Claude Skills Guide

Claude Code for LSP Server Implementation Workflow

The Language Server Protocol (LSP) has become the standard for enabling language features like autocomplete, goto definition, and refactoring across code editors. Implementing an LSP server from scratch, however, can be a daunting task involving complex JSON-RPC messaging, abstract syntax tree (AST) traversal, and editor integration. This is where Claude Code transforms the development experience—turning what could be weeks of work into a streamlined, iterative process.

This guide walks you through a practical workflow for implementing LSP servers using Claude Code, complete with code examples and actionable advice to help you build robust language servers efficiently.

Understanding the LSP Implementation Challenge

An LSP server communicates with editors via JSON-RPC messages, handling requests like textDocument/completion, textDocument/definition, and textDocument/didChange. The challenge isn’t just the protocol—it’s also parsing source code, building semantic understanding, and responding within the performance constraints that editors expect.

Traditionally, implementing an LSP server requires:

Claude Code accelerates this by generating boilerplate, explaining complex concepts, and helping you implement features incrementally. Let’s explore the workflow.

Setting Up Your LSP Project

Start by creating a new project for your LSP server. Claude Code can help scaffold the entire structure:

mkdir my-language-lsp && cd my-language-lsp
npm init -y
npm install vscode-json-rpc language-server-types

Create a skill that encapsulates LSP implementation knowledge:

---
name: lsp-implementer
description: Assists with LSP server implementation
tools: [Read, Write, Bash, Glob]
---

You are an expert in Language Server Protocol implementation.
Help me build a complete LSP server with these requirements:
1. JSON-RPC message handling
2. Document synchronization
3. Code completion
4. Definition and reference providers

Use best practices and explain each component.

Invoke this skill when working on your LSP project to get contextual guidance throughout development.

Implementing Core LSP Handlers

The heart of any LSP server is its request handler. Here’s a practical implementation pattern that Claude Code can help you build:

import { LanguageServer } from './language-server';
import { TextDocument } from './text-document';
import {
  InitializeRequest,
  InitializeResult,
  ServerCapabilities
} from 'vscode-languageserver-types';

export class LSPHandler {
  private server: LanguageServer;
  private documents: Map<string, TextDocument> = new Map();

  constructor() {
    this.server = new LanguageServer();
    this.setupHandlers();
  }

  private setupHandlers(): void {
    // Initialize handler - the entry point for LSP clients
    this.server.onRequest(
      'initialize',
      (params: InitializeRequest): InitializeResult => {
        return {
          capabilities: {
            textDocumentSync: 1, // Full document sync
            completionProvider: { triggerCharacters: ['.'] },
            definitionProvider: true,
            referencesProvider: true,
          } as ServerCapabilities
        };
      }
    );

    // Document change notification
    this.server.onNotification(
      'textDocument/didChange',
      (params: { textDocument: { uri: string; version: number }; contentChanges: Array<{ text: string }> }) => {
        const doc = this.documents.get(params.textDocument.uri);
        if (doc) {
          doc.update(params.contentChanges[0].text);
        }
      }
    );
  }

  start(port: number): void {
    this.server.listen(port);
  }
}

Claude Code can explain each component and help you extend this skeleton with features specific to your language.

Handling Document Synchronization

Document synchronization is critical—editors send content changes, and your server must maintain an accurate in-memory representation. Here’s how to implement it properly:

export class TextDocumentManager {
  private documents = new Map<string, TextDocument>();

  openDocument(uri: string, content: string, version: number): void {
    const doc = new TextDocument(uri, content, version);
    this.documents.set(uri, doc);
    this.parseDocument(doc);
  }

  updateDocument(uri: string, changes: ContentChange[], version: number): void {
    const doc = this.documents.get(uri);
    if (!doc) return;

    for (const change of changes) {
      doc.applyChange(change);
    }
    doc.setVersion(version);
    this.parseDocument(doc);
  }

  private parseDocument(doc: TextDocument): void {
    // Parse and build AST, then update indices
    // This is where you'd integrate your parser
    const ast = this.languageParser.parse(doc.getText());
    this.symbolIndex.update(doc.getUri(), ast);
  }

  getDocument(uri: string): TextDocument | undefined {
    return this.documents.get(uri);
  }
}

Ask Claude Code to explain how to integrate your specific parser (tree-sitter, ANTLR, etc.) into this pattern.

Implementing Code Completion

Code completion is often the most visible LSP feature. Here’s a practical approach:

export class CompletionProvider {
  provideCompletions(
    uri: string,
    position: Position
  ): CompletionItem[] {
    const doc = this.docManager.getDocument(uri);
    if (!doc) return [];

    const line = doc.getLine(position.line);
    const prefix = this.getWordPrefix(line, position.character);
    
    // Get completions from your language's analysis
    const symbols = this.symbolIndex.getSymbols(prefix);
    
    return symbols.map(symbol => ({
      label: symbol.name,
      kind: this.mapSymbolKind(symbol.kind),
      detail: symbol.detail,
      documentation: symbol.documentation,
      insertText: this.getInsertText(symbol),
    }));
  }

  private getWordPrefix(line: string, char: number): string {
    let start = char;
    while (start > 0 && /[a-zA-Z0-9_]/.test(line[start - 1])) {
      start--;
    }
    return line.substring(start, char);
  }
}

Claude Code can help you integrate this with your language’s type system and generate context-aware completions.

Testing Your LSP Server

Testing is crucial for LSP servers since they bridge multiple systems. Claude Code can help you write integration tests:

import { describe, it } from 'mocha';
import { LSPClient } from './test-utils/lsp-client';

describe('LSP Server', () => {
  let client: LSPClient;

  beforeEach(async () => {
    client = await LSPClient.start();
  });

  afterEach(async () => {
    await client.stop();
  });

  it('should respond to initialize request', async () => {
    const result = await client.sendRequest('initialize', {
      processId: process.pid,
      rootUri: '/test-project',
      capabilities: {}
    });
    
    expect(result.capabilities.completionProvider).to.not.be.undefined;
    expect(result.capabilities.definitionProvider).to.not.be.undefined;
  });

  it('should provide completions', async () => {
    await client.openDocument('test.lang', 'func myFunc', 1);
    const completions = await client.sendRequest('textDocument/completion', {
      textDocument: { uri: 'test.lang' },
      position: { line: 0, character: 5 }
    });
    
    expect(completions).to.not.be.empty;
  });
});

Run tests with npm test to verify your implementation before editor integration.

Best Practices for LSP Development

Based on implementing multiple language servers, here are key recommendations:

  1. Start with document sync and diagnostics - Get the basic notification flow working first before tackling complex features.

  2. Separate parsing from serving - Keep your language parser independent from the LSP layer for easier testing.

  3. Cache aggressively - Re-parsing on every request kills performance. Update caches incrementally on document changes.

  4. Handle cancellation - Long-running operations should check for cancellation tokens from the client.

  5. Log comprehensively - LSP errors are hard to debug without detailed logs of requests and responses.

Claude Code excels at helping you implement each of these practices iteratively.

Conclusion

Implementing an LSP server becomes significantly more manageable with Claude Code guiding the process. By following this workflow—scaffolding the project, implementing core handlers incrementally, and testing continuously—you can build production-ready language servers in a fraction of the traditional time.

The key is treating Claude Code as a pair programmer who understands the LSP specification deeply and can explain complex patterns while generating working code. Invoke your implementation skill early and often, and you’ll have a robust language server that provides excellent editor integration for your target language.

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