
+  t.after(async () => {
+    try { await ctx.dispose() } catch {}
+    rmSync(home, { recursive: true, force: true, maxRetries: 5 })
+  })
+  return rt
+}
+
+test('a non-numeric or missing updatedAt keeps workerSeconds finite (#2588)', async (t) => {
+  for (const updatedAt of ['not-a-number', undefined, null]) {
+    const rt = await boot(t, acceptedCard(updatedAt))
+    const counts = rt.taskCounters()
+    const workerSeconds = counts.roles.Dev.workerSeconds
+    assert.equal(Number.isFinite(workerSeconds), true, `updatedAt=${String(updatedAt)} gave workerSeconds=${workerSeconds}`)
+    const throughJson = JSON.parse(JSON.stringify(counts)).roles.Dev.workerSeconds
+    assert.notEqual(throughJson, null, `updatedAt=${String(updatedAt)} serialised to null`)
+  }
+})
+
+test('a numeric updatedAt still measures the lease window (#2588)', async (t) => {
+  const rt = await boot(t, acceptedCard(TAKEN_AT + 90_000))
+  assert.equal(rt.taskCounters().roles.Dev.workerSeconds, 90)
+})