diff --git a/scripts/run-tests.cjs b/scripts/run-tests.cjs
index 1e065d986..53f4a8964 100644
--- a/scripts/run-tests.cjs
+++ b/scripts/run-tests.cjs
@@ -126,9 +126,23 @@ function main() {
       .join(' ')}`,
   );
 
+  // Default concurrency: 4 on Linux/macOS, 2 on Windows.
+  //
+  // Windows has significantly higher per-subprocess overhead than Linux/macOS:
+  //   - Windows Defender scans each spawned process
+  //   - NTFS has higher file-system latency under concurrent access
+  //   - synckit worker_threads (used by the SDK bridge in gsd-tools.cjs) spawn
+  //     native threads that contend on SharedArrayBuffer + Atomics.wait; under
+  //     Node 24 on Windows, 4-way concurrent gsd-tools invocations (each spawning
+  //     a synckit worker) caused intermittent process crashes with empty stderr —
+  //     a signature of OS-level resource exhaustion killing worker threads before
+  //     they could flush. Reducing to 2 halves the peak concurrent worker count.
+  //
+  // Operator override via TEST_CONCURRENCY env var for local debugging.
+  const defaultConcurrency = process.platform === 'win32' ? 2 : 4;
   const concurrency = process.env.TEST_CONCURRENCY
     ? `--test-concurrency=${process.env.TEST_CONCURRENCY}`
-    : '--test-concurrency=4';
+    : `--test-concurrency=${defaultConcurrency}`;
 
   // Windows `CreateProcess` caps the full command line at 32,767 chars
   // (lpCommandLine). With 500+ test paths the spawn fails instantly with no
diff --git a/tests/helpers.cjs b/tests/helpers.cjs
index 109c9838a..9c9869dfc 100644
--- a/tests/helpers.cjs
+++ b/tests/helpers.cjs
@@ -62,10 +62,18 @@ function runGsdTools(args, cwd = process.cwd(), env = {}) {
     }
     return { success: true, output: result.trim(), exitCode: 0 };
   } catch (err) {
+    const stderrRaw = err.stderr?.toString().trim() || '';
+    // Prefer actual stderr content; fall back to err.message (which contains
+    // the command invocation). If stderr is empty, append a note so CI logs
+    // show "stderr: (empty)" rather than silently losing the fact that the
+    // child process produced no error output — empty stderr with a non-zero
+    // exit code is a signal of OS-level crash (OOM kill, worker thread fatal
+    // error) rather than a gsd-tools application error.
+    const error = stderrRaw || `${err.message} [stderr: (empty) exit:${err.status ?? 1}]`;
     return {
       success: false,
       output: err.stdout?.toString().trim() || '',
-      error: err.stderr?.toString().trim() || err.message,
+      error,
       exitCode: err.status ?? 1,
     };
   }