diff --git a/src/channels/slack.ts b/src/channels/slack.ts
index 587a426..cd4f4c5 100644
--- a/src/channels/slack.ts
+++ b/src/channels/slack.ts
@@ -43,6 +43,7 @@ export class SlackChannel implements Channel {
 	private ownerUserId: string | null;
 	private phantomName: string;
 	private rejectedUsers = new Set<string>();
+	private participatedThreads = new Set<string>();
 
 	constructor(config: SlackChannelConfig) {
 		this.app = new App({
@@ -150,6 +151,11 @@ export class SlackChannel implements Channel {
 			lastTs = result.ts ?? "";
 		}
 
+		// Track thread participation so we can respond to replies without @ mention
+		if (replyThreadTs) {
+			this.participatedThreads.add(`${channel}:${replyThreadTs}`);
+		}
+
 		return {
 			id: lastTs || randomUUID(),
 			channelId: this.id,
@@ -282,6 +288,10 @@ export class SlackChannel implements Channel {
 		}
 	}
 
+	trackThreadParticipation(channelId: string, threadTs: string): void {
+		this.participatedThreads.add(`${channelId}:${threadTs}`);
+	}
+
 	private registerEventHandlers(): void {
 		this.app.event("app_mention", async ({ event, client: _client }) => {
 			if (!this.messageHandler) return;
@@ -334,7 +344,13 @@ export class SlackChannel implements Channel {
 			if (this.botUserId && userId === this.botUserId) return;
 
 			const channelType = msg.channel_type as string | undefined;
-			if (channelType !== "im") return;
+			if (channelType !== "im") {
+				// In channels, only respond to thread replies in threads we've participated in
+				const incomingThreadTs = msg.thread_ts as string | undefined;
+				if (!incomingThreadTs) return;
+				const threadKey = `${msg.channel as string}:${incomingThreadTs}`;
+				if (!this.participatedThreads.has(threadKey)) return;
+			}
 
 			if (userId && !this.isOwner(userId)) {
 				console.log(`[slack] Ignoring DM from non-owner: ${userId}`);
diff --git a/src/index.ts b/src/index.ts
index a6e0066..f049178 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -450,12 +450,16 @@ async function main(): Promise<void> {
 		if (progressStream) {
 			// Slack: update the progress message with the final response + feedback buttons
 			await progressStream.finish(response.text);
+			if (slackChannel && slackChannelId && slackThreadTs) {
+				slackChannel.trackThreadParticipation(slackChannelId, slackThreadTs);
+			}
 		} else if (isSlack && slackChannel && slackChannelId && slackThreadTs) {
 			// Slack fallback: send direct reply with feedback
 			const thinkingTs = await slackChannel.postThinking(slackChannelId, slackThreadTs);
 			if (thinkingTs) {
 				await slackChannel.updateWithFeedback(slackChannelId, thinkingTs, response.text);
 			}
+			slackChannel.trackThreadParticipation(slackChannelId, slackThreadTs);
 		} else {
 			// All other channels: send via router
 			await router.send(msg.channelId, msg.conversationId, {