Vercel Workflow DevKit

Build durable, resumable workflows that survive restarts, deployments, and failures using TypeScript directives.

Quick Start (Next.js)

npm i workflow

// next.config.ts
import { withWorkflow } from "workflow/next";
export default withWorkflow({});

// workflows/signup.ts
import { sleep, FatalError } from "workflow";

export async function signupWorkflow(email: string) {
  "use workflow";

  const user = await createUser(email);
  await sendWelcome(user.id, email);
  await sleep("3d");
  await sendFollowUp(user.id);
  return { success: true };
}

async function createUser(email: string) {
  "use step";
  return { id: crypto.randomUUID(), email };
}

async function sendWelcome(userId: string, email: string) {
  "use step";
  const res = await fetch("https://api.email.com/send", {
    method: "POST",
    body: JSON.stringify({ to: email, template: "welcome" })
  });
  if (!res.ok) throw new Error("Failed"); // Auto-retried
}

async function sendFollowUp(userId: string) {
  "use step";
  // FatalError = no retry
  if (!userId) throw new FatalError("Missing userId");
  // ... send email
}

// app/api/signup/route.ts
import { start } from "workflow/api";
import { signupWorkflow } from "@/workflows/signup";

export async function POST(request: Request) {
  const { email } = await request.json();
  const run = await start(signupWorkflow, [email]);
  return Response.json({ runId: run.id });
}

Core Concepts

The Two Directives

Directive	Purpose	Rules
"use workflow"	Orchestrates steps, sleeps, suspends	Deterministic: No side effects, no Node.js modules, no global fetch
"use step"	Contains business logic, I/O, side effects	Runs on separate request: Auto-retries on failure

Why These Restrictions?

Workflow DevKit uses event sourcing - state changes are stored as events and replayed to reconstruct state. Workflows must be deterministic so replays produce identical step sequences. Steps run on separate requests, so data passes by value (mutations don't affect workflow variables).

Data Flow: Pass-by-Value

// WRONG - mutations lost
async function workflow() {
  "use workflow";
  const user = { name: "Alice" };
  await updateUser(user);
  console.log(user.name); // Still "Alice"!
}

// CORRECT - return modified data
async function workflow() {
  "use workflow";
  let user = { name: "Alice" };
  user = await updateUser(user);
  console.log(user.name); // "Bob"
}

async function updateUser(user: User) {
  "use step";
  user.name = "Bob";
  return user; // Must return!
}

API Quick Reference

workflow package

import {
  sleep,              // Suspend for duration/until date
  fetch,              // HTTP with auto-retry (use in workflows)
  FatalError,         // Non-retryable error
  RetryableError,     // Explicit retry with delay
  createHook,         // Receive external payloads
  createWebhook,      // Receive HTTP requests
  defineHook,         // Type-safe hooks with validation
  getWritable,        // Stream output
  getWorkflowMetadata,// { workflowRunId, workflowStartedAt, url }
  getStepMetadata     // { stepId } - use for idempotency keys
} from "workflow";

workflow/api package

import {
  start,          // Start workflow run
  getRun,         // Get run status
  resumeHook,     // Resume via hook token
  resumeWebhook,  // Resume via webhook token
  getHookByToken  // Get hook details
} from "workflow/api";

Sleep & Scheduling

await sleep("10s");   // seconds
await sleep("5m");    // minutes
await sleep("2h");    // hours
await sleep("1d");    // days
await sleep("2w");    // weeks
await sleep(5000);    // milliseconds
await sleep(new Date("2025-12-25")); // until date

Error Handling

import { FatalError, RetryableError } from "workflow";

async function processPayment(amount: number) {
  "use step";

  try {
    return await paymentAPI.charge(amount);
  } catch (error) {
    // Don't retry invalid requests
    if (error.code === "INVALID_CARD") {
      throw new FatalError("Invalid card");
    }
    // Explicit retry with delay
    if (error.code === "RATE_LIMITED") {
      throw new RetryableError("Rate limited", { retryAfter: "5m" });
    }
    // Other errors auto-retry with default backoff
    throw error;
  }
}

Hooks (External Events)

import { createHook, defineHook } from "workflow";
import { resumeHook } from "workflow/api";
import { z } from "zod";

// In workflow - create hook and wait
export async function approvalWorkflow(orderId: string) {
  "use workflow";

  const hook = createHook<{ approved: boolean; comment: string }>({
    token: `approval:${orderId}` // Custom token for deterministic recovery
  });

  await notifyReviewer(orderId, hook.token);
  const result = await hook; // Suspends until resumed

  if (result.approved) await processOrder(orderId);
}

// In API route - resume with payload
export async function POST(request: Request) {
  const { token, approved, comment } = await request.json();
  await resumeHook(token, { approved, comment });
  return Response.json({ success: true });
}

// Type-safe hooks with validation
const approvalHook = defineHook(z.object({
  approved: z.boolean(),
  comment: z.string().max(500)
}));

// Use: approvalHook.create(), approvalHook.resume(token, payload)

Idempotency

Use stepId for idempotency keys - stable across retries, unique per step:

import { getStepMetadata } from "workflow";

async function chargeCustomer(customerId: string, amount: number) {
  "use step";
  const { stepId } = getStepMetadata();

  return stripe.charges.create({
    customer: customerId,
    amount,
    idempotency_key: `charge:${stepId}`
  });
}

Streaming

import { getWritable } from "workflow";

export async function streamingWorkflow() {
  "use workflow";
  const writable = getWritable<{ progress: number }>();
  await streamProgress(writable);
  await closeStream(writable);
}

// IMPORTANT: All stream operations must happen in steps
async function streamProgress(writable: WritableStream) {
  "use step";
  const writer = writable.getWriter();
  try {
    for (let i = 0; i <= 100; i += 10) {
      await writer.write({ progress: i });
    }
  } finally {
    writer.releaseLock(); // Always release!
  }
}

async function closeStream(writable: WritableStream) {
  "use step";
  await writable.close();
}

// Consume in API route
export async function GET() {
  const run = await start(streamingWorkflow);
  return new Response(run.readable);
}

// Namespaced streams
const dataStream = getWritable({ namespace: "data" });
const logsStream = getWritable({ namespace: "logs" });

AI Agents with @workflow/ai

import { DurableAgent } from "@workflow/ai";
import { fetch, getWritable } from "workflow";
import { z } from "zod";

export async function chatWorkflow(messages: UIMessage[]) {
  "use workflow";

  // CRITICAL: Enable fetch for AI SDK
  globalThis.fetch = fetch;

  const writable = getWritable<UIMessageChunk>();

  const agent = new DurableAgent({
    model: "anthropic/claude-haiku-4.5",
    system: "You are a helpful assistant.",
    tools: {
      searchWeb: {
        description: "Search the web",
        parameters: z.object({ query: z.string() }),
        execute: async ({ query }) => {
          "use step";
          return await searchAPI(query);
        }
      }
    }
  });

  const result = await agent.stream({
    messages,
    writable,
    maxSteps: 10
  });

  return result.messages;
}

Observability & Debugging

npx workflow web              # Visual web UI
npx workflow inspect runs     # List recent runs
npx workflow inspect run <id> # Detailed run info
npx workflow inspect steps <id> # List steps

# Production (Vercel)
npx workflow inspect runs --backend vercel --env production

Deployment

Vercel (recommended): vercel deploy - no config needed

Other backends: Set WORKFLOW_TARGET_WORLD:

• @workflow/world-local (default dev)
• @workflow-worlds/postgres
• @workflow-worlds/turso
• @workflow-worlds/mongodb
• @workflow-worlds/redis

Common Patterns

See references/patterns.md for:

• Parallel execution (Promise.all)
• Saga pattern (compensating transactions)
• Circuit breaker
• Scheduled/recurring workflows
• Human-in-the-loop
• Single vs multi-turn chat sessions

Debugging & Errors

See references/debugging.md for:

• All error types and solutions
• Logging best practices
• Testing strategies
• Performance tips
• Middleware configuration