kumo
A lightweight AWS service emulator written in Go
The Lens
By Erik Loyd, SaaS CEO and former COO/CFO of an AWS Premier Partner.
Updated May 2026
Kumo is a single-binary AWS service emulator written in Go. It mocks 76 AWS services (S3, DynamoDB, Lambda, SQS, KMS, IAM) for local development and CI testing. No authentication required, AWS SDK v2 compatible, optional persistence. The pitch is LocalStack with less weight and no Docker dependency.
Setup is downloading a binary or running a container. Point your AWS SDK at the local endpoint and you get an emulator that does not ask for credentials. CI integration is straightforward because there is no auth dance and the binary boots fast. Set `KUMO_DATA_DIR` to keep state across restarts.
Solo developers: use it for CI tests and local dev without an AWS sandbox account. Small and mid-sized teams: a free way to keep AWS-dependent tests fast and predictable. Large teams: LocalStack Pro is still ahead on coverage breadth and edge case fidelity, so this is more of a niche pick than a wholesale replacement.
The catch: 76 services sounds like a lot, but coverage depth varies. Kumo is solid for the common cases (S3, DynamoDB, SQS) and weaker on niche APIs. Verify your specific use case before committing.
Free vs Self-Hosted vs Paid
fully freeFree Tier
The full tool. MIT license, no feature gates, no paid tier.
Self-Hosted
Single Go binary or Docker image. Starts in milliseconds. Optional persistence directory for state across restarts. CI usage is straightforward because there's no auth and no setup beyond pointing your SDK endpoint.
Comparison: LocalStack
LocalStack Free covers fewer services than LocalStack Pro ($35/user/mo or higher for teams). Kumo's coverage falls between the two on services but does not match LocalStack Pro's depth on niche AWS APIs. For 80% of CI cases, kumo is enough; for the last 20%, LocalStack Pro is the safer call.
When the math works
Teams paying for LocalStack Pro that only use mainstream services (S3, DynamoDB, SQS, Lambda, IAM) can save the seat costs by switching.
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License: MIT License
Use freely, including commercial. Just keep the license.
Commercial use: ✓ Yes
About
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- Takuma Shibuya (User)
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