From 5c8b5d53ba5d568c52834e41659b32cddadc952f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kat=20March=C3=A1n?= <kat.marchan@fastly.com>
Date: Mon, 23 Feb 2026 15:15:34 -0800
Subject: [PATCH] fix(early-hints): early hints don't need to be sync inside a
 FetchEvent handler

Fixes: https://github.com/fastly/js-compute-runtime/issues/1322
---
 .../fixtures/app/src/early-hints.js           | 61 +++++++++++++++++++
 .../js-compute/fixtures/app/tests.json        | 54 ++++++++++++++++
 runtime/fastly/builtins/fetch-event.cpp       |  5 --
 3 files changed, 115 insertions(+), 5 deletions(-)

diff --git a/integration-tests/js-compute/fixtures/app/src/early-hints.js b/integration-tests/js-compute/fixtures/app/src/early-hints.js
index 817de3264f..2e8b702a9c 100644
--- a/integration-tests/js-compute/fixtures/app/src/early-hints.js
+++ b/integration-tests/js-compute/fixtures/app/src/early-hints.js
@@ -11,11 +11,56 @@ routes.set('/early-hints/manual-response', (event) => {
   event.respondWith(new Response('ok'));
 });
 
+routes.set('/early-hints/manual-response-late', (event) => {
+  event.respondWith(new Response('ok'));
+  assertThrows(() => {
+    event.respondWith(
+      new Response(null, {
+        status: 103,
+        headers: { link: '</style.css>; rel=preload; as=style' },
+      }),
+    );
+  });
+});
+
+routes.set('/early-hints/manual-response-async', async (event) => {
+  await (async () => {
+    event.respondWith(
+      new Response(null, {
+        status: 103,
+        headers: { link: '</style.css>; rel=preload; as=style' },
+      }),
+    );
+    event.respondWith(new Response('ok'));
+  })();
+});
+
+routes.set('/early-hints/manual-response-late-async', async (event) => {
+  await (async () => {
+    event.respondWith(new Response('ok'));
+    assertThrows(() => {
+      event.respondWith(
+        new Response(null, {
+          status: 103,
+          headers: { link: '</style.css>; rel=preload; as=style' },
+        }),
+      );
+    });
+  })();
+});
+
 routes.set('/early-hints/send-early-hints', (event) => {
   event.sendEarlyHints({ link: '</style.css>; rel=preload; as=style' });
   event.respondWith(new Response('ok'));
 });
 
+routes.set('/early-hints/send-early-hints-late', (event) => {
+  event.respondWith(new Response('ok'));
+  assertThrows(() => {
+    event.sendEarlyHints({ link: '</style.css>; rel=preload; as=style' });
+  });
+});
+
 routes.set('/early-hints/send-early-hints-multiple-headers', (event) => {
   event.sendEarlyHints([
     ['link', '</style.css>; rel=preload; as=style'],
@@ -23,3 +68,19 @@ routes.set('/early-hints/send-early-hints-multiple-headers', (event) => {
   ]);
   event.respondWith(new Response('ok'));
 });
+
+routes.set('/early-hints/send-early-hints-async', async (event) => {
+  await (async () => {
+    event.sendEarlyHints({ link: '</style.css>; rel=preload; as=style' });
+    event.respondWith(new Response('ok'));
+  })();
+});
+
+routes.set('/early-hints/send-early-hints-late-async', async (event) => {
+  await (async () => {
+    event.respondWith(new Response('ok'));
+    assertThrows(() => {
+      event.sendEarlyHints({ link: '</style.css>; rel=preload; as=style' });
+    });
+  })();
+});
diff --git a/integration-tests/js-compute/fixtures/app/tests.json b/integration-tests/js-compute/fixtures/app/tests.json
index aa71d1c882..f6161f41d4 100644
--- a/integration-tests/js-compute/fixtures/app/tests.json
+++ b/integration-tests/js-compute/fixtures/app/tests.json
@@ -3142,6 +3142,33 @@
       }
     }
   },
+  "GET /early-hints/manual-response-late": {
+    "environments": ["compute"],
+    "downstream_response": {
+      "body": "ok",
+      "status": 200
+    }
+  },
+  "GET /early-hints/manual-response-async": {
+    "environments": ["compute"],
+    "downstream_response": {
+      "body": "ok",
+      "status": 200
+    },
+    "downstream_info": {
+      "status": 103,
+      "headers": {
+        "link": "</style.css>; rel=preload; as=style"
+      }
+    }
+  },
+  "GET /early-hints/manual-response-late-async": {
+    "environments": ["compute"],
+    "downstream_response": {
+      "body": "ok",
+      "status": 200
+    }
+  },
   "GET /early-hints/send-early-hints": {
     "environments": ["compute"],
     "downstream_response": {
@@ -3155,6 +3182,13 @@
       }
     }
   },
+  "GET /early-hints/send-early-hints-late": {
+    "environments": ["compute"],
+    "downstream_response": {
+      "body": "ok",
+      "status": 200
+    }
+  },
   "GET /early-hints/send-early-hints-multiple-headers": {
     "environments": ["compute"],
     "downstream_response": {
@@ -3169,6 +3203,26 @@
       ]
     }
   },
+  "GET /early-hints/send-early-hints-async": {
+    "environments": ["compute"],
+    "downstream_response": {
+      "body": "ok",
+      "status": 200
+    },
+    "downstream_info": {
+      "status": 103,
+      "headers": {
+        "link": "</style.css>; rel=preload; as=style"
+      }
+    }
+  },
+  "GET /early-hints/send-early-hints-late-async": {
+    "environments": ["compute"],
+    "downstream_response": {
+      "body": "ok",
+      "status": 200
+    }
+  },
   "GET /shielding/invalid-shield": {
     "environments": ["compute"]
   },
diff --git a/runtime/fastly/builtins/fetch-event.cpp b/runtime/fastly/builtins/fetch-event.cpp
index e130df8824..efc5181ef1 100644
--- a/runtime/fastly/builtins/fetch-event.cpp
+++ b/runtime/fastly/builtins/fetch-event.cpp
@@ -908,11 +908,6 @@ bool FetchEvent::sendEarlyHints(JSContext *cx, unsigned argc, JS::Value *vp) {
   METHOD_HEADER(1)
   MOZ_RELEASE_ASSERT(state(self) == State::unhandled || state(self) == State::waitToRespond);
 
-  if (!is_dispatching(self)) {
-    JS_ReportErrorUTF8(cx, "FetchEvent#sendEarlyHints must be called synchronously from "
-                           "within a FetchEvent handler");
-    return false;
-  }
   if (state(self) != State::unhandled && state(self) != State::waitToRespond) {
     JS_ReportErrorUTF8(
         cx, "FetchEvent#sendEarlyHints can't be called after the main response has been sent");