diff --git a/packages/coding-agent/src/core/subagent-runner.ts b/packages/coding-agent/src/core/subagent-runner.ts
index 598144f2e..1aa2b24df 100644
--- a/packages/coding-agent/src/core/subagent-runner.ts
+++ b/packages/coding-agent/src/core/subagent-runner.ts
@@ -84,6 +84,16 @@ function createSubagentUIContext(): ExtensionUIContext {
 	};
 }
 
+function isMeaningfulSubagentOutputEvent(event: AgentSessionEvent): boolean {
+	if (String(event.type) === "runtime_heartbeat") return false;
+	if (event.type === "message_update") {
+		const streamEvent = event.assistantMessageEvent;
+		if (streamEvent.type === "text_delta") return streamEvent.delta.length > 0;
+		return true;
+	}
+	return true;
+}
+
 export interface RunSubagentOptions {
 	signal?: AbortSignal;
 	timeoutMs?: number;
@@ -321,7 +331,9 @@ export async function runSubagent(
 					options = {
 						...options,
 						onEvent: (event) => {
-							if (!noOutputTimedOut) armNoOutputTimer();
+							if (!noOutputTimedOut && isMeaningfulSubagentOutputEvent(event)) {
+								armNoOutputTimer();
+							}
 							previousOnEvent?.(event);
 						},
 					};