diff --git a/lib/util.js b/lib/util.js
index a36c4804..daecfd15 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -35,15 +35,17 @@ async function validateWebhook(requestData, secret) {
   const signingSecret = secret || requestData.secret;
 
   if (requestData && requestData.headers && requestData.body) {
-    id =
-      requestData.headers["webhook-id"] ||
-      requestData.headers.get?.("webhook-id");
-    timestamp =
-      requestData.headers["webhook-timestamp"] ||
-      requestData.headers.get?.("webhook-timestamp");
-    signature =
-      requestData.headers["webhook-signature"] ||
-      requestData.headers.get?.("webhook-signature");
+    if (typeof requestData.headers.get === "function") {
+      // Headers object (e.g. Fetch API Headers)
+      id = requestData.headers.get("webhook-id");
+      timestamp = requestData.headers.get("webhook-timestamp");
+      signature = requestData.headers.get("webhook-signature");
+    } else {
+      // Plain object with header key-value pairs
+      id = requestData.headers["webhook-id"];
+      timestamp = requestData.headers["webhook-timestamp"];
+      signature = requestData.headers["webhook-signature"];
+    }
     body = requestData.body;
   }
 
@@ -75,11 +77,18 @@ async function validateWebhook(requestData, secret) {
 
   const signedContent = `${id}.${timestamp}.${body}`;
 
-  const computedSignature = await createHMACSHA256(signingSecret.split("_").pop(), signedContent);
+  const computedSignature = await createHMACSHA256(
+    signingSecret.split("_").pop(),
+    signedContent
+  );
 
-  const expectedSignatures = signature.split(" ").map((sig) => sig.split(",")[1]);
+  const expectedSignatures = signature
+    .split(" ")
+    .map((sig) => sig.split(",")[1]);
 
-  return expectedSignatures.some((expectedSignature) => expectedSignature === computedSignature);
+  return expectedSignatures.some(
+    (expectedSignature) => expectedSignature === computedSignature
+  );
 }
 
 /**
@@ -106,9 +115,13 @@ async function createHMACSHA256(secret, data) {
     crypto = require.call(null, "node:crypto").webcrypto;
   }
 
-  const key = await crypto.subtle.importKey("raw", base64ToBytes(secret), { name: "HMAC", hash: "SHA-256" }, false, [
-    "sign",
-  ]);
+  const key = await crypto.subtle.importKey(
+    "raw",
+    base64ToBytes(secret),
+    { name: "HMAC", hash: "SHA-256" },
+    false,
+    ["sign"]
+  );
 
   const signature = await crypto.subtle.sign("HMAC", key, encoder.encode(data));
   return bytesToBase64(signature);
@@ -232,7 +245,11 @@ async function transformFileInputs(client, inputs, strategy) {
       try {
         return await transformFileInputsToReplicateFileURLs(client, inputs);
       } catch (error) {
-        if (error instanceof ApiError && error.response.status >= 400 && error.response.status < 500) {
+        if (
+          error instanceof ApiError &&
+          error.response.status >= 400 &&
+          error.response.status < 500
+        ) {
           throw error;
         }
         return await transformFileInputsToBase64EncodedDataURIs(inputs);
@@ -296,7 +313,7 @@ async function transformFileInputsToBase64EncodedDataURIs(inputs) {
     totalBytes += buffer.byteLength;
     if (totalBytes > MAX_DATA_URI_SIZE) {
       throw new Error(
-        `Combined filesize of prediction ${totalBytes} bytes exceeds 10mb limit for inline encoding, please provide URLs instead`,
+        `Combined filesize of prediction ${totalBytes} bytes exceeds 10mb limit for inline encoding, please provide URLs instead`
       );
     }
 
@@ -354,11 +371,14 @@ function isPlainObject(value) {
   if (proto === null) {
     return true;
   }
-  const Ctor = Object.prototype.hasOwnProperty.call(proto, "constructor") && proto.constructor;
+  const Ctor =
+    Object.prototype.hasOwnProperty.call(proto, "constructor") &&
+    proto.constructor;
   return (
     typeof Ctor === "function" &&
     Ctor instanceof Ctor &&
-    Function.prototype.toString.call(Ctor) === Function.prototype.toString.call(Object)
+    Function.prototype.toString.call(Ctor) ===
+      Function.prototype.toString.call(Object)
   );
 }