feat(lsp): add Language Server Protocol server (glint lsp)

New internal/lsp package implements a minimal JSON-RPC 2.0 LSP server
over stdin/stdout with Content-Length framing. Supported lifecycle:
initialize → initialized → shutdown → exit. Document sync: Full (sends
complete text on every change). Handles textDocument/didOpen,
didChange, didSave, didClose; publishes textDocument/publishDiagnostics
after every change. Rule IDs surface as the diagnostic `code` field
with `"glint"` as source. Parse errors produce an Error diagnostic at
the top of the document. Include resolution is best-effort (GITLAB_TOKEN
env var; ~/.cache/glint default cache). CLI: glint lsp [--token]
[--gitlab-url] [--cache-dir] [--offline].

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 This project uses [Semantic Versioning](https://semver.org).
 
+## [0.2.29] - 2026-06-26
+
+### Added
+
+- **LSP server** (`glint lsp`) — new `internal/lsp` package and `glint lsp` subcommand that starts a Language Server Protocol server over stdin/stdout using Content-Length–framed JSON-RPC 2.0. Editors (VS Code, Neovim, Emacs, JetBrains, etc.) can connect with any generic LSP client configuration. Supported methods: `initialize`, `initialized`, `shutdown`, `exit`, `textDocument/didOpen`, `textDocument/didChange`, `textDocument/didSave`, `textDocument/didClose`. On every document open or change the server runs the full glint lint pipeline and publishes diagnostics via `textDocument/publishDiagnostics`; each diagnostic carries the rule ID as its `code` field and `"glint"` as `source`. Parse errors are surfaced as an Error diagnostic at the top of the document. Include resolution is best-effort (uses `GITLAB_TOKEN` / `GITLAB_URL` env vars; default cache dir `~/.cache/glint`). CLI flags: `--token`, `--gitlab-url`, `--cache-dir`, `--offline`.
+
 ## [0.2.28] - 2026-06-26
 
 ### Added

README.md

@@ -6,7 +6,7 @@
 
 <p align="center">
   <a href="LICENSE"><img src="https://img.shields.io/badge/license-Apache%202.0-blue.svg" alt="License"></a>
-  <a href="CHANGELOG.md"><img src="https://img.shields.io/badge/release-v0.2.28-blue.svg" alt="Release"></a>
+  <a href="CHANGELOG.md"><img src="https://img.shields.io/badge/release-v0.2.29-blue.svg" alt="Release"></a>
 </p>
 
 > **Disclaimer:** This tool was built through iterative AI-assisted development with [Claude](https://claude.ai). It is experimental, incomplete, and not intended for production use. Coverage of GitLab CI keywords is best-effort and may lag behind GitLab's evolving spec. Use it at your own discretion — no correctness guarantees are made. Contributions and bug reports are welcome.
@@ -21,6 +21,7 @@ A local tool to validate and lint `.gitlab-ci.yml` pipelines without needing a G
 - **Multiple output formats** — `--format text` (default, ruff-style), `json`, `sarif` (GitHub Code Scanning / GitLab SAST), `junit`, `github` (PR annotations)
 - **Project config** — `.glint.yml` for rule suppression, severity overrides, token/URL defaults; `# glint: ignore RULE` for per-job inline suppression
 - **Graph visualization** — `glint graph` prints a terminal job tree; `glint graph pipeline` renders a GitLab CI-style SVG/PNG; `--format mermaid` emits a Mermaid flowchart; `--format html` produces a self-contained HTML file with pan/zoom and a job-detail sidebar; context flags grey out skipped jobs
+- **LSP server** — `glint lsp` starts a Language Server Protocol server over stdin/stdout; connect with any LSP client to get inline diagnostics (rule ID as code, error/warning severity) in VS Code, Neovim, Emacs, JetBrains, etc.
 
 See [FEATURES.md](FEATURES.md) for the complete feature reference and lint rules table, and [ROADMAP.md](ROADMAP.md) for planned improvements.
 
@@ -52,6 +53,7 @@ Commands:
   check    Lint a pipeline file — exits 0 (clean) or 1 (errors found)
   graph    Visualise the pipeline as a job tree or Mermaid graph
   explain  Print description and fix for a lint rule
+  lsp      Start a Language Server Protocol server (stdin/stdout)
 ```
 
 Run `glint <command> --help` for all flags. See [USAGE.md](USAGE.md) for full

ROADMAP.md

@@ -97,7 +97,7 @@ The SVG renderer and terminal tree cover the basic layout. These would bring it
 - [x] **GitLab CI template** — shipped v0.2.28; `templates/check.yml` is a GitLab CI/CD Catalog component with `spec:` inputs for stage, file, version, allow_failure, and extra args; also usable as a plain local/remote include
 - [x] **GitHub Actions action** — shipped v0.2.28; `action.yml` composite action downloads the glint Linux binary and runs `glint check`; mirror to GitHub as `k3nny/glint` to reference as `uses: k3nny/glint@v0.2.28`
 - [x] **Pre-commit hook** — shipped v0.2.28; `.pre-commit-hooks.yaml` defines `language: golang` hook; pre-commit builds glint from source on first run and re-runs on staged `.gitlab-ci.yml` changes
-- [ ] **LSP server** — `glint lsp` mode exposing diagnostics over the Language Server Protocol; enables inline squiggles in VS Code, JetBrains, Neovim, etc. without a dedicated extension
+- [x] **LSP server** — shipped v0.2.29; `glint lsp` runs a JSON-RPC 2.0 LSP server over stdin/stdout; `textDocument/didOpen`, `didChange`, `didSave`, `didClose` all publish diagnostics; rule IDs appear as the diagnostic `code`; include resolution is best-effort using env-var token and default cache dir
 - [ ] **VS Code extension** — thin wrapper around the LSP server with syntax highlighting for `.gitlab-ci.yml`
 
 ---

cmd/glint/lsp.go

@@ -0,0 +1,71 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"git.k3nny.fr/glint/internal/fetcher"
+	"git.k3nny.fr/glint/internal/lsp"
+)
+
+func cmdLSP(args []string) {
+	fs := flag.NewFlagSet("glint lsp", flag.ExitOnError)
+	token := fs.String("token", "", "GitLab personal access token (overrides GITLAB_TOKEN)")
+	gitlabURL := fs.String("gitlab-url", "", "GitLab instance URL (overrides CI_SERVER_URL / GITLAB_URL)")
+	cacheDir := fs.String("cache-dir", "", "directory for caching fetched remote includes")
+	offline := fs.Bool("offline", false, "skip all network calls; serve only from --cache-dir")
+	fs.Usage = func() {
+		fmt.Fprintf(os.Stderr, "glint %s\n\n", version)
+		fmt.Fprint(os.Stderr, `Start a Language Server Protocol server for .gitlab-ci.yml files.
+
+Reads JSON-RPC 2.0 messages from stdin and writes responses to stdout using
+the standard Content-Length framing. Connect with any LSP client (VS Code,
+Neovim, Emacs, etc.).
+
+Usage: glint lsp [OPTIONS]
+
+Options:
+      --token <TOKEN>
+          GitLab personal access token used for resolving project: and
+          component: includes. Defaults to GITLAB_TOKEN env var.
+
+      --gitlab-url <URL>
+          GitLab instance URL for resolving remote includes.
+          [env: CI_SERVER_URL | GITLAB_URL] [default: https://gitlab.com]
+
+      --cache-dir <DIR>
+          Cache directory for fetched remote includes. Defaults to
+          ~/.cache/glint so subsequent opens are served from cache.
+
+      --offline
+          Do not make any network calls; resolve only local includes.
+          Implies --cache-dir default (~/.cache/glint) when not set.
+
+  -h, --help
+          Print help
+
+Examples:
+  glint lsp
+  glint lsp --token glpat-xxxx --cache-dir ~/.cache/glint
+  glint lsp --offline
+`)
+	}
+	_ = fs.Parse(args)
+
+	resolvedCacheDir := *cacheDir
+	if resolvedCacheDir == "" {
+		resolvedCacheDir = defaultCacheDir()
+	}
+	if *offline && resolvedCacheDir == "" {
+		resolvedCacheDir = defaultCacheDir()
+	}
+
+	cfg := fetcher.AutoConfig().WithOverrides(*gitlabURL, *token, resolvedCacheDir, *offline)
+
+	srv := lsp.New(os.Stdin, os.Stdout, cfg, version)
+	if err := srv.Run(); err != nil {
+		fmt.Fprintf(os.Stderr, "glint lsp: %v\n", err)
+		exit(2)
+	}
+}

cmd/glint/main.go

@@ -69,6 +69,7 @@ Commands:
   check    Lint a pipeline file — exits 0 (clean) or 1 (errors found)
   graph    Visualise the pipeline as a job tree or Mermaid graph
   explain  Show description and fix for a lint rule (e.g. glint explain GL007)
+  lsp      Start a Language Server Protocol server (stdin/stdout)
 
 Options:
   -h, --help     Print help
@@ -90,6 +91,8 @@ func main() {
 		cmdGraph(os.Args[2:])
 	case "explain":
 		cmdExplain(os.Args[2:])
+	case "lsp":
+		cmdLSP(os.Args[2:])
 	case "-h", "--help", "help":
 		fmt.Fprintf(os.Stderr, "glint %s\n\n", version)
 		fmt.Fprint(os.Stderr, globalUsage)

internal/lsp/server.go

@@ -0,0 +1,332 @@
+package lsp
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"git.k3nny.fr/glint/internal/fetcher"
+	"git.k3nny.fr/glint/internal/linter"
+	"git.k3nny.fr/glint/internal/model"
+	"git.k3nny.fr/glint/internal/resolver"
+)
+
+// Server is a minimal Language Server Protocol server that publishes glint
+// diagnostics for .gitlab-ci.yml files opened in an editor.
+//
+// Transport: JSON-RPC 2.0 over stdin/stdout with Content-Length framing.
+// Sync mode: Full — the client sends the complete document text on every change.
+type Server struct {
+	in      *bufio.Reader
+	out     io.Writer
+	cfg     fetcher.GitLabConfig
+	version string
+	docs    map[string]string // uri → current document text
+
+	// Exit is called with the process exit code when the LSP client sends the
+	// "exit" notification. Defaults to os.Exit; replace in tests.
+	Exit func(int)
+
+	shutdownReceived bool
+}
+
+// New creates a Server reading from r and writing to w.
+func New(r io.Reader, w io.Writer, cfg fetcher.GitLabConfig, version string) *Server {
+	return &Server{
+		in:      bufio.NewReader(r),
+		out:     w,
+		cfg:     cfg,
+		version: version,
+		docs:    make(map[string]string),
+		Exit:    os.Exit,
+	}
+}
+
+// Run processes LSP messages until the connection closes or a fatal error occurs.
+// It returns nil on a clean EOF (client disconnected) and a non-nil error for
+// unrecoverable protocol failures.
+func (s *Server) Run() error {
+	for {
+		msg, err := s.readMessage()
+		if err == io.EOF {
+			return nil
+		}
+		if err != nil {
+			return fmt.Errorf("reading LSP message: %w", err)
+		}
+		if err := s.dispatch(msg); err != nil {
+			return err
+		}
+	}
+}
+
+// readMessage reads one Content-Length–framed JSON-RPC message from the stream.
+func (s *Server) readMessage() (*Message, error) {
+	var contentLength int
+	for {
+		line, err := s.in.ReadString('\n')
+		if err != nil {
+			if err == io.EOF && line == "" {
+				return nil, io.EOF
+			}
+			return nil, err
+		}
+		line = strings.TrimRight(line, "\r\n")
+		if line == "" {
+			break // blank line separates headers from body
+		}
+		if strings.HasPrefix(line, "Content-Length: ") {
+			n, parseErr := strconv.Atoi(strings.TrimPrefix(line, "Content-Length: "))
+			if parseErr != nil {
+				return nil, fmt.Errorf("invalid Content-Length: %w", parseErr)
+			}
+			contentLength = n
+		}
+	}
+	if contentLength == 0 {
+		return nil, fmt.Errorf("missing or zero Content-Length header")
+	}
+
+	body := make([]byte, contentLength)
+	if _, err := io.ReadFull(s.in, body); err != nil {
+		return nil, fmt.Errorf("reading message body: %w", err)
+	}
+
+	var msg Message
+	if err := json.Unmarshal(body, &msg); err != nil {
+		return nil, fmt.Errorf("unmarshalling message: %w", err)
+	}
+	return &msg, nil
+}
+
+// writeMessage encodes v as JSON and sends it with a Content-Length header.
+func (s *Server) writeMessage(v any) error {
+	body, err := json.Marshal(v)
+	if err != nil {
+		return err
+	}
+	header := fmt.Sprintf("Content-Length: %d\r\n\r\n", len(body))
+	if _, err := io.WriteString(s.out, header); err != nil {
+		return err
+	}
+	_, err = s.out.Write(body)
+	return err
+}
+
+func (s *Server) respond(id json.RawMessage, result any) error {
+	raw, err := json.Marshal(result)
+	if err != nil {
+		return err
+	}
+	return s.writeMessage(struct {
+		JSONRPC string          `json:"jsonrpc"`
+		ID      json.RawMessage `json:"id"`
+		Result  json.RawMessage `json:"result"`
+	}{"2.0", id, raw})
+}
+
+func (s *Server) respondError(id json.RawMessage, code int, message string) error {
+	return s.writeMessage(struct {
+		JSONRPC string          `json:"jsonrpc"`
+		ID      json.RawMessage `json:"id"`
+		Error   RPCError        `json:"error"`
+	}{"2.0", id, RPCError{Code: code, Message: message}})
+}
+
+func (s *Server) notify(method string, params any) error {
+	raw, err := json.Marshal(params)
+	if err != nil {
+		return err
+	}
+	return s.writeMessage(struct {
+		JSONRPC string          `json:"jsonrpc"`
+		Method  string          `json:"method"`
+		Params  json.RawMessage `json:"params"`
+	}{"2.0", method, raw})
+}
+
+// isRequest reports whether msg is a JSON-RPC request (has a non-null id).
+func isRequest(msg *Message) bool {
+	return len(msg.ID) > 0 && string(msg.ID) != "null"
+}
+
+func (s *Server) dispatch(msg *Message) error {
+	switch msg.Method {
+	case "initialize":
+		return s.handleInitialize(msg)
+	case "initialized":
+		return nil // notification; no response required
+	case "shutdown":
+		s.shutdownReceived = true
+		if isRequest(msg) {
+			return s.respond(msg.ID, nil)
+		}
+		return nil
+	case "exit":
+		code := 1
+		if s.shutdownReceived {
+			code = 0
+		}
+		s.Exit(code)
+		return nil
+	case "textDocument/didOpen":
+		return s.handleDidOpen(msg)
+	case "textDocument/didChange":
+		return s.handleDidChange(msg)
+	case "textDocument/didSave":
+		return s.handleDidSave(msg)
+	case "textDocument/didClose":
+		return s.handleDidClose(msg)
+	default:
+		if isRequest(msg) {
+			return s.respondError(msg.ID, -32601, "method not found: "+msg.Method)
+		}
+		return nil
+	}
+}
+
+func (s *Server) handleInitialize(msg *Message) error {
+	return s.respond(msg.ID, InitializeResult{
+		Capabilities: ServerCapabilities{TextDocumentSync: 1},
+		ServerInfo:   &ServerInfo{Name: "glint", Version: s.version},
+	})
+}
+
+func (s *Server) handleDidOpen(msg *Message) error {
+	var p DidOpenTextDocumentParams
+	if err := json.Unmarshal(msg.Params, &p); err != nil {
+		return nil // ignore malformed notifications
+	}
+	s.docs[p.TextDocument.URI] = p.TextDocument.Text
+	return s.lintAndPublish(p.TextDocument.URI, p.TextDocument.Text)
+}
+
+func (s *Server) handleDidChange(msg *Message) error {
+	var p DidChangeTextDocumentParams
+	if err := json.Unmarshal(msg.Params, &p); err != nil {
+		return nil
+	}
+	if len(p.ContentChanges) == 0 {
+		return nil
+	}
+	// Full sync: the last change event holds the complete new text.
+	text := p.ContentChanges[len(p.ContentChanges)-1].Text
+	s.docs[p.TextDocument.URI] = text
+	return s.lintAndPublish(p.TextDocument.URI, text)
+}
+
+func (s *Server) handleDidSave(msg *Message) error {
+	var p DidSaveTextDocumentParams
+	if err := json.Unmarshal(msg.Params, &p); err != nil {
+		return nil
+	}
+	text := s.docs[p.TextDocument.URI]
+	if p.Text != nil {
+		text = *p.Text
+		s.docs[p.TextDocument.URI] = text
+	}
+	if text == "" {
+		return nil
+	}
+	return s.lintAndPublish(p.TextDocument.URI, text)
+}
+
+func (s *Server) handleDidClose(msg *Message) error {
+	var p DidCloseTextDocumentParams
+	if err := json.Unmarshal(msg.Params, &p); err != nil {
+		return nil
+	}
+	delete(s.docs, p.TextDocument.URI)
+	// Clear diagnostics so the editor doesn't show stale squiggles.
+	return s.notify("textDocument/publishDiagnostics", PublishDiagnosticsParams{
+		URI:         p.TextDocument.URI,
+		Diagnostics: []Diagnostic{},
+	})
+}
+
+func (s *Server) lintAndPublish(uri, text string) error {
+	diags := s.lintDocument(uri, text)
+	return s.notify("textDocument/publishDiagnostics", PublishDiagnosticsParams{
+		URI:         uri,
+		Diagnostics: diags,
+	})
+}
+
+// lintDocument parses text and runs all lint rules, returning LSP Diagnostics.
+// Findings that originate from included files (not the root document) are
+// excluded; their URIs are not tracked so line numbers would be incorrect.
+func (s *Server) lintDocument(uri, text string) []Diagnostic {
+	path := uriToPath(uri)
+	if path == "" {
+		return []Diagnostic{}
+	}
+	rootDir := filepath.Dir(filepath.Clean(path))
+
+	pipeline, err := model.ParseBytes([]byte(text))
+	if err != nil {
+		return []Diagnostic{{
+			Range:    Range{Start: Position{}, End: Position{}},
+			Severity: 1,
+			Source:   "glint",
+			Message:  "YAML parse error: " + err.Error(),
+		}}
+	}
+	pipeline.SourceFile = path
+	pipeline.SetJobOrigin(path)
+
+	// Include resolution is best-effort: network failures produce warnings that
+	// are intentionally discarded here. The linter operates on whatever was
+	// successfully resolved.
+	_, _ = resolver.ResolveIncludes(pipeline, s.cfg, rootDir)
+	_, _ = resolver.Resolve(pipeline)
+
+	findings := linter.Lint(pipeline, nil)
+
+	diags := make([]Diagnostic, 0, len(findings))
+	for _, f := range findings {
+		// Skip findings from included files — their line numbers reference
+		// a different document URI that the server has not opened.
+		if f.File != path && f.File != "" {
+			continue
+		}
+		line := 0
+		if f.Line > 0 {
+			line = f.Line - 1 // glint uses 1-based lines; LSP uses 0-based
+		}
+		sev := 1 // DiagnosticSeverity: Error
+		if f.Severity == linter.Warning {
+			sev = 2 // DiagnosticSeverity: Warning
+		}
+		msg := f.Message
+		if f.Job != "" {
+			msg = fmt.Sprintf("job %q: %s", f.Job, f.Message)
+		}
+		diags = append(diags, Diagnostic{
+			Range: Range{
+				Start: Position{Line: line},
+				End:   Position{Line: line},
+			},
+			Severity: sev,
+			Code:     f.Rule,
+			Source:   "glint",
+			Message:  msg,
+		})
+	}
+	return diags
+}
+
+// uriToPath converts a file:// URI to a local filesystem path.
+// Returns an empty string for non-file URIs or on parse error.
+func uriToPath(uri string) string {
+	u, err := url.Parse(uri)
+	if err != nil || u.Scheme != "file" {
+		return ""
+	}
+	return filepath.FromSlash(u.Path)
+}

internal/lsp/server_test.go

@@ -0,0 +1,383 @@
+package lsp
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+	"testing"
+
+	"git.k3nny.fr/glint/internal/fetcher"
+)
+
+// frame encodes v as a Content-Length–framed LSP message.
+func frame(t *testing.T, v any) []byte {
+	t.Helper()
+	body, err := json.Marshal(v)
+	if err != nil {
+		t.Fatal(err)
+	}
+	hdr := fmt.Sprintf("Content-Length: %d\r\n\r\n", len(body))
+	return append([]byte(hdr), body...)
+}
+
+// readMsg reads one Content-Length–framed JSON object from r.
+func readMsg(t *testing.T, r *bufio.Reader) map[string]json.RawMessage {
+	t.Helper()
+	var contentLength int
+	for {
+		line, err := r.ReadString('\n')
+		if err != nil {
+			t.Fatalf("reading header: %v", err)
+		}
+		line = strings.TrimRight(line, "\r\n")
+		if line == "" {
+			break
+		}
+		if strings.HasPrefix(line, "Content-Length: ") {
+			n, err := strconv.Atoi(strings.TrimPrefix(line, "Content-Length: "))
+			if err != nil {
+				t.Fatalf("invalid Content-Length: %v", err)
+			}
+			contentLength = n
+		}
+	}
+	body := make([]byte, contentLength)
+	if _, err := io.ReadFull(r, body); err != nil {
+		t.Fatalf("reading body: %v", err)
+	}
+	var m map[string]json.RawMessage
+	if err := json.Unmarshal(body, &m); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	return m
+}
+
+// newTestServer returns a Server with a captured exit code and a bufio.Reader
+// wrapping the output buffer so tests can read back server messages.
+func newTestServer(input []byte) (*Server, *bytes.Buffer, *int) {
+	var out bytes.Buffer
+	exitCode := -1
+	srv := New(bytes.NewReader(input), &out, fetcher.GitLabConfig{}, "test")
+	srv.Exit = func(code int) { exitCode = code }
+	return srv, &out, &exitCode
+}
+
+func TestServer_Initialize(t *testing.T) {
+	input := frame(t, map[string]any{
+		"jsonrpc": "2.0",
+		"id":      1,
+		"method":  "initialize",
+		"params":  map[string]any{},
+	})
+	srv, out, _ := newTestServer(input)
+	srv.Run() //nolint:errcheck
+
+	resp := readMsg(t, bufio.NewReader(out))
+	if string(resp["id"]) != "1" {
+		t.Errorf("response id = %s; want 1", resp["id"])
+	}
+
+	var result InitializeResult
+	if err := json.Unmarshal(resp["result"], &result); err != nil {
+		t.Fatalf("unmarshal result: %v", err)
+	}
+	if result.Capabilities.TextDocumentSync != 1 {
+		t.Errorf("textDocumentSync = %d; want 1", result.Capabilities.TextDocumentSync)
+	}
+	if result.ServerInfo == nil || result.ServerInfo.Name != "glint" {
+		t.Errorf("serverInfo.name = %v; want glint", result.ServerInfo)
+	}
+}
+
+func TestServer_ShutdownExit(t *testing.T) {
+	var buf bytes.Buffer
+	buf.Write(frame(t, map[string]any{
+		"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": map[string]any{},
+	}))
+	buf.Write(frame(t, map[string]any{
+		"jsonrpc": "2.0", "method": "initialized", "params": map[string]any{},
+	}))
+	buf.Write(frame(t, map[string]any{
+		"jsonrpc": "2.0", "id": 2, "method": "shutdown",
+	}))
+	buf.Write(frame(t, map[string]any{
+		"jsonrpc": "2.0", "method": "exit",
+	}))
+
+	srv, out, exitCode := newTestServer(buf.Bytes())
+	srv.Run() //nolint:errcheck
+
+	r := bufio.NewReader(out)
+	initResp := readMsg(t, r)
+	if string(initResp["id"]) != "1" {
+		t.Errorf("init response id = %s; want 1", initResp["id"])
+	}
+	shutResp := readMsg(t, r)
+	if string(shutResp["id"]) != "2" {
+		t.Errorf("shutdown response id = %s; want 2", shutResp["id"])
+	}
+	if string(shutResp["result"]) != "null" {
+		t.Errorf("shutdown result = %s; want null", shutResp["result"])
+	}
+	if *exitCode != 0 {
+		t.Errorf("exit code = %d; want 0", *exitCode)
+	}
+}
+
+func TestServer_ExitWithoutShutdown(t *testing.T) {
+	input := frame(t, map[string]any{
+		"jsonrpc": "2.0", "method": "exit",
+	})
+	srv, _, exitCode := newTestServer(input)
+	srv.Run() //nolint:errcheck
+
+	if *exitCode != 1 {
+		t.Errorf("exit code = %d; want 1 (no prior shutdown)", *exitCode)
+	}
+}
+
+func TestServer_MethodNotFound(t *testing.T) {
+	input := frame(t, map[string]any{
+		"jsonrpc": "2.0", "id": 99, "method": "workspace/unknownMethod",
+	})
+	srv, out, _ := newTestServer(input)
+	srv.Run() //nolint:errcheck
+
+	resp := readMsg(t, bufio.NewReader(out))
+	if resp["error"] == nil {
+		t.Errorf("expected error response for unknown method, got: %v", resp)
+	}
+	var rpcErr RPCError
+	if err := json.Unmarshal(resp["error"], &rpcErr); err != nil {
+		t.Fatalf("unmarshal error: %v", err)
+	}
+	if rpcErr.Code != -32601 {
+		t.Errorf("error code = %d; want -32601", rpcErr.Code)
+	}
+}
+
+func TestServer_UnknownNotificationIgnored(t *testing.T) {
+	// Notifications (no id) for unknown methods must be silently ignored.
+	input := frame(t, map[string]any{
+		"jsonrpc": "2.0", "method": "$/setTrace", "params": map[string]any{"value": "off"},
+	})
+	srv, out, _ := newTestServer(input)
+	srv.Run() //nolint:errcheck
+
+	if out.Len() > 0 {
+		t.Errorf("server wrote %d bytes for unknown notification; want 0", out.Len())
+	}
+}
+
+func TestServer_DidOpen_CleanPipeline(t *testing.T) {
+	yaml := `stages: [build]
+
+build-job:
+  stage: build
+  script: echo hello
+`
+	// Use a pseudo file:// URI that maps to the tmp path; include resolution
+	// will fail silently (no network, no local includes) which is fine.
+	uri := "file:///tmp/test.gitlab-ci.yml"
+
+	input := frame(t, map[string]any{
+		"jsonrpc": "2.0",
+		"method":  "textDocument/didOpen",
+		"params": map[string]any{
+			"textDocument": map[string]any{
+				"uri": uri, "languageId": "yaml", "version": 1, "text": yaml,
+			},
+		},
+	})
+	srv, out, _ := newTestServer(input)
+	srv.Run() //nolint:errcheck
+
+	notif := readMsg(t, bufio.NewReader(out))
+	if string(notif["method"]) != `"textDocument/publishDiagnostics"` {
+		t.Fatalf("method = %s; want textDocument/publishDiagnostics", notif["method"])
+	}
+	var params PublishDiagnosticsParams
+	if err := json.Unmarshal(notif["params"], &params); err != nil {
+		t.Fatalf("unmarshal params: %v", err)
+	}
+	if params.URI != uri {
+		t.Errorf("uri = %q; want %q", params.URI, uri)
+	}
+	// A clean pipeline should produce no diagnostics (or only warnings from
+	// include resolution being skipped — but those are filtered since they
+	// originate from a different file path).
+	for _, d := range params.Diagnostics {
+		if d.Severity == 1 {
+			t.Errorf("unexpected error diagnostic: %s", d.Message)
+		}
+	}
+}
+
+func TestServer_DidOpen_WithErrors(t *testing.T) {
+	// A pipeline with a job in an undeclared stage triggers GL004.
+	yaml := `stages: [build]
+
+bad-job:
+  stage: missing-stage
+  script: echo hi
+`
+	uri := "file:///tmp/bad.gitlab-ci.yml"
+	input := frame(t, map[string]any{
+		"jsonrpc": "2.0",
+		"method":  "textDocument/didOpen",
+		"params": map[string]any{
+			"textDocument": map[string]any{
+				"uri": uri, "languageId": "yaml", "version": 1, "text": yaml,
+			},
+		},
+	})
+	srv, out, _ := newTestServer(input)
+	srv.Run() //nolint:errcheck
+
+	notif := readMsg(t, bufio.NewReader(out))
+	var params PublishDiagnosticsParams
+	if err := json.Unmarshal(notif["params"], &params); err != nil {
+		t.Fatalf("unmarshal params: %v", err)
+	}
+	if len(params.Diagnostics) == 0 {
+		t.Error("expected diagnostics for pipeline with unknown stage, got none")
+	}
+	found := false
+	for _, d := range params.Diagnostics {
+		if d.Code == "GL004" {
+			found = true
+			if d.Severity != 1 {
+				t.Errorf("GL004 severity = %d; want 1 (Error)", d.Severity)
+			}
+		}
+	}
+	if !found {
+		t.Errorf("expected GL004 diagnostic, got: %v", params.Diagnostics)
+	}
+}
+
+func TestServer_DidOpen_ParseError(t *testing.T) {
+	uri := "file:///tmp/broken.gitlab-ci.yml"
+	input := frame(t, map[string]any{
+		"jsonrpc": "2.0",
+		"method":  "textDocument/didOpen",
+		"params": map[string]any{
+			"textDocument": map[string]any{
+				"uri": uri, "languageId": "yaml", "version": 1,
+				"text": "?", // bare ? yields empty job name → parse error
+			},
+		},
+	})
+	srv, out, _ := newTestServer(input)
+	srv.Run() //nolint:errcheck
+
+	notif := readMsg(t, bufio.NewReader(out))
+	var params PublishDiagnosticsParams
+	if err := json.Unmarshal(notif["params"], &params); err != nil {
+		t.Fatalf("unmarshal params: %v", err)
+	}
+	if len(params.Diagnostics) == 0 {
+		t.Fatal("expected parse-error diagnostic, got none")
+	}
+	d := params.Diagnostics[0]
+	if d.Severity != 1 {
+		t.Errorf("severity = %d; want 1 (Error)", d.Severity)
+	}
+	if !strings.Contains(d.Message, "YAML parse error") {
+		t.Errorf("message = %q; want YAML parse error", d.Message)
+	}
+}
+
+func TestServer_DidChange(t *testing.T) {
+	uri := "file:///tmp/ci.gitlab-ci.yml"
+	var buf bytes.Buffer
+	// Open with clean content.
+	buf.Write(frame(t, map[string]any{
+		"jsonrpc": "2.0", "method": "textDocument/didOpen",
+		"params": map[string]any{"textDocument": map[string]any{
+			"uri": uri, "languageId": "yaml", "version": 1,
+			"text": "stages: [build]\nbuild: {stage: build, script: echo}\n",
+		}},
+	}))
+	// Change to content with an error.
+	buf.Write(frame(t, map[string]any{
+		"jsonrpc": "2.0", "method": "textDocument/didChange",
+		"params": map[string]any{
+			"textDocument": map[string]any{"uri": uri, "version": 2},
+			"contentChanges": []map[string]any{
+				{"text": "stages: [build]\nbad: {stage: gone, script: hi}\n"},
+			},
+		},
+	}))
+
+	srv, out, _ := newTestServer(buf.Bytes())
+	srv.Run() //nolint:errcheck
+
+	r := bufio.NewReader(out)
+	_ = readMsg(t, r) // first publishDiagnostics (clean)
+	second := readMsg(t, r)
+
+	var params PublishDiagnosticsParams
+	if err := json.Unmarshal(second["params"], &params); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if len(params.Diagnostics) == 0 {
+		t.Error("expected diagnostics after change to broken content, got none")
+	}
+}
+
+func TestServer_DidClose_ClearsdiAgnostics(t *testing.T) {
+	uri := "file:///tmp/toclose.gitlab-ci.yml"
+	var buf bytes.Buffer
+	buf.Write(frame(t, map[string]any{
+		"jsonrpc": "2.0", "method": "textDocument/didOpen",
+		"params": map[string]any{"textDocument": map[string]any{
+			"uri": uri, "languageId": "yaml", "version": 1,
+			"text": "stages: [build]\nj: {stage: build, script: echo}\n",
+		}},
+	}))
+	buf.Write(frame(t, map[string]any{
+		"jsonrpc": "2.0", "method": "textDocument/didClose",
+		"params": map[string]any{"textDocument": map[string]any{"uri": uri}},
+	}))
+
+	srv, out, _ := newTestServer(buf.Bytes())
+	srv.Run() //nolint:errcheck
+
+	r := bufio.NewReader(out)
+	_ = readMsg(t, r) // publishDiagnostics from didOpen
+
+	closeNotif := readMsg(t, r)
+	var params PublishDiagnosticsParams
+	if err := json.Unmarshal(closeNotif["params"], &params); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if params.URI != uri {
+		t.Errorf("uri = %q; want %q", params.URI, uri)
+	}
+	if len(params.Diagnostics) != 0 {
+		t.Errorf("expected empty diagnostics on close, got %v", params.Diagnostics)
+	}
+}
+
+func TestServer_UriToPath(t *testing.T) {
+	tests := []struct {
+		uri  string
+		want string
+	}{
+		{"file:///tmp/ci.yml", "/tmp/ci.yml"},
+		{"file:///home/user/project/.gitlab-ci.yml", "/home/user/project/.gitlab-ci.yml"},
+		{"https://example.com/file.yml", ""},
+		{"not-a-uri", ""},
+	}
+	for _, tc := range tests {
+		got := uriToPath(tc.uri)
+		if got != tc.want {
+			t.Errorf("uriToPath(%q) = %q; want %q", tc.uri, got, tc.want)
+		}
+	}
+}

internal/lsp/types.go

@@ -0,0 +1,112 @@
+// Package lsp implements a minimal Language Server Protocol server for glint.
+package lsp
+
+import "encoding/json"
+
+// Message is a JSON-RPC 2.0 message (request, response, or notification).
+type Message struct {
+	JSONRPC string          `json:"jsonrpc"`
+	ID      json.RawMessage `json:"id,omitempty"`
+	Method  string          `json:"method,omitempty"`
+	Params  json.RawMessage `json:"params,omitempty"`
+	Result  json.RawMessage `json:"result,omitempty"`
+	Error   *RPCError       `json:"error,omitempty"`
+}
+
+// RPCError is a JSON-RPC 2.0 error object.
+type RPCError struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+}
+
+// InitializeResult is the server's response to the initialize request.
+type InitializeResult struct {
+	Capabilities ServerCapabilities `json:"capabilities"`
+	ServerInfo   *ServerInfo        `json:"serverInfo,omitempty"`
+}
+
+// ServerCapabilities advertises what the server supports.
+type ServerCapabilities struct {
+	// TextDocumentSync: 1 = Full (send entire document on every change).
+	TextDocumentSync int `json:"textDocumentSync"`
+}
+
+// ServerInfo identifies the server to the client.
+type ServerInfo struct {
+	Name    string `json:"name"`
+	Version string `json:"version,omitempty"`
+}
+
+// TextDocumentItem is a text document opened by the client.
+type TextDocumentItem struct {
+	URI        string `json:"uri"`
+	LanguageID string `json:"languageId"`
+	Version    int    `json:"version"`
+	Text       string `json:"text"`
+}
+
+// TextDocumentIdentifier references a text document by URI.
+type TextDocumentIdentifier struct {
+	URI string `json:"uri"`
+}
+
+// VersionedTextDocumentIdentifier includes a version number.
+type VersionedTextDocumentIdentifier struct {
+	URI     string `json:"uri"`
+	Version int    `json:"version"`
+}
+
+// TextDocumentContentChangeEvent is a single content change event.
+// With Full sync the Text field contains the complete new document text.
+type TextDocumentContentChangeEvent struct {
+	Text string `json:"text"`
+}
+
+// DidOpenTextDocumentParams is the params for textDocument/didOpen.
+type DidOpenTextDocumentParams struct {
+	TextDocument TextDocumentItem `json:"textDocument"`
+}
+
+// DidChangeTextDocumentParams is the params for textDocument/didChange.
+type DidChangeTextDocumentParams struct {
+	TextDocument   VersionedTextDocumentIdentifier  `json:"textDocument"`
+	ContentChanges []TextDocumentContentChangeEvent `json:"contentChanges"`
+}
+
+// DidSaveTextDocumentParams is the params for textDocument/didSave.
+type DidSaveTextDocumentParams struct {
+	TextDocument TextDocumentIdentifier `json:"textDocument"`
+	Text         *string                `json:"text,omitempty"`
+}
+
+// DidCloseTextDocumentParams is the params for textDocument/didClose.
+type DidCloseTextDocumentParams struct {
+	TextDocument TextDocumentIdentifier `json:"textDocument"`
+}
+
+// PublishDiagnosticsParams is the params for textDocument/publishDiagnostics.
+type PublishDiagnosticsParams struct {
+	URI         string       `json:"uri"`
+	Diagnostics []Diagnostic `json:"diagnostics"`
+}
+
+// Diagnostic is a lint finding expressed in LSP terms.
+type Diagnostic struct {
+	Range    Range  `json:"range"`
+	Severity int    `json:"severity"` // 1=Error, 2=Warning, 3=Information, 4=Hint
+	Code     string `json:"code,omitempty"`
+	Source   string `json:"source,omitempty"`
+	Message  string `json:"message"`
+}
+
+// Range is a zero-based line/character range within a text document.
+type Range struct {
+	Start Position `json:"start"`
+	End   Position `json:"end"`
+}
+
+// Position is a zero-based line and character offset.
+type Position struct {
+	Line      int `json:"line"`
+	Character int `json:"character"`
+}