diff --git a/.changeset/hungry-spoons-attack.md b/.changeset/hungry-spoons-attack.md
new file mode 100644
index 0000000000..3a0b1b8304
--- /dev/null
+++ b/.changeset/hungry-spoons-attack.md
@@ -0,0 +1,6 @@
+---
+"@workflow/core": patch
+"workflow": patch
+---
+
+Refactor builtin step functions to use `this` value serialization instead of explicit parameter passing. Remove unused duplicate builtins file from `@workflow/core`.
diff --git a/packages/core/package.json b/packages/core/package.json
index a698622340..2145ab08b8 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -52,10 +52,6 @@
       "types": "./dist/class-serialization.d.ts",
       "default": "./dist/class-serialization.js"
     },
-    "./builtins": {
-      "types": "./dist/builtins.d.ts",
-      "default": "./dist/builtins.js"
-    },
     "./serialization": {
       "types": "./dist/serialization.d.ts",
       "default": "./dist/serialization.js"
diff --git a/packages/core/src/builtins.ts b/packages/core/src/builtins.ts
deleted file mode 100644
index 56a30a1ad2..0000000000
--- a/packages/core/src/builtins.ts
+++ /dev/null
@@ -1,14 +0,0 @@
-export async function __builtin_response_array_buffer(res: Response) {
-  'use step';
-  return res.arrayBuffer();
-}
-
-export async function __builtin_response_json(res: Response) {
-  'use step';
-  return res.json();
-}
-
-export async function __builtin_response_text(res: Response) {
-  'use step';
-  return res.text();
-}
diff --git a/packages/core/src/workflow.ts b/packages/core/src/workflow.ts
index b84bb6e85a..5d18c085b4 100644
--- a/packages/core/src/workflow.ts
+++ b/packages/core/src/workflow.ts
@@ -454,29 +454,34 @@ export async function runWorkflow(
       blob!: () => Promise<Blob>;
       formData!: () => Promise<FormData>;
 
-      async arrayBuffer() {
-        return resArrayBuffer(this);
-      }
+      arrayBuffer!: () => Promise<ArrayBuffer>;
+      json!: () => Promise<any>;
+      text!: () => Promise<string>;
 
       async bytes() {
-        return new Uint8Array(await resArrayBuffer(this));
-      }
-
-      async json() {
-        return resJson(this);
-      }
-
-      async text() {
-        return resText(this);
+        return new Uint8Array(await this.arrayBuffer());
       }
     }
     vmGlobalThis.Request = Request;
 
-    const resJson = useStep<[any], any>('__builtin_response_json');
-    const resText = useStep<[any], string>('__builtin_response_text');
-    const resArrayBuffer = useStep<[any], ArrayBuffer>(
-      '__builtin_response_array_buffer'
-    );
+    Object.defineProperties(Request.prototype, {
+      arrayBuffer: {
+        value: useStep<[], ArrayBuffer>('__builtin_response_array_buffer'),
+        writable: true,
+        configurable: true,
+      },
+      json: {
+        value: useStep<[], any>('__builtin_response_json'),
+        writable: true,
+        configurable: true,
+      },
+      text: {
+        value: useStep<[], string>('__builtin_response_text'),
+        writable: true,
+        configurable: true,
+      },
+    });
+
     class Response implements globalThis.Response {
       type!: globalThis.Response['type'];
       url!: string;
@@ -534,16 +539,12 @@ export async function runWorkflow(
         return false;
       }
 
-      async arrayBuffer() {
-        return resArrayBuffer(this);
-      }
+      arrayBuffer!: () => Promise<ArrayBuffer>;
+      json!: () => Promise<any>;
+      text!: () => Promise<string>;
 
       async bytes() {
-        return new Uint8Array(await resArrayBuffer(this));
-      }
-
-      async json() {
-        return resJson(this);
+        return new Uint8Array(await this.arrayBuffer());
       }
 
       static json(data: any, init?: ResponseInit): Response {
@@ -555,10 +556,6 @@ export async function runWorkflow(
         return new Response(body, { ...init, headers });
       }
 
-      async text() {
-        return resText(this);
-      }
-
       static error(): Response {
         ENOTSUP();
       }
@@ -589,6 +586,24 @@ export async function runWorkflow(
     }
     vmGlobalThis.Response = Response;
 
+    Object.defineProperties(Response.prototype, {
+      arrayBuffer: {
+        value: useStep<[], ArrayBuffer>('__builtin_response_array_buffer'),
+        writable: true,
+        configurable: true,
+      },
+      json: {
+        value: useStep<[], any>('__builtin_response_json'),
+        writable: true,
+        configurable: true,
+      },
+      text: {
+        value: useStep<[], string>('__builtin_response_text'),
+        writable: true,
+        configurable: true,
+      },
+    });
+
     class ReadableStream<T> implements globalThis.ReadableStream<T> {
       constructor() {
         ENOTSUP();
diff --git a/packages/workflow/src/internal/builtins.ts b/packages/workflow/src/internal/builtins.ts
index 5510c1fad0..886686e50e 100644
--- a/packages/workflow/src/internal/builtins.ts
+++ b/packages/workflow/src/internal/builtins.ts
@@ -4,17 +4,19 @@
  * alongside user defined steps. They are used internally by the runtime
  */
 
-export async function __builtin_response_array_buffer(res: Response) {
+export async function __builtin_response_array_buffer(
+  this: Request | Response
+) {
   'use step';
-  return res.arrayBuffer();
+  return this.arrayBuffer();
 }
 
-export async function __builtin_response_json(res: Response) {
+export async function __builtin_response_json(this: Request | Response) {
   'use step';
-  return res.json();
+  return this.json();
 }
 
-export async function __builtin_response_text(res: Response) {
+export async function __builtin_response_text(this: Request | Response) {
   'use step';
-  return res.text();
+  return this.text();
 }