http4k is a lightweight but fully-featured HTTP toolkit written in pure Kotlin that enables the serving and consuming of HTTP services in a functional and consistent way. http4k applications are just Kotlin functions. For example, here's a simple echo server:

val app: HttpHandler = { request: Request -> Response(OK).body(request.body) }
val server = app.asServer(SunHttp(8000)).start()

You can read about the rationale and ethos of http4k here

The http4k platform consists of the following main ecosystems, all released under a single version:

• http4k-core consists of a lightweight core library providing a base HTTP implementation and Server/Client implementations based on the JDK classes. Further servers, clients, serverless, templating, websockets capabilities are then implemented in add-on modules. http4k apps can be simply mounted into a running Server, Serverless platform, or compiled to GraalVM and run as a super-lightweight binary.
• http4k-connect is a lightweight API Client toolkit which includes libraries for connecting to popular third-party cloud services and AI backends using http4k compatible APIs, along with Fake implementations for usage during local testing.
• http4k-ai provides universal LLM adapters for OpenAI, Anthropic, Gemini, Azure, and GitHub Models, along with LangChain4J integration and an agent framework - all testable in-memory with Fake implementations.
• http4k-pro is a set of premium, commercially licensed modules that solve recurring enterprise problems - including Wiretap (OpenTelemetry capture and developer console), X402/MPP (machine-to-machine payment protocols), Verify (build-time supply-chain verification), Hot Reload, and pro-tier MCP and A2A integrations. Distributed under the org.http4k.pro Maven group.
• http4k Enterprise delivers long-term stability (up to 24-month LTS), supply-chain security (SLSA provenance, signed SBOMs, cosign signatures), priority support, license reporting, and source access - everything organisations need to run http4k in regulated, mission-critical environments.

Sounds cool! Where can I find out more?

You can find out all about the project on the http4k site.

Installation

dependencies {
    // install the platform...
    implementation(platform("org.http4k:http4k-bom:<LATEST_VERSION>"))

    // ...then choose any moduless but at least the core
    implementation("org.http4k:http4k-core")
}

Release channels

http4k is published to Maven Central and to maven.http4k.org. From 1 October 2026, Sonatype publishing limits will mean releases reach Maven Central approximately quarterly, while maven.http4k.org continues on the normal 1-2 week cadence. Both channels are supported, and the Community Edition remains free, Apache-2.0 and on Maven Central.

For details, see http4k.org/distribution.

Acknowledgments

• Dan Bodart's utterlyidle
• Ivan Moore for pairing on the original hackday project - Barely Magical.
• You can see the amazing people and companies who have helped us to make http4k here.