diff --git a/src/resources/extensions/sf/tests/uok-message-bus.test.mjs b/src/resources/extensions/sf/tests/uok-message-bus.test.mjs
index a44dc1906..97bfe700b 100644
--- a/src/resources/extensions/sf/tests/uok-message-bus.test.mjs
+++ b/src/resources/extensions/sf/tests/uok-message-bus.test.mjs
@@ -143,6 +143,30 @@ test("messageBus_compact_removes_old_messages", () => {
 	assert.equal(inbox.list().length, 0);
 });
 
+test("messageBus_send_when_threshold_reached_auto_compacts_old_messages", () => {
+	const root = makeProject();
+	const bus = new MessageBus(root, {
+		retentionDays: 1,
+		autoCompactThreshold: 1,
+	});
+	insertUokMessage({
+		id: "msg-old-auto",
+		from: "agent-a",
+		to: "agent-b",
+		body: "old",
+		sentAt: new Date(Date.now() - 2 * 24 * 60 * 60 * 1000).toISOString(),
+		deliveredAt: new Date(Date.now() - 2 * 24 * 60 * 60 * 1000).toISOString(),
+		metadata: {},
+	});
+
+	bus.send("agent-a", "agent-b", "fresh");
+
+	const inbox = bus.getInbox("agent-b");
+	inbox.refresh();
+	const bodies = inbox.list().map((m) => m.body);
+	assert.deepEqual(bodies, ["fresh"]);
+});
+
 test("messageBus_getConversation_filters_by_pair", () => {
 	const root = makeProject();
 	const bus = new MessageBus(root);
@@ -214,6 +238,32 @@ test("getUokMessageBusMetrics_counts_messages_and_unread", () => {
 	assert.equal(m2.totalUnread, 2);
 });
 
+test("messageBus_autoCompact_triggers_when_threshold_exceeded", () => {
+	const root = makeProject();
+	const bus = new MessageBus(root, {
+		autoCompactThreshold: 5,
+		retentionDays: 1,
+	});
+	// Insert old messages directly to bypass auto-compact on send
+	for (let i = 0; i < 4; i++) {
+		insertUokMessage({
+			id: `msg-old-${i}`,
+			from: "agent-a",
+			to: "agent-b",
+			body: `old-${i}`,
+			sentAt: new Date(Date.now() - 2 * 24 * 60 * 60 * 1000).toISOString(),
+			metadata: {},
+		});
+	}
+	let m = getUokMessageBusMetrics();
+	assert.equal(m.totalMessages, 4);
+
+	// Sending one more message should trigger auto-compact (5 >= threshold)
+	bus.send("agent-a", "agent-b", "new");
+	m = getUokMessageBusMetrics();
+	assert.equal(m.totalMessages, 1); // old messages compacted, only new remains
+});
+
 test("getUokMessageBusMetrics_ignores_reads_by_non_recipient_for_unread_count", () => {
 	const root = makeProject();
 	const bus = new MessageBus(root);
diff --git a/src/resources/extensions/sf/uok/message-bus.js b/src/resources/extensions/sf/uok/message-bus.js
index 804c11eb5..e80d72b5c 100644
--- a/src/resources/extensions/sf/uok/message-bus.js
+++ b/src/resources/extensions/sf/uok/message-bus.js
@@ -16,6 +16,7 @@ import { sfRoot } from "../paths.js";
 import {
 	compactUokMessages,
 	getUokConversation,
+	getUokMessageBusMetrics,
 	getUokMessageReadIds,
 	getUokMessagesForAgent,
 	insertUokMessage,
@@ -110,15 +111,27 @@ export class AgentInbox {
 	}
 }
 
+const DEFAULT_AUTO_COMPACT_THRESHOLD = 10_000;
+
 export class MessageBus {
 	constructor(basePath, options = {}) {
 		this.basePath = basePath;
 		this.retentionDays = options.retentionDays ?? DEFAULT_RETENTION_DAYS;
 		this.maxInboxSize = options.maxInboxSize ?? DEFAULT_MAX_INBOX_SIZE;
+		this.autoCompactThreshold =
+			options.autoCompactThreshold ?? DEFAULT_AUTO_COMPACT_THRESHOLD;
 		this.inboxes = new Map();
 		ensureDb(basePath);
 	}
 
+	_maybeAutoCompact() {
+		if (this.autoCompactThreshold <= 0) return;
+		const metrics = getUokMessageBusMetrics();
+		if (metrics.totalMessages >= this.autoCompactThreshold) {
+			compactUokMessages(this.retentionDays);
+		}
+	}
+
 	_getOrCreateInbox(agentId) {
 		if (!this.inboxes.has(agentId)) {
 			this.inboxes.set(
@@ -150,6 +163,7 @@ export class MessageBus {
 		if (!alreadyHas) {
 			targetInbox.receive(message);
 		}
+		this._maybeAutoCompact();
 
 		return message.id;
 	}
@@ -177,6 +191,7 @@ export class MessageBus {
 		insertUokMessage(message);
 		const targetInbox = this._getOrCreateInbox(to);
 		targetInbox.refresh();
+		this._maybeAutoCompact();
 		return message.id;
 	}