feat(slack): follow all replies in threads Cyrus is bound to

After an initial @mention binds a Slack thread, plain follow-up messages in
that thread are now fed into the same agent session — no re-mention needed.

- SlackEventTransport accepts "message" events, drops bot/own messages,
  subtype events, and non-threaded messages, and de-dupes the app_mention +
  message double-delivery on (channel, ts).
- ChatPlatformAdapter gains optional isSessionInitiatingEvent; ChatSessionHandler
  refuses to start a new session from a non-initiating event, so a plain reply
  only continues an already-bound thread.
- SlackChatAdapter marks app_mention as initiating, message as follow-up.
- Slack setup manifest subscribes to message.channels/groups/mpim/im.

CYPACK-1267

Author: cyrusagent

CHANGELOG.md

@@ -8,6 +8,7 @@ All notable changes to this project will be documented in this file.
 - Updated `@anthropic-ai/claude-agent-sdk` from 0.3.154 to 0.3.156 and `@anthropic-ai/sdk` from 0.100.0 to 0.100.1. See the [SDK changelog](https://github.com/anthropics/claude-agent-sdk-typescript/blob/main/CHANGELOG.md) for details. ([CYPACK-1265](https://linear.app/ceedar/issue/CYPACK-1265), [#1270](https://github.com/cyrusagents/cyrus/pull/1270))
 
 ### Added
+- Cyrus now follows along in Slack threads it's been pulled into: once you @mention it in a thread, every later reply in that thread is fed to the same session automatically — no need to re-mention it each time. Replies that do @mention it still work as before, and Cyrus ignores its own messages, edits, and plain channel chatter outside threads it's part of. **Existing Slack apps must add the `message.channels`, `message.groups`, `message.mpim`, and `message.im` bot events in their Slack app's Event Subscriptions for this to take effect.** ([CYPACK-1267](https://linear.app/ceedar/issue/CYPACK-1267))
 - A repository can now ship its own skills: any skill directories under `<repo>/.claude/skills/` are automatically discovered and made available to the agent whenever Cyrus works in that repo — for single-repo issues, multi-repo issues (skills from every participating repo are combined), and GitHub/GitLab mentions alike. ([CYPACK-1261](https://linear.app/ceedar/issue/CYPACK-1261), [#1268](https://github.com/cyrusagents/cyrus/pull/1268))
 
 ## [0.2.60] - 2026-05-28

packages/edge-worker/src/ChatSessionHandler.ts

@@ -28,6 +28,17 @@ export interface ChatPlatformAdapter<TEvent> {
 	/** Extract the user's task text from the raw event */
 	extractTaskInstructions(event: TEvent): string;
 
+	/**
+	 * Whether this event is allowed to *start* a brand-new session for its
+	 * thread. Events that may only continue an already-bound thread (e.g. a
+	 * plain Slack message that isn't an @mention) return false, so the handler
+	 * ignores them when no session exists yet.
+	 *
+	 * Optional — when omitted, every event is treated as session-initiating
+	 * (the behaviour for platforms where every event is an explicit invocation).
+	 */
+	isSessionInitiatingEvent?(event: TEvent): boolean;
+
 	/** Derive a unique thread key for session tracking (e.g., "C123:1704110400.000100") */
 	getThreadKey(event: TEvent): string;
 
@@ -199,6 +210,20 @@ export class ChatSessionHandler<TEvent> {
 				);
 			}
 
+			// No session exists for this thread. Only events explicitly allowed to
+			// start a session may do so — e.g. a Slack @mention. A plain follow-up
+			// message in an unbound thread must be ignored, otherwise every message
+			// in any channel Cyrus can see would spin up a session.
+			if (
+				!existingSessionId &&
+				this.adapter.isSessionInitiatingEvent?.(event) === false
+			) {
+				this.logger.info(
+					`Ignoring non-initiating ${this.adapter.platformName} event for unbound thread ${threadKey}`,
+				);
+				return;
+			}
+
 			// Create an empty workspace directory for this thread
 			const workspace = await this.createWorkspace(threadKey);
 			if (!workspace) {

packages/edge-worker/src/SlackChatAdapter.ts

@@ -74,6 +74,16 @@ export class SlackChatAdapter
 		);
 	}
 
+	/**
+	 * Only an explicit @mention may start a new session. Plain `message` events
+	 * are follow-ups: they continue a thread Cyrus is already bound to, but
+	 * never spin up a session on their own (otherwise every message in a watched
+	 * channel would start one).
+	 */
+	isSessionInitiatingEvent(event: SlackWebhookEvent): boolean {
+		return event.eventType === "app_mention";
+	}
+
 	getThreadKey(event: SlackWebhookEvent): string {
 		const threadTs = event.payload.thread_ts || event.payload.ts;
 		return `${event.payload.channel}:${threadTs}`;

packages/edge-worker/test/chat-sessions.test.ts

@@ -150,6 +150,79 @@ describe("ChatSessionHandler chat session permissions", () => {
 	});
 });
 
+describe("ChatSessionHandler session-initiation gate", () => {
+	function buildHandler(adapter: ChatPlatformAdapter<TestEvent>) {
+		const createRunner = vi.fn(
+			() =>
+				({
+					supportsStreamingInput: false,
+					start: vi.fn().mockResolvedValue({ sessionId: "session-1" }),
+					stop: vi.fn(),
+					isRunning: vi.fn().mockReturnValue(false),
+					isStreaming: vi.fn().mockReturnValue(false),
+					addStreamMessage: vi.fn(),
+					getMessages: vi.fn().mockReturnValue([]),
+				}) as any,
+		);
+		const handler = new ChatSessionHandler(adapter, {
+			cyrusHome: TEST_CYRUS_CHAT,
+			chatRepositoryProvider: createStaticProvider([]),
+			runnerConfigBuilder: createMockRunnerConfigBuilder(),
+			createRunner,
+			onWebhookStart: vi.fn(),
+			onWebhookEnd: vi.fn(),
+			onStateChange: vi.fn().mockResolvedValue(undefined),
+			onClaudeError: vi.fn(),
+		});
+		return { handler, createRunner };
+	}
+
+	it("ignores a non-initiating event when no session exists for the thread", async () => {
+		const adapter: ChatPlatformAdapter<TestEvent> = new TestChatAdapter(
+			"unbound-thread",
+		);
+		// Mark this event as a follow-up that must not start a session.
+		adapter.isSessionInitiatingEvent = () => false;
+
+		const { handler, createRunner } = buildHandler(adapter);
+		await handler.handleEvent({
+			eventId: "follow-up",
+			threadKey: "unbound-thread",
+		} as any);
+
+		expect(createRunner).not.toHaveBeenCalled();
+		expect(handler.listThreads()).toHaveLength(0);
+	});
+
+	it("starts a session for an initiating event", async () => {
+		const adapter: ChatPlatformAdapter<TestEvent> = new TestChatAdapter(
+			"bound-thread",
+		);
+		adapter.isSessionInitiatingEvent = () => true;
+
+		const { handler, createRunner } = buildHandler(adapter);
+		await handler.handleEvent({
+			eventId: "mention",
+			threadKey: "bound-thread",
+		} as any);
+
+		expect(createRunner).toHaveBeenCalledTimes(1);
+		expect(handler.listThreads()).toHaveLength(1);
+	});
+});
+
+describe("SlackChatAdapter session initiation", () => {
+	it("treats app_mention as session-initiating and message as a follow-up", () => {
+		const adapter = new SlackChatAdapter(createStaticProvider([]));
+		expect(
+			adapter.isSessionInitiatingEvent({ eventType: "app_mention" } as any),
+		).toBe(true);
+		expect(
+			adapter.isSessionInitiatingEvent({ eventType: "message" } as any),
+		).toBe(false);
+	});
+});
+
 describe("SlackChatAdapter system prompt", () => {
 	it("includes configured repository context and git pull instructions", () => {
 		const repositoryPaths = ["/repo/chat-one", "/repo/chat-two"];

packages/slack-event-transport/src/SlackEventTransport.ts

@@ -8,6 +8,7 @@ import type {
 	SlackEventEnvelope,
 	SlackEventTransportConfig,
 	SlackEventTransportEvents,
+	SlackMessageEvent,
 	SlackVerificationMode,
 	SlackWebhookEvent,
 } from "./types.js";
@@ -33,13 +34,28 @@ export declare interface SlackEventTransport {
  * and verifies incoming webhooks using Bearer token authentication.
  *
  * Supported Slack event types:
- * - app_mention: When the bot is mentioned with @ in a channel or thread
+ * - app_mention: When the bot is mentioned with @ in a channel or thread.
+ *   Always emitted — starts or resumes a session for the thread.
+ * - message: A plain message in a channel/thread the bot can see. Emitted only
+ *   for threaded replies that aren't the bot's own message and aren't a
+ *   subtype event (edits, joins, etc.). Whether it actually does anything is
+ *   decided downstream (ChatSessionHandler only continues already-bound
+ *   threads). Slack delivers both an `app_mention` AND a `message` event for a
+ *   message that mentions the bot, so identical `(channel, ts)` pairs are
+ *   de-duplicated here to avoid double-prompting.
  */
 export class SlackEventTransport extends EventEmitter {
 	private config: SlackEventTransportConfig;
 	private logger: ILogger;
 	private messageTranslator: SlackMessageTranslator;
 	private translationContext: TranslationContext;
+	/**
+	 * Recently emitted `channel:ts` keys, used to collapse Slack's
+	 * double-delivery of app_mention + message for the same underlying message.
+	 * Maps key → epoch ms first seen; pruned by TTL.
+	 */
+	private recentMessageKeys: Map<string, number> = new Map();
+	private static readonly DEDUP_TTL_MS = 10 * 60 * 1000;
 
 	constructor(
 		config: SlackEventTransportConfig,
@@ -271,29 +287,55 @@ export class SlackEventTransport extends EventEmitter {
 
 		const event = envelope.event;
 
-		if (!event || event.type !== "app_mention") {
+		// Slack sends many event types at runtime; the envelope type only models
+		// the two we handle, so widen to string for the membership check.
+		const eventType = event?.type as string | undefined;
+		if (eventType !== "app_mention" && eventType !== "message") {
 			this.logger.debug(
-				`Ignoring unsupported event type: ${event?.type ?? "unknown"}`,
+				`Ignoring unsupported event type: ${eventType ?? "unknown"}`,
 			);
 			reply.code(200).send({ success: true, ignored: true });
 			return;
 		}
 
+		// `message` events fire for every message in every channel the bot can
+		// see, so apply cheap structural filters before doing any work. Anything
+		// that gets through here is a candidate follow-up prompt; the binding
+		// check (is this thread actually bound to Cyrus?) happens downstream.
+		if (event.type === "message" && !this.shouldEmitMessageEvent(event)) {
+			reply.code(200).send({ success: true, ignored: true });
+			return;
+		}
+
+		// Slack delivers both an app_mention and a message event for a single
+		// message that mentions the bot. De-duplicate on (channel, ts) so the
+		// thread only gets prompted once. The first event to arrive wins; both
+		// carry identical text.
+		const dedupKey = `${event.channel}:${event.ts}`;
+		if (this.isDuplicateMessage(dedupKey)) {
+			this.logger.debug(
+				`Ignoring duplicate Slack event for ${dedupKey} (already processed)`,
+			);
+			reply.code(200).send({ success: true, ignored: true });
+			return;
+		}
+		this.rememberMessage(dedupKey);
+
 		// Token may be undefined during startup transitions (e.g. switching runtimes)
 		// when the env update hasn't been processed yet. Downstream consumers
 		// (SlackChatAdapter) fall back to process.env.SLACK_BOT_TOKEN at usage time.
 		const slackBotToken = this.getSlackBotToken();
 
 		const webhookEvent: SlackWebhookEvent = {
-			eventType: "app_mention",
+			eventType: event.type,
 			eventId: envelope.event_id,
 			payload: event,
 			slackBotToken,
 			teamId: envelope.team_id,
 		};
 
 		this.logger.info(
-			`Received app_mention webhook (event: ${envelope.event_id}, channel: ${event.channel})`,
+			`Received ${event.type} webhook (event: ${envelope.event_id}, channel: ${event.channel})`,
 		);
 
 		// Emit "event" for transport-level listeners
@@ -305,6 +347,53 @@ export class SlackEventTransport extends EventEmitter {
 		reply.code(200).send({ success: true });
 	}
 
+	/**
+	 * Decide whether a `message` event is a candidate follow-up prompt.
+	 *
+	 * Drops the bot's own messages (which would otherwise loop), edited/deleted
+	 * and other subtype events, and top-level (non-threaded) messages — only a
+	 * threaded reply can belong to a thread Cyrus is already bound to.
+	 */
+	private shouldEmitMessageEvent(event: SlackMessageEvent): boolean {
+		if (event.bot_id) {
+			this.logger.debug(
+				`Ignoring Slack message from bot ${event.bot_id} (channel ${event.channel})`,
+			);
+			return false;
+		}
+		if (event.subtype) {
+			this.logger.debug(
+				`Ignoring Slack message with subtype "${event.subtype}" (channel ${event.channel})`,
+			);
+			return false;
+		}
+		if (!event.thread_ts) {
+			this.logger.debug(
+				`Ignoring non-threaded Slack message (channel ${event.channel})`,
+			);
+			return false;
+		}
+		return true;
+	}
+
+	private isDuplicateMessage(key: string): boolean {
+		this.pruneRecentMessageKeys();
+		return this.recentMessageKeys.has(key);
+	}
+
+	private rememberMessage(key: string): void {
+		this.recentMessageKeys.set(key, Date.now());
+	}
+
+	private pruneRecentMessageKeys(): void {
+		const now = Date.now();
+		for (const [key, seenAt] of this.recentMessageKeys) {
+			if (now - seenAt > SlackEventTransport.DEDUP_TTL_MS) {
+				this.recentMessageKeys.delete(key);
+			}
+		}
+	}
+
 	/**
 	 * Translate and emit an internal message from a webhook event.
 	 * Only emits if translation succeeds; logs debug message on failure.

packages/slack-event-transport/src/SlackMessageTranslator.ts

@@ -53,7 +53,7 @@ export class SlackMessageTranslator
 
 		return (
 			typeof e.eventType === "string" &&
-			e.eventType === "app_mention" &&
+			(e.eventType === "app_mention" || e.eventType === "message") &&
 			typeof e.eventId === "string" &&
 			e.payload !== null &&
 			typeof e.payload === "object"
@@ -75,6 +75,13 @@ export class SlackMessageTranslator
 			return this.translateAppMention(event, context);
 		}
 
+		// A plain `message` event is always a follow-up in an existing thread —
+		// it can only reach here for a thread Cyrus is already bound to, so it
+		// maps to a user prompt rather than a session start.
+		if (event.eventType === "message") {
+			return this.translateAppMentionAsUserPrompt(event, context);
+		}
+
 		return {
 			success: false,
 			reason: `Unsupported Slack event type: ${event.eventType}`,
@@ -90,7 +97,7 @@ export class SlackMessageTranslator
 		event: SlackWebhookEvent,
 		context?: TranslationContext,
 	): TranslationResult {
-		if (event.eventType === "app_mention") {
+		if (event.eventType === "app_mention" || event.eventType === "message") {
 			return this.translateAppMentionAsUserPrompt(event, context);
 		}
 

packages/slack-event-transport/src/index.ts

@@ -15,9 +15,11 @@ export type {
 	SlackAppMentionEvent,
 	SlackChannel,
 	SlackEventEnvelope,
+	SlackEventPayload,
 	SlackEventTransportConfig,
 	SlackEventTransportEvents,
 	SlackEventType,
+	SlackMessageEvent,
 	SlackUser,
 	SlackVerificationMode,
 	SlackWebhookEvent,