Handling File Uploads in AWS Lambda with Powertools OpenAPI (From Limitation to Production Feature)

Introduction Handling file uploads in serverless APIs sounds simple until you actually try to do it. If you're building APIs with AWS Lambda Powertools and OpenAPI validation, you quickly run into a limitation: multipart/form-data isn’t natively supported in the same way as JSON or form-encoded requests. That gap forces teams into workarounds: Manual multipart parsing Base64 hacks Disabling validation entirely None of which are ideal in production systems. This article walks through: The real problem How the feature was designed How you can now use it in practice The Problem: File Uploads Break the Abstraction Before this feature, Powertools handled: JSON payloads Query parameters Headers Form data (application/x-www-form-urlencoded) But not: multipart/form-data (file uploads) That meant: @app.post("/upload") def upload(file: bytes): ... simply didn’t work with OpenAPI validation. Instead, developers had to: Parse raw request bodies manually Disable validation middleware Or redesign AP