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Request Metadata and Files

Verified status as of March 28, 2026. Runtime note: this page documents current FastFN behavior, including raw-body handling for form and multipart requests.

Complexity

Intermediate

Time

20-30 minutes

Outcome

You understand how headers, cookies, query values, JSON body, forms, and file-upload inputs arrive in FastFN today, and where parsing is explicit.

Validation

  1. Start local runtime: fastfn dev examples/functions.
  2. Run request metadata check: 
    curl -sS http://127.0.0.1:8080/request-inspector \
  -H "x-request-id: req-123" \
  -H "Cookie: session_id=abc123; theme=dark"
  3. Run JSON body check: 
    curl -sS -X POST http://127.0.0.1:8080/tasks \
  -H "Content-Type: application/json" \
  -d '{"title":"Write docs","priority":"2"}'
  4. Run form-urlencoded raw-body check: 
    curl -sS -X POST http://127.0.0.1:8080/contact \
  -H "Content-Type: application/x-www-form-urlencoded" \
  --data 'name=Misael&role=admin'

Troubleshooting

  • If headers are missing, inspect casing and reverse-proxy forwarding rules.
  • If cookies are empty, confirm the Cookie header reaches FastFN.
  • If JSON parse fails, verify Content-Type and payload validity.
  • For multipart parsing needs, treat current support as raw-body only.

Support matrix

Input type Current posture How it arrives
Headers Supported event.headers
Cookies Supported event.session.cookies
Query string Supported event.query
JSON body Supported with explicit parsing event.body raw string
Plain text body Supported event.body raw string
application/x-www-form-urlencoded Raw-only event.body raw string
multipart/form-data Raw-only (no first-class parser) event.body raw string
Binary request payloads Limited avoid assuming automatic structured parsing

Headers

event.headers

Python: 

def handler(event):
    headers = event.get("headers") or {}
    return {
        "status": 200,
        "body": {
            "request_id": headers.get("x-request-id"),
            "authorized": bool(headers.get("x-api-key")),
        },
    }

Node: 

exports.handler = async (event) => {
  const headers = event.headers || {};
  return {
    status: 200,
    body: {
      request_id: headers["x-request-id"] || null,
      authorized: Boolean(headers["x-api-key"] || null),
    },
  };
};

Cookies

FastFN exposes parsed cookies under event.session.cookies.

{
  "session": {
    "id": "abc123",
    "cookies": {
      "session_id": "abc123",
      "theme": "dark"
    }
  }
}

Python: 

def handler(event):
    session = event.get("session") or {}
    cookies = session.get("cookies") or {}
    return {"status": 200, "body": {"theme": cookies.get("theme", "light")}}

JSON and plain-text bodies

Body is raw text. Parse explicitly.

Python: 

import json


def handler(event):
    payload = json.loads(event.get("body") or "{}")
    return {
        "status": 200,
        "body": {
            "title": payload.get("title"),
            "priority": int(payload.get("priority", 1)),
        },
    }

Node: 

exports.handler = async (event) => {
  const payload = JSON.parse(event.body || "{}");
  return {
    status: 200,
    body: {
      title: payload.title || null,
      priority: Number(payload.priority || 1),
    },
  };
};

Forms and multipart

  • application/x-www-form-urlencoded: available as raw string in event.body.
  • multipart/form-data: currently raw-only in gateway contract; no built-in field/file abstraction.

Practical recommendation: 1. Prefer JSON for API contracts. 2. Keep multipart parsing explicit in runtime code only when you fully control format. 3. For heavy upload workflows, use specialized upstream components.

Limits that matter

  • Request size is constrained by max_body_bytes.
  • Per-function policy still applies before handler execution.
  • Response metadata (cookies/headers) remains controlled by response envelope fields.
Last reviewed: March 28, 2026 · Docs on fastfn.dev