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..4a93e4b2f 100644
--- a/packages/cli/src/commands/authCommand.ts
+++ b/packages/cli/src/commands/authCommand.ts
@@ -112,6 +112,8 @@ export abstract class AuthCommand extends BaseCommand {
     // Constructs validate against account-specific limits and have no access to
     // the command instance.
     Session.accountFeatures = this.#account?.features ?? []
+    Session.accountId = this.#account?.id
+    Session.accountConnectedIntegrations = this.#account?.connectedIntegrations
   }
 
   protected async validateProject (
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..ae59d5857
--- /dev/null
+++ b/packages/cli/src/constructs/__tests__/slack-app-alert-channel.spec.ts
@@ -0,0 +1,70 @@
+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'
+
+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',
+    })
+    Session.accountId = 'acc-1'
+  })
+
+  afterEach(() => {
+    Session.accountId = undefined
+    Session.accountConnectedIntegrations = undefined
+  })
+
+  it('refuses the channel when the account has no Checkly Slack app', async () => {
+    Session.accountConnectedIntegrations = []
+
+    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.accountConnectedIntegrations = ['SLACK']
+
+    const diagnostics = await validate(['#ops'])
+
+    expect(diagnostics.isFatal()).toBe(false)
+  })
+
+  it('skips the check when the API does not report connected integrations', async () => {
+    Session.accountConnectedIntegrations = undefined
+
+    const diagnostics = await validate(['#ops'])
+
+    expect(diagnostics.isFatal()).toBe(false)
+  })
+
+  it('accepts a channel without Slack destinations, which needs no Slack app', async () => {
+    Session.accountConnectedIntegrations = []
+
+    const diagnostics = await validate([])
+
+    expect(diagnostics.isFatal()).toBe(false)
+  })
+})
diff --git a/packages/cli/src/constructs/session.ts b/packages/cli/src/constructs/session.ts
index a918d9d65..113152e02 100644
--- a/packages/cli/src/constructs/session.ts
+++ b/packages/cli/src/constructs/session.ts
@@ -90,6 +90,12 @@ export class Session {
   static loadingChecklyConfigFile: boolean
   static checklyConfigFileConstructs?: Construct[]
   static accountFeatures: string[] = []
+  static accountId?: string
+  /**
+   * The integrations the authenticated account has connected, or undefined
+   * when unknown (not authenticated, or an API that does not report them).
+   */
+  static accountConnectedIntegrations?: string[]
   static privateLocations: PrivateLocationApi[]
   static parsers = new Map<string, Parser>()
   static playwrightProjectBundler?: PlaywrightProjectBundler
@@ -124,6 +130,8 @@ export class Session {
     this.loadingChecklyConfigFile = false
     this.checklyConfigFileConstructs = undefined
     this.accountFeatures = []
+    this.accountId = undefined
+    this.accountConnectedIntegrations = 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..dd10554c0 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 account's integrations are unknown.
+   */
+  async validate (diagnostics: Diagnostics): Promise<void> {
+    await super.validate(diagnostics)
+
+    const connected = Session.accountConnectedIntegrations
+    if (connected === undefined || connected.includes(SLACK_INTEGRATION) || !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.accountId}/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..3e2431894 100644
--- a/packages/cli/src/rest/accounts.ts
+++ b/packages/cli/src/rest/accounts.ts
@@ -17,6 +17,11 @@ export interface Account {
    * in which case standard limits apply.
    */
   features?: string[]
+  /**
+   * The integrations the account has connected, e.g. `SLACK` for the Checkly
+   * Slack app. Absent when the API predates it.
+   */
+  connectedIntegrations?: string[]
 }
 
 class Accounts {