How to build a Language Server Protocol (LSP) plugin for Claude Code

CircleCI demonstrates how to wrap its open-source YAML language server as a Claude Code plugin, giving Claude real-time diagnostics, best-practice hints, and schema hover documentation while editing `.circleci/config.yml` files. The plugin uses a verified launcher that downloads and checksum-validates a pinned server binary, plus a JSON-RPC proxy that limits the CircleCI-specific server to appropriate configuration paths so unrelated YAML files are not incorrectly analyzed. A separate hover proxy supplies documentation generated from CircleCI’s configuration schema, while a maintenance pipeline monitors upstream releases, refreshes binary hashes and schema data, runs tests, and opens reviewable update pull requests. In an 84-run benchmark across Claude Opus 4.8 and Sonnet 5, all generated configurations compiled successfully with or without the plugin, but models instructed to act on LSP guidance more often adopted nonrequired best practices, including storing test results and using current orb versions. The example is open source and is presented as a reusable pattern for connecting other domain-specific language servers to Claude Code, though it is not an officially supported CircleCI product.

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