diff --git a/static/web_serial_readuntil_test.mjs b/static/web_serial_readuntil_test.mjs
new file mode 100644
index 0000000..c7170a9
--- /dev/null
+++ b/static/web_serial_readuntil_test.mjs
@@ -0,0 +1,69 @@
+/**
+ * Node regression tests for Web Serial readUntil buffer handling.
+ * Run: node led-tool/static/web_serial_readuntil_test.mjs
+ */
+
+function bytesIndexOf(buf, suffix) {
+  if (!suffix.length) return 0;
+  if (buf.length < suffix.length) return -1;
+  for (let i = 0; i <= buf.length - suffix.length; i += 1) {
+    let ok = true;
+    for (let j = 0; j < suffix.length; j += 1) {
+      if (buf[i + j] !== suffix[j]) {
+        ok = false;
+        break;
+      }
+    }
+    if (ok) return i;
+  }
+  return -1;
+}
+
+function readUntilConsume(rxBuf, suffixBytes) {
+  const idx = bytesIndexOf(rxBuf, suffixBytes);
+  if (idx < 0) return null;
+  const end = idx + suffixBytes.length;
+  const matched = rxBuf.slice(0, end);
+  rxBuf.splice(0, end);
+  return matched;
+}
+
+function enc(s) {
+  return [...new TextEncoder().encode(s)];
+}
+
+function assert(cond, msg) {
+  if (!cond) throw new Error(msg);
+}
+
+const RAW = 'raw REPL; CTRL-B to exit\r\n';
+const SOFT = 'soft reboot\r\n';
+
+// Old bug: one chunk with soft reboot + banner; suffix-at-end + clear-all loses banner.
+{
+  const rx = enc(`MPY: ${SOFT}${RAW}boot.py line\r\n`);
+  const soft = enc(SOFT);
+  const m1 = readUntilConsume(rx, soft);
+  assert(m1 !== null, 'soft reboot should match');
+  const banner = enc(RAW);
+  const m2 = readUntilConsume(rx, banner);
+  assert(m2 !== null, 'banner must remain in buffer after soft reboot match');
+  assert(rx.length > 0, 'boot.py tail should remain after banner');
+}
+
+// Banner anywhere in buffer, not only at end.
+{
+  const rx = enc(`noise${RAW}>>> `);
+  const banner = enc(RAW);
+  const m = readUntilConsume(rx, banner);
+  assert(m !== null, 'banner should match mid-buffer');
+}
+
+// MPY: soft reboot marker
+{
+  const rx = enc(`MPY: ${SOFT}`);
+  const soft = enc('MPY: soft reboot\r\n');
+  assert(readUntilConsume(rx, soft) !== null, 'MPY soft reboot marker');
+}
+
+console.log('web_serial_readuntil_test: ok');