Overcast is alpha — behaviour and APIs may change between releases. Pin your version and read the changelog before upgrading.

overcast

alpha local cloud emulation for developers

Develop against AWS-compatible services on your laptop and in CI.

Overcast is a free, MIT-licensed local emulator for AWS APIs. Install it with one line or run the Docker image, point supported AWS CLI, CDK, or SDK workflows at localhost:4566, and inspect what your app creates in the included web UI.

Read the docs Download
services registered
50
implemented operations
1341
current release
v0.0.1-alpha.42

  1. 01 — install it, or run the image

    >_ install 
    curl -fsSL https://overcast.sh/install.sh | sh

  2. 02 — point a client at it

    >_ first call 
    aws --endpoint-url http://localhost:4566 \
  s3 mb s3://hello-overcast

Use familiar AWS tooling

Keep the CLI, CDK, SDKs, and Terraform workflows where Overcast already supports the services you need. Start by changing the endpoint URL, then check the support matrix for gaps.

No paid feature gates

Use implemented features without an auth token, telemetry, usage limits, or a paid tier. The support matrix shows what works today and what intentionally returns 501.

The same image in CI

One process, one endpoint, no sign-in. A slim headless image drops the console for build agents that only need the API.

service implementation

Check listed operations before you depend on them

All 50 services
S3
45/53

listed operations implemented
Lambda
61/62

listed operations implemented
DynamoDB
21/28

listed operations implemented
SQS
19/21

listed operations implemented
SNS
22/30

listed operations implemented
API Gateway
104/106

listed operations implemented
CloudFormation
25/53

listed operations implemented
IAM
74/74

listed operations implemented

web console

Port 4567 serves a management console.

It ships in the full image and updates as your app runs. Three parts get the most use:

  • Inbox — captures the mail your app sends through SES, SNS, and Cognito, plus the SNS SMS and webhook deliveries that have nowhere to go locally.
  • Request traces — one PutObject and everything it set off, with bodies at each hop and a copy-as-curl on every row.
  • System map — your topology drawn from what the emulator actually holds, with edges lighting up as data moves.
Tour the console
The Overcast console dashboard: emulated AWS services as cards, split into fully emulated, partially emulated, and not emulated. The Overcast console dashboard: emulated AWS services as cards, split into fully emulated, partially emulated, and not emulated.

where to start

Pick the path you're on.

First time here

One command to start it, one call to prove it works, then the guides.

Wiring up an app

Endpoint, credentials, and addressing for the AWS CLI and every SDK.

Deploying with CDK

Bootstrap, deploy, and destroy stacks against the local endpoint.

Evaluating for a team

Per-service coverage, the gaps behind it, and what alpha commits to.

Coming from LocalStack

The image line to change, the environment variables Overcast reads, and the behavioural differences to expect. No auth token to carry over.

Contributing

Where the source lives, how to build it, and how to report a gap.