refactor(ui): model Slack channel and test-message state as unions

- Collapse the channel-list fetch state into one discriminated union
- Collapse the test-message flag and outcome into one status union
- Group the saved-channel mirror and sync marker into channel refs

ui/__tests__/msw/handlers/slack.fixtures.ts

@@ -48,6 +48,36 @@ export interface SlackConnectionFixture {
   error: string | null;
 }
 
+/** A channel the listing endpoint offers for the picker. */
+export interface SlackChannelFixture {
+  id: string;
+  name: string;
+  /** Private channels are listed only where `@Prowler` has been invited. */
+  isPrivate: boolean;
+}
+
+export interface SlackTestMessageFixture {
+  accepted: boolean;
+  /**
+   * Why it did not: the reason `code` would carry, or prose — the contract
+   * leaves the task result's shape open.
+   */
+  error: string | null;
+}
+
+/**
+ * A refusal as the API sends one: the machine-readable reason in `code`, human
+ * copy in `detail`, and — for a `429` — the wait in `Retry-After`.
+ */
+export interface SlackRefusalFixture {
+  status: number;
+  /** Slack's stable reason. `null` for the failures classified by status. */
+  code: string | null;
+  detail: string;
+  /** Seconds `Retry-After` asked for; only a `429` carries one. */
+  retryAfterSeconds: number | null;
+}
+
 export interface SlackFixture {
   /**
    * The deployment has `SLACK_CLIENT_ID` / `SLACK_CLIENT_SECRET` /
@@ -72,9 +102,32 @@ export interface SlackFixture {
    * transport failures. Distinct from `appConfigured: false`, which is a `503`.
    */
   oauthUpstreamError: boolean;
+  channels: SlackChannelFixture[];
+  /**
+   * Small on purpose: the default workspace spans two pages, so a UI that
+   * stopped at `data` instead of following `links.next` would lose channels.
+   */
+  channelsPageSize: number;
+  /** Slack refused the listing outright, with the reason named in `code`. */
+  channelsRefusal: SlackRefusalFixture | null;
+  /**
+   * The cursor the refusal starts at. Absent, the whole read fails; a page
+   * size serves the first page and refuses the second — the partial read.
+   */
+  channelsRefusalFromCursor?: number;
+  /**
+   * Slack refused the chosen channel when the `PATCH` validated it — the
+   * listing itself answered fine.
+   */
+  channelSaveRefusal: SlackRefusalFixture | null;
+  testMessage: SlackTestMessageFixture;
 }
 
-export const SLACK_INTEGRATION_ID = "slack-integration-1";
+/**
+ * A UUID, as the API's ids are: it travels in the URL of every Slack call and
+ * the actions accept no other shape.
+ */
+export const SLACK_INTEGRATION_ID = "7c9e6a1b-2d3f-4e5a-8b6c-9d0e1f2a3b4c";
 
 /** The scopes the channel picker and the posting need (design D2). */
 export const SLACK_BOT_SCOPES = [
@@ -128,6 +181,19 @@ export const SLACK_RATE_LIMITED_DETAIL =
  * is for the user to act on, so the UI answers a server error in its own words.
  */
 export const INTEGRATIONS_SERVER_ERROR_DETAIL = "A server error occurred.";
+export const SLACK_MISSING_SCOPE_DETAIL =
+  "Slack refused the request: missing_scope.";
+/**
+ * The same sentence for "it is gone" and "the app was removed from it": only
+ * `code` separates them, which is why a client must read `code`.
+ */
+export const SLACK_UNKNOWN_CHANNEL_DETAIL =
+  "That channel is not one Prowler can post to.";
+export const SLACK_NO_DEFAULT_CHANNEL_DETAIL =
+  "No default channel is recorded on this integration.";
+/** A task result that reports the refusal as prose instead of as a reason. */
+export const SLACK_TEST_MESSAGE_REFUSED_DETAIL =
+  "Slack rejected the message: the channel is archived.";
 
 /**
  * A `200` challenge page from a proxy or WAF that took the call instead of the
@@ -141,17 +207,104 @@ export const PROXY_CHALLENGE_PAGE = [
 ].join("\n");
 
 /**
- * A wire value, spelled out rather than imported from the UI's own mapping, so
- * a rename on our side fails these tests instead of agreeing with itself.
+ * The `code` values the refusals below are named by. Wire values, spelled out
+ * rather than imported from the UI's own mapping: a rename on our side must
+ * fail these tests, not quietly agree with itself.
  */
 export const SLACK_WORKSPACE_CONFLICT_CODE = "slack_workspace_conflict";
+export const SLACK_MISSING_SCOPE_CODE = "missing_scope";
+export const SLACK_CHANNEL_NOT_FOUND_CODE = "channel_not_found";
+export const SLACK_NOT_IN_CHANNEL_CODE = "not_in_channel";
+/**
+ * A reason Slack really sends that the UI's mapping does not cover — the set is
+ * open-ended, so having no copy for one is the ordinary case.
+ */
+export const SLACK_UNMAPPED_REASON_CODE = "is_archived";
 
 export const SLACK_RETRY_AFTER_SECONDS = 30;
 
-export const SLACK_DEFAULT_CHANNEL = {
+/** The install never granted a scope the call needs: actionable, so a `400`. */
+export const SLACK_MISSING_SCOPE_REFUSAL: SlackRefusalFixture = {
+  status: 400,
+  code: SLACK_MISSING_SCOPE_CODE,
+  detail: SLACK_MISSING_SCOPE_DETAIL,
+  retryAfterSeconds: null,
+};
+
+/**
+ * Where this really happens is the channel listing: `conversations.list` is
+ * tier 2 and paginated.
+ */
+export const SLACK_RATE_LIMITED_REFUSAL: SlackRefusalFixture = {
+  status: 429,
+  code: null,
+  detail: SLACK_RATE_LIMITED_DETAIL,
+  retryAfterSeconds: SLACK_RETRY_AFTER_SECONDS,
+};
+
+/** Slack-side or transport failure — a `502` naming no reason at all. */
+export const SLACK_UPSTREAM_REFUSAL: SlackRefusalFixture = {
+  status: 502,
+  code: null,
+  detail: SLACK_UPSTREAM_DETAIL,
+  retryAfterSeconds: null,
+};
+
+/** The chosen channel is archived, deleted, or was never in the workspace. */
+export const SLACK_CHANNEL_NOT_FOUND_REFUSAL: SlackRefusalFixture = {
+  status: 400,
+  code: SLACK_CHANNEL_NOT_FOUND_CODE,
+  detail: SLACK_UNKNOWN_CHANNEL_DETAIL,
+  retryAfterSeconds: null,
+};
+
+/**
+ * The channel is fine, the Prowler app is simply not in it — fixed with
+ * `/invite @Prowler`. Identical `detail` to the refusal above, deliberately.
+ */
+export const SLACK_NOT_IN_CHANNEL_REFUSAL: SlackRefusalFixture = {
+  status: 400,
+  code: SLACK_NOT_IN_CHANNEL_CODE,
+  detail: SLACK_UNKNOWN_CHANNEL_DETAIL,
+  retryAfterSeconds: null,
+};
+
+/**
+ * Two public channels and one private the Prowler app was invited to, ordered
+ * so the private one lands on the second cursor page.
+ */
+export const SLACK_PUBLIC_CHANNEL: SlackChannelFixture = {
   id: "C0123AB",
   name: "security",
-} as const;
+  isPrivate: false,
+};
+
+export const SLACK_SECOND_PUBLIC_CHANNEL: SlackChannelFixture = {
+  id: "C0789EF",
+  name: "platform",
+  isPrivate: false,
+};
+
+export const SLACK_PRIVATE_CHANNEL: SlackChannelFixture = {
+  id: "C0456CD",
+  name: "security-alerts",
+  isPrivate: true,
+};
+
+export const SLACK_CHANNELS: SlackChannelFixture[] = [
+  SLACK_PUBLIC_CHANNEL,
+  SLACK_SECOND_PUBLIC_CHANNEL,
+  SLACK_PRIVATE_CHANNEL,
+];
+
+/** Two channels per page, so `SLACK_CHANNELS` spans exactly two pages. */
+export const SLACK_CHANNELS_PAGE_SIZE = 2;
+
+/**
+ * The first channel the picker offers, so an install seeded with it always
+ * points at a channel the listing really has.
+ */
+export const SLACK_DEFAULT_CHANNEL = SLACK_PUBLIC_CHANNEL;
 
 const PROWLER_HQ: SlackWorkspaceFixture = {
   teamId: "T01PROWLER",
@@ -171,6 +324,11 @@ export const slackFixture = (
   listServerError: false,
   authorizeUrlUnreadable: false,
   oauthUpstreamError: false,
+  channels: SLACK_CHANNELS.map((channel) => ({ ...channel })),
+  channelsPageSize: SLACK_CHANNELS_PAGE_SIZE,
+  channelsRefusal: null,
+  channelSaveRefusal: null,
+  testMessage: { accepted: true, error: null },
   ...overrides,
 });
 
@@ -193,25 +351,28 @@ export const connectedSlackFixture = (
     ...overrides,
   });
 
-const configuredInstall = (): SlackInstallFixture => ({
+const configuredInstall = (
+  channel: SlackChannelFixture = SLACK_DEFAULT_CHANNEL,
+): SlackInstallFixture => ({
   id: SLACK_INTEGRATION_ID,
   connected: true,
   connectionLastCheckedAt: "2026-08-10T09:30:00Z",
   workspace: {
     ...PROWLER_HQ,
-    channelId: SLACK_DEFAULT_CHANNEL.id,
-    channelName: SLACK_DEFAULT_CHANNEL.name,
+    channelId: channel.id,
+    channelName: channel.name,
   },
 });
 
 /**
- * A workspace connected *and* a channel on record. Anything the API refuses
- * until a channel exists (the connection check) needs this fixture.
+ * The same tenant with a destination channel already on record: the state a
+ * second visit starts from.
  */
-export const configuredSlackFixture = (
+export const slackFixtureWithDefaultChannel = (
+  channel: SlackChannelFixture = SLACK_PUBLIC_CHANNEL,
   overrides: Partial<SlackFixture> = {},
 ): SlackFixture =>
-  connectedSlackFixture({ install: configuredInstall(), ...overrides });
+  connectedSlackFixture({ install: configuredInstall(channel), ...overrides });
 
 /**
  * The same finished setup, with a check time no parser can read: a zero date
@@ -225,3 +386,25 @@ export const unreadableCheckTimeSlackFixture = (): SlackFixture =>
       connectionLastCheckedAt: "0000-00-00T00:00:00Z",
     },
   });
+
+/**
+ * The first cursor page is served and Slack rate limits the second: what is
+ * already read stays usable, the refusal only says why the list is short.
+ */
+export const partiallyReadSlackFixture = (
+  overrides: Partial<SlackFixture> = {},
+): SlackFixture =>
+  slackFixtureWithDefaultChannel(SLACK_PUBLIC_CHANNEL, {
+    channelsRefusal: SLACK_RATE_LIMITED_REFUSAL,
+    channelsRefusalFromCursor: SLACK_CHANNELS_PAGE_SIZE,
+    ...overrides,
+  });
+
+/**
+ * A workspace connected *and* a channel on record. Anything the API refuses
+ * until a channel exists (the connection check) needs this fixture.
+ */
+export const configuredSlackFixture = (
+  overrides: Partial<SlackFixture> = {},
+): SlackFixture =>
+  slackFixtureWithDefaultChannel(SLACK_DEFAULT_CHANNEL, overrides);

ui/__tests__/msw/handlers/slack.ts

@@ -18,10 +18,11 @@ import {
   SLACK_INTEGRATION_ID,
   SLACK_INVALID_CODE_DETAIL,
   SLACK_NO_CHANNEL_DETAIL,
-  SLACK_RATE_LIMITED_DETAIL,
+  SLACK_NO_DEFAULT_CHANNEL_DETAIL,
+  SLACK_RATE_LIMITED_REFUSAL,
   SLACK_REFUSED_STATE_DETAIL,
-  SLACK_RETRY_AFTER_SECONDS,
   SLACK_UNCONFIGURED_DETAIL,
+  SLACK_UNKNOWN_CHANNEL_DETAIL,
   SLACK_UPSTREAM_DETAIL,
   SLACK_UPSTREAM_ERROR_CODE,
   SLACK_WORKSPACE_CONFLICT_CODE,
@@ -30,12 +31,17 @@ import type {
   SlackExchangeOutcome,
   SlackFixture,
   SlackInstallFixture,
+  SlackRefusalFixture,
 } from "./slack.fixtures";
 
 const API = process.env.UI_API_BASE_URL;
 const TS = "2026-08-10T09:00:00Z";
 
 const CONNECTION_TASK_PREFIX = "slack-conn-task-";
+const TEST_MESSAGE_TASK_PREFIX = "slack-test-message-task-";
+
+/** Opaque to the UI, which only ever follows `links.next` (design D6). */
+const CHANNEL_CURSOR_PARAM = "page[cursor]";
 
 /**
  * `status` is a string, per the JSON:API spec. `source.pointer` is `/data` even
@@ -52,6 +58,21 @@ const errorBody = (detail: string, status: number, code?: string) => ({
   ],
 });
 
+/**
+ * Answer a fixture's refusal as the API would: its own status, its `code`
+ * when it names one, and `Retry-After` only where the status carries a wait.
+ */
+const refuse = (refusal: SlackRefusalFixture) =>
+  HttpResponse.json(
+    errorBody(refusal.detail, refusal.status, refusal.code ?? undefined),
+    {
+      status: refusal.status,
+      ...(refusal.retryAfterSeconds === null
+        ? {}
+        : { headers: { "Retry-After": String(refusal.retryAfterSeconds) } }),
+    },
+  );
+
 const configuration = (workspace: SlackInstallFixture["workspace"]) => ({
   team_id: workspace.teamId,
   team_name: workspace.teamName,
@@ -119,11 +140,7 @@ export const handlersForSlack = (fx: SlackFixture) => {
       status: 503,
     });
 
-  const rateLimited = () =>
-    HttpResponse.json(errorBody(SLACK_RATE_LIMITED_DETAIL, 429), {
-      status: 429,
-      headers: { "Retry-After": String(SLACK_RETRY_AFTER_SECONDS) },
-    });
+  const rateLimited = () => refuse(SLACK_RATE_LIMITED_REFUSAL);
 
   /** A `502` per the contract's taxonomy: a server fault, not a Slack state. */
   const upstreamError = () =>
@@ -241,6 +258,16 @@ export const handlersForSlack = (fx: SlackFixture) => {
     ),
 
     http.get<{ taskId: string }>(`${API}/tasks/:taskId`, ({ params }) => {
+      // The test message settles as its own task (design D9).
+      if (params.taskId.startsWith(TEST_MESSAGE_TASK_PREFIX)) {
+        const { accepted, error } = fx.testMessage;
+        return HttpResponse.json(
+          taskResource(params.taskId, accepted ? "completed" : "failed", {
+            error,
+          }),
+        );
+      }
+
       const { connected, error } = fx.connection;
       if (install && params.taskId.startsWith(CONNECTION_TASK_PREFIX)) {
         install.connected = connected;
@@ -250,5 +277,94 @@ export const handlersForSlack = (fx: SlackFixture) => {
         taskResource(params.taskId, "completed", { connected, error }),
       );
     }),
+
+    // --- Channels ----------------------------------------------------------
+    http.get<{ id: string }>(
+      `${API}/integrations/:id/slack/channels`,
+      ({ params, request }) => {
+        // The UI follows `links.next` opaquely, so the cursor's shape is this
+        // fixture's business alone. Read first: the page decides the refusal.
+        const cursor = Number(
+          new URL(request.url).searchParams.get(CHANNEL_CURSOR_PARAM) ?? "0",
+        );
+
+        // An endpoint-specific refusal wins over the blanket rate limiting,
+        // and applies from the named cursor, so a partial read is expressible.
+        if (
+          fx.channelsRefusal &&
+          cursor >= (fx.channelsRefusalFromCursor ?? 0)
+        ) {
+          return refuse(fx.channelsRefusal);
+        }
+        if (fx.rateLimited) return rateLimited();
+
+        const nextCursor = cursor + fx.channelsPageSize;
+        const page = fx.channels.slice(cursor, nextCursor);
+        const hasMore = nextCursor < fx.channels.length;
+
+        return HttpResponse.json({
+          data: page.map((channel) => ({
+            type: "slack-channels",
+            id: channel.id,
+            attributes: { name: channel.name, is_private: channel.isPrivate },
+          })),
+          links: {
+            next: hasMore
+              ? `${API}/integrations/${params.id}/slack/channels` +
+                `?${CHANNEL_CURSOR_PARAM}=${nextCursor}`
+              : null,
+          },
+        });
+      },
+    ),
+
+    /**
+     * The generic PATCH. The UI submits only `channel_id`; the name is derived
+     * from it here, as the API derives it from Slack (design D6).
+     */
+    http.patch(`${API}/integrations/:id`, async ({ request }) => {
+      const body = (await request.json().catch(() => null)) as {
+        data?: { attributes?: { configuration?: { channel_id?: string } } };
+      } | null;
+      const channelId = body?.data?.attributes?.configuration?.channel_id;
+      const channel = fx.channels.find((c) => c.id === channelId);
+
+      if (!install) {
+        return HttpResponse.json(errorBody("Not found.", 404), { status: 404 });
+      }
+      // Checked before the id lookup: the picker did offer this channel, and
+      // Slack refused it anyway when the API validated it.
+      if (fx.channelSaveRefusal) return refuse(fx.channelSaveRefusal);
+      if (!channel) {
+        return HttpResponse.json(errorBody(SLACK_UNKNOWN_CHANNEL_DETAIL, 400), {
+          status: 400,
+        });
+      }
+
+      install.workspace.channelId = channel.id;
+      install.workspace.channelName = channel.name;
+      return HttpResponse.json({ data: integrationResource(install) });
+    }),
+
+    // --- Test message ------------------------------------------------------
+    http.post<{ id: string }>(
+      `${API}/integrations/:id/slack/test-message`,
+      ({ params }) => {
+        if (!install?.workspace.channelId) {
+          return HttpResponse.json(
+            errorBody(SLACK_NO_DEFAULT_CHANNEL_DETAIL, 400),
+            { status: 400 },
+          );
+        }
+        return HttpResponse.json(
+          taskResource(
+            `${TEST_MESSAGE_TASK_PREFIX}${params.id}`,
+            "available",
+            null,
+          ),
+          { status: 202 },
+        );
+      },
+    ),
   ];
 };

ui/actions/integrations/integrations.ts

@@ -388,6 +388,7 @@ export const pollConnectionTestStatus = async (
     revalidatePath("/integrations/amazon-s3");
     revalidatePath("/integrations/aws-security-hub");
     revalidatePath("/integrations/jira");
+    revalidatePath("/integrations/slack");
 
     if ("error" in pollResult) {
       return { success: false, error: pollResult.error };

ui/actions/integrations/slack.test.ts

@@ -1,11 +1,19 @@
 /**
- * Sentry reporting for the Slack OAuth actions. A capture leaves no mark on the
- * DOM, so it cannot be covered from `slack-page.integration.test.tsx`.
+ * What the Slack actions do off the DOM, which
+ * `slack-page.integration.test.tsx` cannot cover: which failures reach Sentry,
+ * and the URLs the channel listing's cursor pagination follows.
  */
 
+import { revalidatePath } from "next/cache";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
-import { SLACK_UNREADABLE_RESULT_MESSAGE } from "@/lib/integrations/slack-errors";
+import {
+  SLACK_ERROR_CODE,
+  SLACK_ERROR_MESSAGES,
+  SLACK_GENERIC_ERROR_MESSAGE,
+  SLACK_PARTIAL_CHANNEL_LIST_MESSAGE,
+  SLACK_UNREADABLE_RESULT_MESSAGE,
+} from "@/lib/integrations/slack-errors";
 import { SentryErrorSource, SentryErrorType } from "@/sentry";
 
 const { captureExceptionMock, captureMessageMock, fetchMock } = vi.hoisted(
@@ -30,6 +38,8 @@ const { captureExceptionMock, captureMessageMock, fetchMock } = vi.hoisted(
 vi.mock("@sentry/nextjs", () => ({
   captureException: captureExceptionMock,
   captureMessage: captureMessageMock,
+  // The task poll leaves breadcrumbs on every read it makes.
+  addBreadcrumb: vi.fn(),
 }));
 
 vi.mock("next/cache", () => ({
@@ -50,7 +60,13 @@ vi.mock("@/lib", () => ({
   parseStringify: (value: unknown) => value,
 }));
 
-import { exchangeSlackOAuthCode, getSlackAuthorizeUrl } from "./slack";
+import {
+  exchangeSlackOAuthCode,
+  getSlackAuthorizeUrl,
+  getSlackChannels,
+  sendSlackTestMessage,
+  setSlackDefaultChannel,
+} from "./slack";
 
 /** The status the contract reserves for an upstream Slack failure. */
 const UPSTREAM_STATUS = 502;
@@ -248,10 +264,19 @@ describe("exchangeSlackOAuthCode result shape", () => {
     ["an array", []],
     ["a bare string", "invalid"],
     ["a resource with no id", { type: "integrations", attributes: {} }],
+    ["a resource with an empty id", { ...INTEGRATION, id: "" }],
+    ["a resource of another type", { ...INTEGRATION, type: "tasks" }],
     [
       "a resource with no attributes",
       { id: INTEGRATION.id, type: "integrations" },
     ],
+    [
+      "another kind of integration",
+      {
+        ...INTEGRATION,
+        attributes: { ...INTEGRATION.attributes, integration_type: "jira" },
+      },
+    ],
   ])("cannot confirm the install from %s", async (_label, data) => {
     // Given — a 2xx whose `data` is truthy but is not an integration resource.
     fetchMock.mockResolvedValue(exchangeResponse(data));
@@ -275,3 +300,452 @@ describe("exchangeSlackOAuthCode result shape", () => {
     expect(await exchange()).toEqual({ integration: INTEGRATION });
   });
 });
+
+/** The shape the API's integration ids have, which is the only shape accepted. */
+const SLACK_INTEGRATION_ID = "b2c7fd0a-3e51-4d8f-9a6c-1f0e2d3c4b5a";
+
+const CHANNELS_URL =
+  `https://api.test/api/v1/integrations/${SLACK_INTEGRATION_ID}` +
+  "/slack/channels";
+
+const FIRST_CHANNEL = { id: "C0123AB", name: "security" };
+const SECOND_CHANNEL = { id: "C0789EF", name: "platform" };
+
+const channelPage = (
+  channel: { id: string; name: string },
+  next: string | null,
+) =>
+  new Response(
+    JSON.stringify({
+      data: [
+        {
+          type: "slack-channels",
+          id: channel.id,
+          attributes: { name: channel.name, is_private: false },
+        },
+      ],
+      links: { next },
+    }),
+    { status: 200, headers: { "content-type": "application/vnd.api+json" } },
+  );
+
+const channelOption = (channel: { id: string; name: string }) => ({
+  id: channel.id,
+  name: channel.name,
+  is_private: false,
+});
+
+const requestedUrls = (): string[] =>
+  fetchMock.mock.calls.map(([url]) => String(url));
+
+/**
+ * `MAX_CHANNEL_PAGES` in the action, which a `"use server"` module cannot
+ * export: only async functions may leave one.
+ */
+const MAX_CHANNEL_PAGES = 20;
+
+const channelOptions = (count: number) =>
+  Array.from({ length: count }, () => channelOption(FIRST_CHANNEL));
+
+/** What a `429` carrying `Retry-After: 30` is turned into. */
+const RATE_LIMITED_MESSAGE =
+  "Slack is rate limiting Prowler right now. Try again in about 30 seconds.";
+
+describe("getSlackChannels", () => {
+  it("follows a cursor-only `next` on the listing's own URL, not on the API root", async () => {
+    // The link is opaque (design D6), so the API may answer with the cursor
+    // alone; resolved against the API root it loses the listing's own path.
+    fetchMock
+      .mockResolvedValueOnce(channelPage(FIRST_CHANNEL, "?page[cursor]=2"))
+      .mockResolvedValueOnce(channelPage(SECOND_CHANNEL, null));
+
+    const result = await getSlackChannels(SLACK_INTEGRATION_ID);
+
+    expect(requestedUrls()).toEqual([
+      CHANNELS_URL,
+      `${CHANNELS_URL}?page[cursor]=2`,
+    ]);
+    expect(result).toEqual({
+      channels: [channelOption(FIRST_CHANNEL), channelOption(SECOND_CHANNEL)],
+    });
+  });
+
+  it.each([
+    {
+      shape: "an absolute",
+      next: "https://evil.test/api/v1/integrations/x/slack/channels?cursor=2",
+    },
+    { shape: "a protocol-relative", next: "//evil.test/api/v1/channels?c=2" },
+  ])(
+    "stops at $shape off-origin `next` rather than sending the tenant's token to it",
+    async ({ next }) => {
+      // `fetch` strips the tenant's `Authorization` on a redirect that leaves
+      // the origin, but not on a hop the UI makes itself.
+      fetchMock.mockResolvedValueOnce(channelPage(FIRST_CHANNEL, next));
+
+      const result = await getSlackChannels(SLACK_INTEGRATION_ID);
+
+      expect(requestedUrls()).toEqual([CHANNELS_URL]);
+      expect(result).toEqual({
+        channels: [channelOption(FIRST_CHANNEL)],
+        incomplete: SLACK_PARTIAL_CHANNEL_LIST_MESSAGE,
+      });
+    },
+  );
+
+  it("answers an unreadable page as no channels rather than parser prose", async () => {
+    fetchMock.mockResolvedValueOnce(unreadableOk(HTML_INTERSTITIAL));
+
+    const result = await getSlackChannels(SLACK_INTEGRATION_ID);
+
+    expect(result).toEqual({ channels: [] });
+    expectNoParserProse(result);
+  });
+
+  it("says the list is short of the workspace when the page budget runs out", async () => {
+    // The budget exists because `conversations.list` is tier 2 and a workspace
+    // can outgrow it (design.md, Risks). A fresh `Response` per call: one
+    // instance is already consumed on its second read.
+    fetchMock.mockImplementation(() =>
+      Promise.resolve(channelPage(FIRST_CHANNEL, "?page[cursor]=next")),
+    );
+
+    const result = await getSlackChannels(SLACK_INTEGRATION_ID);
+
+    expect(fetchMock).toHaveBeenCalledTimes(MAX_CHANNEL_PAGES);
+    expect(result).toEqual({
+      channels: channelOptions(MAX_CHANNEL_PAGES),
+      incomplete: SLACK_PARTIAL_CHANNEL_LIST_MESSAGE,
+    });
+  });
+
+  it("says nothing about a short list for a workspace that just fits the budget", async () => {
+    let page = 0;
+    fetchMock.mockImplementation(() => {
+      page += 1;
+      return Promise.resolve(
+        channelPage(
+          FIRST_CHANNEL,
+          page < MAX_CHANNEL_PAGES ? `?page[cursor]=${page}` : null,
+        ),
+      );
+    });
+
+    const result = await getSlackChannels(SLACK_INTEGRATION_ID);
+
+    expect(fetchMock).toHaveBeenCalledTimes(MAX_CHANNEL_PAGES);
+    expect(result).toEqual({ channels: channelOptions(MAX_CHANNEL_PAGES) });
+    expect(result).not.toHaveProperty("incomplete");
+  });
+
+  it("keeps the pages it read when a later one is refused, saying why the list stops", async () => {
+    fetchMock
+      .mockResolvedValueOnce(channelPage(FIRST_CHANNEL, "?page[cursor]=2"))
+      .mockResolvedValueOnce(rateLimitedResponse());
+
+    const result = await getSlackChannels(SLACK_INTEGRATION_ID);
+
+    expect(result).toEqual({
+      channels: [channelOption(FIRST_CHANNEL)],
+      incomplete: RATE_LIMITED_MESSAGE,
+    });
+  });
+
+  it("answers a refusal on the first page as a failure, having nothing to show", async () => {
+    fetchMock.mockResolvedValueOnce(rateLimitedResponse());
+
+    const result = await getSlackChannels(SLACK_INTEGRATION_ID);
+
+    expect(result).toEqual({ error: RATE_LIMITED_MESSAGE });
+  });
+});
+
+/**
+ * A `2xx` whose body is not JSON:API: an empty answer, or the HTML a proxy or
+ * WAF puts in front of one. The raw `SyntaxError` survives
+ * `sanitizeErrorMessage` (V8 truncates the snippet to ten characters, so its
+ * `<!doctype html>` branch never matches) and would be shown verbatim.
+ */
+const HTML_INTERSTITIAL =
+  "<!DOCTYPE html><html><body><h1>Checking your browser</h1></body></html>";
+
+const unreadableOk = (body: string) =>
+  new Response(body, {
+    status: 200,
+    headers: { "content-type": body ? "text/html" : "application/json" },
+  });
+
+/** V8's parser wording, which no user should ever be shown. */
+const PARSER_PROSE = /unexpected (token|end of json)|not valid json/i;
+
+const expectNoParserProse = (result: unknown) => {
+  const message = (result as { error?: string }).error ?? "";
+  expect(message).not.toMatch(PARSER_PROSE);
+};
+
+const INTEGRATION_URL = `https://api.test/api/v1/integrations/${SLACK_INTEGRATION_ID}`;
+
+const saveChannel = () =>
+  setSlackDefaultChannel(SLACK_INTEGRATION_ID, FIRST_CHANNEL.id);
+
+const expectIntegrationsRevalidated = () => {
+  expect(vi.mocked(revalidatePath).mock.calls).toEqual([
+    ["/integrations"],
+    ["/integrations/slack"],
+  ]);
+};
+
+describe("setSlackDefaultChannel", () => {
+  it("returns the saved integration and revalidates the pages listing it", async () => {
+    fetchMock.mockResolvedValueOnce(
+      new Response(
+        JSON.stringify({
+          data: {
+            type: "integrations",
+            id: SLACK_INTEGRATION_ID,
+            attributes: {
+              integration_type: "slack",
+              configuration: {
+                channel_id: FIRST_CHANNEL.id,
+                channel_name: FIRST_CHANNEL.name,
+              },
+            },
+          },
+        }),
+        {
+          status: 200,
+          headers: { "content-type": "application/vnd.api+json" },
+        },
+      ),
+    );
+
+    const result = await saveChannel();
+
+    expect(requestedUrls()).toEqual([INTEGRATION_URL]);
+    expect(result).toMatchObject({
+      integration: {
+        attributes: { configuration: { channel_name: FIRST_CHANNEL.name } },
+      },
+    });
+    expectIntegrationsRevalidated();
+  });
+
+  it.each([
+    { shape: "empty", body: "" },
+    { shape: "an HTML interstitial", body: HTML_INTERSTITIAL },
+  ])(
+    "answers a $shape `200` as an unread result, not as a failed save",
+    async ({ body }) => {
+      fetchMock.mockResolvedValueOnce(unreadableOk(body));
+
+      const result = await saveChannel();
+
+      expect(result).toEqual({ error: SLACK_UNREADABLE_RESULT_MESSAGE });
+      expectNoParserProse(result);
+      // The API recorded the channel before answering, so both pages refresh.
+      expectIntegrationsRevalidated();
+    },
+  );
+
+  // The caller reads `integration.attributes.configuration`, so a shallower
+  // guard lets the miss surface later as the manager's generic catch.
+  it.each([
+    { shape: "no `data`", body: {} },
+    { shape: "a null `data`", body: { data: null } },
+    { shape: "a `data` with no configuration", body: { data: {} } },
+  ])(
+    "answers a `200` carrying $shape as an unread result",
+    async ({ body }) => {
+      fetchMock.mockResolvedValueOnce(
+        new Response(JSON.stringify(body), {
+          status: 200,
+          headers: { "content-type": "application/vnd.api+json" },
+        }),
+      );
+
+      const result = await saveChannel();
+
+      expect(result).toEqual({ error: SLACK_UNREADABLE_RESULT_MESSAGE });
+      expectNoParserProse(result);
+      expectIntegrationsRevalidated();
+    },
+  );
+});
+
+/** The `202` that hands back the task the post is reported on (design D9). */
+const TEST_MESSAGE_TASK_ID = "5f8b1c2d-7e64-4a90-8c31-2b7d6e5f4a90";
+
+const testMessageAccepted = () =>
+  new Response(JSON.stringify({ data: { id: TEST_MESSAGE_TASK_ID } }), {
+    status: 202,
+    headers: { "content-type": "application/vnd.api+json" },
+  });
+
+/** The task read the poll makes, already settled on its first look. */
+const settledTask = (state: string, result: unknown) =>
+  new Response(
+    JSON.stringify({
+      data: {
+        type: "tasks",
+        id: TEST_MESSAGE_TASK_ID,
+        attributes: { state, result },
+      },
+    }),
+    { status: 200, headers: { "content-type": "application/vnd.api+json" } },
+  );
+
+describe("sendSlackTestMessage", () => {
+  it("answers an unreadable `202` as no task started, not as parser prose", async () => {
+    fetchMock.mockResolvedValueOnce(
+      new Response(HTML_INTERSTITIAL, {
+        status: 202,
+        headers: { "content-type": "text/html" },
+      }),
+    );
+
+    const result = await sendSlackTestMessage(SLACK_INTEGRATION_ID);
+
+    expect(result).toEqual({ error: "Slack did not start the test message." });
+    expectNoParserProse(result);
+  });
+
+  it("wraps a reason it has no copy for instead of answering with the bare token", async () => {
+    fetchMock
+      .mockResolvedValueOnce(testMessageAccepted())
+      .mockResolvedValueOnce(settledTask("failed", { error: "is_archived" }));
+
+    const result = await sendSlackTestMessage(SLACK_INTEGRATION_ID);
+
+    const error = (result as { error?: string }).error ?? "";
+    expect(error).toMatch(/Slack refused the message/);
+    expect(error).toContain("is_archived");
+    expect(error).not.toBe("is_archived");
+  });
+
+  it("keeps Prowler's own wording for a reason the mapping covers", async () => {
+    fetchMock
+      .mockResolvedValueOnce(testMessageAccepted())
+      .mockResolvedValueOnce(
+        settledTask("failed", { error: "not_in_channel" }),
+      );
+
+    const result = await sendSlackTestMessage(SLACK_INTEGRATION_ID);
+
+    expect(result).toEqual({
+      error: SLACK_ERROR_MESSAGES[SLACK_ERROR_CODE.NOT_IN_CHANNEL],
+    });
+  });
+
+  it("shows a reason the task worded itself as the prose it is", async () => {
+    // Not token-shaped, so nothing is wrapped around it.
+    const prose = "Slack rejected the message: the channel is archived.";
+    fetchMock
+      .mockResolvedValueOnce(testMessageAccepted())
+      .mockResolvedValueOnce(settledTask("failed", { error: prose }));
+
+    const result = await sendSlackTestMessage(SLACK_INTEGRATION_ID);
+
+    expect(result).toEqual({ error: prose });
+  });
+});
+
+/** The calls whose only failure path is one line of copy. */
+const COPY_ONLY_ACTIONS = [
+  {
+    name: "getSlackChannels",
+    call: (id: string) => getSlackChannels(id),
+  },
+  {
+    name: "setSlackDefaultChannel",
+    call: (id: string) => setSlackDefaultChannel(id, FIRST_CHANNEL.id),
+  },
+  {
+    name: "sendSlackTestMessage",
+    call: (id: string) => sendSlackTestMessage(id),
+  },
+];
+
+const rateLimitedResponse = () =>
+  new Response(
+    JSON.stringify({
+      errors: [{ status: "429", detail: "Slack is rate limiting." }],
+    }),
+    {
+      status: 429,
+      headers: {
+        "content-type": "application/vnd.api+json",
+        "Retry-After": "30",
+      },
+    },
+  );
+
+describe.each(COPY_ONLY_ACTIONS)("$name", ({ call }) => {
+  it("reports an upstream Slack failure and still answers in the same words", async () => {
+    fetchMock.mockResolvedValue(
+      errorResponse(UPSTREAM_STATUS, UPSTREAM_DETAIL),
+    );
+
+    const result = await call(SLACK_INTEGRATION_ID);
+
+    // Once, not twice: `handleApiResponse` reports and throws, and the action's
+    // catch sees the mark.
+    expect(captureExceptionMock).toHaveBeenCalledTimes(1);
+    expect(captureExceptionMock.mock.calls[0]?.[1]).toMatchObject({
+      tags: {
+        api_error: true,
+        error_source: SentryErrorSource.HANDLE_API_RESPONSE,
+        error_type: SentryErrorType.SERVER_ERROR,
+        status_code: String(UPSTREAM_STATUS),
+      },
+    });
+    expect(captureMessageMock).not.toHaveBeenCalled();
+
+    expect(result).toEqual({ error: UPSTREAM_DETAIL });
+  });
+
+  it.each([
+    {
+      status: 503,
+      why: "Slack being unavailable, not a fault",
+      response: () => errorResponse(503, "Slack is unavailable."),
+      expected: "Slack is unavailable.",
+    },
+    {
+      status: 429,
+      why: "a wait, not a fault",
+      response: rateLimitedResponse,
+      expected:
+        "Slack is rate limiting Prowler right now. Try again in about 30 seconds.",
+    },
+    {
+      status: 400,
+      why: "a refusal the API meant to give",
+      response: () => errorResponse(400, "No default channel is set."),
+      expected: "No default channel is set.",
+    },
+  ])("reports nothing for a $status: that is $why", async (refusal) => {
+    fetchMock.mockResolvedValue(refusal.response());
+
+    const result = await call(SLACK_INTEGRATION_ID);
+
+    expect(captureExceptionMock).not.toHaveBeenCalled();
+    expect(captureMessageMock).not.toHaveBeenCalled();
+    expect(result).toEqual({ error: refusal.expected });
+  });
+});
+
+/**
+ * The integration id is interpolated into every one of these URLs, so a
+ * malformed one is refused before the request is built.
+ */
+describe.each(COPY_ONLY_ACTIONS)("$name", ({ call }) => {
+  it.each(["../../users", "not-a-uuid", ""])(
+    "asks the API nothing when the integration id is %o",
+    async (id) => {
+      const result = await call(id);
+
+      expect(fetchMock).not.toHaveBeenCalled();
+      expect(result).toEqual({ error: SLACK_GENERIC_ERROR_MESSAGE });
+    },
+  );
+});

ui/actions/integrations/slack.ts

@@ -3,16 +3,24 @@
 import { revalidatePath } from "next/cache";
 import { z } from "zod";
 
+import { pollTaskUntilSettled } from "@/actions/task/poll";
 import { apiBaseUrl, getAuthHeaders, parseStringify } from "@/lib";
 import {
   readSlackFailure,
   SLACK_GENERIC_ERROR_MESSAGE,
+  SLACK_PARTIAL_CHANNEL_LIST_MESSAGE,
+  SLACK_REASON_TOKEN,
   SLACK_UNREADABLE_RESULT_MESSAGE,
   slackErrorMessage,
   slackRateLimitMessage,
+  slackUnknownReasonMessage,
 } from "@/lib/integrations/slack-errors";
 import { handleApiError, handleApiResponse } from "@/lib/server-actions-helper";
-import type { IntegrationProps } from "@/types/integrations";
+import {
+  INTEGRATION_TYPE,
+  type IntegrationProps,
+  type SlackChannelOption,
+} from "@/types/integrations";
 
 interface SlackUnavailable {
   unavailable: true;
@@ -57,6 +65,17 @@ const slackExchangeInputSchema = z.object({
   state: z.string().min(1),
 });
 
+/**
+ * SSRF guard: the integration id is interpolated into the request URL, so only
+ * the shape the API's ids have reaches it.
+ */
+const integrationIdSchema = z.uuid();
+
+const parseIntegrationId = (integrationId: string): string | null => {
+  const parsed = integrationIdSchema.safeParse(integrationId);
+  return parsed.success ? parsed.data : null;
+};
+
 interface SlackExchangeSuccess {
   integration: IntegrationProps;
 }
@@ -98,23 +117,36 @@ const isSlackAuthorizeUrl = (value: string): boolean => {
   }
 };
 
+const INTEGRATIONS_RESOURCE_TYPE = "integrations";
+
 /**
  * The callback names the workspace and redirects on this value alone, so a `2xx`
  * payload that is not a JSON:API resource (`{}`, `[]`, `"invalid"`) must read as
- * unreadable rather than as a connected workspace.
+ * unreadable rather than as a connected workspace. Identity too: a resource
+ * that is not a linkable Slack integration would be shown as the workspace
+ * just installed.
  */
 const isIntegrationResource = (value: unknown): boolean => {
   if (typeof value !== "object" || value === null || Array.isArray(value)) {
     return false;
   }
 
-  const { id, attributes } = value as Record<string, unknown>;
+  const { id, type, attributes } = value as Record<string, unknown>;
+
+  if (
+    typeof id !== "string" ||
+    id === "" ||
+    type !== INTEGRATIONS_RESOURCE_TYPE ||
+    typeof attributes !== "object" ||
+    attributes === null ||
+    Array.isArray(attributes)
+  ) {
+    return false;
+  }
 
   return (
-    typeof id === "string" &&
-    typeof attributes === "object" &&
-    attributes !== null &&
-    !Array.isArray(attributes)
+    (attributes as Record<string, unknown>).integration_type ===
+    INTEGRATION_TYPE.SLACK
   );
 };
 
@@ -143,6 +175,30 @@ const failureFrom = async (
   return { error: slackErrorMessage(failure, fallback) };
 };
 
+/**
+ * `failureFrom` flattened to one line of copy, for the calls whose only
+ * outcome is "it did not work". Rate limiting keeps its own wording:
+ * `conversations.list` is Slack tier 2, so a `429` shows up here (contract,
+ * Errors) and the wait it names is the useful part.
+ */
+const errorMessageFrom = async (
+  response: Response,
+  fallback: string,
+): Promise<string> => {
+  // Same 5xx handling as `failureFrom`, `503` excepted: here too it means Slack
+  // is unavailable. Must run before `readSlackFailure`: a body can only be read
+  // once.
+  if (response.status >= 500 && response.status !== 503) {
+    await handleApiResponse(response);
+  }
+
+  const failure = await readSlackFailure(response);
+
+  return failure.status === RATE_LIMITED_STATUS
+    ? slackRateLimitMessage(failure.retryAfterSeconds)
+    : slackErrorMessage(failure, fallback);
+};
+
 /** Mint an OAuth state and get the consent URL. Creates no integration. */
 export const getSlackAuthorizeUrl =
   async (): Promise<SlackAuthorizeUrlResult> => {
@@ -234,3 +290,259 @@ export const exchangeSlackOAuthCode = async (
     return handleApiError(error);
   }
 };
+
+interface SlackChannelsSuccess {
+  channels: SlackChannelOption[];
+  /**
+   * Present when these channels are only part of the workspace's, carrying the
+   * sentence that says why: a partial read is a success, so the caller renders
+   * the picker *and* the reason.
+   */
+  incomplete?: string;
+}
+
+export type SlackChannelsResult = SlackChannelsSuccess | SlackActionError;
+
+/**
+ * Cursor pages followed before giving up: `conversations.list` is a tier-2,
+ * rate-limited Slack call (design.md, Risks), so the aggregation is bounded
+ * rather than open-ended.
+ */
+const MAX_CHANNEL_PAGES = 20;
+
+/**
+ * Every channel Prowler can post to in the connected workspace — the picker's
+ * options.
+ *
+ * The durable primitive, not the channel stored on the integration (design D6):
+ * a consumer needing a per-rule channel reads the same endpoint. `links.next`
+ * is followed opaquely — the contract does not pin the cursor parameter naming,
+ * so the UI never builds one of its own. An early stop that still read
+ * something reports through `incomplete`, not as a failure.
+ */
+export const getSlackChannels = async (
+  integrationId: string,
+): Promise<SlackChannelsResult> => {
+  const id = parseIntegrationId(integrationId);
+  if (!id) return { error: SLACK_GENERIC_ERROR_MESSAGE };
+
+  const headers = await getAuthHeaders({ contentType: false });
+  const channels: SlackChannelOption[] = [];
+
+  const listing = new URL(`${apiBaseUrl}/integrations/${id}/slack/channels`);
+  let next: string | null = listing.toString();
+  let incomplete: string | null = null;
+
+  try {
+    for (let page = 0; next && page < MAX_CHANNEL_PAGES; page += 1) {
+      const current: string = next;
+      const response: Response = await fetch(current, {
+        method: "GET",
+        headers,
+      });
+
+      if (!response.ok) {
+        const message = await errorMessageFrom(
+          response,
+          `Unable to read the workspace's channels: ${response.statusText}`,
+        );
+
+        return channels.length > 0
+          ? { channels, incomplete: message }
+          : { error: message };
+      }
+
+      // A page that is not JSON reads as no channels, rather than throwing a
+      // parser message the user would be shown verbatim.
+      const body = await response.json().catch(() => null);
+
+      for (const resource of body?.data ?? []) {
+        channels.push({
+          id: resource?.id,
+          name: resource?.attributes?.name ?? "",
+          is_private: Boolean(resource?.attributes?.is_private),
+        });
+      }
+
+      const rawNext = body?.links?.next;
+      const candidate =
+        typeof rawNext === "string" && rawNext.length > 0
+          ? new URL(rawNext, current)
+          : null;
+      // Resolved against the page it arrived on, so a cursor-only `next` keeps
+      // this listing's path. Followed only while it stays on the listing's
+      // origin: every page is fetched with the tenant's token, and an
+      // off-origin hop made here would carry it along.
+      if (candidate === null) {
+        next = null;
+      } else if (candidate.origin === listing.origin) {
+        next = candidate.toString();
+      } else {
+        next = null;
+        incomplete = SLACK_PARTIAL_CHANNEL_LIST_MESSAGE;
+      }
+    }
+
+    // A link still waiting when the budget ran out. Checked rather than assumed
+    // from the page count: a workspace of exactly `MAX_CHANNEL_PAGES` pages was
+    // read to the end.
+    if (next) incomplete = SLACK_PARTIAL_CHANNEL_LIST_MESSAGE;
+
+    return incomplete === null ? { channels } : { channels, incomplete };
+  } catch (error) {
+    return handleApiError(error);
+  }
+};
+
+interface SlackDefaultChannelSuccess {
+  integration: IntegrationProps;
+}
+
+export type SlackDefaultChannelResult =
+  | SlackDefaultChannelSuccess
+  | SlackActionError;
+
+/**
+ * Record the channel Prowler posts to, on the generic integration endpoint.
+ *
+ * A Slack action despite the generic `PATCH`: `channel_not_found` and
+ * `not_in_channel` carry the same `detail`, so only `code` tells them apart,
+ * and the generic action reads `detail` alone. Only `channel_id` travels — the
+ * API derives `channel_name` server-side (design D6).
+ */
+export const setSlackDefaultChannel = async (
+  integrationId: string,
+  channelId: string,
+): Promise<SlackDefaultChannelResult> => {
+  const id = parseIntegrationId(integrationId);
+  if (!id) return { error: SLACK_GENERIC_ERROR_MESSAGE };
+
+  const headers = await getAuthHeaders({ contentType: true });
+  const url = new URL(`${apiBaseUrl}/integrations/${id}`);
+
+  try {
+    const response = await fetch(url.toString(), {
+      method: "PATCH",
+      headers,
+      body: JSON.stringify({
+        data: {
+          type: "integrations",
+          id,
+          attributes: {
+            integration_type: "slack",
+            configuration: { channel_id: channelId },
+          },
+        },
+      }),
+    });
+
+    if (!response.ok) {
+      return {
+        error: await errorMessageFrom(
+          response,
+          `Unable to save the destination channel: ${response.statusText}`,
+        ),
+      };
+    }
+
+    const body = await response.json().catch(() => null);
+
+    // Before the guard and on both paths: the save happened, so a cache still
+    // holding the previous channel would keep showing it.
+    revalidatePath("/integrations");
+    revalidatePath("/integrations/slack");
+
+    // Guarded as deep as the caller reads: it names the saved channel from
+    // `attributes.configuration`.
+    if (!body?.data?.attributes?.configuration) {
+      return { error: SLACK_UNREADABLE_RESULT_MESSAGE };
+    }
+
+    return { integration: parseStringify(body.data) as IntegrationProps };
+  } catch (error) {
+    return handleApiError(error);
+  }
+};
+
+interface SlackTestMessageSuccess {
+  sent: true;
+}
+
+export type SlackTestMessageResult = SlackTestMessageSuccess | SlackActionError;
+
+interface SlackTestMessageTaskResult {
+  error?: string | null;
+}
+
+const TEST_MESSAGE_POLL = { maxAttempts: 20, delayMs: 3000 } as const;
+
+/**
+ * Post the test message to the integration's default channel.
+ *
+ * Async on the API's side — `202` plus a Task (design D9) — so this polls the
+ * same task machinery the connection test uses. A `400` means no default
+ * channel is recorded.
+ */
+export const sendSlackTestMessage = async (
+  integrationId: string,
+): Promise<SlackTestMessageResult> => {
+  const id = parseIntegrationId(integrationId);
+  if (!id) return { error: SLACK_GENERIC_ERROR_MESSAGE };
+
+  const headers = await getAuthHeaders({ contentType: true });
+  const url = new URL(`${apiBaseUrl}/integrations/${id}/slack/test-message`);
+
+  try {
+    const response = await fetch(url.toString(), { method: "POST", headers });
+
+    if (!response.ok) {
+      return {
+        error: await errorMessageFrom(
+          response,
+          `Unable to send the test message: ${response.statusText}`,
+        ),
+      };
+    }
+
+    // As above: an unreadable `202` is "no task to follow", not a parser
+    // message.
+    const body = await response.json().catch(() => null);
+    const taskId = body?.data?.id;
+
+    if (!taskId) {
+      return { error: "Slack did not start the test message." };
+    }
+
+    const settled = await pollTaskUntilSettled<SlackTestMessageTaskResult>(
+      taskId,
+      TEST_MESSAGE_POLL,
+    );
+
+    if (!settled.ok) {
+      return { error: settled.error };
+    }
+
+    // Slack's refusal travels in the task result, not in an HTTP error: the
+    // post happens after the `202`. A known code gets Prowler's own wording, a
+    // code-shaped reason is wrapped in one (contract, test-message), and prose
+    // is shown as it arrived.
+    const reason = settled.result?.error?.trim();
+    if (reason) {
+      return {
+        error: SLACK_REASON_TOKEN.test(reason)
+          ? slackErrorMessage(
+              { code: reason },
+              slackUnknownReasonMessage(reason),
+            )
+          : reason,
+      };
+    }
+    if (settled.state !== "completed") {
+      return { error: "Slack did not accept the test message." };
+    }
+
+    return { sent: true };
+  } catch (error) {
+    return handleApiError(error);
+  }
+};

ui/app/(prowler)/integrations/slack/slack-integration.harness.ts

@@ -14,6 +14,7 @@ import { handlersForSlack } from "@/__tests__/msw/handlers/slack";
 import type { SlackFixture } from "@/__tests__/msw/handlers/slack.fixtures";
 import { worker } from "@/__tests__/msw/worker";
 import { render } from "@/__tests__/render-browser";
+import { setSlackDefaultChannel } from "@/actions/integrations/slack";
 import { SlackCallback } from "@/components/integrations/slack/slack-callback";
 
 import { IntegrationsContent } from "../integrations-content";
@@ -28,6 +29,17 @@ export const CONNECTION_OUTCOME = {
 export type ConnectionOutcome =
   (typeof CONNECTION_OUTCOME)[keyof typeof CONNECTION_OUTCOME];
 
+export const TEST_MESSAGE_OUTCOME = {
+  SENT: "sent",
+  FAILED: "failed",
+} as const;
+
+export type TestMessageOutcome =
+  (typeof TEST_MESSAGE_OUTCOME)[keyof typeof TEST_MESSAGE_OUTCOME];
+
+/** Sentinel: the page settled on "no channel recorded", rather than not yet. */
+const NO_DEFAULT_CHANNEL = "<no channel recorded>";
+
 interface CallbackParams {
   code?: string;
   state?: string;
@@ -63,7 +75,37 @@ export class SlackIntegrationHarness extends BrowserHarness<SlackFixture> {
     window.history.replaceState(null, "", "/integrations/slack");
     this.wireHandlers();
 
-    render(await SlackIntegrationContent());
+    const readsBefore = this.channelListCallCount;
+    this.mounted = render(await SlackIntegrationContent());
+    if (this.fixture.install) await this.waitForChannelsRead(readsBefore);
+  }
+
+  private mounted: ReturnType<typeof render> | null = null;
+
+  /**
+   * Open the management page again, the way a later visit does — the handlers in
+   * place keep serving what the previous visit left behind. Unmounts the previous
+   * render first: two live copies would make every assertion ambiguous.
+   */
+  async revisit(): Promise<void> {
+    (await this.mounted)?.unmount();
+    const readsBefore = this.channelListCallCount;
+    this.mounted = render(await SlackIntegrationContent());
+    await this.mounted;
+    if (this.fixture.install) await this.waitForChannelsRead(readsBefore);
+  }
+
+  /**
+   * Refresh the page's server data under the open card, as `revalidatePath` does
+   * after an action: new props, no unmount, so React state survives — unlike
+   * `revisit()`, which re-seeds everything from scratch.
+   */
+  async refreshPageData(): Promise<void> {
+    const rendered = await this.mounted;
+    if (!rendered) {
+      throw new Error("refreshPageData: the page is not mounted");
+    }
+    await rendered.rerender(await SlackIntegrationContent());
   }
 
   async mountCallback({ code, state, error }: CallbackParams): Promise<void> {
@@ -317,4 +359,320 @@ export class SlackIntegrationHarness extends BrowserHarness<SlackFixture> {
   offersRetry(): boolean {
     return this.backLink() !== null || this.offersInstall();
   }
+
+  // --- Choosing a destination channel --------------------------------------
+
+  /** Channel reads issued — one per cursor page the UI followed. */
+  get channelListCallCount(): number {
+    return this.countRequests("GET", "/slack/channels");
+  }
+
+  /**
+   * Wait for the channel read every connected mount starts, counting from the
+   * reads already issued: one still in flight when the test ends lands in the
+   * middle of the next, against a harness that never asked for it.
+   */
+  private async waitForChannelsRead(readsBefore: number): Promise<void> {
+    await this.waitFor(
+      () => {
+        const refresh = this.buttonByText(/Refresh channels/);
+        return this.channelListCallCount > readsBefore &&
+          refresh !== null &&
+          !refresh.disabled
+          ? true
+          : null;
+      },
+      15000,
+      "the workspace's channels to be read",
+    );
+  }
+
+  /**
+   * Open the picker and hand back its options. A re-render landing mid-gesture
+   * makes Radix drop the open state, so re-open from the keyboard when nothing
+   * mounted at all.
+   */
+  private async openChannelPicker(): Promise<HTMLElement[]> {
+    const mounted = (): HTMLElement[] | null => {
+      const options = Array.from(
+        document.querySelectorAll<HTMLElement>('[role="option"]'),
+      );
+      return options.length > 0 ? options : null;
+    };
+
+    const alreadyOpen = mounted();
+    if (alreadyOpen) return alreadyOpen;
+
+    const trigger = await this.waitFor<HTMLElement>(
+      () => this.q("#slack-channel"),
+      10000,
+      "the channel picker",
+    );
+
+    await this.clickElement(trigger, { fallbackToDomClick: true });
+
+    let options = await this.waitForOrNull(
+      mounted,
+      2000,
+      "the channel options",
+    );
+    if (!options) {
+      await this.user.keyboard("{Enter}");
+      options = await this.waitForOrNull(mounted, 8000, "the channel options");
+    }
+
+    if (!options) {
+      throw new Error("openChannelPicker: the channel picker offered nothing");
+    }
+    return options;
+  }
+
+  private async closeChannelPicker(): Promise<void> {
+    await this.user.keyboard("{Escape}");
+    await this.waitForTransition();
+  }
+
+  /**
+   * Re-read the workspace's channels, the way a user does after inviting
+   * `@Prowler` to one in Slack. Waits for the read to have settled, not for the
+   * click alone.
+   */
+  async refreshChannels(): Promise<void> {
+    const readsBefore = this.channelListCallCount;
+    await this.clickButton(/Refresh channels/);
+
+    await this.waitFor(
+      () => {
+        const button = this.buttonByText(/Refresh channels/);
+        return (
+          this.channelListCallCount > readsBefore &&
+          button !== null &&
+          !button.disabled
+        );
+      },
+      15000,
+      "the workspace's channels to be read again",
+    );
+  }
+
+  /** The channels the workspace offers, in the order the picker lists them. */
+  async channelOptions(): Promise<string[]> {
+    const options = await this.openChannelPicker();
+    const names = options.map(
+      (option) => option.getAttribute("data-channel") ?? "",
+    );
+
+    await this.closeChannelPicker();
+
+    return names;
+  }
+
+  /**
+   * Whether the channel offered under `name` is presented as private — read
+   * from the marker the user sees, not from how the option is wired up.
+   */
+  async isChannelShownAsPrivate(name: string): Promise<boolean> {
+    const options = await this.openChannelPicker();
+    const option = options.find(
+      (element) => element.getAttribute("data-channel") === name,
+    );
+
+    await this.closeChannelPicker();
+
+    return /Private/.test(option?.textContent ?? "");
+  }
+
+  private async pickAndSave(name: string): Promise<void> {
+    const options = await this.openChannelPicker();
+    const option = options.find(
+      (element) => element.getAttribute("data-channel") === name,
+    );
+
+    if (!option) {
+      throw new Error(`pickAndSave: no channel named "${name}" is offered`);
+    }
+
+    await this.user.click(option);
+    await this.waitForTransition();
+    await this.clickButton(/Save channel/);
+  }
+
+  /** Pick a channel, save it, and wait for it to be recorded as the destination. */
+  async chooseChannel(name: string): Promise<void> {
+    await this.pickAndSave(name);
+    await this.waitFor(
+      () => this.defaultChannelName() === name,
+      15000,
+      `#${name} to be recorded as the destination`,
+    );
+  }
+
+  /**
+   * Record a different destination away from this page — a second tab, or someone
+   * else in the tenant. Goes through the same call the page makes, leaving this
+   * page's own copy of it untouched.
+   */
+  async channelRecordedElsewhere(name: string): Promise<void> {
+    const channel = this.fixture.channels.find((c) => c.name === name);
+    if (!channel) {
+      throw new Error(
+        `channelRecordedElsewhere: no channel named "${name}" is offered`,
+      );
+    }
+
+    const integrationId = this.fixture.install?.id;
+    if (!integrationId) {
+      throw new Error("channelRecordedElsewhere: no workspace is connected");
+    }
+
+    const result = await setSlackDefaultChannel(integrationId, channel.id);
+    if ("error" in result) {
+      throw new Error(`channelRecordedElsewhere: ${result.error}`);
+    }
+  }
+
+  /** Whether the picked channel can be saved — false when there is nothing new to save. */
+  offersChannelSave(): boolean {
+    const button = this.buttonByText(/Save channel/);
+    return button !== null && !button.disabled;
+  }
+
+  /**
+   * Try to save a channel the API refuses and hand back what the user is told. A
+   * save that succeeds fails the test rather than timing out.
+   */
+  async refusedChannelSave(name: string): Promise<string> {
+    await this.pickAndSave(name);
+
+    return this.waitFor(
+      () => {
+        if (this.defaultChannelName() === name) {
+          throw new Error(
+            `refusedChannelSave: #${name} was recorded, not refused`,
+          );
+        }
+        return this.toastText(/Could not save the destination channel/);
+      },
+      15000,
+      "the refused channel save",
+    );
+  }
+
+  /**
+   * The text of the toast matching `pattern` — title and message together. Radix
+   * portals each toast into its viewport as an `<li>`, outside the page's markup.
+   */
+  private toastText(pattern: RegExp): string | null {
+    const toast = Array.from(
+      document.querySelectorAll<HTMLElement>("ol li"),
+    ).find((element) => pattern.test(element.textContent ?? ""));
+    return toast ? (toast.textContent ?? "").replace(/\s+/g, " ").trim() : null;
+  }
+
+  private defaultChannelName(): string | null {
+    return (
+      /Prowler posts to #(\S+?)\./.exec(
+        this.container.textContent ?? "",
+      )?.[1] ?? null
+    );
+  }
+
+  /** The channel recorded as the integration's destination, if any. */
+  async defaultChannel(): Promise<string | null> {
+    const settled = await this.waitFor(
+      () =>
+        this.defaultChannelName() ??
+        (this.containsText(/No destination channel recorded yet/)
+          ? NO_DEFAULT_CHANNEL
+          : null),
+      10000,
+      "the recorded destination channel",
+    );
+    return settled === NO_DEFAULT_CHANNEL ? null : settled;
+  }
+
+  /** What the user is told when the workspace exposes no channel at all. */
+  async channelPickerMessage(): Promise<string> {
+    const alert = await this.waitFor(
+      () =>
+        Array.from(
+          this.container.querySelectorAll<HTMLElement>('[data-slot="alert"]'),
+        ).find((element) =>
+          /No channels available yet|Could not read the workspace/.test(
+            element.textContent ?? "",
+          ),
+        ),
+      10000,
+      "the channel picker's message",
+    );
+    return (alert.textContent ?? "").replace(/\s+/g, " ").trim();
+  }
+
+  /**
+   * What the user is told about a list short of the workspace, shown beside a
+   * picker that still works — unlike `channelPickerMessage()`, which replaces it.
+   */
+  partialListNotice(): string | null {
+    const notice = this.q("[data-channels-notice]");
+    return notice
+      ? (notice.textContent ?? "").replace(/\s+/g, " ").trim()
+      : null;
+  }
+
+  /** Whether the picker was replaced by the "could not read them" alert. */
+  saysChannelsUnreadable(): boolean {
+    return this.containsText(/Could not read the workspace/);
+  }
+
+  /** The invite copy that says how to make a private channel appear. */
+  channelInviteHint(): string | null {
+    const hint = Array.from(
+      this.container.querySelectorAll<HTMLElement>("p"),
+    ).find((element) => /invites? @Prowler/.test(element.textContent ?? ""));
+    return hint ? (hint.textContent ?? "").trim() : null;
+  }
+
+  // --- The test message ----------------------------------------------------
+
+  offersTestMessage(): boolean {
+    return this.buttonByText(/Send test message/) !== null;
+  }
+
+  private testMessageAlert(): HTMLElement | null {
+    return (
+      Array.from(
+        this.container.querySelectorAll<HTMLElement>('[data-slot="alert"]'),
+      ).find((element) =>
+        /Test message (sent|failed)/.test(element.textContent ?? ""),
+      ) ?? null
+    );
+  }
+
+  async sendTestMessage(): Promise<TestMessageOutcome> {
+    await this.clickButton(/Send test message/);
+
+    return this.waitFor(
+      () => {
+        const alert = this.testMessageAlert();
+        if (!alert) return null;
+        return /Test message sent/.test(alert.textContent ?? "")
+          ? TEST_MESSAGE_OUTCOME.SENT
+          : TEST_MESSAGE_OUTCOME.FAILED;
+      },
+      15000,
+      "the test message outcome",
+    );
+  }
+
+  async lastTestMessageOutcome(): Promise<string> {
+    const alert = await this.waitFor(
+      () => this.testMessageAlert(),
+      10000,
+      "the test message outcome",
+    );
+    const description = alert.querySelector<HTMLElement>(
+      '[data-slot="alert-description"]',
+    );
+    return (description?.textContent ?? "").trim();
+  }
 }

ui/app/(prowler)/integrations/slack/slack-page.integration.test.tsx

@@ -12,15 +12,36 @@ import {
   configuredSlackFixture,
   connectedSlackFixture,
   INTEGRATIONS_SERVER_ERROR_DETAIL,
+  partiallyReadSlackFixture,
+  SLACK_CHANNEL_NOT_FOUND_REFUSAL,
+  SLACK_MISSING_SCOPE_CODE,
+  SLACK_MISSING_SCOPE_REFUSAL,
+  SLACK_NOT_IN_CHANNEL_CODE,
+  SLACK_NOT_IN_CHANNEL_REFUSAL,
+  SLACK_PRIVATE_CHANNEL,
+  SLACK_PUBLIC_CHANNEL,
+  SLACK_RATE_LIMITED_REFUSAL,
+  SLACK_SECOND_PUBLIC_CHANNEL,
+  SLACK_TEST_MESSAGE_REFUSED_DETAIL,
+  SLACK_UNKNOWN_CHANNEL_DETAIL,
+  SLACK_UNMAPPED_REASON_CODE,
+  SLACK_UPSTREAM_REFUSAL,
   slackFixture,
+  slackFixtureWithDefaultChannel,
   unreadableCheckTimeSlackFixture,
 } from "@/__tests__/msw/handlers/slack.fixtures";
 
 import {
   CONNECTION_OUTCOME,
   SlackIntegrationHarness,
+  TEST_MESSAGE_OUTCOME,
 } from "./slack-integration.harness";
 
+/** The shape the channel save is asserted against — only the id travels. */
+interface PatchIntegrationBody {
+  data: { attributes: { configuration: { channel_id: string } } };
+}
+
 /** The workspace the fixtures connect. */
 const WORKSPACE_NAME = "Prowler HQ";
 
@@ -43,9 +64,9 @@ describe("starting the install", () => {
     expect(`${consentScreen.origin}${consentScreen.pathname}`).toBe(
       "https://slack.com/oauth/v2/authorize",
     );
-    expect((consentScreen.searchParams.get("scope") ?? "").split(",")).toEqual(
-      expect.arrayContaining(REQUIRED_SCOPES),
-    );
+    const scopes = (consentScreen.searchParams.get("scope") ?? "").split(",");
+    expect(scopes).toHaveLength(REQUIRED_SCOPES.length);
+    expect(scopes).toEqual(expect.arrayContaining(REQUIRED_SCOPES));
     // The state is server-minted, binding this install to the session
     // (design D5).
     expect(consentScreen.searchParams.get("state")).toBeTruthy();
@@ -220,3 +241,402 @@ describe("a connected workspace", () => {
     expect(harness.saysChannelIsNextStep()).toBe(true);
   }, 30000);
 });
+
+describe("choosing a destination channel", () => {
+  it("offers the workspace's channels and remembers the one chosen", async () => {
+    // Given — a connected tenant whose channels span two cursor pages.
+    const harness = new SlackIntegrationHarness(connectedSlackFixture());
+    await harness.mount();
+
+    // Then — every channel is offered, so the picker followed `links.next`
+    // rather than stopping at the first page (design D6).
+    expect(await harness.channelOptions()).toEqual([
+      SLACK_PUBLIC_CHANNEL.name,
+      SLACK_SECOND_PUBLIC_CHANNEL.name,
+      SLACK_PRIVATE_CHANNEL.name,
+    ]);
+    expect(harness.channelListCallCount).toBe(2);
+
+    // When
+    await harness.chooseChannel(SLACK_PUBLIC_CHANNEL.name);
+
+    // Then — only the id is submitted: the API derives the name from it.
+    const saved = await harness.lastRequestBody<PatchIntegrationBody>(
+      "PATCH",
+      "/integrations/",
+    );
+    expect(saved?.data.attributes.configuration).toEqual({
+      channel_id: SLACK_PUBLIC_CHANNEL.id,
+    });
+
+    // And — a later visit shows it, under the name the API derived from the id.
+    await harness.revisit();
+    expect(await harness.defaultChannel()).toBe(SLACK_PUBLIC_CHANNEL.name);
+  }, 60000);
+
+  it("offers a private channel the app was invited to, marked as private, and saves it", async () => {
+    // Given — `@Prowler` was invited to one private channel; `groups:read` is
+    // membership-gated (D2).
+    const harness = new SlackIntegrationHarness(connectedSlackFixture());
+    await harness.mount();
+
+    // Then
+    expect(await harness.channelOptions()).toContain(
+      SLACK_PRIVATE_CHANNEL.name,
+    );
+    expect(
+      await harness.isChannelShownAsPrivate(SLACK_PRIVATE_CHANNEL.name),
+    ).toBe(true);
+    expect(
+      await harness.isChannelShownAsPrivate(SLACK_PUBLIC_CHANNEL.name),
+    ).toBe(false);
+
+    // When
+    await harness.chooseChannel(SLACK_PRIVATE_CHANNEL.name);
+
+    // Then
+    expect(await harness.defaultChannel()).toBe(SLACK_PRIVATE_CHANNEL.name);
+  }, 60000);
+
+  it("offers a private channel once @Prowler is invited to it and the list is refreshed", async () => {
+    // Given — a workspace whose only channels are public: `groups:read` is
+    // membership-gated (design D2).
+    const harness = new SlackIntegrationHarness(
+      connectedSlackFixture({
+        channels: [
+          { ...SLACK_PUBLIC_CHANNEL },
+          { ...SLACK_SECOND_PUBLIC_CHANNEL },
+        ],
+      }),
+    );
+    await harness.mount();
+    expect(await harness.channelOptions()).not.toContain(
+      SLACK_PRIVATE_CHANNEL.name,
+    );
+
+    // When — `@Prowler` is invited to a private channel, and the user refreshes
+    // instead of reconnecting the workspace.
+    harness.fixture.channels.push({ ...SLACK_PRIVATE_CHANNEL });
+    await harness.refreshChannels();
+
+    // Then
+    expect(await harness.channelOptions()).toContain(
+      SLACK_PRIVATE_CHANNEL.name,
+    );
+    expect(
+      await harness.isChannelShownAsPrivate(SLACK_PRIVATE_CHANNEL.name),
+    ).toBe(true);
+  }, 60000);
+
+  it("says what to do when the workspace exposes no channel Prowler can post to", async () => {
+    // Given — a connected workspace exposing no channels at all.
+    const harness = new SlackIntegrationHarness(
+      connectedSlackFixture({ channels: [] }),
+    );
+
+    // When
+    await harness.mount();
+
+    // Then — the user is told what to do, not merely that the list is empty.
+    const message = await harness.channelPickerMessage();
+    expect(message).toMatch(/No channels available yet/);
+    expect(message).toMatch(/invite @Prowler/);
+    expect(await harness.defaultChannel()).toBeNull();
+    expect(harness.offersTestMessage()).toBe(false);
+  }, 30000);
+
+  it("offers the connection check as soon as the destination is saved, without a revisit", async () => {
+    // Given — connected with nothing recorded: the check posts to the
+    // destination, so it is not offered yet.
+    const harness = new SlackIntegrationHarness(connectedSlackFixture());
+    await harness.mount();
+    expect(await harness.offersConnectionTest()).toBe(false);
+    expect(harness.saysChannelIsNextStep()).toBe(true);
+
+    // When
+    await harness.chooseChannel(SLACK_PUBLIC_CHANNEL.name);
+
+    // Then — everything waiting on a destination moves with the save, in the
+    // same paint: no reload to find the check on offer.
+    expect(await harness.offersConnectionTest()).toBe(true);
+    expect(harness.saysChannelIsNextStep()).toBe(false);
+    // And — the check really runs.
+    expect(await harness.testConnection()).toBe(CONNECTION_OUTCOME.SUCCESS);
+  }, 60000);
+
+  it("follows the destination recorded elsewhere when the page's data refreshes under it", async () => {
+    // Given — a finished setup, open on screen.
+    const harness = new SlackIntegrationHarness(configuredSlackFixture());
+    await harness.mount();
+    expect(await harness.defaultChannel()).toBe(SLACK_PUBLIC_CHANNEL.name);
+
+    // When — the destination changes elsewhere (a second tab, another user) and
+    // this page's server data refreshes under the open card, as
+    // `revalidatePath` does after an action.
+    await harness.channelRecordedElsewhere(SLACK_SECOND_PUBLIC_CHANNEL.name);
+    await harness.refreshPageData();
+
+    // Then — the card reports what is on record, not the copy it took at mount.
+    expect(await harness.defaultChannel()).toBe(
+      SLACK_SECOND_PUBLIC_CHANNEL.name,
+    );
+    expect(harness.offersTestMessage()).toBe(true);
+    // And — the picker followed too: the superseded destination is not left one
+    // click from being saved back.
+    expect(harness.offersChannelSave()).toBe(false);
+  }, 60000);
+
+  it("says which permission is missing when Slack refuses the channel listing, leaving the recorded channel alone", async () => {
+    // Given — a recorded destination, and an install missing a scope the listing
+    // needs. The API names it in `code` (contract, Errors), not in `detail`.
+    const harness = new SlackIntegrationHarness(
+      slackFixtureWithDefaultChannel(SLACK_PUBLIC_CHANNEL, {
+        channelsRefusal: SLACK_MISSING_SCOPE_REFUSAL,
+      }),
+    );
+
+    // When
+    await harness.mount();
+
+    // Then — the reason, worded as a fix, with the invite copy still beside the
+    // picker.
+    const message = await harness.channelPickerMessage();
+    expect(message).toMatch(/missing a permission it needs in Slack/);
+    expect(message).toMatch(/Connect the workspace again and approve/);
+    // Slack's reason is a protocol token: it travels in `code` and is never
+    // shown.
+    expect(message).not.toMatch(SLACK_MISSING_SCOPE_CODE);
+    expect(harness.channelInviteHint()).toMatch(/invites @Prowler/);
+
+    // And — a listing Prowler could not read says nothing about the channel
+    // already recorded.
+    expect(await harness.defaultChannel()).toBe(SLACK_PUBLIC_CHANNEL.name);
+    expect(harness.offersTestMessage()).toBe(true);
+  }, 30000);
+
+  it("names the wait Slack asked for when it rate limits the channel listing", async () => {
+    // Given — `conversations.list` is Slack tier 2 and paginated (contract,
+    // Errors); the `429` carries the wait in `Retry-After`.
+    const harness = new SlackIntegrationHarness(
+      slackFixtureWithDefaultChannel(SLACK_PUBLIC_CHANNEL, {
+        channelsRefusal: SLACK_RATE_LIMITED_REFUSAL,
+      }),
+    );
+
+    // When
+    await harness.mount();
+
+    // Then — when to come back, not just that it was refused: the wait is
+    // asserted, not only the wording.
+    const message = await harness.channelPickerMessage();
+    expect(message).toMatch(/rate limiting/);
+    expect(message).toMatch(/about 30 seconds/);
+
+    // And — waiting is the fix, so nothing is said about permissions.
+    expect(message).not.toMatch(/permission/);
+    expect(await harness.defaultChannel()).toBe(SLACK_PUBLIC_CHANNEL.name);
+  }, 30000);
+
+  it("keeps the channels it did read on offer when Slack refuses a later page", async () => {
+    // Given — a two-page workspace whose second page is rate limited
+    // (`conversations.list` is tier 2, contract, Errors).
+    const harness = new SlackIntegrationHarness(partiallyReadSlackFixture());
+
+    // When
+    await harness.mount();
+
+    // Then — the picker offers what was read rather than being replaced by the
+    // refusal: every reload re-runs the same reads into the same limit.
+    expect(await harness.channelOptions()).toEqual([
+      SLACK_PUBLIC_CHANNEL.name,
+      SLACK_SECOND_PUBLIC_CHANNEL.name,
+    ]);
+    expect(harness.saysChannelsUnreadable()).toBe(false);
+
+    // And — the wait is still said, as the explanation for the short list.
+    const notice = harness.partialListNotice();
+    expect(notice).toMatch(/rate limiting/);
+    expect(notice).toMatch(/about 30 seconds/);
+
+    // And — a partial read says nothing about the destination already recorded.
+    expect(await harness.defaultChannel()).toBe(SLACK_PUBLIC_CHANNEL.name);
+    expect(harness.offersTestMessage()).toBe(true);
+  }, 60000);
+
+  it("says nothing about a short list when the whole workspace was read", async () => {
+    // Given — the default workspace: two cursor pages, read to the end.
+    const harness = new SlackIntegrationHarness(connectedSlackFixture());
+
+    // When
+    await harness.mount();
+
+    // Then
+    expect(harness.partialListNotice()).toBeNull();
+  }, 30000);
+
+  it("falls back to the API's wording when the listing fails upstream", async () => {
+    // Given — a `502`, which names no `code` because there is nothing to act on
+    // (contract, Errors).
+    const harness = new SlackIntegrationHarness(
+      slackFixtureWithDefaultChannel(SLACK_PUBLIC_CHANNEL, {
+        channelsRefusal: SLACK_UPSTREAM_REFUSAL,
+      }),
+    );
+
+    // When
+    await harness.mount();
+
+    // Then — the API's own `detail`, and not a wait that was never promised.
+    const message = await harness.channelPickerMessage();
+    expect(message).toMatch(/Slack is temporarily unavailable/);
+    expect(message).not.toMatch(/rate limiting/);
+    expect(await harness.defaultChannel()).toBe(SLACK_PUBLIC_CHANNEL.name);
+  }, 30000);
+
+  it("says to invite @Prowler when Slack refuses the channel because the app is not in it", async () => {
+    // Given — a private channel the app was removed from. The API validates the
+    // channel against Slack on the way in and refuses with `not_in_channel`.
+    const harness = new SlackIntegrationHarness(
+      connectedSlackFixture({
+        channelSaveRefusal: SLACK_NOT_IN_CHANNEL_REFUSAL,
+      }),
+    );
+    await harness.mount();
+
+    // When
+    const refusal = await harness.refusedChannelSave(
+      SLACK_PRIVATE_CHANNEL.name,
+    );
+
+    // Then — the one fix the user can carry out themselves, in Slack.
+    expect(refusal).toMatch(/Prowler is not in that channel/);
+    expect(refusal).toMatch(/Invite @Prowler to it in Slack/);
+    expect(refusal).not.toMatch(SLACK_NOT_IN_CHANNEL_CODE);
+
+    // And — nothing was recorded, so nothing is offered to post with.
+    expect(await harness.defaultChannel()).toBeNull();
+    expect(harness.offersTestMessage()).toBe(false);
+  }, 60000);
+
+  it("says the channel is gone, not that @Prowler needs inviting, when Slack no longer has it", async () => {
+    // Given — a channel archived since the listing was read. The API's `detail`
+    // is word-for-word the one for `not_in_channel`, so only `code` tells them
+    // apart.
+    const harness = new SlackIntegrationHarness(
+      connectedSlackFixture({
+        channelSaveRefusal: SLACK_CHANNEL_NOT_FOUND_REFUSAL,
+      }),
+    );
+    await harness.mount();
+
+    // When
+    const refusal = await harness.refusedChannelSave(SLACK_PUBLIC_CHANNEL.name);
+
+    // Then — a different problem, so different copy: nothing to invite to a
+    // channel that no longer exists.
+    expect(refusal).toMatch(/no longer exists in the workspace/);
+    expect(refusal).toMatch(/Choose another one/);
+    expect(refusal).not.toMatch(/Invite @Prowler/);
+    expect(refusal).not.toMatch(SLACK_UNKNOWN_CHANNEL_DETAIL);
+    expect(await harness.defaultChannel()).toBeNull();
+  }, 60000);
+});
+
+describe("sending a test message", () => {
+  it("is not offered until a destination channel is recorded", async () => {
+    // Given — connected, but no channel chosen yet.
+    const harness = new SlackIntegrationHarness(connectedSlackFixture());
+
+    // When
+    await harness.mount();
+
+    // Then
+    expect(await harness.defaultChannel()).toBeNull();
+    expect(harness.offersTestMessage()).toBe(false);
+  }, 30000);
+
+  it("sends a test message to the recorded channel and reports it delivered", async () => {
+    // Given — a tenant that has recorded where Prowler should post.
+    const harness = new SlackIntegrationHarness(connectedSlackFixture());
+    await harness.mount();
+    await harness.chooseChannel(SLACK_PUBLIC_CHANNEL.name);
+
+    // When
+    const outcome = await harness.sendTestMessage();
+
+    // Then — sent, and the user reads which channel it went to.
+    expect(outcome).toBe(TEST_MESSAGE_OUTCOME.SENT);
+    expect(await harness.lastTestMessageOutcome()).toMatch(
+      `#${SLACK_PUBLIC_CHANNEL.name}`,
+    );
+  }, 60000);
+
+  it("surfaces the reason when Slack refuses the test message", async () => {
+    // Given — the post fails, which the API reports on the task it handed back
+    // (design D9), not on the request that started it, using the same stable
+    // reason the synchronous endpoints put in `code`.
+    const harness = new SlackIntegrationHarness(
+      connectedSlackFixture({
+        testMessage: { accepted: false, error: SLACK_NOT_IN_CHANNEL_CODE },
+      }),
+    );
+    await harness.mount();
+    await harness.chooseChannel(SLACK_PUBLIC_CHANNEL.name);
+
+    // When
+    const outcome = await harness.sendTestMessage();
+
+    // Then — the same copy the synchronous refusals get, not the raw token.
+    expect(outcome).toBe(TEST_MESSAGE_OUTCOME.FAILED);
+    const reported = await harness.lastTestMessageOutcome();
+    expect(reported).toMatch(/Prowler is not in that channel/);
+    expect(reported).toMatch(/Invite @Prowler to it in Slack/);
+    expect(reported).not.toMatch(SLACK_NOT_IN_CHANNEL_CODE);
+  }, 60000);
+
+  it("reports a refusal the task words itself, rather than swallowing it", async () => {
+    // Given — a task result carrying prose instead of a stable reason; its exact
+    // shape is the cloud lane's to pin down (contract, test-message).
+    const harness = new SlackIntegrationHarness(
+      connectedSlackFixture({
+        testMessage: {
+          accepted: false,
+          error: SLACK_TEST_MESSAGE_REFUSED_DETAIL,
+        },
+      }),
+    );
+    await harness.mount();
+    await harness.chooseChannel(SLACK_PUBLIC_CHANNEL.name);
+
+    // When
+    const outcome = await harness.sendTestMessage();
+
+    // Then
+    expect(outcome).toBe(TEST_MESSAGE_OUTCOME.FAILED);
+    expect(await harness.lastTestMessageOutcome()).toMatch(
+      SLACK_TEST_MESSAGE_REFUSED_DETAIL,
+    );
+  }, 60000);
+
+  it("keeps a reason it has no copy for inside its own sentence, not as the whole message", async () => {
+    // Given — a real Slack reason this UI has no copy for; Slack's set is
+    // open-ended, so this is the ordinary case.
+    const harness = new SlackIntegrationHarness(
+      connectedSlackFixture({
+        testMessage: { accepted: false, error: SLACK_UNMAPPED_REASON_CODE },
+      }),
+    );
+    await harness.mount();
+    await harness.chooseChannel(SLACK_PUBLIC_CHANNEL.name);
+
+    // When
+    const outcome = await harness.sendTestMessage();
+
+    // Then — Prowler's wording, with Slack's word for it kept for diagnosis.
+    expect(outcome).toBe(TEST_MESSAGE_OUTCOME.FAILED);
+    const reported = await harness.lastTestMessageOutcome();
+    expect(reported).toMatch(/Slack refused the message/);
+    expect(reported).toMatch(SLACK_UNMAPPED_REASON_CODE);
+    expect(reported).not.toBe(SLACK_UNMAPPED_REASON_CODE);
+  }, 60000);
+});

ui/components/icons/services/IconServices.tsx

@@ -764,7 +764,6 @@ export const SlackIcon: React.FC<IconSvgProps> = ({
     fill="none"
     focusable="false"
     height={height ?? size}
-    role="presentation"
     viewBox="0 0 48 48"
     width={width ?? size}
     className={className}

ui/components/integrations/slack/slack-callback.tsx

@@ -12,6 +12,7 @@ import {
   AlertTitle,
   Button,
 } from "@/components/shadcn";
+import { SLACK_REASON_TOKEN } from "@/lib/integrations/slack-errors";
 
 const SLACK_INTEGRATION_PATH = "/integrations/slack";
 
@@ -35,15 +36,13 @@ const FAILURE_TITLE = "Slack workspace not connected";
  */
 const UNCONFIRMED_TITLE = "Slack install not confirmed";
 
-// `error` comes straight off the URL and is interpolated into Prowler's own copy,
-// so gate on the shape of a code: Slack publishes no closed set of values.
-const REASON_TOKEN = /^[a-z0-9_]{1,48}$/;
-
 const describeSlackError = (reason: string): string => {
   if (reason === "access_denied") {
     return "The install was not approved in Slack, so no workspace was connected.";
   }
-  return REASON_TOKEN.test(reason)
+  // `error` comes straight off the URL and is interpolated into Prowler's own
+  // copy, so gate on the shape of a code: Slack publishes no closed set.
+  return SLACK_REASON_TOKEN.test(reason)
     ? `Slack could not complete the install (${reason}).`
     : "Slack could not complete the install.";
 };

ui/components/integrations/slack/slack-channel-selector.tsx

@@ -0,0 +1,132 @@
+"use client";
+
+import { Lock, RefreshCw } from "lucide-react";
+
+import {
+  Alert,
+  AlertDescription,
+  AlertTitle,
+  Badge,
+  Button,
+  Label,
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from "@/components/shadcn";
+import type { SlackChannelOption } from "@/types/integrations";
+
+const INVITE_HINT =
+  "A private channel only appears here after someone invites @Prowler to it in Slack. Invite it, then refresh.";
+
+interface SlackChannelSelectorProps {
+  options: SlackChannelOption[];
+  value: string | null;
+  onChange: (channelId: string) => void;
+  isLoading?: boolean;
+  /** Why the channels could not be read — Slack's own reason, when it gave one. */
+  error?: string | null;
+  /** Why the list is partial. Shown with the picker, not instead of it. */
+  incompleteNotice?: string | null;
+  onRefresh?: () => void;
+  disabled?: boolean;
+}
+
+/** Driven entirely by props (design D13) so the alert-rule form can reuse it. */
+export const SlackChannelSelector = ({
+  options,
+  value,
+  onChange,
+  isLoading = false,
+  error = null,
+  incompleteNotice = null,
+  onRefresh,
+  disabled = false,
+}: SlackChannelSelectorProps) => {
+  const isEmpty = !isLoading && !error && options.length === 0;
+  // `htmlFor` may only name an element that exists, and the trigger is only
+  // rendered in the picker branch.
+  const hasPicker = !error && !isEmpty;
+
+  return (
+    <div className="flex flex-col gap-2">
+      <div className="flex items-center justify-between gap-3">
+        <Label htmlFor={hasPicker ? "slack-channel" : undefined}>
+          Destination channel
+        </Label>
+        {onRefresh && (
+          <Button
+            size="sm"
+            variant="outline"
+            disabled={isLoading}
+            onClick={onRefresh}
+          >
+            <RefreshCw size={14} />
+            {isLoading ? "Refreshing..." : "Refresh channels"}
+          </Button>
+        )}
+      </div>
+
+      {error ? (
+        <Alert variant="error">
+          <AlertTitle>Could not read the workspace&apos;s channels</AlertTitle>
+          <AlertDescription>{error}</AlertDescription>
+        </Alert>
+      ) : isEmpty ? (
+        <Alert variant="info">
+          <AlertTitle>No channels available yet</AlertTitle>
+          <AlertDescription>
+            Prowler cannot see a single channel in this workspace. Create a
+            public channel, or invite @Prowler to a private one in Slack with
+            <span className="font-medium"> /invite @Prowler</span>, then
+            refresh.
+          </AlertDescription>
+        </Alert>
+      ) : (
+        <>
+          {incompleteNotice && (
+            <Alert variant="warning" data-channels-notice>
+              <AlertTitle>Not every channel is listed</AlertTitle>
+              <AlertDescription>{incompleteNotice}</AlertDescription>
+            </Alert>
+          )}
+          <Select
+            value={value ?? undefined}
+            onValueChange={onChange}
+            disabled={disabled || isLoading}
+          >
+            <SelectTrigger id="slack-channel" size="sm">
+              <SelectValue
+                placeholder={
+                  isLoading ? "Reading channels..." : "Choose a channel"
+                }
+              />
+            </SelectTrigger>
+            <SelectContent>
+              {options.map((option) => (
+                <SelectItem
+                  key={option.id}
+                  value={option.id}
+                  // Name hook: the rendered label mixes it with a lock icon
+                  // and a "Private" badge.
+                  data-channel={option.name}
+                >
+                  {option.is_private && <Lock size={14} aria-hidden="true" />}
+                  <span className="min-w-0 truncate">#{option.name}</span>
+                  {option.is_private && (
+                    <Badge variant="tag" size="sm">
+                      Private
+                    </Badge>
+                  )}
+                </SelectItem>
+              ))}
+            </SelectContent>
+          </Select>
+        </>
+      )}
+
+      <p className="text-text-neutral-secondary text-xs">{INVITE_HINT}</p>
+    </div>
+  );
+};

ui/components/integrations/slack/slack-integration-manager.test.tsx

@@ -0,0 +1,66 @@
+/**
+ * The case `slack-page.integration.test.tsx` cannot express: it asserts against
+ * a hydrated, settled page, so it never sees the first frame the user is
+ * served. The effect that reads the channels only runs in the browser, so the
+ * channel state at render time is what the served HTML says until hydration.
+ */
+
+import { renderToString } from "react-dom/server";
+import { describe, expect, it, vi } from "vitest";
+
+import { INTEGRATION_TYPE, type IntegrationProps } from "@/types/integrations";
+
+import { SlackIntegrationManager } from "./slack-integration-manager";
+
+vi.mock("@/actions/integrations/slack", () => ({
+  getSlackChannels: vi.fn(),
+  sendSlackTestMessage: vi.fn(),
+  setSlackDefaultChannel: vi.fn(),
+}));
+
+vi.mock("@/actions/integrations/integrations", () => ({
+  testIntegrationConnection: vi.fn(),
+}));
+
+/**
+ * A connected workspace with no channel recorded, as the contract has it before
+ * a save: with one, the picker would show that channel instead of the
+ * placeholder this test reads.
+ */
+const CONNECTED_WORKSPACE: IntegrationProps = {
+  type: "integrations",
+  id: "slack-integration-1",
+  attributes: {
+    inserted_at: "2026-08-10T09:00:00Z",
+    updated_at: "2026-08-10T09:00:00Z",
+    enabled: true,
+    connected: true,
+    connection_last_checked_at: "2026-08-10T09:05:00Z",
+    integration_type: INTEGRATION_TYPE.SLACK,
+    configuration: {
+      team_id: "T024BE7LD",
+      team_name: "Prowler HQ",
+      bot_user_id: "U0KRQLJ9H",
+    },
+  },
+  links: { self: "/api/v1/integrations/slack-integration-1" },
+};
+
+describe("the first paint of a connected workspace", () => {
+  it("reads as still reading the channels rather than as a workspace with none", () => {
+    // When
+    const serverHtml = renderToString(
+      <SlackIntegrationManager
+        integration={CONNECTED_WORKSPACE}
+        authorizeUrl={null}
+        unavailable={false}
+        rateLimitMessage={null}
+        loadError={null}
+      />,
+    );
+
+    // Then
+    expect(serverHtml).toContain("Reading channels...");
+    expect(serverHtml).not.toContain("No channels available yet");
+  });
+});

ui/components/integrations/slack/slack-integration-manager.tsx

@@ -1,12 +1,18 @@
 "use client";
 
 import { format, isValid, parseISO } from "date-fns";
-import { TestTube } from "lucide-react";
-import { useState } from "react";
+import { Send, TestTube } from "lucide-react";
+import { useEffect, useState } from "react";
 
 import { testIntegrationConnection } from "@/actions/integrations/integrations";
+import {
+  getSlackChannels,
+  sendSlackTestMessage,
+  setSlackDefaultChannel,
+} from "@/actions/integrations/slack";
 import { SlackIcon } from "@/components/icons/services/IconServices";
 import { IntegrationCardHeader } from "@/components/integrations/shared";
+import { SlackChannelSelector } from "@/components/integrations/slack/slack-channel-selector";
 import {
   Alert,
   AlertDescription,
@@ -17,7 +23,76 @@ import {
   CardHeader,
   useToast,
 } from "@/components/shadcn";
-import type { IntegrationProps } from "@/types/integrations";
+import type {
+  IntegrationProps,
+  SlackChannelOption,
+} from "@/types/integrations";
+
+const CHANNELS_STATUS = {
+  LOADING: "loading",
+  ERROR: "error",
+  LOADED: "loaded",
+} as const;
+
+interface ChannelsLoading {
+  status: typeof CHANNELS_STATUS.LOADING;
+}
+
+interface ChannelsFailed {
+  status: typeof CHANNELS_STATUS.ERROR;
+  message: string;
+}
+
+interface ChannelsLoaded {
+  status: typeof CHANNELS_STATUS.LOADED;
+  channels: SlackChannelOption[];
+  // Rides with the list it qualifies, so it can never outlive it.
+  notice: string | null;
+}
+
+type ChannelsState = ChannelsLoading | ChannelsFailed | ChannelsLoaded;
+
+const TEST_MESSAGE_STATUS = {
+  IDLE: "idle",
+  SENDING: "sending",
+  SENT: "sent",
+  FAILED: "failed",
+} as const;
+
+interface TestMessageIdle {
+  status: typeof TEST_MESSAGE_STATUS.IDLE;
+}
+
+interface TestMessageSending {
+  status: typeof TEST_MESSAGE_STATUS.SENDING;
+}
+
+interface TestMessageSent {
+  status: typeof TEST_MESSAGE_STATUS.SENT;
+  detail: string;
+}
+
+interface TestMessageFailed {
+  status: typeof TEST_MESSAGE_STATUS.FAILED;
+  detail: string;
+}
+
+type TestMessageState =
+  | TestMessageIdle
+  | TestMessageSending
+  | TestMessageSent
+  | TestMessageFailed;
+
+// The name may be missing: the id decides what the UI can do with it.
+interface SlackChannelRef {
+  id: string;
+  name: string | null;
+}
+
+const channelRefEquals = (
+  a: SlackChannelRef | null,
+  b: SlackChannelRef | null,
+) => a?.id === b?.id && a?.name === b?.name;
 
 interface SlackIntegrationManagerProps {
   /** At most one exists per tenant (one workspace). */
@@ -39,6 +114,162 @@ export const SlackIntegrationManager = ({
   const [isTesting, setIsTesting] = useState(false);
   const { toast } = useToast();
 
+  const integrationId = integration?.id ?? null;
+  const recordedChannelId =
+    integration?.attributes.configuration.channel_id ?? null;
+  const recordedChannelName =
+    integration?.attributes.configuration.channel_name ?? null;
+
+  const recordedChannel: SlackChannelRef | null = recordedChannelId
+    ? { id: recordedChannelId, name: recordedChannelName }
+    : null;
+
+  // Seeded `loading`, not by the effect: the effect never runs on the server,
+  // so anything else would server-render a "no channels" picker until
+  // hydration.
+  const [channelsState, setChannelsState] = useState<ChannelsState>(
+    integrationId
+      ? { status: CHANNELS_STATUS.LOADING }
+      : { status: CHANNELS_STATUS.LOADED, channels: [], notice: null },
+  );
+  // Bumped by refresh: a channel invited after load only shows on a re-read.
+  const [channelReloads, setChannelReloads] = useState(0);
+  // Local state needed: the pick is buffered until the user saves it.
+  const [selectedChannelId, setSelectedChannelId] = useState<string | null>(
+    recordedChannelId,
+  );
+  // Mirrored in state, not read from the prop, so channel-gated affordances
+  // move on save instead of waiting for the revalidation.
+  const [defaultChannel, setDefaultChannel] = useState(recordedChannel);
+  // The prop the mirror was last taken from: the card never unmounts, so a
+  // mirror seeded only at mount would go stale when the record changes.
+  const [syncedChannel, setSyncedChannel] = useState(recordedChannel);
+  const [isSavingChannel, setIsSavingChannel] = useState(false);
+  const [testMessageState, setTestMessageState] = useState<TestMessageState>({
+    status: TEST_MESSAGE_STATUS.IDLE,
+  });
+
+  if (!channelRefEquals(recordedChannel, syncedChannel)) {
+    const previousSyncedId = syncedChannel?.id ?? null;
+    setSyncedChannel(recordedChannel);
+    setDefaultChannel(recordedChannel);
+    // Follow the record only while the buffered pick still matches it: an
+    // unsaved pick is the user's, not ours to overwrite mid-edit.
+    if (selectedChannelId === previousSyncedId) {
+      setSelectedChannelId(recordedChannel?.id ?? null);
+    }
+  }
+
+  useEffect(() => {
+    if (!integrationId) return;
+
+    let cancelled = false;
+    setChannelsState({ status: CHANNELS_STATUS.LOADING });
+
+    getSlackChannels(integrationId)
+      .then((result) => {
+        if (cancelled) return;
+        setChannelsState(
+          "error" in result
+            ? { status: CHANNELS_STATUS.ERROR, message: result.error }
+            : {
+                status: CHANNELS_STATUS.LOADED,
+                channels: result.channels,
+                notice: result.incomplete ?? null,
+              },
+        );
+      })
+      .catch(() => {
+        if (cancelled) return;
+        setChannelsState({
+          status: CHANNELS_STATUS.ERROR,
+          message: "Could not reach Slack to read the channel list.",
+        });
+      });
+
+    return () => {
+      cancelled = true;
+    };
+  }, [integrationId, channelReloads]);
+
+  const channels =
+    channelsState.status === CHANNELS_STATUS.LOADED
+      ? channelsState.channels
+      : [];
+
+  const handleSaveChannel = async () => {
+    if (!integrationId || !selectedChannelId) return;
+
+    setIsSavingChannel(true);
+    try {
+      // Only the id travels — the API validates it and derives the name
+      // (design D6).
+      const result = await setSlackDefaultChannel(
+        integrationId,
+        selectedChannelId,
+      );
+
+      if ("error" in result) {
+        toast({
+          variant: "destructive",
+          title: "Could not save the destination channel",
+          description: result.error,
+        });
+        return;
+      }
+
+      // Prefer the API's derived name: a channel renamed in Slack since the
+      // list was read would otherwise show its old name.
+      const savedName =
+        result.integration.attributes.configuration.channel_name ??
+        channels.find((channel) => channel.id === selectedChannelId)?.name ??
+        null;
+
+      setDefaultChannel({ id: selectedChannelId, name: savedName });
+      // An outcome about the previous destination would mislead here.
+      setTestMessageState({ status: TEST_MESSAGE_STATUS.IDLE });
+      toast({
+        title: "Destination channel saved",
+        description: savedName
+          ? `Prowler will post to #${savedName}.`
+          : "Prowler will post to the channel you chose.",
+      });
+    } catch (_error) {
+      toast({
+        variant: "destructive",
+        title: "Could not save the destination channel",
+        description: "Something went wrong. Please try again.",
+      });
+    } finally {
+      setIsSavingChannel(false);
+    }
+  };
+
+  const handleSendTestMessage = async () => {
+    if (!integrationId) return;
+
+    setTestMessageState({ status: TEST_MESSAGE_STATUS.SENDING });
+    try {
+      const result = await sendSlackTestMessage(integrationId);
+
+      setTestMessageState(
+        "sent" in result
+          ? {
+              status: TEST_MESSAGE_STATUS.SENT,
+              detail: defaultChannel?.name
+                ? `Prowler posted a test message to #${defaultChannel.name}.`
+                : "Prowler posted a test message to your default channel.",
+            }
+          : { status: TEST_MESSAGE_STATUS.FAILED, detail: result.error },
+      );
+    } catch (_error) {
+      setTestMessageState({
+        status: TEST_MESSAGE_STATUS.FAILED,
+        detail: "Something went wrong. Please try again.",
+      });
+    }
+  };
+
   const handleTestConnection = async (id: string) => {
     setIsTesting(true);
     try {
@@ -68,10 +299,7 @@ export const SlackIntegrationManager = ({
     }
   };
 
-  const configuration = integration?.attributes.configuration;
-  const workspaceName = configuration?.team_name;
-  // Absent until a channel is chosen, never present-and-null.
-  const channelId = configuration?.channel_id ?? null;
+  const workspaceName = integration?.attributes.configuration.team_name;
 
   const checkedAt = integration?.attributes.connection_last_checked_at;
   const checkedOn = checkedAt ? parseISO(checkedAt) : null;
@@ -132,7 +360,7 @@ export const SlackIntegrationManager = ({
                     {lastCheckedOn}
                   </p>
                 )}
-                {!channelId && (
+                {!defaultChannel && (
                   <p>
                     Choosing a destination channel is the next step — the
                     connection is checked against it.
@@ -144,13 +372,92 @@ export const SlackIntegrationManager = ({
               <Button
                 size="sm"
                 variant="outline"
-                disabled={isTesting || !channelId}
+                disabled={isTesting || !defaultChannel}
                 onClick={() => handleTestConnection(integration.id)}
               >
                 <TestTube size={14} />
                 {isTesting ? "Testing..." : "Test connection"}
               </Button>
             </div>
+
+            <div className="border-border-neutral-secondary mt-6 flex flex-col gap-4 border-t pt-6">
+              <SlackChannelSelector
+                options={channels}
+                value={selectedChannelId}
+                onChange={setSelectedChannelId}
+                isLoading={channelsState.status === CHANNELS_STATUS.LOADING}
+                error={
+                  channelsState.status === CHANNELS_STATUS.ERROR
+                    ? channelsState.message
+                    : null
+                }
+                incompleteNotice={
+                  channelsState.status === CHANNELS_STATUS.LOADED
+                    ? channelsState.notice
+                    : null
+                }
+                onRefresh={() => setChannelReloads((reloads) => reloads + 1)}
+                disabled={isSavingChannel}
+              />
+
+              <div className="flex flex-col gap-3 sm:flex-row sm:items-center sm:justify-between">
+                <p className="text-text-neutral-secondary text-xs">
+                  {/* The id decides, not the name: a missing name would deny
+                      a destination the test button posts to. */}
+                  {defaultChannel
+                    ? defaultChannel.name
+                      ? `Prowler posts to #${defaultChannel.name}.`
+                      : "Prowler posts to the channel you saved."
+                    : "No destination channel recorded yet."}
+                </p>
+                <div className="flex gap-2">
+                  <Button
+                    size="sm"
+                    disabled={
+                      !selectedChannelId ||
+                      selectedChannelId === (defaultChannel?.id ?? null) ||
+                      isSavingChannel
+                    }
+                    onClick={handleSaveChannel}
+                  >
+                    {isSavingChannel ? "Saving..." : "Save channel"}
+                  </Button>
+                  {defaultChannel && (
+                    <Button
+                      size="sm"
+                      variant="outline"
+                      disabled={
+                        testMessageState.status === TEST_MESSAGE_STATUS.SENDING
+                      }
+                      onClick={handleSendTestMessage}
+                    >
+                      <Send size={14} />
+                      {testMessageState.status === TEST_MESSAGE_STATUS.SENDING
+                        ? "Sending..."
+                        : "Send test message"}
+                    </Button>
+                  )}
+                </div>
+              </div>
+
+              {(testMessageState.status === TEST_MESSAGE_STATUS.SENT ||
+                testMessageState.status === TEST_MESSAGE_STATUS.FAILED) && (
+                <Alert
+                  variant={
+                    testMessageState.status === TEST_MESSAGE_STATUS.SENT
+                      ? "success"
+                      : "error"
+                  }
+                >
+                  <AlertTitle>
+                    {testMessageState.status === TEST_MESSAGE_STATUS.SENT
+                      ? "Test message sent"
+                      : "Test message failed"}
+                  </AlertTitle>
+                  <AlertDescription>{testMessageState.detail}</AlertDescription>
+                </Alert>
+              )}
+            </div>
           </CardContent>
         </Card>
       ) : (

ui/lib/integrations/slack-errors.test.ts

@@ -11,10 +11,12 @@ import {
   SLACK_ERROR_MESSAGES,
   SLACK_GENERIC_ERROR_MESSAGE,
   SLACK_RATE_LIMITED_MESSAGE,
+  SLACK_REASON_TOKEN,
   SLACK_TOKEN_ERROR_CODES,
   readSlackFailure,
   slackErrorMessage,
   slackRateLimitMessage,
+  slackUnknownReasonMessage,
 } from "./slack-errors";
 
 describe("slackErrorMessage", () => {
@@ -91,6 +93,63 @@ describe("slackErrorMessage", () => {
   });
 });
 
+describe("slackUnknownReasonMessage", () => {
+  /** A real Slack reason this UI has no copy of its own for. */
+  const UNMAPPED_REASON = "is_archived";
+
+  it("keeps an unmapped reason diagnosable without letting it be the message", () => {
+    const message = slackUnknownReasonMessage(UNMAPPED_REASON);
+
+    expect(message).toMatch(/Slack refused the message/);
+    expect(message).toContain(UNMAPPED_REASON);
+    expect(message).not.toBe(UNMAPPED_REASON);
+    expect(message).toMatch(/Choose another channel/);
+  });
+
+  it("is only reached for a code the mapping does not cover", () => {
+    expect(
+      slackErrorMessage(
+        { code: SLACK_ERROR_CODE.NOT_IN_CHANNEL },
+        slackUnknownReasonMessage(SLACK_ERROR_CODE.NOT_IN_CHANNEL),
+      ),
+    ).toBe(SLACK_ERROR_MESSAGES[SLACK_ERROR_CODE.NOT_IN_CHANNEL]);
+
+    // No `detail`: one holding the same token would make the raw token the
+    // whole message again.
+    expect(
+      slackErrorMessage(
+        { code: UNMAPPED_REASON },
+        slackUnknownReasonMessage(UNMAPPED_REASON),
+      ),
+    ).toBe(slackUnknownReasonMessage(UNMAPPED_REASON));
+  });
+});
+
+describe("SLACK_REASON_TOKEN", () => {
+  it("recognises a reason code and refuses anything that reads as a sentence", () => {
+    // Slack publishes no closed set of reasons, so the guard is on shape rather
+    // than an allowlist.
+    for (const reason of [
+      "is_archived",
+      "restricted_action",
+      "team_access_not_granted",
+      "ekm_access_denied",
+      "messages_tab_disabled",
+    ]) {
+      expect(SLACK_REASON_TOKEN.test(reason)).toBe(true);
+    }
+
+    for (const prose of [
+      "Slack rejected the message: the channel is archived.",
+      "). Contact support at +1-555-0100 (",
+      "",
+      "a".repeat(49),
+    ]) {
+      expect(SLACK_REASON_TOKEN.test(prose)).toBe(false);
+    }
+  });
+});
+
 describe("slackRateLimitMessage", () => {
   it("names the wait Slack asked for", () => {
     expect(slackRateLimitMessage(30)).toMatch(/about 30 seconds/);

ui/lib/integrations/slack-errors.ts

@@ -39,6 +39,13 @@ export const SLACK_GENERIC_ERROR_MESSAGE =
 export const SLACK_RATE_LIMITED_MESSAGE =
   "Slack is rate limiting Prowler right now. Try again in a few moments.";
 
+/**
+ * For a channel list that stopped short of the workspace: the page budget ran
+ * out, or `links.next` left the API's origin.
+ */
+export const SLACK_PARTIAL_CHANNEL_LIST_MESSAGE =
+  "This workspace has more channels than Prowler reads in one go, so this list is not all of them. A channel missing from it is not necessarily one @Prowler has to be invited to.";
+
 /**
  * For a `2xx` the UI could not read. Not phrased as a failure: the install
  * happened, only the workspace cannot be named.
@@ -46,6 +53,19 @@ export const SLACK_RATE_LIMITED_MESSAGE =
 export const SLACK_UNREADABLE_RESULT_MESSAGE =
   "Prowler could not read the result of the install. Open the Slack integration page to see the workspace — if none is listed there, start the install again.";
 
+/**
+ * The shape of a Slack reason code, as opposed to a sentence: the set is
+ * open-ended, so a reason is gated on its shape before being interpolated.
+ */
+export const SLACK_REASON_TOKEN = /^[a-z0-9_]{1,48}$/;
+
+/**
+ * Copy for a reason code Prowler has no wording of its own for — the ordinary
+ * case, since the set is open-ended.
+ */
+export const slackUnknownReasonMessage = (reason: string): string =>
+  `Slack refused the message (${reason}). Choose another channel, or check the channel in Slack.`;
+
 const RECONNECT = "Connect the workspace again to restore access.";
 
 export const SLACK_ERROR_MESSAGES = {

ui/types/integrations.ts

@@ -113,6 +113,16 @@ export interface IntegrationProps {
   links: { self: string };
 }
 
+/**
+ * A channel Prowler can post to: every active public channel, plus the private
+ * ones `@Prowler` was invited to. `is_private` keeps the API's own naming.
+ */
+export interface SlackChannelOption {
+  id: string;
+  name: string;
+  is_private: boolean;
+}
+
 // Jira dispatch types
 export interface JiraDispatchRequest {
   data: {