AWS released AWS Emulator on Wednesday 11 November. The same day, Matthew Goldberg, the AWS engineer who led the project, described on an Amazon blog how agents built it. Autonomous agents wrote the whole emulator, Goldberg said: ‘Our team hasn’t written or reviewed any of the code.’

The emulator is a local program that responds to AWS API calls. When the AWS CLI or SDK is pointed at it, an application’s requests go to the emulator rather than to AWS. It can run on a developer’s own machine, on a development server or in a CI pipeline for automated testing.

GeekWire reported that the emulator covers more than 200 AWS services. Goldberg said it is built on 438 AWS API models. ITmedia, however, reported that the depth of implementation varies by service and operation. With each release, AWS publishes per-service fidelity scores, which show how closely the emulator mimics AWS.

Three agent teams and a review gate

Goldberg said three teams of agents build the emulator. The first writes tests from public API models and checks that each one works on real AWS. A test might, for example, create a queue, send a message and delete the queue. The second team runs the tests against the emulator and logs failures to be fixed. In the third team, each agent fixes one issue.

A change reaches the main branch only once the code compiles. Reviewing agents must also confirm that it follows coding conventions, is secure and does not reduce fidelity to AWS. Hundreds of agents fix thousands of issues a day, Goldberg said. Simon Willison wrote in a blog post that the project accumulated 29,684 commits in its first three weeks.

The project began as a weekend experiment by Goldberg. It was inspired by an experiment by Anthropic researchers in which agents built a C compiler autonomously. Goldberg said Amazon’s Kiro coding agent produced a viable first version of the Amazon SQS queue service in 15 minutes.

Not for production or security testing

AWS advises against using the emulator in production or for security testing. ITmedia reported that the emulator does not handle IAM and resource policies, which define access permissions, the same way AWS does. Resources are held in memory and disappear when the emulator is stopped.

GeekWire reported that IDC analyst Devin Pratt told InfoWorld the emulator does not yet mimic request throttling, outages or regional differences in the availability of services and features. Pratt also said it does not isolate the code it runs. Flaws the agents have not yet fixed are publicly listed in the repository’s issues/things-that-do-not-work directory.

Availability and setup

The emulator needs no AWS account, carries no extra charge, and its code is on GitHub, mostly under a CC0 licence. GeekWire reported that LocalStack, the best-known alternative, this year limited its free version to non-commercial use and began requiring its users to have an account.

On Linux, the emulator can be installed from an RPM or Debian package; on macOS and Windows it runs in a Docker container. Telemetry, which sends usage data to AWS, is on by default. Instructions for turning it off and for other setup are in the README file on GitHub and in the AWS user guide.