diff --git a/examples/advanced-project-js/src/__checks__/utils/alert-channels.js b/examples/advanced-project-js/src/__checks__/utils/alert-channels.js
index 251cfdabe..4002a88ae 100644
--- a/examples/advanced-project-js/src/__checks__/utils/alert-channels.js
+++ b/examples/advanced-project-js/src/__checks__/utils/alert-channels.js
@@ -2,7 +2,6 @@ const { URL } = require('node:url')
 
 const {
   EmailAlertChannel,
-  SlackAppAlertChannel,
   WebhookAlertChannel,
 } = require('checkly/constructs')
 
@@ -25,10 +24,13 @@ const emailChannel = new EmailAlertChannel('email-channel-1', {
   ...sendDefaults,
 })
 
-const slackChannel = new SlackAppAlertChannel('slack-channel-1', {
-  slackChannels: ['#ops'],
-  ...sendDefaults,
-})
+// A Slack channel needs the Checkly Slack app connected to your account first
+// (https://www.checklyhq.com/docs/integrations/alerts/slack/). Once it is, import
+// SlackAppAlertChannel from 'checkly/constructs' and uncomment:
+// const slackChannel = new SlackAppAlertChannel('slack-channel-1', {
+//   slackChannels: ['#ops'],
+//   ...sendDefaults,
+// })
 
 const webhookChannel = new WebhookAlertChannel('webhook-channel-1', {
   name: 'Pushover webhook',
@@ -49,6 +51,5 @@ const webhookChannel = new WebhookAlertChannel('webhook-channel-1', {
 
 module.exports = {
   emailChannel,
-  slackChannel,
   webhookChannel,
 }
diff --git a/examples/advanced-project/src/__checks__/utils/alert-channels.ts b/examples/advanced-project/src/__checks__/utils/alert-channels.ts
index 730f41733..d33cd06a5 100644
--- a/examples/advanced-project/src/__checks__/utils/alert-channels.ts
+++ b/examples/advanced-project/src/__checks__/utils/alert-channels.ts
@@ -1,7 +1,6 @@
 import { URL } from 'node:url'
 import {
   EmailAlertChannel,
-  SlackAppAlertChannel,
   WebhookAlertChannel
 } from 'checkly/constructs'
 
@@ -25,10 +24,13 @@ export const emailChannel = new EmailAlertChannel('email-channel-1', {
   ...sendDefaults
 })
 
-export const slackChannel = new SlackAppAlertChannel('slack-channel-1', {
-  slackChannels: ['#ops'],
-  ...sendDefaults
-})
+// A Slack channel needs the Checkly Slack app connected to your account first
+// (https://www.checklyhq.com/docs/integrations/alerts/slack/). Once it is, import
+// SlackAppAlertChannel from 'checkly/constructs' and uncomment:
+// export const slackChannel = new SlackAppAlertChannel('slack-channel-1', {
+//   slackChannels: ['#ops'],
+//   ...sendDefaults
+// })
 
 export const webhookChannel = new WebhookAlertChannel('webhook-channel-1', {
   name: 'Pushover webhook',
diff --git a/packages/cli/src/ai-context/references/configure-alert-channels.md b/packages/cli/src/ai-context/references/configure-alert-channels.md
index 287c0efb7..cc04c2c43 100644
--- a/packages/cli/src/ai-context/references/configure-alert-channels.md
+++ b/packages/cli/src/ai-context/references/configure-alert-channels.md
@@ -9,7 +9,7 @@ All available alerts are described in the [Checkly docs](https://www.checklyhq.c
 
 *Important*: Don't make up email addresses, phone numbers, Slack channel names or similar static values. Scan the project to discover a valid configuration or ask what the values should be.
 
-For new Slack notifications, prefer `SlackAppAlertChannel`. It uses the Checkly Slack App and only needs Slack `#channel` names or `@user` handles in `slackChannels`.
+For new Slack notifications, prefer `SlackAppAlertChannel`. It uses the Checkly Slack App and only needs Slack `#channel` names or `@user` handles in `slackChannels`. It needs the Checkly Slack App connected to the Checkly account: without it, `npx checkly validate`, `test` and `deploy` fail with "Checkly Slack app not connected". If you can't confirm the app is connected, ask before adding one.
 
 ## Email Alert Channel
 
diff --git a/packages/cli/src/commands/authCommand.ts b/packages/cli/src/commands/authCommand.ts
index 123c7fa06..434377e88 100644
--- a/packages/cli/src/commands/authCommand.ts
+++ b/packages/cli/src/commands/authCommand.ts
@@ -111,7 +111,7 @@ export abstract class AuthCommand extends BaseCommand {
     config.pinEnvironment()
     // Constructs validate against account-specific limits and have no access to
     // the command instance.
-    Session.accountFeatures = this.#account?.features ?? []
+    Session.account = this.#account
   }
 
   protected async validateProject (
diff --git a/packages/cli/src/constructs/__tests__/account-features.spec.ts b/packages/cli/src/constructs/__tests__/account-features.spec.ts
index 5aa6375f5..e9288fc1f 100644
--- a/packages/cli/src/constructs/__tests__/account-features.spec.ts
+++ b/packages/cli/src/constructs/__tests__/account-features.spec.ts
@@ -4,6 +4,7 @@ import { ApiCheck, TcpMonitor, TcpRequest, UrlMonitor } from '../index.js'
 import { Diagnostics } from '../diagnostics.js'
 import { Project } from '../project.js'
 import { Session } from '../session.js'
+import type { Account } from '../../rest/accounts.js'
 
 const EXTENDED_RESPONSE_TIME_LIMITS = 'EXTENDED_RESPONSE_TIME_LIMITS'
 
@@ -19,6 +20,13 @@ const newProject = () => {
   })
 }
 
+const withFeatures = (features: string[]): Account => ({
+  id: 'acc-1',
+  name: 'Test Account',
+  runtimeId: '2025.04',
+  features,
+})
+
 const validate = async (construct: { validate: (diagnostics: Diagnostics) => Promise<void> }) => {
   const diagnostics = new Diagnostics()
   await construct.validate(diagnostics)
@@ -27,12 +35,12 @@ const validate = async (construct: { validate: (diagnostics: Diagnostics) => Pro
 
 describe('account-aware response time limits', () => {
   afterEach(() => {
-    Session.accountFeatures = []
+    Session.account = undefined
   })
 
   it('rejects an extended response time without the feature', async () => {
     newProject()
-    Session.accountFeatures = []
+    Session.account = withFeatures([])
 
     const diagnostics = await validate(new TcpMonitor('tcp-standard', {
       name: 'Test Check',
@@ -50,7 +58,7 @@ describe('account-aware response time limits', () => {
 
   it('accepts an extended response time with the feature', async () => {
     newProject()
-    Session.accountFeatures = [EXTENDED_RESPONSE_TIME_LIMITS]
+    Session.account = withFeatures([EXTENDED_RESPONSE_TIME_LIMITS])
 
     const diagnostics = await validate(new TcpMonitor('tcp-extended', {
       name: 'Test Check',
@@ -64,7 +72,7 @@ describe('account-aware response time limits', () => {
 
   it('defers values above 45s to the API with the feature', async () => {
     newProject()
-    Session.accountFeatures = [EXTENDED_RESPONSE_TIME_LIMITS]
+    Session.account = withFeatures([EXTENDED_RESPONSE_TIME_LIMITS])
 
     const diagnostics = await validate(new TcpMonitor('tcp-over-extended', {
       name: 'Test Check',
@@ -77,7 +85,7 @@ describe('account-aware response time limits', () => {
 
   it('keeps the degraded <= max cross-check with the feature', async () => {
     newProject()
-    Session.accountFeatures = [EXTENDED_RESPONSE_TIME_LIMITS]
+    Session.account = withFeatures([EXTENDED_RESPONSE_TIME_LIMITS])
 
     const diagnostics = await validate(new TcpMonitor('tcp-degraded-above-max', {
       name: 'Test Check',
@@ -96,7 +104,7 @@ describe('account-aware response time limits', () => {
 
   it('raises the API check limit above its 30s standard', async () => {
     newProject()
-    Session.accountFeatures = [EXTENDED_RESPONSE_TIME_LIMITS]
+    Session.account = withFeatures([EXTENDED_RESPONSE_TIME_LIMITS])
 
     const diagnostics = await validate(new ApiCheck('api-extended', {
       name: 'Test Check',
@@ -109,7 +117,7 @@ describe('account-aware response time limits', () => {
 
   it('raises the URL monitor limit above its 30s standard', async () => {
     newProject()
-    Session.accountFeatures = [EXTENDED_RESPONSE_TIME_LIMITS]
+    Session.account = withFeatures([EXTENDED_RESPONSE_TIME_LIMITS])
 
     const diagnostics = await validate(new UrlMonitor('url-extended', {
       name: 'Test Check',
diff --git a/packages/cli/src/constructs/__tests__/slack-app-alert-channel.spec.ts b/packages/cli/src/constructs/__tests__/slack-app-alert-channel.spec.ts
new file mode 100644
index 000000000..e86fa714b
--- /dev/null
+++ b/packages/cli/src/constructs/__tests__/slack-app-alert-channel.spec.ts
@@ -0,0 +1,84 @@
+import { describe, it, expect, afterEach, beforeEach } from 'vitest'
+
+import { SlackAppAlertChannel } from '../index.js'
+import { Diagnostics } from '../diagnostics.js'
+import { Project } from '../project.js'
+import { Session } from '../session.js'
+import type { Account, AccountIntegration } from '../../rest/accounts.js'
+
+const withIntegrations = (integrations?: AccountIntegration[]): Account => ({
+  id: 'acc-1',
+  name: 'Test Account',
+  runtimeId: '2025.04',
+  integrations,
+})
+
+const validate = async (slackChannels: string[]) => {
+  const diagnostics = new Diagnostics()
+  await new SlackAppAlertChannel('ops-slack', { slackChannels }).validate(diagnostics)
+  return diagnostics
+}
+
+describe('SlackAppAlertChannel', () => {
+  beforeEach(() => {
+    Session.project = new Project('project-id', {
+      name: 'Test Project',
+      repoUrl: 'https://github.com/checkly/checkly-cli',
+    })
+  })
+
+  afterEach(() => {
+    Session.account = undefined
+  })
+
+  it('refuses the channel when the account has no Checkly Slack app', async () => {
+    Session.account = withIntegrations([{ type: 'SLACK', connected: false }])
+
+    const diagnostics = await validate(['#ops'])
+
+    expect(diagnostics.isFatal()).toBe(true)
+    expect(diagnostics.observations).toEqual([
+      expect.objectContaining({
+        title: 'Checkly Slack app not connected',
+        message: expect.stringContaining(
+          `SlackAppAlertChannel 'ops-slack' needs the Checkly Slack app, which is not connected to this account, so its alerts cannot be delivered.`,
+        ),
+      }),
+    ])
+    expect(diagnostics.observations[0].message).toContain(
+      'https://app.checklyhq.com/accounts/acc-1/settings/account/integrations',
+    )
+  })
+
+  it('accepts the channel when the Checkly Slack app is connected', async () => {
+    Session.account = withIntegrations([{ type: 'SLACK', connected: true }])
+
+    const diagnostics = await validate(['#ops'])
+
+    expect(diagnostics.isFatal()).toBe(false)
+  })
+
+  it('skips the check when the API does not report integrations', async () => {
+    Session.account = withIntegrations(undefined)
+
+    const diagnostics = await validate(['#ops'])
+
+    expect(diagnostics.isFatal()).toBe(false)
+  })
+
+  it('skips the check when the API does not report the Slack app', async () => {
+    Session.account = withIntegrations([{ type: 'OTHER', connected: false }])
+
+    const diagnostics = await validate(['#ops'])
+
+    expect(diagnostics.isFatal()).toBe(false)
+  })
+
+  it('accepts a channel without Slack destinations, which needs no Slack app', async () => {
+    Session.account = withIntegrations([{ type: 'SLACK', connected: false }])
+
+    const diagnostics = await validate([])
+
+    expect(diagnostics.isFatal()).toBe(false)
+  })
+})
diff --git a/packages/cli/src/constructs/internal/account-features.ts b/packages/cli/src/constructs/internal/account-features.ts
index 03961b671..7f2dd9a4b 100644
--- a/packages/cli/src/constructs/internal/account-features.ts
+++ b/packages/cli/src/constructs/internal/account-features.ts
@@ -16,7 +16,7 @@ const EXTENDED_RESPONSE_TIME_LIMITS = 'EXTENDED_RESPONSE_TIME_LIMITS'
  * the API does not report features at all (older or self-hosted deployments).
  */
 export function responseTimeLimits (standard: number): ResponseTimeLimits {
-  const limit = Session.accountFeatures.includes(EXTENDED_RESPONSE_TIME_LIMITS)
+  const limit = Session.account?.features?.includes(EXTENDED_RESPONSE_TIME_LIMITS)
     ? Number.MAX_SAFE_INTEGER
     : standard
 
diff --git a/packages/cli/src/constructs/session.ts b/packages/cli/src/constructs/session.ts
index a918d9d65..764719c27 100644
--- a/packages/cli/src/constructs/session.ts
+++ b/packages/cli/src/constructs/session.ts
@@ -6,6 +6,7 @@ import { type EngineDetectionResult } from '../services/engine-detector.js'
 import { Parser } from '../services/check-parser/parser.js'
 import { Construct } from './construct.js'
 import { PrivateLocationApi } from '../rest/private-locations.js'
+import type { Account } from '../rest/accounts.js'
 import {
   FileLoader,
   JitiFileLoader,
@@ -89,7 +90,11 @@ export class Session {
   static verifyRuntimeDependencies = true
   static loadingChecklyConfigFile: boolean
   static checklyConfigFileConstructs?: Construct[]
-  static accountFeatures: string[] = []
+  /**
+   * The authenticated account, for constructs that validate against it.
+   * Undefined when the command does not authenticate.
+   */
+  static account?: Account
   static privateLocations: PrivateLocationApi[]
   static parsers = new Map<string, Parser>()
   static playwrightProjectBundler?: PlaywrightProjectBundler
@@ -123,7 +128,7 @@ export class Session {
     this.verifyRuntimeDependencies = true
     this.loadingChecklyConfigFile = false
     this.checklyConfigFileConstructs = undefined
-    this.accountFeatures = []
+    this.account = undefined
     this.privateLocations = []
     this.parsers = new Map<string, Parser>()
     this.playwrightProjectBundler = undefined
diff --git a/packages/cli/src/constructs/slack-app-alert-channel.ts b/packages/cli/src/constructs/slack-app-alert-channel.ts
index bcd9240d9..60451b5fa 100644
--- a/packages/cli/src/constructs/slack-app-alert-channel.ts
+++ b/packages/cli/src/constructs/slack-app-alert-channel.ts
@@ -1,6 +1,9 @@
 import { AlertChannel, AlertChannelProps } from './alert-channel.js'
+import { Diagnostics, ErrorDiagnostic } from './diagnostics.js'
 import { Session } from './session.js'
 
+const SLACK_INTEGRATION = 'SLACK'
+
 export interface SlackAppAlertChannelProps extends AlertChannelProps {
   /**
    * List of Slack #channels or @handles to notify.
@@ -38,6 +41,33 @@ export class SlackAppAlertChannel extends AlertChannel {
     return `SlackAppAlertChannel:${this.logicalId}`
   }
 
+  /**
+   * Refuses the channel when the account has no Checkly Slack app: its alerts
+   * cannot be delivered, whether the channel is new or was deployed before the
+   * app was disconnected. Skipped when the API does not report the Slack app.
+   */
+  async validate (diagnostics: Diagnostics): Promise<void> {
+    await super.validate(diagnostics)
+
+    const slack = Session.account?.integrations?.find(({ type }) => type === SLACK_INTEGRATION)
+    if (slack === undefined || slack.connected || !this.slackChannels?.length) {
+      return
+    }
+    const error = new Error(
+      `SlackAppAlertChannel '${this.logicalId}' needs the Checkly Slack app, which is not connected to this account,`
+      + ` so its alerts cannot be delivered.`,
+    )
+    diagnostics.add(new ErrorDiagnostic({
+      title: 'Checkly Slack app not connected',
+      message:
+        `${error.message}\n\n`
+        + `Hint: Connect the Checkly Slack app at`
+        + ` https://app.checklyhq.com/accounts/${Session.account?.id}/settings/account/integrations,`
+        + ` or remove the channel.`,
+      error,
+    }))
+  }
+
   synthesize () {
     return {
       ...super.synthesize(),
diff --git a/packages/cli/src/rest/accounts.ts b/packages/cli/src/rest/accounts.ts
index 13326d1ef..c3a180469 100644
--- a/packages/cli/src/rest/accounts.ts
+++ b/packages/cli/src/rest/accounts.ts
@@ -17,6 +17,17 @@ export interface Account {
    * in which case standard limits apply.
    */
   features?: string[]
+  /**
+   * Every integration type the API reports, each marked connected or not, e.g.
+   * `SLACK` for the Checkly Slack app. A type missing from the list is one the
+   * API does not report. Absent when the API predates it.
+   */
+  integrations?: AccountIntegration[]
+}
+
+export interface AccountIntegration {
+  type: string
+  connected: boolean
 }
 
 class Accounts {