feat(channels): add thread @-mentions and an Activity page

packages/api-client/src/posthog-client.ts

@@ -49,6 +49,7 @@ import type {
   DismissalArtefact,
   LineReferenceArtefact,
   NoteArtefact,
+  OrganizationMemberBasic,
   PriorityJudgmentArtefact,
   RepoSelectionArtefact,
   SafetyJudgmentArtefact,
@@ -72,6 +73,7 @@ import type {
   SuggestedReviewerWriteEntry,
   Task,
   TaskChannel,
+  TaskMention,
   TaskRun,
   TaskRunArtefact,
   TaskThreadMessage,
@@ -2304,6 +2306,25 @@ export class PostHogAPIClient {
     return (await response.json()) as TaskChannel;
   }
 
+  // Mentions of the current user across task threads, newest first.
+  async getTaskMentions(options?: { since?: string }): Promise<TaskMention[]> {
+    const teamId = await this.getTeamId();
+    const urlPath = `/api/projects/${teamId}/task_mentions/`;
+    const url = new URL(`${this.api.baseUrl}${urlPath}`);
+    if (options?.since) {
+      url.searchParams.set("since", options.since);
+    }
+    const response = await this.api.fetcher.fetch({
+      method: "get",
+      url,
+      path: urlPath,
+    });
+    if (!response.ok) {
+      throw new Error(`Failed to fetch task mentions: ${response.statusText}`);
+    }
+    return (await response.json()) as TaskMention[];
+  }
+
   async getTaskThreadMessages(taskId: string): Promise<TaskThreadMessage[]> {
     const teamId = await this.getTeamId();
     const urlPath = `/api/projects/${teamId}/tasks/${taskId}/thread_messages/`;
@@ -2384,6 +2405,40 @@ export class PostHogAPIClient {
     return (await response.json()) as TaskThreadMessage;
   }
 
+  // Everyone in the current organization — the pool of taggable teammates for
+  // thread @-mentions. Membership churn is slow, so callers cache aggressively.
+  async listOrganizationMembers(): Promise<OrganizationMemberBasic[]> {
+    const ORG_MEMBERS_MAX_PAGES = 20;
+    const ORG_MEMBERS_PAGE_SIZE = 200;
+    const all: OrganizationMemberBasic[] = [];
+    let urlPath = `/api/organizations/@current/members/?limit=${ORG_MEMBERS_PAGE_SIZE}`;
+    for (let i = 0; i < ORG_MEMBERS_MAX_PAGES; i++) {
+      const response = await this.api.fetcher.fetch({
+        method: "get",
+        url: new URL(`${this.api.baseUrl}${urlPath}`),
+        path: urlPath,
+      });
+      if (!response.ok) {
+        throw new Error(
+          `Failed to fetch organization members: ${response.statusText}`,
+        );
+      }
+      const page = (await response.json()) as {
+        results: OrganizationMemberBasic[];
+        next: string | null;
+      };
+      all.push(...page.results);
+      if (!page.next) return all;
+      const nextUrl = new URL(page.next);
+      urlPath = `${nextUrl.pathname}${nextUrl.search}`;
+    }
+    log.warn(
+      `listOrganizationMembers hit MAX_PAGES (${ORG_MEMBERS_MAX_PAGES}); returning partial results`,
+      { returned: all.length },
+    );
+    return all;
+  }
+
   async sendRunCommand(
     taskId: string,
     runId: string,

packages/core/src/canvas/mentionActivity.test.ts

@@ -0,0 +1,135 @@
+import type { TaskMention, UserBasic } from "@posthog/shared/domain-types";
+import { describe, expect, it } from "vitest";
+import {
+  countUnseenActivity,
+  mergeTaskMentions,
+  toMentionActivityItems,
+} from "./mentionActivity";
+
+const ann: UserBasic = {
+  id: 2,
+  uuid: "ann-uuid",
+  email: "ann@posthog.com",
+  first_name: "Ann",
+};
+
+function mention(overrides: Partial<TaskMention> = {}): TaskMention {
+  return {
+    id: "mention-1",
+    message_id: "m1",
+    task_id: "t1",
+    task_title: "Task t1",
+    channel_id: "c1",
+    channel_name: "general",
+    author: ann,
+    content: "ping @[Me](me@posthog.com)",
+    created_at: "2026-07-01T10:00:00Z",
+    ...overrides,
+  };
+}
+
+describe("toMentionActivityItems", () => {
+  it("maps mention DTOs to feed items", () => {
+    expect(toMentionActivityItems([mention()])).toEqual([
+      {
+        messageId: "m1",
+        taskId: "t1",
+        taskTitle: "Task t1",
+        channelId: "c1",
+        channelName: "general",
+        author: ann,
+        content: "ping @[Me](me@posthog.com)",
+        createdAt: "2026-07-01T10:00:00Z",
+      },
+    ]);
+  });
+
+  it("labels untitled tasks and tolerates missing channel and author", () => {
+    const items = toMentionActivityItems([
+      mention({
+        task_title: "",
+        channel_id: null,
+        channel_name: null,
+        author: null,
+      }),
+    ]);
+    expect(items[0]).toMatchObject({
+      taskTitle: "Untitled task",
+      channelId: null,
+      channelName: null,
+      author: null,
+    });
+  });
+});
+
+describe("countUnseenActivity", () => {
+  const items = toMentionActivityItems([
+    mention({ message_id: "m2", created_at: "2026-07-03T10:00:00Z" }),
+    mention({ message_id: "m1", created_at: "2026-07-01T10:00:00Z" }),
+  ]);
+
+  it("counts everything when never seen", () => {
+    expect(countUnseenActivity(items, null)).toBe(2);
+  });
+
+  it("counts only items after the last-seen timestamp", () => {
+    expect(countUnseenActivity(items, "2026-07-02T00:00:00Z")).toBe(1);
+    expect(countUnseenActivity(items, "2026-07-04T00:00:00Z")).toBe(0);
+  });
+});
+
+describe("mergeTaskMentions", () => {
+  it("prepends newly-fetched mentions ahead of the previous page", () => {
+    const previous = [
+      mention({ message_id: "m1", created_at: "2026-07-01T10:00:00Z" }),
+    ];
+    const incoming = [
+      mention({ message_id: "m2", created_at: "2026-07-02T10:00:00Z" }),
+    ];
+    expect(
+      mergeTaskMentions(previous, incoming).map((m) => m.message_id),
+    ).toEqual(["m2", "m1"]);
+  });
+
+  it("replaces a mention that was re-fetched instead of duplicating it", () => {
+    const previous = [
+      mention({
+        message_id: "m1",
+        content: "old",
+        created_at: "2026-07-01T10:00:00Z",
+      }),
+    ];
+    const incoming = [
+      mention({
+        message_id: "m1",
+        content: "edited",
+        created_at: "2026-07-01T10:00:00Z",
+      }),
+    ];
+    const merged = mergeTaskMentions(previous, incoming);
+    expect(merged).toHaveLength(1);
+    expect(merged[0].content).toBe("edited");
+  });
+
+  it("returns the previous page unchanged when there is nothing new", () => {
+    const previous = [
+      mention({ message_id: "m1", created_at: "2026-07-01T10:00:00Z" }),
+    ];
+    expect(mergeTaskMentions(previous, [])).toEqual(previous);
+  });
+
+  it("caps the merged result so a long session can't grow it unbounded", () => {
+    const previous = Array.from({ length: 300 }, (_, i) =>
+      mention({
+        message_id: `old-${i}`,
+        created_at: `2026-06-01T${String(i % 24).padStart(2, "0")}:00:00Z`,
+      }),
+    );
+    const incoming = [
+      mention({ message_id: "newest", created_at: "2026-07-05T10:00:00Z" }),
+    ];
+    const merged = mergeTaskMentions(previous, incoming);
+    expect(merged).toHaveLength(300);
+    expect(merged[0].message_id).toBe("newest");
+  });
+});

packages/core/src/canvas/mentionActivity.ts

@@ -0,0 +1,68 @@
+import type { TaskMention, UserBasic } from "@posthog/shared/domain-types";
+
+/**
+ * The Activity feed — thread messages that @-mention the current user — as
+ * served by the backend mentions index (`getTaskMentions`). Mentions are
+ * extracted server-side at write time, so the client only maps DTOs to items.
+ */
+
+export interface MentionActivityItem {
+  messageId: string;
+  taskId: string;
+  taskTitle: string;
+  /** Backend channel (tasks product Channel UUID); null for channel-less tasks. */
+  channelId: string | null;
+  /** Backend channel name, for the "#channel" label. */
+  channelName: string | null;
+  author: UserBasic | null;
+  content: string;
+  createdAt: string;
+}
+
+/** Map mention DTOs (already newest-first from the backend) to feed items. */
+export function toMentionActivityItems(
+  mentions: readonly TaskMention[],
+): MentionActivityItem[] {
+  return mentions.map((mention) => ({
+    messageId: mention.message_id,
+    taskId: mention.task_id,
+    taskTitle: mention.task_title || "Untitled task",
+    channelId: mention.channel_id ?? null,
+    channelName: mention.channel_name ?? null,
+    author: mention.author ?? null,
+    content: mention.content,
+    createdAt: mention.created_at,
+  }));
+}
+
+/** How many items arrived after the viewer last opened the Activity page. */
+export function countUnseenActivity(
+  items: readonly MentionActivityItem[],
+  lastSeenAt: string | null,
+): number {
+  if (!lastSeenAt) return items.length;
+  return items.filter((item) => item.createdAt > lastSeenAt).length;
+}
+
+// Bounds the cache so a long-running session's accumulated feed can't grow
+// without limit.
+const MAX_CACHED_MENTIONS = 300;
+
+/**
+ * Fold a page of freshly-fetched mentions into the previously cached set —
+ * dedupe by message, keep newest first. Lets repolls fetch only what's new
+ * (via `since`) instead of re-fetching the whole top page every time.
+ */
+export function mergeTaskMentions(
+  previous: readonly TaskMention[],
+  incoming: readonly TaskMention[],
+): TaskMention[] {
+  if (incoming.length === 0) return [...previous];
+  const byMessageId = new Map(
+    previous.map((mention) => [mention.message_id, mention]),
+  );
+  for (const mention of incoming) byMessageId.set(mention.message_id, mention);
+  return Array.from(byMessageId.values())
+    .sort((a, b) => (a.created_at < b.created_at ? 1 : -1))
+    .slice(0, MAX_CACHED_MENTIONS);
+}

packages/shared/src/analytics-events.ts

@@ -832,7 +832,8 @@ export type ChannelsSurface =
   | "dashboards_grid"
   | "canvas"
   | "context"
-  | "thread_panel";
+  | "thread_panel"
+  | "activity";
 
 export type ChannelActionType =
   | "enter_space"
@@ -860,7 +861,10 @@ export type ChannelActionType =
   | "open_task"
   | "collapse_thread"
   | "expand_thread"
-  | "copy_link";
+  | "copy_link"
+  | "mention_member"
+  | "view_activity"
+  | "open_mention";
 
 export interface ChannelActionProperties {
   action_type: ChannelActionType;
@@ -871,8 +875,10 @@ export interface ChannelActionProperties {
   task_id?: string;
   /** For file_task: destination channel when different from `channel_id`. */
   target_channel_id?: string;
-  /** For nav_click: which destination ("home"|"inbox"|"canvas"|"agents"|"files"|"settings"). */
+  /** For nav_click: which destination ("home"|"activity"|"inbox"|"canvas"|"agents"|"files"|"settings"). */
   nav_target?: string;
+  /** For mention_member: the tagged teammate's user uuid. */
+  mentioned_user_id?: string;
   /** For new_task_suggestion: the starter-prompt card label. */
   suggestion_label?: string;
   /** Whether the underlying mutation resolved successfully. */

packages/shared/src/domain-types.ts

@@ -36,6 +36,12 @@ export interface UserBasic {
   is_email_verified?: boolean | null;
 }
 
+/** One row from the org members list; trimmed to what mention pickers need. */
+export interface OrganizationMemberBasic {
+  id: string;
+  user: UserBasic;
+}
+
 export interface Task {
   id: string;
   task_number: number | null;
@@ -86,6 +92,22 @@ export interface TaskThreadMessage {
   forwarded_by?: UserBasic | null;
 }
 
+/**
+ * One @-mention of the current user in a task's thread, from the backend
+ * mentions index (`/task_mentions/`). Mirrors `TaskMentionDTO`.
+ */
+export interface TaskMention {
+  id: string;
+  message_id: string;
+  task_id: string;
+  task_title: string;
+  channel_id?: string | null;
+  channel_name?: string | null;
+  author?: UserBasic | null;
+  content: string;
+  created_at: string;
+}
+
 export type TaskRunStatus =
   | "not_started"
   | "queued"

packages/shared/src/index.ts

@@ -119,6 +119,12 @@ export {
 export { buildDiscussReportPrompt } from "./inbox-prompts";
 export type { AvailableSuggestedReviewer, SourceProduct } from "./inbox-types";
 export { EXTERNAL_LINKS } from "./links";
+export {
+  formatMention,
+  type MentionSegment,
+  mentionsToPlainText,
+  splitMentionSegments,
+} from "./mentions";
 export {
   getOauthClientIdFromRegion,
   OAUTH_SCOPE_VERSION,