[gonzaloriestra]

WHY are these changes introduced?

Fixes https://github.com/shop/issues-develop/issues/24319.

The theme language server already outputs JSON through LSP, but does not accept --json or declare its output schema. CLI errors can also corrupt its stdout when JSON output is enabled globally.

WHAT is this pull request doing?

Support --json (-j) while preserving native LSP messages and Content-Length framing. Keep CLI diagnostics and argument/startup errors on stderr, and make the shared fatal-error handler respect the command's explicit output context.

Register a schema for JSON-RPC requests, notifications, successful responses, and protocol errors. --json-schema prints the contract and exits without starting the server. Method-specific payloads follow LSP. Refresh generated help, remove the lint exception, and add schema and protocol regression coverage.

How to manually test your changes?

1. Run pnpm shopify theme language-server --json-schema. Confirm that it prints a JSON Schema and exits without starting the server. Repeat with --json --json-schema, then with SHOPIFY_FLAG_JSON_SCHEMA=1 pnpm shopify theme language-server.
2. Configure an LSP client/editor to launch pnpm shopify theme language-server. Initialize the session, make an editor request, then send shutdown followed by exit.
3. Repeat with pnpm shopify theme language-server --json, then with SHOPIFY_FLAG_JSON=1. Confirm that the client receives the same framed JSON-RPC messages on stdout.
4. Run pnpm shopify theme language-server --json --invalid > lsp.stdout 2> lsp.stderr. Confirm that the argument error appears on stderr and stdout stays empty.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump types (minor for CLI flag support and patch for CLI Kit error routing) and added a changeset.

[gonzaloriestra]

/snapit

[github-actions]

🫰✨ Thanks @gonzaloriestra! Your snapshot has been published to npm.

Built from 3963afa8c5da9f4104ece54404b7e8609ba6f320. Workflow run.

Test the snapshot by installing your package globally:

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Caution

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Use which shopify to find out which one you are running and uninstall it.