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Part 1: Setup and Your First Route

Verified status as of March 28, 2026. Runtime note: FastFN auto-installs function-local dependencies from requirements.txt / package.json; host runtimes are required in fastfn dev --native, while fastfn dev depends on a running Docker daemon.

Quick View

  • Complexity: Beginner
  • Typical time: 15-20 minutes
  • Outcome: clean project with one GET /tasks endpoint and OpenAPI entry

1. Clean-room setup

mkdir -p task-manager-api/functions/tasks
cd task-manager-api

2. Implement the first route (choose one runtime)

File: functions/tasks/handler.js

exports.handler = async () => ({
  status: 200,
  body: [
    { id: 1, title: "Learn FastFN", completed: false },
    { id: 2, title: "Ship first endpoint", completed: false }
  ]
});

File: functions/tasks/handler.py

def handler(_event):
    return {
        "status": 200,
        "body": [
            {"id": 1, "title": "Learn FastFN", "completed": False},
            {"id": 2, "title": "Ship first endpoint", "completed": False},
        ],
    }

File: functions/tasks/handler.rs

use serde_json::json;

pub fn handler(_event: serde_json::Value) -> serde_json::Value {
    json!({
        "status": 200,
        "body": [
            { "id": 1, "title": "Learn FastFN", "completed": false },
            { "id": 2, "title": "Ship first endpoint", "completed": false }
        ]
    })
}

File: functions/tasks/handler.php

<?php

function handler(array $event): array {
    return [
        'status' => 200,
        'body' => [
            ['id' => 1, 'title' => 'Learn FastFN', 'completed' => false],
            ['id' => 2, 'title' => 'Ship first endpoint', 'completed' => false],
        ],
    ];
}

3. Run locally

fastfn dev functions

4. Validate first request (per runtime)

curl -sS 'http://127.0.0.1:8080/tasks'

curl -sS 'http://127.0.0.1:8080/tasks'

curl -sS 'http://127.0.0.1:8080/tasks'

curl -sS 'http://127.0.0.1:8080/tasks'

Expected body shape:

[
  { "id": 1, "title": "...", "completed": false },
  { "id": 2, "title": "...", "completed": false }
]

5. Validate OpenAPI visibility

curl -sS 'http://127.0.0.1:8080/openapi.json' | jq '.paths | has("/tasks")'

Expected output:

true

Browser showing JSON response at /tasks

Troubleshooting

  • 503: check /_fn/health and runtime dependencies
  • route not found: confirm handler path under functions/tasks/
  • OpenAPI missing path: run curl -X POST http://127.0.0.1:8080/_fn/reload

Next step

Go to Part 2: Routing and Data

Last reviewed: March 28, 2026 · Docs on fastfn.dev