Genkit Fastify Plugin

This plugin provides utilities for conveniently exposing Genkit flows and actions via a Fastify HTTP server as REST APIs.

Fastify is not Web Fetch native and has no built-in Genkit integration, so wiring a flow into Fastify by hand requires bridging Fastify's request/reply to the Genkit action protocol (including Server-Sent Events for streaming). This plugin does that bridging for you, so a flow mounts in one line, just like @genkit-ai/express does for Express.

See the official documentation for more.

Installation

npm i @genkit-ai/fastify

Usage

Mount a single flow with fastifyHandler:

import Fastify from 'fastify';
import { fastifyHandler } from '@genkit-ai/fastify';

const simpleFlow = ai.defineFlow('simpleFlow', async (input, { sendChunk }) => {
  const { text } = await ai.generate({
    model: googleAI.model('gemini-2.5-flash'),
    prompt: input,
    onChunk: (c) => sendChunk(c.text),
  });
  return text;
});

const app = Fastify();

app.post('/simpleFlow', fastifyHandler(simpleFlow));

await app.listen({ port: 8080 });

The handler reads { data } from the JSON body, runs the flow, and streams chunks back as Server-Sent Events when the caller sends Accept: text/event-stream (or ?stream=true) — exactly what the runFlow and streamFlow clients in genkit/beta/client expect.

Expose multiple flows with the plugin

Register genkitFastify to mount several flows at once. Each flow is exposed at /<flowName> (override with withFlowOptions):

import Fastify from 'fastify';
import { genkitFastify, withFlowOptions } from '@genkit-ai/fastify';

const app = Fastify();

await app.register(genkitFastify, {
  pathPrefix: '/api',
  flows: [
    simpleFlow,
    withFlowOptions(secureFlow, { contextProvider }),
  ],
});

await app.listen({ port: 8080 });

Handle auth with context providers

import { UserFacingError } from 'genkit';
import { ContextProvider, RequestData } from 'genkit/context';

const contextProvider: ContextProvider<Context> = (req: RequestData) => {
  if (req.headers['authorization'] !== 'open sesame') {
    throw new UserFacingError('PERMISSION_DENIED', 'not authorized');
  }
  return { auth: { user: 'Ali Baba' } };
};

app.post('/secureFlow', fastifyHandler(secureFlow, { contextProvider }));

The sources for this package are in the main Genkit repo. Please file issues and pull requests against that repo.

Usage information and reference details can be found in Genkit documentation.

License: Apache 2.0