diff --git a/apps/api/src/handlers/mcp/__tests__/slack-thread-reply-quotes.test.ts b/apps/api/src/handlers/mcp/__tests__/slack-thread-reply-quotes.test.ts
index cb9d378e3..78505ed91 100644
--- a/apps/api/src/handlers/mcp/__tests__/slack-thread-reply-quotes.test.ts
+++ b/apps/api/src/handlers/mcp/__tests__/slack-thread-reply-quotes.test.ts
@@ -11,6 +11,7 @@ const {
   getTaskChannelBindingsMock,
   maybeSendCommunicationThreadReplyMock,
   postMessageMock,
+  postTopLevelMessageMock,
   slackInstallationFindFirstMock,
   taskRunFindFirstMock,
 } = vi.hoisted(() => ({
@@ -21,6 +22,7 @@ const {
   getTaskChannelBindingsMock: vi.fn(),
   maybeSendCommunicationThreadReplyMock: vi.fn(),
   postMessageMock: vi.fn(),
+  postTopLevelMessageMock: vi.fn(),
   slackInstallationFindFirstMock: vi.fn(),
   taskRunFindFirstMock: vi.fn(),
 }));
@@ -74,6 +76,7 @@ vi.mock('@roomote/slack', () => ({
   SlackNotifier: vi.fn(
     class {
       postMessage = postMessageMock;
+      postTopLevelMessage = postTopLevelMessageMock;
     },
   ),
   trackLatestUserMessageForSlackQuote: vi.fn(),
@@ -178,6 +181,7 @@ describe('Slack thread reply quotes', () => {
       },
     ]);
     postMessageMock.mockResolvedValue('333.444');
+    postTopLevelMessageMock.mockResolvedValue({ messageTs: '333.444' });
     clearLatestUserMessageForReplyQuoteIfIdMock.mockResolvedValue(true);
   });
 
@@ -213,6 +217,32 @@ describe('Slack thread reply quotes', () => {
     );
   });
 
+  it('preserves Slack API errors when a late-bound automation root cannot be posted', async () => {
+    taskRunFindFirstMock.mockResolvedValue({
+      id: 42,
+      actingUserId: null,
+      taskId: 'task-1',
+      payload: { channel: 'C123', customAutomationId: 'automation-1' },
+    });
+    getTaskChannelBindingsMock.mockResolvedValue({
+      slackChannelId: 'C123',
+      slackThreadTs: null,
+    });
+    buildThreadReplyImageBlocksMock.mockResolvedValue([]);
+    postTopLevelMessageMock.mockResolvedValue({ error: 'not_in_channel' });
+
+    const response = await createApp().request('/mcp/thread_reply', {
+      method: 'POST',
+      headers: { 'content-type': 'application/json' },
+      body: JSON.stringify({ text: 'done' }),
+    });
+
+    expect(response.status).toBe(502);
+    await expect(response.json()).resolves.toEqual({
+      error: 'Slack chat.postMessage failed: not_in_channel',
+    });
+  });
+
   it('consumes the exact pending quote after an image-only reply without rendering it', async () => {
     const response = await createApp().request('/mcp/thread_reply', {
       method: 'POST',
diff --git a/apps/api/src/handlers/mcp/slack.ts b/apps/api/src/handlers/mcp/slack.ts
index ac8a52421..88339302b 100644
--- a/apps/api/src/handlers/mcp/slack.ts
+++ b/apps/api/src/handlers/mcp/slack.ts
@@ -971,16 +971,19 @@ slackMcp.post('/thread_reply', async (c) => {
     blocks.push(...imageBlocks);
     blocks.push(...rootFooterBlocks);
 
-    const rootMessageTs = await slack.postMessage({
+    const rootPost = await slack.postTopLevelMessage({
       channel: slackReplyTarget.channel,
       text: getSlackFallbackText(fallbackText, imageBlocks.length),
       unfurl_links: false,
       unfurl_media: false,
       blocks,
     });
+    const rootMessageTs = rootPost.messageTs;
 
     if (!rootMessageTs) {
-      throw new Error('Slack chat.postMessage returned no message timestamp');
+      throw new Error(
+        `Slack chat.postMessage failed${rootPost.error ? `: ${rootPost.error}` : ' without a message timestamp'}`,
+      );
     }
 
     // The root message is already visible in Slack; failing the reply here
@@ -1343,11 +1346,8 @@ slackMcp.post('/thread_reply', async (c) => {
       );
     }
 
-    if (message === 'Slack chat.postMessage returned no message timestamp') {
-      return c.json(
-        { error: 'Slack chat.postMessage returned no message timestamp' },
-        502,
-      );
+    if (message.startsWith('Slack chat.postMessage failed')) {
+      return c.json({ error: message }, 502);
     }
 
     if (message === 'Slack thread source message no longer exists') {
diff --git a/packages/sdk/src/server/automations/__tests__/custom-automations.test.ts b/packages/sdk/src/server/automations/__tests__/custom-automations.test.ts
index a0a3349f2..9a918a9c7 100644
--- a/packages/sdk/src/server/automations/__tests__/custom-automations.test.ts
+++ b/packages/sdk/src/server/automations/__tests__/custom-automations.test.ts
@@ -1,5 +1,9 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 
+const { isAppInChannelMock } = vi.hoisted(() => ({
+  isAppInChannelMock: vi.fn(async () => true),
+}));
+
 vi.mock('@roomote/cloud-agents/server', () => ({
   enqueueTask: vi.fn(),
 }));
@@ -33,6 +37,12 @@ vi.mock('../destination', () => ({
   listConnectedCommunicationProviders: vi.fn(async () => ['slack', 'teams']),
 }));
 
+vi.mock('../../lib/communication-providers', () => ({
+  getCommunicationProviderAdapter: vi.fn(async () => ({
+    isAppInChannel: isAppInChannelMock,
+  })),
+}));
+
 vi.mock('../scheduling-utils', () => ({
   DAILY_WEEKLY_SCHEDULE_HOUR_LOCAL: 3,
   isRunDue: vi.fn(),
@@ -112,6 +122,7 @@ describe('customAutomationsJob', () => {
     vi.mocked(enqueueTask).mockResolvedValue({
       taskId: 'task_abc',
     } as never);
+    isAppInChannelMock.mockResolvedValue(true);
   });
 
   it('launches a StandardTask for due automations', async () => {
@@ -221,6 +232,25 @@ describe('customAutomationsJob', () => {
     );
   });
 
+  it('fails before launch when the Slack app cannot access the report channel', async () => {
+    isAppInChannelMock.mockResolvedValue(false);
+
+    const result = await customAutomationsJob();
+
+    expect(result.launchedTaskId).toBeNull();
+    expect(enqueueTask).not.toHaveBeenCalled();
+    expect(recordCustomAutomationRunOutcome).toHaveBeenCalledWith(
+      db,
+      expect.objectContaining({
+        id: automation.id,
+        status: 'failed',
+        error: expect.stringContaining(
+          'Slack app cannot access report channel C123',
+        ),
+      }),
+    );
+  });
+
   it('launches without channel anchoring when no report channel is configured', async () => {
     vi.mocked(listEnabledCustomAutomations).mockResolvedValue([
       { ...automation, target: {} } as never,
@@ -339,6 +369,7 @@ describe('customAutomationsJob', () => {
 describe('runCustomAutomationNow', () => {
   beforeEach(() => {
     vi.clearAllMocks();
+    isAppInChannelMock.mockResolvedValue(true);
     vi.mocked(getCustomAutomationById).mockResolvedValue(automation as never);
     vi.mocked(getCustomAutomationFrequency).mockReturnValue('daily');
     vi.mocked(tryClaimCustomAutomationLaunch).mockResolvedValue(new Date());
diff --git a/packages/sdk/src/server/automations/custom-automations.ts b/packages/sdk/src/server/automations/custom-automations.ts
index 51bd30882..78ed91a09 100644
--- a/packages/sdk/src/server/automations/custom-automations.ts
+++ b/packages/sdk/src/server/automations/custom-automations.ts
@@ -26,6 +26,7 @@ import {
   listConnectedCommunicationProviders,
   type ResolvedAutomationDestination,
 } from './destination';
+import { getCommunicationProviderAdapter } from '../lib/communication-providers';
 import {
   isCronRunDue,
   resolveDeploymentTimeZone,
@@ -246,6 +247,28 @@ async function launchCustomAutomationRow(
       });
       return result;
     }
+
+    if (destination.provider === 'slack') {
+      const slack = await getCommunicationProviderAdapter('slack');
+      const isMember =
+        slack?.provider === 'slack'
+          ? await slack.isAppInChannel(destination.channelId)
+          : null;
+      if (isMember !== true) {
+        const message =
+          isMember === false
+            ? `Slack app cannot access report channel ${destination.channelId}. Invite the app to the channel or choose another destination.`
+            : `Slack report channel ${destination.channelId} could not be verified. Try again before running this automation.`;
+        result.skippedReason = message;
+        result.errors.push(message);
+        await recordCustomAutomationRunOutcome(db, {
+          id: automation.id,
+          status: 'failed',
+          error: message,
+        });
+        return result;
+      }
+    }
   }
 
   // The short claim fence prevents concurrent launchers from double-launching
diff --git a/packages/slack/src/__tests__/slack-notifier.test.ts b/packages/slack/src/__tests__/slack-notifier.test.ts
index 780b0d3cf..39e0eb36b 100644
--- a/packages/slack/src/__tests__/slack-notifier.test.ts
+++ b/packages/slack/src/__tests__/slack-notifier.test.ts
@@ -131,6 +131,20 @@ describe('SlackNotifier', () => {
       expect(ts).toBeUndefined();
     });
 
+    it('preserves Slack API errors for top-level message callers', async () => {
+      getGlobalWithFetch().fetch = vi.fn().mockResolvedValue({
+        ok: true,
+        json: async () => ({ ok: false, error: 'not_in_channel' }),
+      });
+
+      const result = await notifier.postTopLevelMessage({
+        channel: 'C123',
+        text: 'failure case',
+      });
+
+      expect(result).toEqual({ error: 'not_in_channel' });
+    });
+
     it('does not change unfurl behavior for plain text messages', async () => {
       getGlobalWithFetch().fetch = vi.fn().mockResolvedValue({
         ok: true,
diff --git a/packages/slack/src/slack-notifier.ts b/packages/slack/src/slack-notifier.ts
index 3771e43ba..b99fbb2ec 100644
--- a/packages/slack/src/slack-notifier.ts
+++ b/packages/slack/src/slack-notifier.ts
@@ -886,6 +886,26 @@ export class SlackNotifier {
     return response?.ts;
   }
 
+  /**
+   * Posts a top-level message while preserving Slack's API error code for
+   * callers that need to report why a new conversation could not be created.
+   */
+  public async postTopLevelMessage(message: SlackMessage): Promise<{
+    messageTs?: string;
+    error?: string;
+  }> {
+    const response = await this.sendMessage(
+      'chat.postMessage',
+      message,
+      'regular',
+    );
+
+    return {
+      ...(response?.ts ? { messageTs: response.ts } : {}),
+      ...(response?.error ? { error: response.error } : {}),
+    };
+  }
+
   public async postEphemeralMessage(message: SlackMessage & { user: string }) {
     const response = await this.sendMessage(
       'chat.postEphemeral',