Recently when working on a TypeScript project using AWS CDK inside a Turborepo monorepo, I needed a way to test an API Gateway setup locally without redeploying every time I tweaked a Lambda function code.

While tools like LocalStack can simulate multiple AWS services, I wanted to keep things simple and stick with the AWS SAM CLI. With a little bit of setup, I got a workflow that lets me edit Lambda code, save, and instantly test again — no redeploy required.

The Project Setup

My repo was organized with a lambdas folder containing multiple Lambda functions. These functions were referenced in the CDK stack as handlers for the API Gateway routes.

To enable hot-reload testing, I made sure I had the AWS SAM CLI installed, then added two scripts in my package.json and installed nodemon as a development dependency:

{
  "synth": "cdk synth",
  "startapi": "sam local start-api -t cdk.out/stack.template.json --skip-pull-image --warm-containers EAGER",
  "watch": "nodemon --watch ../../lambdas -e ts --exec 'pnpm run synth'"
}

Here’s what they do:

• startapi — Runs sam local start-api, using the synthesized CloudFormation template from CDK (e.g: cdk.out/stack.template.json).

  • --skip-pull-image — avoids pulling the latest Lambda image every time.
  • --warm-containers EAGER — containers for all functions are loaded at startup and persist between invocations.

• watch — Uses nodemon to watch for changes in the lambdas folder.

  • On every save, it runs npm run synth:local to regenerate the CDK template.
  • SAM picks up the updated template and reloads automatically.

How It Works

With this setup running:

1. Start SAM in one terminal:

pnpm startapi

2. Run the watcher in another terminal:

pnpm watch

3. Make a change to a Lambda function, save, and hit your local API endpoint again — your changes are now live.