The Language Server Protocol in Swift

What's New

Support Other Platforms


Fixes compile issues on iOS, tvOS and watchOS


👩🏻‍🚀 This project is still a tad experimental. Contributors and pioneers welcome!


SwiftLSP offers a quite dynamic Swift representation of the LSP (Language Server Protocol) and helps with many related use cases. It is foundational for LSPService and LSPServiceKit.

Since the LSP standard defines a complex amorphous multitude of valid JSON objects, it doesn't exactly lend itself to being represented as a strict type system that would mirror the standard down to every permutation and property. So SwiftLSP is strictly typed at the higher level of LSP messages but falls back onto a more dynamic and flexible JSON representation for the details. The strict typing can easily be expanded on client demand.


Some of these examples build upon preceding ones, so it's best to read them from the beginning.

Create Messages

let myRequest = LSP.Request(method: "myMethod", params: nil)
let myRequestMessage = LSP.Message.request(myRequest)

let myNotification = LSP.Notification(method: "myMethod", params: nil)
let myNotificationMessage = LSP.Message.notification(myNotification)

Encode and Decode Messages

SwiftLSP encodes LSP messages with the LSP-conform JSON-RPC encoding.

let myRequestMessageEncoded = try myRequestMessage.encode()  // Data
let myRequestMessageDecoded = try LSP.Message(myRequestMessageEncoded)

Wrap Messages in Packets

To send LSP messages via data channels, the standard defines how to wrap each message in what we call an LSP.Packet, which holds the Data of its header- and content part.

let myRequestMessagePacket = try LSP.Packet(myRequestMessage)
let packetHeader = myRequestMessagePacket.header     // Data
let packetContent = myRequestMessagePacket.content   // Data
let packetTotalData = myRequestMessagePacket.data    // Data

Extract Messages From Packets

let myRequestMessageUnpacked = try myRequestMessagePacket.message()  // LSP.Message

Extract Packets From Data

A client talking to an LSP server might need to extract LSP.Packets from the server's output Data stream.

SwiftLSP can parse an LSP.Packet from the beginning of a Data instance:

let dataStartingWithPacket = packetTotalData + "Some other data".data(using: .utf8)!
let detectedPacket = try LSP.Packet(parsingPrefixOf: dataStartingWithPacket)

// now detectedPacket == myRequestMessagePacket

SwiftLSP also offers the LSP.PacketDetector for parsing a stream of Data incrementally:

var streamedPacket: LSP.Packet? = nil
let detector = LSP.PacketDetector { packet in
    streamedPacket = packet

for byte in dataStartingWithPacket {

// now streamedPacket == myRequestMessagePacket

More Use Cases

Beyond what the examples above have touched, SwiftLSP also helps with:

  • Creating messages for specific use cases (initialize server, request symbols, request references ...)
  • Launching an LSP server executable
  • Matching response messages to request messages
  • Making requests to an LSP Server through async functions
  • Using an LSP Server via WebSocket


Some context and essential types:

architecture architecture

Internal architecture (composition and essential dependencies) of the top-level source folder:

The above image was generated with Codeface.

Development Status

From version/tag 0.1.0 on, SwiftLSP adheres to semantic versioning. So until it has reached 1.0.0, its API may still break frequently, but this will be expressed in version bumps.

SwiftLSP is already being used in production, but Codeface is still its primary client. SwiftLSP will move to version 1.0.0 as soon as its basic practicality and conceptual soundness have been validated by serving multiple real-world clients.


  • Swift Tools 5.6.0
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Last updated: Fri Nov 25 2022 00:26:08 GMT-0500 (GMT-05:00)