scalameta / lsp4s

Scala implementation for JSON-RPC and Language Server Protocol


Build Status Maven Central


This is a Scala implementation for JSON-RPC and the Language Server Protocol with a emphasis on compile-time safety and extensibility. This project contains two modules:

  • jsonrpc: core data structures and client/server implementations for JSON-RPC with additional support for cancellation via method $/cancelRequest.
  • lsp4s: data structures for the Language Server Protocol.

The jsonrpc module has the following dependencies:

  • Monix: for asynchronous programming primitives Task and Observable. Monix Task was chosen over the standard library Future in order to support cancellation, which is an important part of the Language Server Protocol. Monix Observable is also a good fit the stream-based nature of the JSON-RPC protocol.
  • Circe: for JSON serialization and de-serialization. Our requirements for JSON include a parser, printer, custom decoding/encoding APIs as well as automatic (or semi-automatic) derivation for case classes. Circe was chosen over alternative JSON libraries for its high quality error handling and type-safe APIs with strong compile-time guarantees.
  • Scribe: for logging. Scribe was chosen over alternative logging frameworks (such as slf4j) for its usage of compile-time reflection over runtime reflection in addition to a fully programmable configuration API avoiding the need for XML.

The lsp4s module has the following dependencies:

  • jsonrpc: the module in this project
  • Enumeratum: for reflection-free and boilerplate-free enumerations.

Getting Started

Maven Central

To implement a language server with lsp4s, add the following dependency:

libraryDependencies += "org.scalameta" %% "lsp4s" % "VERSION"

In the following example, we are going to implement a handler for the LSP textDocument/hover request. In addition to the request handler, we need to handle the lifecycle requests and notifications initialize, initialized, shutdown and exit.

import io.circe.Json        // to construct Json values
import scala.meta.jsonrpc._ // for JSON-RPC APIs
import scala.meta.lsp._     // for LSP endpoints
import scribe._             // for logging

def myServices(logger: LoggerSupport, client: LanguageClient): Services = {
    .request(Lifecycle.initialize) { params =>
      val capabilities = ServerCapabilities(
        hoverProvider = true
    .notification(Lifecycle.initialized) { _ =>

      // NOTE: It's possible to send a notification to the client at any point"Hello world!")(client)

      // notification handlers can't respond so return Unit"Client is initialized")
    .request(Lifecycle.shutdown) { _ =>"Client is about to call exit soon")
    .notification(Lifecycle.exit) { _ =>"Goodbye!")
    .request(TextDocument.hover) { params =>

      // Publish a dummy error message, normally diagnostics are published  after textDocument/didChange
      // or textDocument/didSave with error messages from the compiler or build tool.
          uri = params.textDocument.uri,
          diagnostics = List(
              range = Range(start = params.position, end = params.position),
              severity = Some(DiagnosticSeverity.Error),
              code = Some("hover-error"),
              source = Some("my-project"),
              message = "This is an example red error message!"

      // Return a dummy response for this example, normally the implementation will call the compiler
      // for type information here and display a type signature and docstrings.
      Hover(List(MarkdownString("**Hello** from server!")), range = None)

Now that we have implemented our services, let's wire everything together

val scheduler =

// Construct an InputStream and OutputStream pair. For LSP, this is normally and
// System.out but to connect with for example sbt server you use sockets:
val io = new InputOutput(, System.out)

// Construct logger that appends to file, don't log to stdout since that is reserved for
// JSON-RPC communication.
val logPath = java.nio.file.Paths.get("my-logger.log").toAbsolutePath
val fileWriter = scribe.writer.FileWriter().path(timestamp => logPath).autoFlush
val logger = Logger("my-logger").orphan().withHandler(writer = fileWriter)

// Establish a connection with the client, this fires up the server and starts listening for
// requests and notifications until the input stream closes.
val connection =
  Connection(io, serverLogger = logger, clientLogger = logger) { client =>
    myServices(logger, client)

// Wait forever until the client closes the connection.

For a full executable example, see PingPongSuite.

It's possible to use only the JSON-RPC parts of this library without the LSP endpoints. Instead of depending on the lsp4s module, use the following module instead:

libraryDependencies += "org.scalameta" %% "jsonrpc" % "VERSION"

Used by

The modules lsp4s and jsonrpc are used in the following projects:

  • Metals: a language server for Scala that uses lsp4s to communicate with editors and jsonrpc to communicate with sbt server as a client.
  • IntelliJ Scala: the most widely used IDE for Scala uses jsonrpc to communicate with build tools through the Build Server Protocol.
  • Bloop: Scala build server and command-line tool for fast compile and test workflows that uses jsonrpc to communicate with IDEs through the Build Server Protocol.
  • Build Server Protocol: a complementary protocol to the Language Server Protocol targeted for build tools that uses jsonrpc to provide a bsp Scala library similar to the lsp4s module in this project.

Are you using lsp4s or jsonrpc? Submit a pull request adding your project to the list :)


The current maintainers (people who can merge pull requests) are:

An up-to-date list of contributors is available here:


We recommend looking into lsp4j if you want to implement a language server or do JSON-RPC on the JVM. It's a pure Java implementation with a smaller dependency footprint than lsp4s. It's used by the official Java Language Server and is actively maintained. Before starting lsp4s, we made several attempts to use lsp4j but encountered cryptic runtime-reflection errors that we struggled to resolve. The emphasis in lsp4s on compile-time safety is partly motivated by this experience.

For a Scala alternative, there is dragos-vscode-scala that builds on top of scala-json-rpc and uses the standard library Future. We used dragos-vscode-scala for several months and it was helpful in getting us off-the-ground early on. In fact, the lsp4s JSON-RPC header parser is a fork of the header parser in dragos-scode-scala (see NOTICE). As our usage of dragos-vscode-scala grew we found it difficult to add new LSP endpoints such as workspace/applyEdit or build new protocols such as BSP. We encountered several times cryptic runtime errors during manual integration testing, which hurt our productivity. The lsp4s Endpoint abstraction builds on top of this experience by reducing ceremony for adding new endpoints while lowering the risk of serialization errors at runtime.