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Nitro (UnJS)

Nitro (nitro.build) is an open-source JavaScript/TypeScript server toolkit from the UnJS ecosystem. It provides file-based routing, a build system, and virtual handlers — the three primitives needed to integrate a build-system-aware SDK with JavaScript frameworks that don't ship with a bundler.

Distinct from AWS Nitro (the EC2 hypervisor / offload-card family — see AWS Nitro); this is the UnJS server toolkit.

Stub

This page covers only what currently-ingested sources name about Nitro. Broader Nitro internals (H3 HTTP framework, Nuxt relationship, preset deployment targets) are out of scope until a source discusses them.

Relevance to the wiki (2026-04-21)

Canonical framework-shim substrate in Vercel Workflow DevKit (WDK)'s integrations for bundlerless frameworks (Express, Hono).

From Vercel's 2026-04-21 WDK post verbatim: "For frameworks without a bundler like Express or Hono, we use Nitro instead. Nitro is a server toolkit that provides file-based routing, a build system, and other quality-of-life features such as virtual handlers that can be mounted to a server at runtime. This brings the same workflow capabilities to bare HTTP servers."

Nitro was also one of two launch-support frameworks for WDK (alongside Next.js) — "At launch, WDK supported Next.js and Nitro."

Relevant properties (named by ingested sources)

• File-based routing — directory structure → HTTP endpoint auto-discovery. The property bundlerless frameworks lack and Nitro backfills.
• Build system — esbuild-orchestration for packaging server bundles. Used by WDK to bundle step + workflow handlers.
• Virtual handlers — in-memory HTTP routes that aren't files on disk; Nitro mounts them by injecting into the wrapped HTTP server at runtime. This is the mechanism WDK uses to expose workflow endpoints in Express/Hono apps.
• Server wrapping — Nitro wraps the user's HTTP server (Express app, Hono app, etc.) and injects additional handlers.

Seen in

• sources/2026-04-21-vercel-inside-workflow-devkit-how-framework-integrations-work — canonical wiki disclosure. Nitro named as launch-support framework + as the integration substrate for Express / Hono via the virtual-handler pattern.

Related

• systems/vercel-workflow — the 2026-04-21 WDK integration this page anchors on.
• systems/express, systems/hono — the bundlerless frameworks Nitro shims for.
• systems/esbuild — Nitro's underlying bundler.
• systems/vite — sibling JS build tool in the Vite-based framework branch of the WDK integration map.
• concepts/file-based-routing-vs-bare-http-framework-taxonomy — the taxonomy Nitro rebalances.
• patterns/virtual-handler-via-nitro-for-bundlerless-frameworks — the canonical pattern Nitro enables.