diff --git a/image_pusher/TCPStreamPusher.cpp b/image_pusher/TCPStreamPusher.cpp
index 8d2fe829..2fd529ff 100644
--- a/image_pusher/TCPStreamPusher.cpp
+++ b/image_pusher/TCPStreamPusher.cpp
@@ -866,9 +866,11 @@ void TCPStreamPusher::Preflight(StartMessage& message) {
     for (size_t i = 0; i < local_connections.size(); i++) {
         auto& c = *local_connections[i];
 
-        // Same assignment StartDataCollection makes, and it has to be the same one: only the
-        // writer holding the master file checks the output files, so a different index 0 here
-        // than there would check the run on the wrong writer.
+        // The same assignment StartDataCollection makes, so exactly one writer runs the check. It
+        // does NOT have to fall on the same connection there as here - the pool can lose a writer in
+        // between, and START re-derives index 0 after detaching the dead ones. Every writer of a run
+        // works on the same filesystem, so any of them answers the same question about the same
+        // files; which one is asked does not change the answer.
         message.socket_number = static_cast<int64_t>(c.socket_number);
         message.write_master_file = (i == 0);
 
diff --git a/writer/StreamWriter.cpp b/writer/StreamWriter.cpp
index 7303ec19..806a8662 100644
--- a/writer/StreamWriter.cpp
+++ b/writer/StreamWriter.cpp
@@ -99,10 +99,16 @@ void StreamWriter::ProcessPreflight() {
     run_number = msg.run_number;
     socket_number = msg.socket_number.value_or(0);
 
+    // std::exception, not JFJochException: the answer matters more than the diagnosis. The pusher is
+    // blocked on this ACK and reads its absence as a dead writer, so anything that escapes here costs
+    // the caller the real reason and hands it a five-second timeout instead. std::filesystem::exists
+    // is the concrete leak - it throws filesystem_error, not JFJochException, when the output path
+    // cannot be walked at all (a directory component with no search permission, a symlink loop),
+    // which is exactly the kind of misconfiguration the pre-flight exists to report before arming.
     try {
         FileWriter::Preflight(msg);
         NotifyTcpAck(TCPFrameType::PREFLIGHT, true, false, TCPAckCode::None);
-    } catch (const JFJochException &e) {
+    } catch (const std::exception &e) {
         logger.Warning("Pre-flight check failed: {}", e.what());
         // Not fatal: nothing was started, so the connection stays usable for the next attempt.
         NotifyTcpAck(TCPFrameType::PREFLIGHT, false, false, TCPAckCode::StartFailed, e.what());