diff --git a/cmd/mdt/main.go b/cmd/mdt/main.go
new file mode 100644
index 0000000..4d1b2d5
--- /dev/null
+++ b/cmd/mdt/main.go
@@ -0,0 +1,113 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+
+	"github.com/marte-dev/marte-dev-tools/internal/builder"
+	"github.com/marte-dev/marte-dev-tools/internal/index"
+	"github.com/marte-dev/marte-dev-tools/internal/lsp"
+	"github.com/marte-dev/marte-dev-tools/internal/parser"
+	"github.com/marte-dev/marte-dev-tools/internal/validator"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Println("Usage: mdt <command> [arguments]")
+		fmt.Println("Commands: lsp, build, check, fmt")
+		os.Exit(1)
+	}
+
+	command := os.Args[1]
+	switch command {
+	case "lsp":
+		runLSP()
+	case "build":
+		runBuild(os.Args[2:])
+	case "check":
+		runCheck(os.Args[2:])
+	case "fmt":
+		runFmt()
+	default:
+		fmt.Printf("Unknown command: %s\n", command)
+		os.Exit(1)
+	}
+}
+
+func runLSP() {
+	lsp.RunServer()
+}
+
+func runBuild(args []string) {
+	if len(args) < 1 {
+		fmt.Println("Usage: mdt build <input_files...>")
+		os.Exit(1)
+	}
+	
+	outputDir := "build"
+	os.MkdirAll(outputDir, 0755)
+
+	b := builder.NewBuilder(args)
+	err := b.Build(outputDir)
+	if err != nil {
+		fmt.Printf("Build failed: %v\n", err)
+		os.Exit(1)
+	}
+	fmt.Println("Build successful. Output in", outputDir)
+}
+
+func runCheck(args []string) {
+	if len(args) < 1 {
+		fmt.Println("Usage: mdt check <input_files...>")
+		os.Exit(1)
+	}
+
+	idx := index.NewIndex()
+	configs := make(map[string]*parser.Configuration)
+
+	for _, file := range args {
+		content, err := ioutil.ReadFile(file)
+		if err != nil {
+			fmt.Printf("Error reading %s: %v\n", file, err)
+			continue
+		}
+
+		p := parser.NewParser(string(content))
+		config, err := p.Parse()
+		if err != nil {
+			fmt.Printf("%s: Grammar error: %v\n", file, err)
+			continue
+		}
+
+		configs[file] = config
+		idx.IndexConfig(file, config)
+	}
+
+	idx.ResolveReferences()
+	v := validator.NewValidator(idx)
+
+	for file, config := range configs {
+		v.Validate(file, config)
+	}
+	v.CheckUnused()
+
+	for _, diag := range v.Diagnostics {
+		level := "ERROR"
+		if diag.Level == validator.LevelWarning {
+			level = "WARNING"
+		}
+		fmt.Printf("%s:%d:%d: %s: %s\n", diag.File, diag.Position.Line, diag.Position.Column, level, diag.Message)
+	}
+
+	if len(v.Diagnostics) > 0 {
+		fmt.Printf("\nFound %d issues.\n", len(v.Diagnostics))
+	} else {
+		fmt.Println("No issues found.")
+	}
+}
+
+func runFmt() {
+	fmt.Println("Formatting files...")
+	// TODO: Implement fmt
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..c6f6ec6
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module github.com/marte-dev/marte-dev-tools
+
+go 1.25.6
diff --git a/internal/builder/builder.go b/internal/builder/builder.go
new file mode 100644
index 0000000..f3c8095
--- /dev/null
+++ b/internal/builder/builder.go
@@ -0,0 +1,111 @@
+package builder
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/marte-dev/marte-dev-tools/internal/parser"
+)
+
+type Builder struct {
+	Files []string
+}
+
+func NewBuilder(files []string) *Builder {
+	return &Builder{Files: files}
+}
+
+func (b *Builder) Build(outputDir string) error {
+	packages := make(map[string]*parser.Configuration)
+
+	for _, file := range b.Files {
+		content, err := ioutil.ReadFile(file)
+		if err != nil {
+			return err
+		}
+
+		p := parser.NewParser(string(content))
+		config, err := p.Parse()
+		if err != nil {
+			return fmt.Errorf("error parsing %s: %v", file, err)
+		}
+
+		pkgURI := ""
+		if config.Package != nil {
+			pkgURI = config.Package.URI
+		}
+
+		if existing, ok := packages[pkgURI]; ok {
+			existing.Definitions = append(existing.Definitions, config.Definitions...)
+		} else {
+			packages[pkgURI] = config
+		}
+	}
+
+	for pkg, config := range packages {
+		if pkg == "" {
+			continue // Or handle global package
+		}
+		
+		outputPath := filepath.Join(outputDir, pkg+".marte")
+		err := b.writeConfig(outputPath, config)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (b *Builder) writeConfig(path string, config *parser.Configuration) error {
+	f, err := os.Create(path)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	for _, def := range config.Definitions {
+		b.writeDefinition(f, def, 0)
+	}
+	return nil
+}
+
+func (b *Builder) writeDefinition(f *os.File, def parser.Definition, indent int) {
+	indentStr := strings.Repeat("    ", indent)
+	switch d := def.(type) {
+	case *parser.Field:
+		fmt.Fprintf(f, "%s%s = %s\n", indentStr, d.Name, b.formatValue(d.Value))
+	case *parser.ObjectNode:
+		fmt.Fprintf(f, "%s%s = {\n", indentStr, d.Name)
+		for _, subDef := range d.Subnode.Definitions {
+			b.writeDefinition(f, subDef, indent+1)
+		}
+		fmt.Fprintf(f, "%s}\n", indentStr)
+	}
+}
+
+func (b *Builder) formatValue(val parser.Value) string {
+	switch v := val.(type) {
+	case *parser.StringValue:
+		return fmt.Sprintf("\"%s\"", v.Value)
+	case *parser.IntValue:
+		return v.Raw
+	case *parser.FloatValue:
+		return v.Raw
+	case *parser.BoolValue:
+		return fmt.Sprintf("%v", v.Value)
+	case *parser.ReferenceValue:
+		return v.Value
+	case *parser.ArrayValue:
+		elements := []string{}
+		for _, e := range v.Elements {
+			elements = append(elements, b.formatValue(e))
+		}
+		return fmt.Sprintf("{%s}", strings.Join(elements, " "))
+	default:
+		return ""
+	}
+}
diff --git a/internal/index/index.go b/internal/index/index.go
new file mode 100644
index 0000000..7224d9e
--- /dev/null
+++ b/internal/index/index.go
@@ -0,0 +1,125 @@
+package index
+
+import (
+	"github.com/marte-dev/marte-dev-tools/internal/parser"
+)
+
+type SymbolType int
+
+const (
+	SymbolObject SymbolType = iota
+	SymbolSignal
+	SymbolDataSource
+	SymbolGAM
+)
+
+type Symbol struct {
+	Name     string
+	Type     SymbolType
+	Position parser.Position
+	File     string
+	Doc      string
+	Class    string
+	Parent   *Symbol
+}
+
+type Reference struct {
+	Name     string
+	Position parser.Position
+	File     string
+	Target   *Symbol
+}
+
+type Index struct {
+	Symbols    map[string]*Symbol
+	References []Reference
+	Packages   map[string][]string // pkgURI -> list of files
+}
+
+func NewIndex() *Index {
+	return &Index{
+		Symbols:  make(map[string]*Symbol),
+		Packages: make(map[string][]string),
+	}
+}
+
+func (idx *Index) IndexConfig(file string, config *parser.Configuration) {
+	pkgURI := ""
+	if config.Package != nil {
+		pkgURI = config.Package.URI
+	}
+	idx.Packages[pkgURI] = append(idx.Packages[pkgURI], file)
+
+	for _, def := range config.Definitions {
+		idx.indexDefinition(file, "", nil, def)
+	}
+}
+
+func (idx *Index) indexDefinition(file string, path string, parent *Symbol, def parser.Definition) {
+	switch d := def.(type) {
+	case *parser.ObjectNode:
+		name := d.Name
+		fullPath := name
+		if path != "" {
+			fullPath = path + "." + name
+		}
+		
+		class := ""
+		for _, subDef := range d.Subnode.Definitions {
+			if f, ok := subDef.(*parser.Field); ok && f.Name == "Class" {
+				if s, ok := f.Value.(*parser.StringValue); ok {
+					class = s.Value
+				} else if r, ok := f.Value.(*parser.ReferenceValue); ok {
+					class = r.Value
+				}
+			}
+		}
+
+		symType := SymbolObject
+		// Simple heuristic for GAM or DataSource if class name matches or node name starts with +/$
+		// In a real implementation we would check the class against known MARTe classes
+
+		sym := &Symbol{
+			Name:     fullPath,
+			Type:     symType,
+			Position: d.Position,
+			File:     file,
+			Class:    class,
+			Parent:   parent,
+		}
+		idx.Symbols[fullPath] = sym
+
+		for _, subDef := range d.Subnode.Definitions {
+			idx.indexDefinition(file, fullPath, sym, subDef)
+		}
+
+	case *parser.Field:
+		idx.indexValue(file, d.Value)
+	}
+}
+
+func (idx *Index) indexValue(file string, val parser.Value) {
+	switch v := val.(type) {
+	case *parser.ReferenceValue:
+		idx.References = append(idx.References, Reference{
+			Name:     v.Value,
+			Position: v.Position,
+			File:     file,
+		})
+	case *parser.ArrayValue:
+		for _, elem := range v.Elements {
+			idx.indexValue(file, elem)
+		}
+	}
+}
+
+func (idx *Index) ResolveReferences() {
+	for i := range idx.References {
+		ref := &idx.References[i]
+		if sym, ok := idx.Symbols[ref.Name]; ok {
+			ref.Target = sym
+		} else {
+			// Try relative resolution?
+		}
+	}
+}
\ No newline at end of file
diff --git a/internal/lsp/server.go b/internal/lsp/server.go
new file mode 100644
index 0000000..ec11d48
--- /dev/null
+++ b/internal/lsp/server.go
@@ -0,0 +1,109 @@
+package lsp
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+)
+
+type JsonRpcMessage struct {
+	Jsonrpc string          `json:"jsonrpc"`
+	Method  string          `json:"method,omitempty"`
+	Params  json.RawMessage `json:"params,omitempty"`
+	ID      interface{}     `json:"id,omitempty"`
+	Result  interface{}     `json:"result,omitempty"`
+	Error   *JsonRpcError   `json:"error,omitempty"`
+}
+
+type JsonRpcError struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+}
+
+func RunServer() {
+	reader := bufio.NewReader(os.Stdin)
+	for {
+		msg, err := readMessage(reader)
+		if err != nil {
+			if err == io.EOF {
+				break
+			}
+			fmt.Fprintf(os.Stderr, "Error reading message: %v\n", err)
+			continue
+		}
+
+		handleMessage(msg)
+	}
+}
+
+func readMessage(reader *bufio.Reader) (*JsonRpcMessage, error) {
+	// LSP uses Content-Length header
+	var contentLength int
+	for {
+		line, err := reader.ReadString('\n')
+		if err != nil {
+			return nil, err
+		}
+		if line == "\r\n" {
+			break
+		}
+		if _, err := fmt.Sscanf(line, "Content-Length: %d", &contentLength); err == nil {
+			continue
+		}
+	}
+
+	body := make([]byte, contentLength)
+	_, err := io.ReadFull(reader, body)
+	if err != nil {
+		return nil, err
+	}
+
+	var msg JsonRpcMessage
+	err = json.Unmarshal(body, &msg)
+	return &msg, err
+}
+
+func handleMessage(msg *JsonRpcMessage) {
+	switch msg.Method {
+	case "initialize":
+		respond(msg.ID, map[string]interface{}{
+			"capabilities": map[string]interface{}{
+				"textDocumentSync": 1, // Full sync
+				"hoverProvider":    true,
+				"definitionProvider": true,
+				"referencesProvider": true,
+				"completionProvider": map[string]interface{}{
+					"triggerCharacters": []string{"=", ".", "{", "+", "$"},
+				},
+			},
+		})
+	case "initialized":
+		// Do nothing
+	case "shutdown":
+		respond(msg.ID, nil)
+	case "exit":
+		os.Exit(0)
+	case "textDocument/didOpen":
+		// Handle file open
+	case "textDocument/didChange":
+		// Handle file change
+	case "textDocument/hover":
+		// Handle hover
+	}
+}
+
+func respond(id interface{}, result interface{}) {
+	msg := JsonRpcMessage{
+		Jsonrpc: "2.0",
+		ID:      id,
+		Result:  result,
+	}
+	send(msg)
+}
+
+func send(msg interface{}) {
+	body, _ := json.Marshal(msg)
+	fmt.Printf("Content-Length: %d\r\n\r\n%s", len(body), body)
+}
diff --git a/internal/parser/ast.go b/internal/parser/ast.go
new file mode 100644
index 0000000..ac0e766
--- /dev/null
+++ b/internal/parser/ast.go
@@ -0,0 +1,116 @@
+package parser
+
+type Node interface {
+	Pos() Position
+}
+
+type Position struct {
+	Line   int
+	Column int
+}
+
+type Configuration struct {
+	Definitions []Definition
+	Package     *Package
+}
+
+type Definition interface {
+	Node
+	isDefinition()
+}
+
+type Field struct {
+	Position Position
+	Name     string
+	Value    Value
+}
+
+func (f *Field) Pos() Position { return f.Position }
+func (f *Field) isDefinition() {}
+
+type ObjectNode struct {
+	Position Position
+	Name     string // includes + or $
+	Subnode  Subnode
+}
+
+func (o *ObjectNode) Pos() Position { return o.Position }
+func (o *ObjectNode) isDefinition() {}
+
+type Subnode struct {
+	Position    Position
+	Definitions []Definition
+}
+
+type Value interface {
+	Node
+	isValue()
+}
+
+type StringValue struct {
+	Position Position
+	Value    string
+}
+
+func (v *StringValue) Pos() Position { return v.Position }
+func (v *StringValue) isValue()      {}
+
+type IntValue struct {
+	Position Position
+	Value    int64
+	Raw      string
+}
+
+func (v *IntValue) Pos() Position { return v.Position }
+func (v *IntValue) isValue()      {}
+
+type FloatValue struct {
+	Position Position
+	Value    float64
+	Raw      string
+}
+
+func (v *FloatValue) Pos() Position { return v.Position }
+func (v *FloatValue) isValue()      {}
+
+type BoolValue struct {
+	Position Position
+	Value    bool
+}
+
+func (v *BoolValue) Pos() Position { return v.Position }
+func (v *BoolValue) isValue()      {}
+
+type ReferenceValue struct {
+	Position Position
+	Value    string
+}
+
+func (v *ReferenceValue) Pos() Position { return v.Position }
+func (v *ReferenceValue) isValue()      {}
+
+type ArrayValue struct {
+	Position Position
+	Elements []Value
+}
+
+func (v *ArrayValue) Pos() Position { return v.Position }
+func (v *ArrayValue) isValue()      {}
+
+type Package struct {
+	Position Position
+	URI      string
+}
+
+func (p *Package) Pos() Position { return p.Position }
+
+type Comment struct {
+	Position Position
+	Text     string
+	Doc      bool // true if starts with //#
+}
+
+type Pragma struct {
+	Position Position
+	Text     string
+}
diff --git a/internal/parser/lexer.go b/internal/parser/lexer.go
new file mode 100644
index 0000000..300a1ee
--- /dev/null
+++ b/internal/parser/lexer.go
@@ -0,0 +1,233 @@
+package parser
+
+import (
+	"unicode"
+	"unicode/utf8"
+)
+
+type TokenType int
+
+const (
+	TokenError TokenType = iota
+	TokenEOF
+	TokenIdentifier
+	TokenObjectIdentifier // +$
+	TokenEqual
+	TokenLBrace
+	TokenRBrace
+	TokenString
+	TokenNumber
+	TokenBool
+	TokenPackage
+	TokenPragma
+	TokenComment
+	TokenDocstring
+)
+
+type Token struct {
+	Type     TokenType
+	Value    string
+	Position Position
+}
+
+type Lexer struct {
+	input    string
+	start    int
+	pos      int
+	width    int
+	line     int
+	lineStart int
+}
+
+func NewLexer(input string) *Lexer {
+	return &Lexer{
+		input: input,
+		line:  1,
+	}
+}
+
+func (l *Lexer) next() rune {
+	if l.pos >= len(l.input) {
+		l.width = 0
+		return -1
+	}
+	r, w := utf8.DecodeRuneInString(l.input[l.pos:])
+	l.width = w
+	l.pos += l.width
+	if r == '\n' {
+		l.line++
+		l.lineStart = l.pos
+	}
+	return r
+}
+
+func (l *Lexer) backup() {
+	l.pos -= l.width
+	if l.width > 0 {
+		r, _ := utf8.DecodeRuneInString(l.input[l.pos:])
+		if r == '\n' {
+			l.line--
+			// This is tricky, we'd need to find the previous line start
+			// For simplicity, let's just not backup over newlines or handle it better
+		}
+	}
+}
+
+func (l *Lexer) peek() rune {
+	r := l.next()
+	l.backup()
+	return r
+}
+
+func (l *Lexer) emit(t TokenType) Token {
+	tok := Token{
+		Type: t,
+		Value: l.input[l.start:l.pos],
+		Position: Position{
+			Line:   l.line,
+			Column: l.start - l.lineStart + 1,
+		},
+	}
+	l.start = l.pos
+	return tok
+}
+
+func (l *Lexer) NextToken() Token {
+	for {
+		r := l.next()
+		if r == -1 {
+			return l.emit(TokenEOF)
+		}
+
+		if unicode.IsSpace(r) {
+			l.start = l.pos
+			continue
+		}
+
+		switch r {
+		case '=':
+			return l.emit(TokenEqual)
+		case '{':
+			return l.emit(TokenLBrace)
+		case '}':
+			return l.emit(TokenRBrace)
+		case '"':
+			return l.lexString()
+		case '/':
+			return l.lexComment()
+		case '#':
+			return l.lexPackage()
+		case '!':
+			// Might be part of pragma //! 
+			// But grammar says pragma is //!
+			// So it should start with //
+		case '+':
+			fallthrough
+		case '$':
+			return l.lexObjectIdentifier()
+		}
+
+		if unicode.IsLetter(r) {
+			return l.lexIdentifier()
+		}
+
+		if unicode.IsDigit(r) || r == '-' {
+			return l.lexNumber()
+		}
+
+		return l.emit(TokenError)
+	}
+}
+
+func (l *Lexer) lexIdentifier() Token {
+	for {
+		r := l.next()
+		if unicode.IsLetter(r) || unicode.IsDigit(r) || r == '_' || r == '-' {
+			continue
+		}
+		l.backup()
+		val := l.input[l.start:l.pos]
+		if val == "true" || val == "false" {
+			return l.emit(TokenBool)
+		}
+		return l.emit(TokenIdentifier)
+	}
+}
+
+func (l *Lexer) lexObjectIdentifier() Token {
+	for {
+		r := l.next()
+		if unicode.IsLetter(r) || unicode.IsDigit(r) || r == '_' || r == '-' {
+			continue
+		}
+		l.backup()
+		return l.emit(TokenObjectIdentifier)
+	}
+}
+
+func (l *Lexer) lexString() Token {
+	for {
+		r := l.next()
+		if r == '"' {
+			return l.emit(TokenString)
+		}
+		if r == -1 {
+			return l.emit(TokenError)
+		}
+	}
+}
+
+func (l *Lexer) lexNumber() Token {
+	// Simple number lexing, could be improved for hex, binary, float
+	for {
+		r := l.next()
+		if unicode.IsDigit(r) || r == '.' || r == 'x' || r == 'b' || r == 'e' || r == '-' {
+			continue
+		}
+		l.backup()
+		return l.emit(TokenNumber)
+	}
+}
+
+func (l *Lexer) lexComment() Token {
+	r := l.next()
+	if r == '/' {
+		// It's a comment, docstring or pragma
+		r = l.next()
+		if r == '#' {
+			return l.lexUntilNewline(TokenDocstring)
+		}
+		if r == '!' {
+			return l.lexUntilNewline(TokenPragma)
+		}
+		return l.lexUntilNewline(TokenComment)
+	}
+	l.backup()
+	return l.emit(TokenError)
+}
+
+func (l *Lexer) lexUntilNewline(t TokenType) Token {
+	for {
+		r := l.next()
+		if r == '\n' || r == -1 {
+			return l.emit(t)
+		}
+	}
+}
+
+func (l *Lexer) lexPackage() Token {
+	// #package
+	l.start = l.pos - 1 // Include '#'
+	for {
+		r := l.next()
+		if unicode.IsLetter(r) {
+			continue
+		}
+		l.backup()
+		break
+	}
+	if l.input[l.start:l.pos] == "#package" {
+		return l.lexUntilNewline(TokenPackage)
+	}
+	return l.emit(TokenError)
+}
diff --git a/internal/parser/parser.go b/internal/parser/parser.go
new file mode 100644
index 0000000..35e0bef
--- /dev/null
+++ b/internal/parser/parser.go
@@ -0,0 +1,176 @@
+package parser
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+type Parser struct {
+	lexer *Lexer
+	tok   Token
+	peeked bool
+}
+
+func NewParser(input string) *Parser {
+	return &Parser{
+		lexer: NewLexer(input),
+	}
+}
+
+func (p *Parser) next() Token {
+	if p.peeked {
+		p.peeked = false
+		return p.tok
+	}
+	p.tok = p.lexer.NextToken()
+	return p.tok
+}
+
+func (p *Parser) peek() Token {
+	if p.peeked {
+		return p.tok
+	}
+	p.tok = p.lexer.NextToken()
+	p.peeked = true
+	return p.tok
+}
+
+func (p *Parser) Parse() (*Configuration, error) {
+	config := &Configuration{}
+	for {
+		tok := p.peek()
+		if tok.Type == TokenEOF {
+			break
+		}
+		if tok.Type == TokenPackage {
+			p.next()
+			config.Package = &Package{
+				Position: tok.Position,
+				URI:      strings.TrimSpace(strings.TrimPrefix(tok.Value, "#package")),
+			}
+			continue
+		}
+		
+		// Skip comments, pragmas, docstrings for now in AST
+		if tok.Type == TokenComment || tok.Type == TokenDocstring || tok.Type == TokenPragma {
+			p.next()
+			continue
+		}
+
+		def, err := p.parseDefinition()
+		if err != nil {
+			return nil, err
+		}
+		config.Definitions = append(config.Definitions, def)
+	}
+	return config, nil
+}
+
+func (p *Parser) parseDefinition() (Definition, error) {
+	tok := p.next()
+	switch tok.Type {
+	case TokenIdentifier:
+		// field = value
+		name := tok.Value
+		if p.next().Type != TokenEqual {
+			return nil, fmt.Errorf("%d:%d: expected =", p.tok.Position.Line, p.tok.Position.Column)
+		}
+		val, err := p.parseValue()
+		if err != nil {
+			return nil, err
+		}
+		return &Field{
+			Position: tok.Position,
+			Name:     name,
+			Value:    val,
+		}, nil
+	case TokenObjectIdentifier:
+		// node = subnode
+		name := tok.Value
+		if p.next().Type != TokenEqual {
+			return nil, fmt.Errorf("%d:%d: expected =", p.tok.Position.Line, p.tok.Position.Column)
+		}
+		sub, err := p.parseSubnode()
+		if err != nil {
+			return nil, err
+		}
+		return &ObjectNode{
+			Position: tok.Position,
+			Name:     name,
+			Subnode:  sub,
+		}, nil
+	default:
+		return nil, fmt.Errorf("%d:%d: unexpected token %v", tok.Position.Line, tok.Position.Column, tok.Value)
+	}
+}
+
+func (p *Parser) parseSubnode() (Subnode, error) {
+	tok := p.next()
+	if tok.Type != TokenLBrace {
+		return Subnode{}, fmt.Errorf("%d:%d: expected {", tok.Position.Line, tok.Position.Column)
+	}
+	sub := Subnode{Position: tok.Position}
+	for {
+		t := p.peek()
+		if t.Type == TokenRBrace {
+			p.next()
+			break
+		}
+		if t.Type == TokenEOF {
+			return sub, fmt.Errorf("%d:%d: unexpected EOF, expected }", t.Position.Line, t.Position.Column)
+		}
+		if t.Type == TokenComment || t.Type == TokenDocstring || t.Type == TokenPragma {
+			p.next()
+			continue
+		}
+		def, err := p.parseDefinition()
+		if err != nil {
+			return sub, err
+		}
+		sub.Definitions = append(sub.Definitions, def)
+	}
+	return sub, nil
+}
+
+func (p *Parser) parseValue() (Value, error) {
+	tok := p.next()
+	switch tok.Type {
+	case TokenString:
+		return &StringValue{
+			Position: tok.Position,
+			Value:    strings.Trim(tok.Value, "\""),
+		}, nil
+	case TokenNumber:
+		// Simplistic handling
+		if strings.Contains(tok.Value, ".") || strings.Contains(tok.Value, "e") {
+			f, _ := strconv.ParseFloat(tok.Value, 64)
+			return &FloatValue{Position: tok.Position, Value: f, Raw: tok.Value}, nil
+		}
+		i, _ := strconv.ParseInt(tok.Value, 0, 64)
+		return &IntValue{Position: tok.Position, Value: i, Raw: tok.Value}, nil
+	case TokenBool:
+		return &BoolValue{Position: tok.Position, Value: tok.Value == "true"}, nil
+	case TokenIdentifier:
+		// reference?
+		return &ReferenceValue{Position: tok.Position, Value: tok.Value}, nil
+	case TokenLBrace:
+		// array
+		arr := &ArrayValue{Position: tok.Position}
+		for {
+			t := p.peek()
+			if t.Type == TokenRBrace {
+				p.next()
+				break
+			}
+			val, err := p.parseValue()
+			if err != nil {
+				return nil, err
+			}
+			arr.Elements = append(arr.Elements, val)
+		}
+		return arr, nil
+	default:
+		return nil, fmt.Errorf("%d:%d: unexpected value token %v", tok.Position.Line, tok.Position.Column, tok.Value)
+	}
+}
diff --git a/internal/parser/parser_test.go b/internal/parser/parser_test.go
new file mode 100644
index 0000000..83d6833
--- /dev/null
+++ b/internal/parser/parser_test.go
@@ -0,0 +1,38 @@
+package parser
+
+import (
+	"testing"
+)
+
+func TestParseBasic(t *testing.T) {
+	input := `
+#package PROJECT.SUB
+// comment
++Node1 = {
+    Class = MyClass
+    Field1 = "value"
+    Field2 = 123
+    Field3 = true
+    +SubNode = {
+        Class = OtherClass
+    }
+}
+$Node2 = {
+    Class = AppClass
+    Array = {1 2 3}
+}
+`
+	p := NewParser(input)
+	config, err := p.Parse()
+	if err != nil {
+		t.Fatalf("Parse error: %v", err)
+	}
+
+	if config.Package == nil || config.Package.URI != "PROJECT.SUB" {
+		t.Errorf("Expected package PROJECT.SUB, got %v", config.Package)
+	}
+
+	if len(config.Definitions) != 2 {
+		t.Errorf("Expected 2 definitions, got %d", len(config.Definitions))
+	}
+}
diff --git a/internal/validator/validator.go b/internal/validator/validator.go
new file mode 100644
index 0000000..204220d
--- /dev/null
+++ b/internal/validator/validator.go
@@ -0,0 +1,95 @@
+package validator
+
+import (
+	"fmt"
+	"github.com/marte-dev/marte-dev-tools/internal/parser"
+	"github.com/marte-dev/marte-dev-tools/internal/index"
+)
+
+type DiagnosticLevel int
+
+const (
+	LevelError DiagnosticLevel = iota
+	LevelWarning
+)
+
+type Diagnostic struct {
+	Level    DiagnosticLevel
+	Message  string
+	Position parser.Position
+	File     string
+}
+
+type Validator struct {
+	Diagnostics []Diagnostic
+	Index       *index.Index
+}
+
+func NewValidator(idx *index.Index) *Validator {
+	return &Validator{Index: idx}
+}
+
+func (v *Validator) Validate(file string, config *parser.Configuration) {
+	for _, def := range config.Definitions {
+		v.validateDefinition(file, "", config, def)
+	}
+}
+
+func (v *Validator) validateDefinition(file string, path string, config *parser.Configuration, def parser.Definition) {
+	switch d := def.(type) {
+	case *parser.ObjectNode:
+		name := d.Name
+		fullPath := name
+		if path != "" {
+			fullPath = path + "." + name
+		}
+
+		// Check for mandatory 'Class' field for +/$ nodes
+		if d.Name != "" && (d.Name[0] == '+' || d.Name[0] == '$') {
+			hasClass := false
+			for _, subDef := range d.Subnode.Definitions {
+				if f, ok := subDef.(*parser.Field); ok && f.Name == "Class" {
+					hasClass = true
+					break
+				}
+			}
+			if !hasClass {
+				v.Diagnostics = append(v.Diagnostics, Diagnostic{
+					Level:    LevelError,
+					Message:  fmt.Sprintf("Node %s is an object and must contain a 'Class' field", d.Name),
+					Position: d.Position,
+					File:     file,
+				})
+			}
+		}
+
+		// GAM specific validation
+		// (This is a placeholder, real logic would check if it's a GAM)
+
+		for _, subDef := range d.Subnode.Definitions {
+			v.validateDefinition(file, fullPath, config, subDef)
+		}
+	}
+}
+
+func (v *Validator) CheckUnused() {
+	if v.Index == nil {
+		return
+	}
+
+	referencedSymbols := make(map[*index.Symbol]bool)
+	for _, ref := range v.Index.References {
+		if ref.Target != nil {
+			referencedSymbols[ref.Target] = true
+		}
+	}
+
+	for _, sym := range v.Index.Symbols {
+		// Heuristic: if it's a GAM or Signal, check if referenced
+		// (Refining this later with proper class checks)
+		if !referencedSymbols[sym] {
+			// Logic to determine if it should be warned as unused
+			// e.g. if sym.Class is a GAM or if it's a signal in a DataSource
+		}
+	}
+}
diff --git a/mdt b/mdt
new file mode 100755
index 0000000..9b4d6ab
Binary files /dev/null and b/mdt differ
diff --git a/test_error.marte b/test_error.marte
new file mode 100644
index 0000000..6fe8521
--- /dev/null
+++ b/test_error.marte
@@ -0,0 +1,3 @@
++NodeWithoutClass = {
+    Field = 1
+}