diff --git a/src/agents/pi-embedded-runner/session-manager-init.ts b/src/agents/pi-embedded-runner/session-manager-init.ts
index ef795718320..95c699947bd 100644
--- a/src/agents/pi-embedded-runner/session-manager-init.ts
+++ b/src/agents/pi-embedded-runner/session-manager-init.ts
@@ -43,7 +43,7 @@ export async function prepareSessionManagerForRun(params: {
 
   if (params.hadSessionFile && header && !hasAssistant) {
     // Reset file so the first assistant flush includes header+user+assistant in order.
-    await fs.writeFile(params.sessionFile, "", { encoding: "utf-8", mode: 0o600 });
+    await fs.writeFile(params.sessionFile, "", "utf-8");
     sm.fileEntries = [header];
     sm.byId?.clear?.();
     sm.labelsById?.clear?.();
diff --git a/src/auto-reply/reply/session.ts b/src/auto-reply/reply/session.ts
index f03de1c3161..cd0ccf75bd8 100644
--- a/src/auto-reply/reply/session.ts
+++ b/src/auto-reply/reply/session.ts
@@ -89,10 +89,7 @@ function forkSessionFromParent(params: {
       cwd: manager.getCwd(),
       parentSession: parentSessionFile,
     };
-    fs.writeFileSync(sessionFile, `${JSON.stringify(header)}\n`, {
-      encoding: "utf-8",
-      mode: 0o600,
-    });
+    fs.writeFileSync(sessionFile, `${JSON.stringify(header)}\n`, "utf-8");
     return { sessionId, sessionFile };
   } catch {
     return null;
diff --git a/src/gateway/server-methods/sessions.ts b/src/gateway/server-methods/sessions.ts
index ab37ff7bc49..ffe69ba88f4 100644
--- a/src/gateway/server-methods/sessions.ts
+++ b/src/gateway/server-methods/sessions.ts
@@ -471,10 +471,7 @@ export const sessionsHandlers: GatewayRequestHandlers = {
 
     const archived = archiveFileOnDisk(filePath, "bak");
     const keptLines = lines.slice(-maxLines);
-    fs.writeFileSync(filePath, `${keptLines.join("\n")}\n`, {
-      encoding: "utf-8",
-      mode: 0o600,
-    });
+    fs.writeFileSync(filePath, `${keptLines.join("\n")}\n`, "utf-8");
 
     await updateSessionStore(storePath, (store) => {
       const entryKey = compactTarget.primaryKey;