WDK Sleep, Webhooks & Hooks
These are the primitives that make durable workflows worth their cost. Anything you'd previously implement with a cron job, a state column, a polling loop, or a queue + scheduled job collapses into ordinary linear code. Human-in-the-loop, long-running approval flows, multi-day onboarding sequences, and "call a tool then wait for the user" agent patterns all fall out of `sleep` + `createWebhook` for free.
What It Defines
Suspension primitives that let a workflow pause without consuming resources. `sleep("7 days")` is a durable timer — the workflow is unloaded entirely and the runtime wakes it on schedule. `createWebhook()` returns a URL plus a promise; the workflow suspends until an external system POSTs to that URL, then resumes with the request body. Hooks are the same idea exposed to other workflows or app code. Combined with the replay model these primitives make patterns like "send onboarding email, wait 3 days, check engagement, branch" or "launch agent, suspend until human approval, then continue" trivial — and crash-safe.
Canonical (Normative)
Convenient (Practical)
Frontend cloud platform and the steward of the open-source Workflow DevKit (Apache-2.0). Vercel publishes the WDK spec and reference implementation ("use workflow" / "use step" directives, durable timers, hooks, streaming) and runs the integrated workflow runtime on its platform. WDK can also be self-hosted on Docker, AWS, or DigitalOcean. Vercel also publishes the AI SDK and the Vercel Functions runtime spec.
Related Specs
WDK is the durable-execution model arriving inside the JavaScript ecosystem proper: instead of writing a state machine, you write `async` code with two extra string directives and you get crash-safe, resumable, retry-aware, observable workflows. If you're building AI agents, multi-step background jobs, human-in-the-loop flows, or anything that previously needed BullMQ + a state machine + careful idempotency, WDK collapses that into a single mental model. Even if you don't pick WDK specifically, the directives + replay model are the durable-execution pattern Temporal pioneered and Cloudflare/Restate/DBOS now ship — knowing one teaches you the rest.
If you remember one thing about durable execution, remember this: workflow code runs many times and the event log decides what's real. That single fact explains every WDK rule — why steps must be idempotent, why you can't read `Date.now()` directly in a workflow, why parameters are passed by value, why mutating an object inside a step doesn't propagate, and why bundlers occasionally bite you. Internalize the replay model and the rest of WDK (and Temporal, and Cloudflare Workflows) becomes obvious.
AI chat UIs and agent dashboards need two things at once: streaming responses for perceived speed, and durable state so a reload doesn't lose the conversation. Workflow DevKit's resumable streams give you both — the same writer that powers `streamText`-style UX is also the workflow's persisted output. If you've ever lost an in-progress LLM response to a refresh, this is the fix.
Temporal is the prior art every durable-execution framework is measured against. If you're picking a workflow runtime, evaluating WDK, or reading a paper on durable execution, the Temporal model — workflows + activities + signals + timers + event history — is the vocabulary. Even if you choose WDK or Step Functions for a specific stack, understanding Temporal makes their tradeoffs legible (sandbox restrictions vs full Node, hosted runtime vs self-host, JSON DSL vs code).