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Fastify 5 routes with JSON Schema validation: working versions (tested 2026-10-15)

Fastify 5 route validation and response serialization with JSON Schema, tested via app.inject().

passed2 passed · 0 failed2026-10-15 12:53 UTC
Status
passed
Last passed
2026-10-15 12:53 UTC
Tests
2 / 2
Language
TypeScript
Runtime
node v22.23.3
pinned versions3
fastify@5.12.7tsx@4.23.15@types/node@24.19.2
run it
npm install --no-audit --no-fund --ignore-scripts=false
npm test
test environment
node v22.23.3
linux 6.17.0-1022-azure
x64
runner: github-actions 38737894352

The code

Exactly the files that passed. Your agent can pull them with get_recipe("fastify-json-schema") (install), or see them on GitHub.

package.json
{
  "name": "fastify-json-schema",
  "private": true,
  "type": "module",
  "scripts": {
    "test": "node --import tsx --test test/*.test.ts"
  },
  "dependencies": {
    "fastify": "5.12.7"
  },
  "devDependencies": {
    "tsx": "4.23.15",
    "@types/node": "24.19.2"
  }
}
src/app.ts
import Fastify from "fastify";

export function build() {
  const app = Fastify();
  app.post("/items", {
    schema: {
      body: { type: "object", required: ["name", "qty"], additionalProperties: false,
        properties: { name: { type: "string", minLength: 1 }, qty: { type: "integer", minimum: 1 } } },
      response: { 201: { type: "object", properties: { id: { type: "integer" }, name: { type: "string" }, qty: { type: "integer" } } } },
    },
  }, async (req, reply) => {
    const body = req.body as { name: string; qty: number };
    return reply.code(201).send({ id: 1, ...body, secret: "never serialized" });
  });
  return app;
}
test/app.test.ts
import { test } from "node:test";
import assert from "node:assert/strict";
import { build } from "../src/app.ts";

test("valid body, response schema strips extra fields", async () => {
  const res = await build().inject({ method: "POST", url: "/items", payload: { name: "bolt", qty: 3 } });
  assert.equal(res.statusCode, 201);
  assert.deepEqual(res.json(), { id: 1, name: "bolt", qty: 3 });
});
test("invalid body is 400", async () => {
  const res = await build().inject({ method: "POST", url: "/items", payload: { name: "", qty: 0 } });
  assert.equal(res.statusCode, 400);
});

Connect to ProvenPin

Add the free MCP server so your agent can pull this recipe and others like it. Same setup as the docs.

terminal
claude mcp add --transport http provenpin https://mcp.provenpin.com/mcp

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