From 3f6b481464bfb6d14592f91d33d76d6d481daed6 Mon Sep 17 00:00:00 2001
From: Peter Steinberger <steipete@gmail.com>
Date: Wed, 6 May 2026 04:06:23 +0100
Subject: [PATCH] fix: serialize concurrent transcript appends

---
 src/config/sessions/transcript-append.ts | 49 ++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/src/config/sessions/transcript-append.ts b/src/config/sessions/transcript-append.ts
index 7b4959d3f35..9f88b786427 100644
--- a/src/config/sessions/transcript-append.ts
+++ b/src/config/sessions/transcript-append.ts
@@ -13,6 +13,7 @@ const SESSION_MANAGER_APPEND_MAX_BYTES = 8 * 1024 * 1024;
 
 let piCodingAgentModulePromise: Promise<typeof import("@mariozechner/pi-coding-agent")> | null =
   null;
+const transcriptAppendQueues = new Map<string, Promise<void>>();
 
 async function loadCurrentSessionVersion(): Promise<number> {
   piCodingAgentModulePromise ??= import("@mariozechner/pi-coding-agent");
@@ -194,6 +195,40 @@ async function ensureTranscriptHeader(
   });
 }
 
+async function resolveTranscriptAppendQueueKey(transcriptPath: string): Promise<string> {
+  const resolvedTranscriptPath = path.resolve(transcriptPath);
+  const transcriptDir = path.dirname(resolvedTranscriptPath);
+  await fs.mkdir(transcriptDir, { recursive: true });
+  try {
+    return path.join(await fs.realpath(transcriptDir), path.basename(resolvedTranscriptPath));
+  } catch {
+    return resolvedTranscriptPath;
+  }
+}
+
+async function withTranscriptAppendQueue<T>(
+  transcriptPath: string,
+  fn: () => Promise<T>,
+): Promise<T> {
+  const queueKey = await resolveTranscriptAppendQueueKey(transcriptPath);
+  const previous = transcriptAppendQueues.get(queueKey) ?? Promise.resolve();
+  let releaseCurrent!: () => void;
+  const current = new Promise<void>((resolve) => {
+    releaseCurrent = resolve;
+  });
+  const tail = previous.catch(() => undefined).then(() => current);
+  transcriptAppendQueues.set(queueKey, tail);
+  await previous.catch(() => undefined);
+  try {
+    return await fn();
+  } finally {
+    releaseCurrent();
+    if (transcriptAppendQueues.get(queueKey) === tail) {
+      transcriptAppendQueues.delete(queueKey);
+    }
+  }
+}
+
 export async function appendSessionTranscriptMessage(params: {
   transcriptPath: string;
   message: unknown;
@@ -202,6 +237,20 @@ export async function appendSessionTranscriptMessage(params: {
   cwd?: string;
   useRawWhenLinear?: boolean;
   config?: SessionWriteLockAcquireTimeoutConfig;
+}): Promise<{ messageId: string }> {
+  return await withTranscriptAppendQueue(params.transcriptPath, () =>
+    appendSessionTranscriptMessageLocked(params),
+  );
+}
+
+async function appendSessionTranscriptMessageLocked(params: {
+  transcriptPath: string;
+  message: unknown;
+  now?: number;
+  sessionId?: string;
+  cwd?: string;
+  useRawWhenLinear?: boolean;
+  config?: SessionWriteLockAcquireTimeoutConfig;
 }): Promise<{ messageId: string }> {
   const lock = await acquireSessionWriteLock({
     sessionFile: params.transcriptPath,