mom: add [SILENT] response marker for periodic events

When mom responds with just [SILENT], the status message is deleted
and nothing is posted to Slack. Useful for periodic events that check
for activity but find nothing to report.

packages/mom/CHANGELOG.md

@@ -9,6 +9,7 @@
   - One-shot events: trigger at specific time (for reminders)
   - Periodic events: trigger on cron schedule (for recurring tasks)
 - `SlackBot.enqueueEvent()` for queueing events (max 5 per channel)
+- `[SILENT]` response marker: deletes status message, posts nothing to Slack (for periodic events with nothing to report)
 - Events documentation in `docs/events.md`
 - System prompt section explaining events to mom
 

packages/mom/docs/events.md

@@ -130,9 +130,16 @@ When an event is dequeued and executes:
    - For one-shot: `[EVENT:dentist.json:one-shot:2025-12-15T09:00:00+01:00] Remind Mario`
    - For periodic: `[EVENT:daily-inbox.json:periodic:0 9 * * 1-5] Check inbox`
 3. After execution:
+   - If response is `[SILENT]`: delete status message, post nothing to Slack
    - Immediate and one-shot: delete the event file
    - Periodic: keep the file, event will trigger again on schedule
 
+## Silent Completion
+
+For periodic events that check for activity (inbox, notifications, etc.), mom may find nothing to report. To avoid spamming the channel, mom can respond with just `[SILENT]`. This deletes the "Starting event..." status message and posts nothing to Slack.
+
+Example: A periodic event checks for new emails every 15 minutes. If there are no new emails, mom responds `[SILENT]`. If there are new emails, mom posts a summary.
+
 ## File Naming
 
 Event files should have descriptive names ending in `.json`:

packages/mom/src/agent.ts

@@ -211,6 +211,9 @@ You receive a message like:
 \`\`\`
 Immediate and one-shot events auto-delete after triggering. Periodic events persist until you delete them.
 
+### Silent Completion
+For periodic events where there's nothing to report, respond with just \`[SILENT]\` (no other text). This deletes the status message and posts nothing to Slack. Use this to avoid spamming the channel when periodic checks find nothing actionable.
+
 ### Debouncing
 When writing programs that create immediate events (email watchers, webhook handlers, etc.), always debounce. If 50 emails arrive in a minute, don't create 50 immediate events. Instead collect events over a window and create ONE immediate event summarizing what happened, or just signal "new activity, check inbox" rather than per-item events. Or simpler: use a periodic event to check for new items every N minutes instead of immediate events.
 
@@ -687,7 +690,16 @@ function createRunner(sandboxConfig: SandboxConfig, channelId: string, channelDi
 						.map((c) => c.text)
 						.join("\n") || "";
 
-				if (finalText.trim()) {
+				// Check for [SILENT] marker - delete message instead of posting
+				if (finalText.trim() === "[SILENT]" || finalText.trim().startsWith("[SILENT]")) {
+					try {
+						await ctx.deleteMessage();
+						log.logInfo("Silent response - deleted status message");
+					} catch (err) {
+						const errMsg = err instanceof Error ? err.message : String(err);
+						log.logWarning("Failed to delete message for silent response", errMsg);
+					}
+				} else if (finalText.trim()) {
 					try {
 						const mainText =
 							finalText.length > SLACK_MAX_LENGTH

packages/mom/src/main.ts

@@ -207,6 +207,16 @@ function createSlackContext(event: SlackEvent, slack: SlackBot, state: ChannelSt
 			});
 			await updatePromise;
 		},
+
+		deleteMessage: async () => {
+			updatePromise = updatePromise.then(async () => {
+				if (messageTs) {
+					await slack.deleteMessage(event.channel, messageTs);
+					messageTs = null;
+				}
+			});
+			await updatePromise;
+		},
 	};
 }
 

packages/mom/src/slack.ts

@@ -62,6 +62,7 @@ export interface SlackContext {
 	setTyping: (isTyping: boolean) => Promise<void>;
 	uploadFile: (filePath: string, title?: string) => Promise<void>;
 	setWorking: (working: boolean) => Promise<void>;
+	deleteMessage: () => Promise<void>;
 }
 
 export interface MomHandler {
@@ -192,6 +193,10 @@ export class SlackBot {
 		await this.webClient.chat.update({ channel, ts, text });
 	}
 
+	async deleteMessage(channel: string, ts: string): Promise<void> {
+		await this.webClient.chat.delete({ channel, ts });
+	}
+
 	async postInThread(channel: string, threadTs: string, text: string): Promise<void> {
 		await this.webClient.chat.postMessage({ channel, thread_ts: threadTs, text });
 	}