diff --git a/coderd/x/chatd/chatdebug/model.go b/coderd/x/chatd/chatdebug/model.go
index 0ac7326080..e30a8a21e5 100644
--- a/coderd/x/chatd/chatdebug/model.go
+++ b/coderd/x/chatd/chatdebug/model.go
@@ -361,10 +361,20 @@ func launchHeartbeat(ctx context.Context, svc *Service, stepID, runID, chatID uu
 		return
 	}
 	go func() {
+		// Subscribe before reading the interval. The channel invalidates
+		// the interval, so any concurrent SetStaleAfter either happened
+		// before this interval read or will close thresholdCh below.
+		thresholdCh := svc.thresholdChan()
 		interval := svc.heartbeatInterval()
 		ticker := svc.clock.NewTicker(interval, "chatdebug", "heartbeat")
 		defer ticker.Stop()
-		thresholdCh := svc.thresholdChan()
+		resetTicker := func() {
+			if newInterval := svc.heartbeatInterval(); newInterval != interval {
+				interval = newInterval
+				ticker.Reset(interval, "chatdebug", "heartbeat")
+			}
+		}
+
 		for {
 			select {
 			case <-ctx.Done():
@@ -375,10 +385,7 @@ func launchHeartbeat(ctx context.Context, svc *Service, stepID, runID, chatID uu
 				// SetStaleAfter was called; re-read the interval
 				// and reset the ticker immediately.
 				thresholdCh = svc.thresholdChan()
-				if newInterval := svc.heartbeatInterval(); newInterval != interval {
-					interval = newInterval
-					ticker.Reset(interval, "chatdebug", "heartbeat")
-				}
+				resetTicker()
 			case <-ticker.C:
 				if err := svc.TouchStep(ctx, stepID, runID, chatID); err != nil {
 					svc.log.Debug(ctx, "heartbeat touch failed",
@@ -388,10 +395,7 @@ func launchHeartbeat(ctx context.Context, svc *Service, stepID, runID, chatID uu
 				}
 				// Also re-read interval on every tick as a
 				// secondary check.
-				if newInterval := svc.heartbeatInterval(); newInterval != interval {
-					interval = newInterval
-					ticker.Reset(interval, "chatdebug", "heartbeat")
-				}
+				resetTicker()
 			}
 		}
 	}()
diff --git a/coderd/x/chatd/chatdebug/model_internal_test.go b/coderd/x/chatd/chatdebug/model_internal_test.go
index a3386a7058..03bb51cab8 100644
--- a/coderd/x/chatd/chatdebug/model_internal_test.go
+++ b/coderd/x/chatd/chatdebug/model_internal_test.go
@@ -1361,13 +1361,15 @@ func TestLaunchHeartbeat(t *testing.T) {
 		// threshold/2 interval.
 		newCall := tickerTrap.MustWait(ctx)
 		require.Equal(t, 30*time.Second, newCall.Duration)
+
+		// Reduce the threshold while NewTicker is trapped. This
+		// simulates SetStaleAfter racing with heartbeat startup before
+		// the goroutine can select on thresholdCh.
+		svc.SetStaleAfter(10 * time.Second)
 		newCall.MustRelease(ctx)
 
-		// Reducing the threshold must wake the heartbeat via the
-		// thresholdChan close and trigger a ticker reset to
-		// newThreshold/2 without advancing the mock clock.
-		svc.SetStaleAfter(10 * time.Second)
-
+		// The heartbeat must still reset to newThreshold/2 without
+		// advancing the mock clock.
 		resetCall := resetTrap.MustWait(ctx)
 		require.Equal(t, 5*time.Second, resetCall.Duration,
 			"ticker should reset to newThreshold/2 when SetStaleAfter"+