lspeasy-core

description	Documentation site for lspeasy Use when: You are building a browser-based LSP client, a WebSocket-backed language....
name	lspeasy-core

@lspeasy/core

Documentation site for lspeasy

@lspeasy/core is the shared foundation for the lspeasy SDK. It contains everything needed to build custom LSP integrations, and re-exports the most-used pieces from @lspeasy/client and @lspeasy/server.

Key areas

JSON-RPC 2.0 — Message types (RequestMessage, NotificationMessage, ResponseMessage), framing (parseMessage, serializeMessage), and Zod schemas for validation.

Transports — The Transport interface plus browser-compatible implementations: WebSocketTransport, DedicatedWorkerTransport, SharedWorkerTransport. Node.js transports (StdioTransport, TcpTransport, IpcTransport) are in @lspeasy/core/node to avoid importing Node.js builtins in browsers.

Transport Selection Guide

Need	Transport	Critical Gotcha
Spawn server as child process	StdioTransport	ConsoleLogger corrupts stdout — use NullLogger
Browser or remote server	WebSocketTransport	Call send() only after isConnected() is true
Persistent local daemon	TcpTransport	Create a new server instance per client reconnect
In-process browser isolation	DedicatedWorkerTransport	Monitor worker.onerror; crashes are silent
Shared worker, multiple tabs	SharedWorkerTransport	One worker handles all port connections

Middleware — The Middleware pipeline runs on every client-to-server and server-to-client message. Use createScopedMiddleware to limit a middleware to specific methods, and createTypedMiddleware for full param/result type inference.

LSP protocol — LSPRequest and LSPNotification namespaces expose every standard LSP method with its params and result types. LSPRequestMethod / LSPNotificationMethod are the union types for string-literal method names.

Utilities — CancellationTokenSource for request cancellation, DisposableStore for lifecycle management, ResponseError for structured JSON-RPC errors, DocumentVersionTracker for document sync.

When to Use

Use this skill when:

• You are building a browser-based LSP client, a WebSocket-backed language server, or any LSP integration that must run over HTTP/HTTPS infrastructure. → use WebSocketTransport
• You register multiple handlers (hover, completion, definition) that share the same lifetime → use DisposableStore — collect them all into one store and dispose the store on shutdown or feature toggle.
• A request handler needs to reject with a machine-readable error code that the client can act on (e.g. respond with MethodNotFound when a capability was not declared, or InvalidParams when schema validation fails). → use ResponseError
• You are building an LSP client that sends textDocument/didChange notifications and need to track per-document version counters. → use DocumentVersionTracker

Do NOT use when:

• You are building a CLI language server — StdioTransport (from @lspeasy/core/node) is the conventional choice and avoids the overhead of a network stack. For same-process workers prefer DedicatedWorkerTransport or SharedWorkerTransport. (WebSocketTransport)
• You want to log a server-side error without sending an error to the client — throw a plain Error and handle it via server.onError() instead. (ResponseError)

API surface: 62 functions, 11 classes, 122 types, 1 enums, 106 constants

NEVER

• Returns null silently when no lsp.json is found anywhere in the search path (including the global ~/.claude/lsp.json fallback). Callers that skip the null check will silently fail to resolve a server command — for the CLI this means lsproxy exits before the proxy daemon is ever spawned. Create an lsp.json at the workspace root or at ~/.claude/lsp.json for a per-user fallback.
• Returns null silently when no matching entry is found. The proxy daemon's BackendPool calls this function on every new language connection — if lsp.json is absent or omits the requested language, the daemon starts successfully but throws "No LSP server configured for languageId" the moment a client request arrives. An lsp.json must be present in the workspace (or at ~/.claude/lsp.json) before the proxy server can serve any language.
• NEVER set enableReconnect: true in server mode (socket provided) — the option is silently ignored (reconnect has no URL to reconnect to), but the intent is misleading and suggests lifecycle management will be handled when it is not.
• NEVER send messages before isConnected() returns true. In client mode the socket is in CONNECTING state immediately after construction; send() will throw until the open event fires.
• NEVER use ConsoleLogger in a stdio LSP server (StdioTransport) — the LSP base protocol uses stdout as the message channel. Any console.log / console.info / console.debug output will corrupt the stdio stream. Use NullLogger or a file-based logger instead, and send diagnostic messages via window/logMessage notifications.
• NEVER throw ResponseError with a code outside the defined ranges without documenting it. Undocumented codes are opaque to clients and tools.
• NEVER send a textDocument/didChange with the same version number as a previous change for the same document. The server may reject the change as a no-op or apply it out of order, causing text state desync.

Configuration

4 configuration interfaces — see references/config.md for details.

Quick Reference

Key functions: isRequestMessage (Returns true when message is a JSON-RPC request (has id + method)), isNotificationMessage (Returns true when message is a JSON-RPC notification (has method, no id)), isResponseMessage (Returns true when message is a JSON-RPC response (has id, no method)), isSuccessResponse (Returns true when response carries a result (success case)), isErrorResponse (Returns true when response carries an error (error case)), parseMessage (Parses a single framed JSON-RPC 2), serializeMessage (Serializes a JSON-RPC 2), createWebSocketClient (Creates a WebSocket client instance, preferring the native globalThis), composeMiddleware(Combines multiple middleware functions into a single middleware that runs them left-to-right, each delegating to the next vianext()), executeMiddlewarePipeline(Runs the registered middleware chain for a single JSON-RPC message, then callsfinalHandlerif no middleware short-circuits),createScopedMiddleware(Wraps a middleware with a filter so it only runs for matching LSP messages),createTypedMiddleware(Creates a typed, method-scoped middleware with full TypeScript inference for the message params and result),createFullDidChangeParams(BuildsDidChangeTextDocumentParamsfor a full-document text replacement),createIncrementalDidChangeParams(BuildsDidChangeTextDocumentParamsfor an incremental (range-based) document change notification),createProgressBegin(Creates aWorkDoneProgressBeginpayload to start a work-done progress notification),createProgressReport(Creates aWorkDoneProgressReportpayload to update an in-progress work-done notification),buildMethodSets(Builds the full set of LSP methods and the subset that are always allowed (not gated by a capability) for a given capability key),discoverServer(Walk the directory tree fromrootto the filesystem root, checkinglsp), discoverServerByLanguageId (Walk the directory tree from root looking for a lsp) **Key classes:** WebSocketTransport(WebSocket-based transport for LSP communication),DisposableStore(Collects multipleDisposableinstances and releases them together),CancellationTokenSource(Controller that creates and manages aCancellationToken), ConsoleLogger(Logger implementation that writes to the process console with level filtering),NullLogger(No-op logger that silently discards all messages),ResponseError(AnErrorsubclass that maps to a JSON-RPC 2),DocumentVersionTracker` (Tracks monotonically increasing version numbers for open text documents)

302 exports total — see references/ for full API.

References

Load these on demand — do NOT read all at once:

• When calling any function → browse references/functions/ for grouped indexes, full signatures, parameters, and return types
• When using a class → read references/classes.md for properties, methods, and inheritance
• When defining typed variables or function parameters → read references/types.md
• When using exported constants → read references/variables.md
• When configuring options → read references/config.md for all settings and defaults

Links

• Author: Pradeep Mouli [e-mail] (https://github.com/pradeepmouli)