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All function arguments and return values passed between workflow and step functions must be serializable. Workflow DevKit uses a custom serialization system built on top of devalue. This system supports standard JSON types, as well as a few additional popular Web API types.
The serialization system ensures that all data persists correctly across workflow suspensions and resumptions, enabling durable execution.

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Supported Serializable Types

The following types can be serialized and passed through workflow functions: Standard JSON Types:
  • string
  • number
  • boolean
  • null
  • Arrays of serializable values
  • Objects with string keys and serializable values
Extended Types:
  • undefined
  • bigint
  • ArrayBuffer
  • BigInt64Array, BigUint64Array
  • Date
  • Float32Array, Float64Array
  • Int8Array, Int16Array, Int32Array
  • Map<Serializable, Serializable>
  • RegExp
  • Set<Serializable>
  • URL
  • URLSearchParams
  • Uint8Array, Uint8ClampedArray, Uint16Array, Uint32Array
Notable:
These types have special handling and are explained in detail in the sections below.
  • Headers
  • Request
  • Response
  • ReadableStream<Serializable>
  • WritableStream<Serializable>

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Streaming

ReadableStream and WritableStream are supported as serializable types with special handling. These streams can be passed between workflow and step functions while maintaining their streaming capabilities. For complete information about using streams in workflows, including patterns for AI streaming, file processing, and progress updates, see the Streaming Guide.

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Request & Response

The Web API Request and Response APIs are supported by the serialization system, and can be passed around between workflow and step functions similarly to other data types. As a convenience, these two APIs are treated slightly differently when used within a workflow function: calling the text() / json() / arrayBuffer() instance methods is automatically treated as a step function invocation. This allows you to consume the body directly in the workflow context while maintaining proper serialization and caching. For example, consider how receiving a webhook request provides the entire Request instance into the workflow context. You may consume the body of that request directly in the workflow, which will be cached as a step result for future resumptions of the workflow: 
import { createWebhook } from "workflow";

export async function handleWebhookWorkflow() {
  "use workflow";

  const webhook = createWebhook();
  const request = await webhook;

  // The body of the request will only be consumed once
  const body = await request.json();

  // …
}

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Using fetch in Workflows

Because Request and Response are serializable, Workflow DevKit provides a fetch function that can be used directly in workflow functions: 
import { fetch } from "workflow";

export async function apiWorkflow() {
  "use workflow";

  // fetch can be called directly in workflows
  const response = await fetch("https://api.example.com/data");
  const data = await response.json();

  return data;
}
The implementation is straightforward - fetch from workflow is a step function that wraps the standard fetch: 
export async function fetch(...args: Parameters<typeof globalThis.fetch>) {
  "use step";
  return globalThis.fetch(...args);
}
This allows you to make HTTP requests directly in workflow functions while maintaining deterministic replay behavior through automatic caching.

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Pass-by-Value Semantics

Parameters are passed by value, not by reference. Steps receive deserialized copies of data. Mutations inside a step won’t affect the original in the workflow. Incorrect: 
export async function updateUserWorkflow(userId: string) {
  "use workflow";

  let user = { id: userId, name: "John", email: "[email protected]" };
  await updateUserStep(user);

  // user.email is still "[email protected]"
  console.log(user.email);
}

async function updateUserStep(user: { id: string; name: string; email: string }) {
  "use step";
  user.email = "[email protected]"; // Changes are lost
}
Correct - return the modified data: 
export async function updateUserWorkflow(userId: string) {
  "use workflow";

  let user = { id: userId, name: "John", email: "[email protected]" };
  user = await updateUserStep(user); // Reassign the return value

  console.log(user.email); // "[email protected]"
}

async function updateUserStep(user: { id: string; name: string; email: string }) {
  "use step";
  user.email = "[email protected]";
  return user;
}

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