From 8f9799307b59d91e350bf1239ba20ec24a47b58d Mon Sep 17 00:00:00 2001
From: Peter Steinberger <steipete@gmail.com>
Date: Mon, 23 Mar 2026 19:34:48 -0700
Subject: [PATCH] test: print failed test lane output tails

---
 scripts/test-parallel-utils.mjs | 14 ++++++++++++++
 scripts/test-parallel.mjs       |  7 +++++++
 2 files changed, 21 insertions(+)

diff --git a/scripts/test-parallel-utils.mjs b/scripts/test-parallel-utils.mjs
index 3f815a5036d..591be46a34e 100644
--- a/scripts/test-parallel-utils.mjs
+++ b/scripts/test-parallel-utils.mjs
@@ -22,6 +22,20 @@ export function hasFatalTestRunOutput(output) {
   return fatalOutputPatterns.some((pattern) => pattern.test(output));
 }
 
+export function formatCapturedOutputTail(output, maxLines = 60, maxChars = 6000) {
+  const trimmed = output.trim();
+  if (!trimmed) {
+    return "";
+  }
+  const lines = trimmed.split(/\r?\n/u);
+  const tailLines = lines.slice(-maxLines);
+  const tail = tailLines.join("\n");
+  if (tail.length <= maxChars) {
+    return tail;
+  }
+  return tail.slice(-maxChars);
+}
+
 export function resolveTestRunExitCode({ code, signal, output, fatalSeen = false, childError }) {
   if (typeof code === "number" && code !== 0) {
     return code;
diff --git a/scripts/test-parallel.mjs b/scripts/test-parallel.mjs
index ed3cc6ca534..b1b42c7a2f8 100644
--- a/scripts/test-parallel.mjs
+++ b/scripts/test-parallel.mjs
@@ -11,6 +11,7 @@ import {
 } from "./test-parallel-memory.mjs";
 import {
   appendCapturedOutput,
+  formatCapturedOutputTail,
   hasFatalTestRunOutput,
   resolveTestRunExitCode,
 } from "./test-parallel-utils.mjs";
@@ -1296,6 +1297,12 @@ const runOnce = (entry, extraArgs = []) =>
       children.delete(child);
       const resolvedCode = resolveTestRunExitCode({ code, signal, output, fatalSeen, childError });
       logMemoryTraceSummary();
+      if (resolvedCode !== 0) {
+        const failureTail = formatCapturedOutputTail(output);
+        if (failureTail) {
+          console.error(`[test-parallel] failure tail ${entry.name}\n${failureTail}`);
+        }
+      }
       console.log(
         `[test-parallel] done ${entry.name} code=${String(resolvedCode)} elapsed=${formatElapsedMs(Date.now() - startedAt)}`,
       );