Runtime.HandlerNotFound ☁️ AWS

Runtime.HandlerNotFound: index.handler is undefined or not exported

Lambda loaded the file but couldn’t find the function named in the handler setting.

Seen on: AWS

Meaning

The handler setting is file.exportedFunction (Node) or module.function (Python). Renamed functions, default exports, TypeScript compiled to another folder, or ESM files (.mjs) referenced as .js cause it.

Common causes

  • Handler setting doesn’t match file or function name
  • Function not exported (Node: exports.handler / export const handler)
  • Compiled output in dist/ but handler points at the root
  • Default export instead of named export

⚡ Quick fix

  1. Set the handler to <file>.<export> exactly
  2. Export the function with the expected name
  3. Point the handler at the compiled path (dist/index.handler)

Detailed fix by platform

Node.js

  1. javascript
    export const handler = async (event) => {
  return { statusCode: 200, body: "ok" };
};   // handler setting: index.handler

How to diagnose

  1. Setting — Runtime settings → Handler
  2. Export — Name exported by the file
  3. Path — File location in the zip

🔧 Still not fixed?

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