Language Server

Language server (croma-lsp)

croma-lsp is a stdio Language Server Protocol server for ABC 2.1. It is a thin adapter over croma's already-proven core and formatter — it never reparses, reformats, or re-derives diagnostics itself — so its output is byte-identical to the CLI's, by construction. It is a proven, un-gated capability and ships in the default build.

cargo install croma-lsp     # or build from source: cargo build --release

The server is launched by an editor / LSP client, not run by hand: point your client at the croma-lsp binary with stdio transport and associate it with the .abc language. For a ready-made setup, see Editors-and-Zed.

Capabilities

LSP feature	Backed by
publishDiagnostics (push)	croma_core::export_musicxml diagnostics
textDocument/formatting	croma_fmt::format (one full-document edit)
textDocument/semanticTokens/full	the music-token stream (11-type legend)
textDocument/documentSymbol	one symbol per tune, header fields as children
textDocument/foldingRange	one fold per tune
textDocument/hover	static ABC 2.1 field / decoration tables
textDocument/completion	header field keys; decoration names
textDocument/codeAction	croma_fmt::auto_fix as source.fixAll

• Sync: advertises incremental textDocumentSync; the parser is panic-free on malformed / mid-edit input, so every keystroke state is safe to analyze.
• Position encoding: negotiates UTF-8 when offered (then a character offset is a byte offset, matching croma's spans), else falls back to UTF-16.
• Strict-spec: inherits the parser's strict ABC 2.1 default — e.g. the +...+ decoration delimiter is not enabled, so +decoration+ is a malformed token.

Proven over the corpus

The promotion legs run over the full 10k corpus (see How-its-Proven):

Leg	Result
Diagnostics fidelity (LSP path == core)	10,000 / 0 mismatches
Formatting identity (== croma_fmt::format)	10,000 / 0 mismatches
Totality (no panics / no hangs)	10,000 files, 0 panics, 0 hangs
Semantic-token correctness	10,000 / 0 violations
Latency (diagnostics + tokens, real-size)	~1 ms

The croma-lsp deps (lsp-server, lsp-types) live on the server binary only; croma-core stays zero-dependency.

Full reference

Architecture (transport-free analysis layer + thin stdio loop), the full semantic-token legend, the promotion harness, and the layout are documented in docs/lsp.md.

Latency percentiles (p50/p95/p99 per request type): Benchmarks.