diff --git a/src/config/sessions/transcript.ts b/src/config/sessions/transcript.ts
index c00a0e36b760..87b6e2bded63 100644
--- a/src/config/sessions/transcript.ts
+++ b/src/config/sessions/transcript.ts
@@ -300,7 +300,7 @@ export async function appendExactAssistantMessageToSessionTranscript(params: {
   }
 
   let transcriptMarkerUpdatedAt: number | undefined;
-  const result = await runWithOwnedSessionTranscriptWriteLock(
+  const result = await runWithOwnedSessionTranscriptWriteLock<SessionTranscriptAppendResult>(
     { sessionFile, sessionKey: resolved.normalizedKey },
     async () => {
       const explicitIdempotencyKey =