AWS Emulator gives coding agents a local copy of AWS

AWS has released AWS Emulator, a free offline tool that gives coding agents a local copy of AWS to test against. The launch connects to early distributed systems work at Amazon, including the Alf lock service, and reflects a shift in how engineers build guardrails when agents write the code.

From Alf to emulator

About twenty years ago, Swaminathan Sivasubramanian worked with Al Vermeulen on Alf, a distributed lock service built on Paxos. Alf powers many complex distributed systems, such as S3 and DynamoDB, and was tested by simulating its environment with high fidelity. Today AWS has over 200 services, so building test environments for each one by hand would take years of engineering.

How agents build the emulator

The idea came from Matt Goldberg, a Senior Principal Engineer on the AWS Developer Tools team. Agents use public API models written in Smithy and fidelity tests based on customer scenarios, then check them against real AWS. Matt Goldberg gave Kiro one prompt describing this approach and got a passable first version of an SQS emulator in about 15 minutes. All of AWS Emulator's code was written and tested by agents, and it is built from more than 400 API models covering more than 200 AWS services.

The engineer's role now

When agents write and check their own code, engineers build the specs, tests, and merge gates that decide when the work is correct. Swaminathan Sivasubramanian learned this when rebuilding a replication engine with Kiro: the rebuild took about two hours once a spec and test environment were available, after taking about four months the first time. AWS Emulator includes fidelity tests verified against real AWS, a merge gate with security agents and a general code reviewer, and deterministic checks. An internal version came together over a weekend and started gaining adoption within a month. The emulator code is released under CC0, and AWS will share more at re:Invent.

Sources: All Things Distributed · Simon Willison · The New Stack