diff --git a/moli-benchmark/moli_benchmark/wpt_cross/server.py b/moli-benchmark/moli_benchmark/wpt_cross/server.py
index fee63469a1..830d3b0925 100644
--- a/moli-benchmark/moli_benchmark/wpt_cross/server.py
+++ b/moli-benchmark/moli_benchmark/wpt_cross/server.py
@@ -193,6 +193,7 @@ FETCH_PREFLIGHT_RESOURCE_PATHS = {
     "/fetch/api/resources/clean-stash.py",
 }
 
+FORM_ECHO_PATH = "/html/semantics/forms/form-submission-0/form-echo.py"
 FORM_SUBMISSION_PATH = (
     "/html/semantics/forms/form-submission-0/resources/form-submission.py"
 )
@@ -1959,6 +1960,10 @@ def _workers_url_encoding_response(query: str) -> bytes:
     return b"PASS" if value == "å" else b"FAIL"
 
 
+def _form_echo_response(body: bytes) -> bytes:
+    return b" ".join(f"{byte:02x}".encode("ascii") for byte in body)
+
+
 def _make_handler(
     wpt_root: Path,
     results_store: "ResultsStore",
@@ -2263,6 +2268,11 @@ requestExecutor("{executor_uuid}", {start_on_js});
             self.send_error(404)
 
         def do_POST(self) -> None:  # noqa: N802
+            if unquote(urlsplit(self.path).path) == FORM_ECHO_PATH:
+                raw = self._read_content_length_request_body()
+                if raw is not None:
+                    self._send_bytes("text/plain", _form_echo_response(raw), emit_body=True)
+                return
             if unquote(urlsplit(self.path).path) == FETCH_INSPECT_HEADERS_PATH:
                 self._serve_fetch_inspect_headers(
                     urlsplit(self.path).query, emit_body=self.command != "HEAD",
@@ -2687,6 +2697,9 @@ requestExecutor("{executor_uuid}", {start_on_js});
                 self.send_error(500, "Invalid WPT template or pipe")
 
         def _serve_response(self, *, emit_body: bool) -> None:
+            if unquote(urlsplit(self.path).path) == FORM_ECHO_PATH:
+                self._send_bytes("text/plain", _form_echo_response(b""), emit_body=emit_body)
+                return
             if unquote(urlsplit(self.path).path) == "/workers/semantics/encodings/003-1.py":
                 self._send_bytes(
                     "text/plain; charset=utf-8",
diff --git a/moli-benchmark/tests/test_wpt_cross_fixture_responses.py b/moli-benchmark/tests/test_wpt_cross_fixture_responses.py
index cb9206b3f9..acf5836e9f 100644
--- a/moli-benchmark/tests/test_wpt_cross_fixture_responses.py
+++ b/moli-benchmark/tests/test_wpt_cross_fixture_responses.py
@@ -605,3 +605,41 @@ class WptCrossFixtureResponsesTests(WptCrossTestCase):
                 ("HEAD", 200, b"", "text/plain; charset=utf-8"),
             ],
         )
+
+    def test_fixture_server_models_form_echo_handler(self) -> None:
+        with tempfile.TemporaryDirectory() as root:
+            root_path = Path(root)
+            (root_path / "resources").mkdir()
+            (root_path / "resources" / "testharness.js").write_text(
+                "// testharness", encoding="utf-8"
+            )
+            with WptFixtureServer(root_path) as server:
+                url = (
+                    f"{server.base_url}/html/semantics/forms/"
+                    "form-submission-0/form-echo.py"
+                )
+                responses = []
+                for method, body in (
+                    ("GET", None),
+                    ("HEAD", None),
+                    ("POST", b"\x00\x09\x7f\x80\xff"),
+                ):
+                    request = Request(url, data=body, method=method)
+                    with urlopen(request, timeout=2) as response:
+                        responses.append(
+                            (
+                                method,
+                                response.status,
+                                response.read(),
+                                response.headers["Content-Type"],
+                            )
+                        )
+
+        self.assertEqual(
+            responses,
+            [
+                ("GET", 200, b"", "text/plain"),
+                ("HEAD", 200, b"", "text/plain"),
+                ("POST", 200, b"00 09 7f 80 ff", "text/plain"),
+            ],
+        )