diff --git a/test/napi/napi-app/main.js b/test/napi/napi-app/main.js
index 9c11801727f4..8d0b3c2e79bb 100644
--- a/test/napi/napi-app/main.js
+++ b/test/napi/napi-app/main.js
@@ -1,3 +1,8 @@
+// Node writes to a piped stdout asynchronously on POSIX: after one EAGAIN every
+// console.log is queued until the event loop runs while the addons' printf keeps
+// writing to the fd directly. Make it blocking so output stays in program order.
+if (!process.isBun) process.stdout._handle?.setBlocking?.(true);
+
 const tests = require("./module");
 if (process.argv[2] === "self") {
   console.log(
diff --git a/test/napi/napi-app/utils.h b/test/napi/napi-app/utils.h
index 0711999a329e..3a0f69854b20 100644
--- a/test/napi/napi-app/utils.h
+++ b/test/napi/napi-app/utils.h
@@ -13,18 +13,21 @@ class BlockingStdoutScope {
   BlockingStdoutScope() {
     original = fcntl(1, F_GETFL);
     fcntl(1, F_SETFL, original & ~O_NONBLOCK);
-    setvbuf(stdout, nullptr, _IOFBF, 8192);
     fflush(stdout);
+    // Both runtimes start with stdout unbuffered. glibc keeps the existing
+    // 1-byte buffer when setvbuf is given a null buffer, so pass a real one.
+    setvbuf(stdout, buffer, _IOFBF, sizeof(buffer));
   }
 
   ~BlockingStdoutScope() {
     fflush(stdout);
     fcntl(1, F_SETFL, original);
-    setvbuf(stdout, nullptr, _IOLBF, 0);
+    setvbuf(stdout, nullptr, _IONBF, 0);
   }
 
 private:
   int original;
+  inline static char buffer[8192];
 };
 
 #endif