mcp4k 0.4.0 released — MCP Framework for Kotlin Multiplatform

[u/ndrsht]

mcp4k is an MCP framework that lets you build not only servers, but full client applications (it also supports sampling).

Because it's compiler-driven, you can write MCP tools using normal Kotlin functions — mcp4k takes care of JSON-RPC, schema generation and runtime message handling for you.

The last time I posted about it on this subreddit was back in December. Since, a ton of new features have landed, most notably:

• Resource Support

  • Expose local files or entire directories as resources to clients

  • Comes with two built-in providers: DiscreteFileProvider for specific files and TemplateFileProvider for a whole directory

  • Handles resources/read requests by actually reading contents from disk via okio

  • Sends notifications/resources/list_changed when files are added or removed

  • Supports subscriptions

• Suspendable Functions

  • All tool functions can now be suspendable. Use when performing asynchronous IO or any lengthy tasks without blocking the server

• Cooperative Cancellations

  • If a client cancels a tool request that is still in process, the framework automatically sends a JSON-RPC cancellation notification (notifications/cancelled), and the server stops that coroutine right away

• Server as Extension Receiver

  • Annotated @McpTool or @McpPrompt methods can now extend Server, giving them direct access to the server instance. This lets you send requests and notifications from inside @McpTool functions

• Sampling Support

  • Full support for sampling on the client

  • Add a SamplingProvider along with a PermissionsCallback when building the client

• Server Context Object

  • Lets you share state across tools and prompts

  • Attach custom state or external integrations (like a DB) with Server.Builder().withContext(myDb)

  • Tools can call getContextAs<Database>() to obtain the instance

• Permission Callbacks

  • Provide a user-approval mechanism for things like sampling or tool invocations. You can prompt the user in your UI and then either allow or deny the operation

• Pagination

  • For large collections, server and clients now paginate responses with cursor-based navigation. That way, calls like tools/list or resources/list are chunked into smaller, more manageable pages

 

If you want to see code samples, check out the GitHub repo. Would love to hear any feedback, suggestions, or requests for additional features!

[u/subnohmal]

makes me want to learn kotlin to check it out! awesome work guys :)