Viljami.io / Open source
Viljami Kuosmanen
Open-source developer & maintainer · Tampere, Finland
I'm Viljami Kuosmanen (@anttiviljami), a Finnish open-source developer based in the Tampere region. I'm the creator and maintainer of OpenAPI Stack, a family of MIT-licensed TypeScript libraries for API-first development with OpenAPI: openapi-backend, openapi-client-axios and openapicmd.
I started the project in 2018 and have maintained it since. Today the stack gets over 1.8 million npm downloads a month and is used by teams at AWS, Microsoft, GitHub, IBM, Intel, SAP, Red Hat and Notion, as well as Finland's own Opetushallitus. 67+ people have contributed to it.
By day I'm a Distinguished Engineer at epilot, one of the companies running OpenAPI Stack in production. There I established and run the epilot open source program: publishing our SDKs and tools, sending fixes upstream on work time, and sponsoring the maintainers we depend on.
Projects
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openapi-client-axios
Typesafe JavaScript client for any OpenAPI-enabled API. No codegen required.
Creator & maintainer · since 2018 · 1.05M downloads/month · 617 ★
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openapi-backend
Build, validate, route, authenticate and mock a backend from an OpenAPI spec. Runs on Express, Hapi, Koa, AWS Lambda and more.
Creator & maintainer · since 2018 · 555K downloads/month · 687 ★
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openapicmd
CLI to bundle, lint, mock, test and generate types for OpenAPI specs.
Creator & maintainer · since 2019 · 111K downloads/month · 73 ★
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dereference-json-schema
Dereference
$refpointers in JSON Schema and OpenAPI documents. Used by openapi-backend and openapi-client-axios.Creator & maintainer · since 2023 · 1.41M downloads/month
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mock-json-schema
Mock example objects from JSON Schema definitions. Powers the mock responses in openapi-backend.
Creator & maintainer · since 2018 · 546K downloads/month
npm downloads for 19 Sep – 18 Oct 2026 from npmjs.com; GitHub stars as of 21 Oct 2026. The 1.8M figure counts openapi-backend, openapi-client-axios, openapicmd and openapi-client-axios-typegen. Public adopters are listed at openapistack.co/users.
epilot open source program
I established and run opensource.epilot.cloud, epilot's open source program. It publishes 88 npm packages under @epilot, including the epilot SDK, an agent toolkit and npm supply chain tooling, and has pledged 24 000 € in 2027 to sponsor the maintainers of projects epilot depends on.
Security research
In 2017 I published browser-autofill-phishing (1.5k ★), a proof of concept showing how browser autofill could leak personal data through hidden form fields. The Guardian and Forbes covered it.
Writing & talks
- Why we design APIs first · 2020
- We made coding agents actually reliable by fixing one thing · 2026
- SDLCAI 2026, Espoo: “I Let Non-Engineers Vibe-Code Our Production Codebase”
- Open Source – The Good Parts of WordPress · 2016 talk on the GPL and WordPress
Organising a meetup in Tampere or elsewhere in Finland? I'm happy to talk about OpenAPI, API-first development or maintaining open source. Get in touch.