diff --git a/Cargo.lock b/Cargo.lock
index 4ed4393..ffe211f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -17,6 +17,16 @@ version = "3.20.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "72f5acc6cb2ba439de613abc23857ec3d78374d8ed5ac84e9d11336e87da8649"
 
+[[package]]
+name = "cc"
+version = "1.2.66"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f5d6cac793997bd970000024b2934968efe83b382de4fdcf4fcb46b6ee4ad996"
+dependencies = [
+ "find-msvc-tools",
+ "shlex",
+]
+
 [[package]]
 name = "cfg-if"
 version = "1.0.4"
@@ -48,6 +58,17 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "decodal-language-tools"
+version = "0.1.2"
+dependencies = [
+ "decodal",
+ "serde_json",
+ "tree-sitter",
+ "tree-sitter-decodal",
+ "wasm-bindgen",
+]
+
 [[package]]
 name = "decodal-wasm"
 version = "0.1.2"
@@ -57,6 +78,12 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "find-msvc-tools"
+version = "0.1.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5baebc0774151f905a1a2cc41989300b1e6fbb29aff0ceffa1064fdd3088d582"
+
 [[package]]
 name = "itoa"
 version = "1.0.18"
@@ -170,6 +197,12 @@ dependencies = [
  "zmij",
 ]
 
+[[package]]
+name = "shlex"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f8fadd59c855ef2080decdef8ff161eb6661b86933c9d82e5ba29dc602a55aba"
+
 [[package]]
 name = "syn"
 version = "2.0.117"
@@ -181,6 +214,24 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "tree-sitter"
+version = "0.22.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "df7cc499ceadd4dcdf7ec6d4cbc34ece92c3fa07821e287aedecd4416c516dca"
+dependencies = [
+ "cc",
+ "regex",
+]
+
+[[package]]
+name = "tree-sitter-decodal"
+version = "0.0.1"
+dependencies = [
+ "cc",
+ "tree-sitter",
+]
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.24"
diff --git a/Cargo.toml b/Cargo.toml
index d3b63c2..7f1149a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,6 +4,7 @@ members = [
   "crates/decodal-cli",
   "crates/decodal-wasm",
   "crates/decodal-derive",
+  "crates/decodal-language-tools",
 ]
 resolver = "2"
 
diff --git a/crates/decodal-language-tools/Cargo.toml b/crates/decodal-language-tools/Cargo.toml
new file mode 100644
index 0000000..6db03b4
--- /dev/null
+++ b/crates/decodal-language-tools/Cargo.toml
@@ -0,0 +1,25 @@
+[package]
+name = "decodal-language-tools"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+repository.workspace = true
+readme.workspace = true
+description = "Source-level language tooling for Decodal."
+keywords = ["decodal", "formatter", "lsp", "tree-sitter"]
+categories = ["development-tools", "text-processing"]
+publish = false
+
+[lib]
+crate-type = ["cdylib", "rlib"]
+
+[dependencies]
+decodal = { version = "0.1.2", path = "../decodal-core" }
+serde_json.workspace = true
+tree-sitter = "0.22.6"
+tree-sitter-decodal = { path = "../../editors/tree-sitter-decodal" }
+wasm-bindgen.workspace = true
+
+[package.metadata.wasm-pack.profile.release]
+wasm-opt = false
diff --git a/crates/decodal-language-tools/src/lib.rs b/crates/decodal-language-tools/src/lib.rs
new file mode 100644
index 0000000..02eaf9c
--- /dev/null
+++ b/crates/decodal-language-tools/src/lib.rs
@@ -0,0 +1,851 @@
+use std::{error::Error, fmt};
+
+#[cfg(not(target_arch = "wasm32"))]
+use tree_sitter::{Node, Parser, TreeCursor};
+use wasm_bindgen::prelude::*;
+
+const INDENT: usize = 4;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct FormatError {
+    message: String,
+}
+
+impl FormatError {
+    fn new(message: impl Into<String>) -> Self {
+        Self {
+            message: message.into(),
+        }
+    }
+
+    pub fn message(&self) -> &str {
+        &self.message
+    }
+}
+
+impl fmt::Display for FormatError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&self.message)
+    }
+}
+
+impl Error for FormatError {}
+
+pub fn format_source(source: &str) -> Result<String, FormatError> {
+    decodal::parse_source(source)
+        .map_err(|diagnostic| FormatError::new(format!("parse error: {}", diagnostic.message)))?;
+    format_source_impl(source)
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+fn format_source_impl(source: &str) -> Result<String, FormatError> {
+    let mut parser = Parser::new();
+    parser
+        .set_language(&tree_sitter_decodal::language())
+        .map_err(|error| {
+            FormatError::new(format!("failed to load tree-sitter grammar: {error}"))
+        })?;
+    let tree = parser
+        .parse(source, None)
+        .ok_or_else(|| FormatError::new("tree-sitter failed to parse source"))?;
+    let root = tree.root_node();
+    if root.has_error() {
+        return Err(FormatError::new(
+            "parse error: source contains tree-sitter errors",
+        ));
+    }
+
+    let mut formatter = Formatter::new(source);
+    let mut out = formatter.format_source_file(root);
+    if !out.ends_with('\n') {
+        out.push('\n');
+    }
+    Ok(out)
+}
+
+#[wasm_bindgen(js_name = formatSource)]
+pub fn format_source_json(source: &str) -> String {
+    match format_source(source) {
+        Ok(formatted) => serde_json::json!({ "ok": true, "source": formatted }).to_string(),
+        Err(error) => serde_json::json!({ "ok": false, "error": error.message() }).to_string(),
+    }
+}
+
+#[cfg(target_arch = "wasm32")]
+fn format_source_impl(source: &str) -> Result<String, FormatError> {
+    Ok(LexicalFormatter::new(source).format())
+}
+
+#[cfg(target_arch = "wasm32")]
+struct LexicalFormatter<'a> {
+    chars: core::iter::Peekable<core::str::Chars<'a>>,
+    out: String,
+    indent: usize,
+    line_start: bool,
+}
+
+#[cfg(target_arch = "wasm32")]
+impl<'a> LexicalFormatter<'a> {
+    fn new(source: &'a str) -> Self {
+        Self {
+            chars: source.chars().peekable(),
+            out: String::new(),
+            indent: 0,
+            line_start: true,
+        }
+    }
+
+    fn format(mut self) -> String {
+        while let Some(ch) = self.chars.next() {
+            match ch {
+                ' ' | '\t' | '\r' | '\n' => self.consume_whitespace(ch),
+                '#' => self.write_comment(),
+                '"' => self.write_string(),
+                '{' => self.open_block('{'),
+                '[' => self.open_block('['),
+                '}' => self.close_block('}'),
+                ']' => self.close_block(']'),
+                ';' => self.end_statement(),
+                ',' => self.write_comma(),
+                '.' => self.write_compact('.'),
+                ':' => self.write_spaced_operator(":"),
+                '(' => self.write_compact('('),
+                ')' => self.write_compact(')'),
+                '+' | '-' | '*' | '/' | '=' | '!' | '&' | '|' | '>' | '<' => {
+                    self.write_operator(ch)
+                }
+                _ => self.write_word(ch),
+            }
+        }
+        self.trim_trailing_spaces();
+        if !self.out.ends_with('\n') {
+            self.out.push('\n');
+        }
+        self.out
+    }
+
+    fn consume_whitespace(&mut self, ch: char) {
+        if ch == '\n' && self.out.ends_with('\n') && !self.out.ends_with("\n\n") {
+            self.out.push('\n');
+            self.line_start = true;
+        }
+    }
+
+    fn write_comment(&mut self) {
+        if self.line_start {
+            self.write_indent();
+        } else {
+            self.ensure_space();
+        }
+        self.out.push('#');
+        for ch in self.chars.by_ref() {
+            if ch == '\n' {
+                break;
+            }
+            self.out.push(ch);
+        }
+        self.trim_trailing_spaces();
+        self.out.push('\n');
+        self.line_start = true;
+    }
+
+    fn write_string(&mut self) {
+        self.write_indent_if_needed();
+        self.ensure_word_boundary();
+        self.out.push('"');
+        let mut escaped = false;
+        for ch in self.chars.by_ref() {
+            self.out.push(ch);
+            if escaped {
+                escaped = false;
+            } else if ch == '\\' {
+                escaped = true;
+            } else if ch == '"' {
+                break;
+            }
+        }
+    }
+
+    fn open_block(&mut self, ch: char) {
+        self.trim_trailing_spaces();
+        self.out.push(' ');
+        self.out.push(ch);
+        self.out.push('\n');
+        self.indent += INDENT;
+        self.line_start = true;
+    }
+
+    fn close_block(&mut self, ch: char) {
+        self.trim_trailing_spaces();
+        if !self.out.ends_with('\n') {
+            self.out.push('\n');
+        }
+        self.indent = self.indent.saturating_sub(INDENT);
+        self.write_indent();
+        self.out.push(ch);
+        self.line_start = false;
+    }
+
+    fn end_statement(&mut self) {
+        self.trim_trailing_spaces();
+        self.out.push(';');
+        self.out.push('\n');
+        self.line_start = true;
+    }
+
+    fn write_comma(&mut self) {
+        self.trim_trailing_spaces();
+        self.out.push(',');
+        self.out.push(' ');
+        self.line_start = false;
+    }
+
+    fn write_operator(&mut self, ch: char) {
+        let mut op = String::new();
+        op.push(ch);
+        if let Some(next) = self.chars.peek().copied() {
+            let two_char = matches!(
+                (ch, next),
+                ('=', '=')
+                    | ('!', '=')
+                    | ('>', '=')
+                    | ('<', '=')
+                    | ('+', '+')
+                    | ('/', '/')
+                    | ('&', '&')
+                    | ('|', '|')
+                    | ('=', '>')
+            );
+            if two_char {
+                op.push(next);
+                self.chars.next();
+            }
+        }
+        self.write_spaced_operator(&op);
+    }
+
+    fn write_spaced_operator(&mut self, op: &str) {
+        self.trim_trailing_spaces();
+        self.ensure_space();
+        self.out.push_str(op);
+        self.out.push(' ');
+        self.line_start = false;
+    }
+
+    fn write_compact(&mut self, ch: char) {
+        self.trim_trailing_spaces();
+        self.write_indent_if_needed();
+        self.out.push(ch);
+        self.line_start = false;
+    }
+
+    fn write_word(&mut self, first: char) {
+        self.write_indent_if_needed();
+        self.ensure_word_boundary();
+        self.out.push(first);
+        while let Some(next) = self.chars.peek().copied() {
+            if next.is_alphanumeric() || next == '_' {
+                self.out.push(next);
+                self.chars.next();
+            } else {
+                break;
+            }
+        }
+        self.line_start = false;
+    }
+
+    fn write_indent_if_needed(&mut self) {
+        if self.line_start {
+            self.write_indent();
+            self.line_start = false;
+        }
+    }
+
+    fn write_indent(&mut self) {
+        for _ in 0..self.indent {
+            self.out.push(' ');
+        }
+    }
+
+    fn ensure_word_boundary(&mut self) {
+        let needs_space = self
+            .out
+            .chars()
+            .last()
+            .is_some_and(|ch| ch.is_alphanumeric() || matches!(ch, '_' | '"'));
+        if needs_space {
+            self.out.push(' ');
+        }
+    }
+
+    fn ensure_space(&mut self) {
+        let needs_space = self
+            .out
+            .chars()
+            .last()
+            .is_some_and(|ch| !matches!(ch, ' ' | '\n' | '.' | '(' | '['));
+        if needs_space {
+            self.out.push(' ');
+        }
+    }
+
+    fn trim_trailing_spaces(&mut self) {
+        while self.out.ends_with(' ') || self.out.ends_with('\t') {
+            self.out.pop();
+        }
+    }
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+struct Formatter<'a> {
+    source: &'a str,
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+impl<'a> Formatter<'a> {
+    fn new(source: &'a str) -> Self {
+        Self { source }
+    }
+
+    fn format_source_file(&mut self, node: Node<'a>) -> String {
+        let mut out = String::new();
+        let children = named_children(node);
+        self.write_statement_list(&mut out, &children, 0);
+        out
+    }
+
+    fn write_statement_list(&mut self, out: &mut String, children: &[Node<'a>], indent: usize) {
+        let mut pending_comments = Vec::new();
+        let mut pending_blank = false;
+        let mut previous_end = 0;
+        let mut previous_statement_end_row = None;
+
+        for &child in children {
+            let is_trailing_comment = child.kind() == "comment"
+                && previous_statement_end_row == Some(child.start_position().row);
+            if previous_statement_end_row.is_some() && !is_trailing_comment {
+                if !out.ends_with('\n') {
+                    out.push('\n');
+                }
+                previous_statement_end_row = None;
+            }
+
+            if previous_end > 0 && has_blank_line(self.slice(previous_end, child.start_byte())) {
+                pending_blank = true;
+            }
+
+            if child.kind() == "comment" {
+                if is_trailing_comment {
+                    if !out.ends_with(' ') {
+                        out.push(' ');
+                    }
+                    out.push_str(self.raw_trimmed(child));
+                    out.push('\n');
+                    previous_statement_end_row = None;
+                } else {
+                    pending_comments.push(child);
+                }
+                previous_end = child.end_byte();
+                continue;
+            }
+
+            if pending_blank && !out.is_empty() && !out.ends_with("\n\n") {
+                out.push('\n');
+            }
+            pending_blank = false;
+
+            for comment in pending_comments.drain(..) {
+                write_indent(out, indent);
+                out.push_str(self.raw_trimmed(comment));
+                out.push('\n');
+            }
+
+            write_indent(out, indent);
+            self.write_statement(out, child, indent);
+            previous_statement_end_row = Some(child.end_position().row);
+            previous_end = child.end_byte();
+        }
+
+        if previous_statement_end_row.is_some() && !out.ends_with('\n') {
+            out.push('\n');
+        }
+
+        if pending_blank && !out.is_empty() && !out.ends_with("\n\n") {
+            out.push('\n');
+        }
+        for comment in pending_comments {
+            write_indent(out, indent);
+            out.push_str(self.raw_trimmed(comment));
+            out.push('\n');
+        }
+    }
+
+    fn write_statement(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        match node.kind() {
+            "field_definition" => {
+                self.write_field_definition(out, node, indent);
+                out.push(';');
+            }
+            _ => {
+                self.write_expr(out, node, indent, 0);
+                out.push(';');
+            }
+        }
+    }
+
+    fn write_field_definition(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        if let Some(path) = node.child_by_field_name("path") {
+            self.write_field_path(out, path);
+        } else {
+            out.push_str(self.raw_trimmed(node));
+            return;
+        }
+        out.push_str(" = ");
+        if let Some(value) = node.child_by_field_name("value") {
+            self.write_expr(out, value, indent, 0);
+        }
+    }
+
+    fn write_field_path(&mut self, out: &mut String, node: Node<'a>) {
+        let identifiers: Vec<_> = named_children(node)
+            .into_iter()
+            .filter(|child| child.kind() == "identifier")
+            .collect();
+        if identifiers.is_empty() {
+            out.push_str(self.raw_trimmed(node));
+            return;
+        }
+        for (index, identifier) in identifiers.into_iter().enumerate() {
+            if index > 0 {
+                out.push('.');
+            }
+            out.push_str(self.raw_trimmed(identifier));
+        }
+    }
+
+    fn write_expr(&mut self, out: &mut String, node: Node<'a>, indent: usize, parent_prec: u8) {
+        if expression_contains_comment(node)
+            && !matches!(
+                node.kind(),
+                "object" | "array" | "let_expression" | "match_expression"
+            )
+        {
+            out.push_str(self.raw_trimmed(node));
+            return;
+        }
+
+        let prec = self.precedence(node);
+        let parenthesize = prec < parent_prec;
+        if parenthesize {
+            out.push('(');
+        }
+
+        match node.kind() {
+            "literal" => {
+                if let Some(child) = named_children(node).first().copied() {
+                    out.push_str(self.raw_trimmed(child));
+                } else {
+                    out.push_str(self.raw_trimmed(node));
+                }
+            }
+            "identifier" | "string" | "integer" | "float" | "boolean" | "regex_literal" => {
+                out.push_str(self.raw_trimmed(node));
+            }
+            "comparison_constraint" => self.write_comparison_constraint(out, node, indent),
+            "object" => self.write_object(out, node, indent),
+            "array" => self.write_array(out, node, indent),
+            "let_expression" => self.write_let(out, node, indent),
+            "function_expression" => self.write_function(out, node, indent),
+            "match_expression" => self.write_match(out, node, indent),
+            "import_expression" => self.write_import(out, node),
+            "parenthesized_expression" => {
+                out.push('(');
+                if let Some(expr) = named_children(node)
+                    .into_iter()
+                    .find(|child| child.kind() != "comment")
+                {
+                    self.write_expr(out, expr, indent, 0);
+                }
+                out.push(')');
+            }
+            "call_expression" => self.write_call(out, node, indent),
+            "path_expression" => self.write_path(out, node, indent),
+            "unary_expression" => self.write_unary(out, node, indent),
+            "binary_expression" | "default_expression" => {
+                self.write_binary_like(out, node, indent, prec)
+            }
+            _ => out.push_str(self.raw_trimmed(node)),
+        }
+
+        if parenthesize {
+            out.push(')');
+        }
+    }
+
+    fn write_object(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        let children = named_children(node);
+        if children.is_empty() {
+            out.push_str("{}");
+            return;
+        }
+        out.push('{');
+        out.push('\n');
+        self.write_statement_list(out, &children, indent + INDENT);
+        write_indent(out, indent);
+        out.push('}');
+    }
+
+    fn write_array(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        let children = named_children(node);
+        if children.is_empty() {
+            out.push_str("[]");
+            return;
+        }
+        let has_comment = children.iter().any(|child| child.kind() == "comment");
+        let inline = !has_comment && children.iter().all(|child| is_inline_expr(*child));
+        if inline {
+            out.push('[');
+            for (index, child) in children.into_iter().enumerate() {
+                if index > 0 {
+                    out.push_str(", ");
+                }
+                self.write_expr(out, child, indent, 0);
+            }
+            out.push(']');
+            return;
+        }
+
+        out.push('[');
+        out.push('\n');
+        let mut pending_comments = Vec::new();
+        for child in children {
+            if child.kind() == "comment" {
+                pending_comments.push(child);
+                continue;
+            }
+            for comment in pending_comments.drain(..) {
+                write_indent(out, indent + INDENT);
+                out.push_str(self.raw_trimmed(comment));
+                out.push('\n');
+            }
+            write_indent(out, indent + INDENT);
+            self.write_expr(out, child, indent + INDENT, 0);
+            out.push_str(",\n");
+        }
+        for comment in pending_comments {
+            write_indent(out, indent + INDENT);
+            out.push_str(self.raw_trimmed(comment));
+            out.push('\n');
+        }
+        write_indent(out, indent);
+        out.push(']');
+    }
+
+    fn write_let(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        let body = node.child_by_field_name("body");
+        out.push_str("let");
+        out.push('\n');
+        let children: Vec<_> = named_children(node)
+            .into_iter()
+            .filter(|child| Some(*child) != body)
+            .collect();
+        self.write_statement_list(out, &children, indent + INDENT);
+        write_indent(out, indent);
+        out.push_str("in");
+        if let Some(body) = body {
+            out.push('\n');
+            write_indent(out, indent + INDENT);
+            self.write_expr(out, body, indent + INDENT, 0);
+        }
+    }
+
+    fn write_function(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        let body = node.child_by_field_name("body");
+        let params: Vec<_> = named_children(node)
+            .into_iter()
+            .filter(|child| child.kind() == "parameter")
+            .collect();
+        out.push('(');
+        for (index, param) in params.into_iter().enumerate() {
+            if index > 0 {
+                out.push_str(", ");
+            }
+            self.write_parameter(out, param, indent);
+        }
+        out.push_str(") =>");
+        if let Some(body) = body {
+            if is_inline_expr(body) {
+                out.push(' ');
+                self.write_expr(out, body, indent, 0);
+            } else {
+                out.push('\n');
+                write_indent(out, indent + INDENT);
+                self.write_expr(out, body, indent + INDENT, 0);
+            }
+        }
+    }
+
+    fn write_parameter(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        if let Some(name) = node.child_by_field_name("name") {
+            out.push_str(self.raw_trimmed(name));
+        }
+        if let Some(constraint) = node.child_by_field_name("constraint") {
+            out.push_str(": ");
+            self.write_expr(out, constraint, indent, 0);
+        }
+    }
+
+    fn write_match(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        out.push_str("match ");
+        if let Some(scrutinee) = node.child_by_field_name("scrutinee") {
+            self.write_expr(out, scrutinee, indent, 0);
+        }
+        out.push_str(" {");
+        let children: Vec<_> = named_children(node)
+            .into_iter()
+            .filter(|child| child.kind() == "match_arm" || child.kind() == "comment")
+            .collect();
+        if !children.is_empty() {
+            out.push('\n');
+            self.write_match_arms(out, &children, indent + INDENT);
+            write_indent(out, indent);
+        }
+        out.push('}');
+    }
+
+    fn write_match_arms(&mut self, out: &mut String, children: &[Node<'a>], indent: usize) {
+        let mut pending_comments = Vec::new();
+        for &child in children {
+            if child.kind() == "comment" {
+                pending_comments.push(child);
+                continue;
+            }
+            for comment in pending_comments.drain(..) {
+                write_indent(out, indent);
+                out.push_str(self.raw_trimmed(comment));
+                out.push('\n');
+            }
+            write_indent(out, indent);
+            if let Some(pattern) = child.child_by_field_name("pattern") {
+                self.write_expr(out, pattern, indent, 0);
+            }
+            out.push_str(": ");
+            if let Some(body) = child.child_by_field_name("body") {
+                self.write_expr(out, body, indent, 0);
+            }
+            out.push_str(";\n");
+        }
+        for comment in pending_comments {
+            write_indent(out, indent);
+            out.push_str(self.raw_trimmed(comment));
+            out.push('\n');
+        }
+    }
+
+    fn write_import(&mut self, out: &mut String, node: Node<'a>) {
+        out.push_str("import ");
+        if let Some(specifier) = node.child_by_field_name("specifier") {
+            out.push_str(self.raw_trimmed(specifier));
+        }
+    }
+
+    fn write_call(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        let callee = node.child_by_field_name("function");
+        if let Some(callee) = callee {
+            self.write_expr(out, callee, indent, self.precedence(node));
+        }
+        out.push('(');
+        let args: Vec<_> = named_children(node)
+            .into_iter()
+            .filter(|child| Some(*child) != callee && child.kind() != "comment")
+            .collect();
+        for (index, arg) in args.into_iter().enumerate() {
+            if index > 0 {
+                out.push_str(", ");
+            }
+            self.write_expr(out, arg, indent, 0);
+        }
+        out.push(')');
+    }
+
+    fn write_path(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        if let Some(object) = node.child_by_field_name("object") {
+            self.write_expr(out, object, indent, self.precedence(node));
+        }
+        out.push('.');
+        if let Some(field) = node.child_by_field_name("field") {
+            out.push_str(self.raw_trimmed(field));
+        }
+    }
+
+    fn write_unary(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        if let Some(operand) = node.child_by_field_name("operand") {
+            let operator = self.slice(node.start_byte(), operand.start_byte()).trim();
+            out.push_str(operator);
+            self.write_expr(out, operand, indent, self.precedence(node));
+        } else {
+            out.push_str(self.raw_trimmed(node));
+        }
+    }
+
+    fn write_comparison_constraint(&mut self, out: &mut String, node: Node<'a>, indent: usize) {
+        if let Some(value) = node.child_by_field_name("value") {
+            let operator = self.slice(node.start_byte(), value.start_byte()).trim();
+            out.push_str(operator);
+            out.push(' ');
+            self.write_expr(out, value, indent, self.precedence(node));
+        } else {
+            out.push_str(self.raw_trimmed(node));
+        }
+    }
+
+    fn write_binary_like(&mut self, out: &mut String, node: Node<'a>, indent: usize, prec: u8) {
+        let left = node
+            .child_by_field_name("left")
+            .or_else(|| node.child_by_field_name("base"));
+        let right = node
+            .child_by_field_name("right")
+            .or_else(|| node.child_by_field_name("fallback"));
+        let (Some(left), Some(right)) = (left, right) else {
+            out.push_str(self.raw_trimmed(node));
+            return;
+        };
+        self.write_expr(out, left, indent, prec);
+        out.push(' ');
+        if node.kind() == "default_expression" {
+            out.push_str("default");
+        } else {
+            out.push_str(self.slice(left.end_byte(), right.start_byte()).trim());
+        }
+        out.push(' ');
+        self.write_expr(out, right, indent, prec + 1);
+    }
+
+    fn precedence(&self, node: Node<'a>) -> u8 {
+        match node.kind() {
+            "default_expression" => 1,
+            "binary_expression" => self.binary_precedence(node),
+            "unary_expression" | "comparison_constraint" => 17,
+            "call_expression" | "path_expression" => 19,
+            _ => 20,
+        }
+    }
+
+    fn binary_precedence(&self, node: Node<'a>) -> u8 {
+        let Some(left) = node.child_by_field_name("left") else {
+            return 10;
+        };
+        let Some(right) = node.child_by_field_name("right") else {
+            return 10;
+        };
+        match self.slice(left.end_byte(), right.start_byte()).trim() {
+            "//" => 3,
+            "&" => 5,
+            "||" => 7,
+            "&&" => 9,
+            "==" | "!=" | ">" | ">=" | "<" | "<=" => 11,
+            "++" => 12,
+            "+" | "-" => 13,
+            "*" | "/" => 15,
+            _ => 10,
+        }
+    }
+
+    fn raw_trimmed(&self, node: Node<'a>) -> &'a str {
+        self.slice(node.start_byte(), node.end_byte()).trim()
+    }
+
+    fn slice(&self, start: usize, end: usize) -> &'a str {
+        &self.source[start..end]
+    }
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+fn named_children<'a>(node: Node<'a>) -> Vec<Node<'a>> {
+    let mut cursor: TreeCursor<'a> = node.walk();
+    node.named_children(&mut cursor).collect()
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+fn expression_contains_comment(node: Node<'_>) -> bool {
+    if node.kind() == "comment" {
+        return true;
+    }
+    named_children(node)
+        .into_iter()
+        .any(expression_contains_comment)
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+fn is_inline_expr(node: Node<'_>) -> bool {
+    match node.kind() {
+        "literal"
+        | "identifier"
+        | "string"
+        | "integer"
+        | "float"
+        | "boolean"
+        | "regex_literal"
+        | "comparison_constraint"
+        | "import_expression" => true,
+        "path_expression"
+        | "call_expression"
+        | "unary_expression"
+        | "binary_expression"
+        | "default_expression"
+        | "parenthesized_expression" => {
+            !expression_contains_comment(node)
+                && named_children(node).into_iter().all(is_inline_expr)
+        }
+        "array" => {
+            !expression_contains_comment(node)
+                && named_children(node).into_iter().all(is_inline_expr)
+        }
+        _ => false,
+    }
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+fn has_blank_line(text: &str) -> bool {
+    text.bytes().filter(|byte| *byte == b'\n').count() >= 2
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+fn write_indent(out: &mut String, indent: usize) {
+    for _ in 0..indent {
+        out.push(' ');
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn formats_fields_and_preserves_comments() {
+        let source =
+            "Server={\n# host\nhost=String default \"localhost\"; # trailing\n\nport=Int&>0;\n};";
+        let formatted = format_source(source).unwrap();
+        assert_eq!(
+            formatted,
+            "Server = {\n    # host\n    host = String default \"localhost\"; # trailing\n\n    port = Int & > 0;\n};\n"
+        );
+    }
+
+    #[test]
+    fn formats_arrays_and_match_comments() {
+        let source = "result=match x{# c\n_: [1,{a=2;}];};";
+        let formatted = format_source(source).unwrap();
+        assert_eq!(
+            formatted,
+            "result = match x {\n    # c\n    _: [\n        1,\n        {\n            a = 2;\n        },\n    ];\n};\n"
+        );
+    }
+
+    #[test]
+    fn wasm_export_returns_json() {
+        let output = format_source_json("value={a=1;};");
+        assert!(output.contains("\"ok\":true"));
+        assert!(output.contains("value = {\\n    a = 1;\\n};\\n"));
+    }
+}
diff --git a/doc/manual/souce/components.md b/doc/manual/souce/components.md
index 83f50b3..92493c8 100644
--- a/doc/manual/souce/components.md
+++ b/doc/manual/souce/components.md
@@ -39,6 +39,24 @@ The JSR package is `@hare/decodal-wasm`.
 The generated files in `packages/decodal-wasm/` are committed so the site can build without requiring every consumer to run `wasm-pack` first.
 The WebAssembly package is for execution, not syntax highlighting.
 
+## Language tools
+
+Source-level tooling lives in the Rust language tools crate and its generated JavaScript/WebAssembly package.
+This component is responsible for operations that must preserve source text details such as comments and whitespace.
+
+Important paths:
+
+```text
+crates/decodal-language-tools/
+packages/decodal-language-tools/
+```
+
+The npm package is `decodal-language-tools`.
+The JSR package is `@hare/decodal-language-tools`.
+
+The current language tools package exposes the formatter.
+Future LSP functionality should build on the same source-level tooling layer rather than on the runtime core.
+
 ## Web editor components
 
 The Web playground editor uses CodeMirror 6 with a generated Lezer parser.
@@ -94,6 +112,7 @@ Consumers that need syntax information should use the component that matches the
 
 - Rust execution and embedding: `decodal`
 - Browser execution: `decodal-wasm`
+- Source formatting and future LSP tooling: `decodal-language-tools`
 - Web editor syntax: Lezer / CodeMirror
 - General editor syntax: Tree-sitter
 
diff --git a/doc/manual/souce/development.md b/doc/manual/souce/development.md
index 514920a..1e92c0a 100644
--- a/doc/manual/souce/development.md
+++ b/doc/manual/souce/development.md
@@ -24,8 +24,8 @@ cargo run -q -p decodal-cli --features regex -- examples/regex/main.dcdl
 
 The primary crates.io package is `decodal`, which contains the embeddable library.
 `decodal-derive` provides optional derive macros for Rust struct integration and is published only when the derive crate changes.
-Workspace support crates such as `decodal-cli` and the Rust source crate `decodal-wasm` are not published to crates.io.
-The generated WebAssembly package under `packages/decodal-wasm/` is published to npm and JSR.
+Workspace support crates such as `decodal-cli`, the Rust source crate `decodal-wasm`, and `decodal-language-tools` are not published to crates.io.
+The generated WebAssembly packages under `packages/decodal-wasm/` and `packages/decodal-language-tools/` are published to npm and JSR.
 The project is dual licensed as `MIT OR Apache-2.0`.
 
 ```sh
@@ -58,7 +58,7 @@ site/decodal-site/
 ```
 
 The site imports Markdown files from `doc/manual/souce/` and renders them as mdBook-style pages.
-The playground loads `decodal-wasm` and evaluates DCDL entirely in the browser.
+The playground loads `decodal-wasm` for evaluation and `decodal-language-tools` for source formatting.
 
 Important files:
 
@@ -74,25 +74,25 @@ packages/decodal-codemirror/src/decodal.js
 packages/decodal-codemirror/src/decodal-parser.js
 packages/decodal-wasm/decodal_wasm.js
 packages/decodal-wasm/decodal_wasm_bg.wasm
+packages/decodal-language-tools/decodal_language_tools.js
+packages/decodal-language-tools/decodal_language_tools_bg.wasm
 crates/decodal-wasm/src/lib.rs
+crates/decodal-language-tools/src/lib.rs
 ```
 
-Build the WebAssembly package before building the site:
+Build the WebAssembly packages before building the site:
 
 ```sh
 cd site/decodal-site
 npm install
-npm run build:wasm
+npm run build:packages
 npm run build
 ```
 
-`npm run build:wasm` writes generated files into:
+`npm run build:wasm` writes generated runtime files into `packages/decodal-wasm/`.
+`npm run build:tools` writes generated language tools files into `packages/decodal-language-tools/`.
 
-```text
-packages/decodal-wasm/
-```
-
-These generated files are committed so the site can be built without requiring every consumer to regenerate the wasm package first.
+These generated files are committed so the site can be built without requiring every consumer to regenerate the wasm packages first.
 
 Publish the generated WebAssembly package from its package directory:
 
@@ -113,6 +113,24 @@ The npm package name is `decodal-wasm`.
 The JSR package name is `@hare/decodal-wasm`.
 JSR currently expects a single SPDX license identifier in `jsr.json`; the WebAssembly package metadata uses `MIT` while the Rust workspace remains dual licensed as `MIT OR Apache-2.0`.
 
+Publish the generated language tools package from its package directory:
+
+```sh
+cd packages/decodal-language-tools
+npm publish
+npx jsr publish
+```
+
+Run dry-runs first when preparing a release:
+
+```sh
+npm pack --dry-run
+npx jsr publish --dry-run
+```
+
+The npm package name is `decodal-language-tools`.
+The JSR package name is `@hare/decodal-language-tools`.
+
 The playground editor uses the `decodal-codemirror` package with the generated Lezer parser in `packages/decodal-codemirror/src/decodal-parser.js`.
 The canonical grammar is documented in `doc/manual/souce/language/grammar.md`; regenerate the Lezer parser when that grammar or `editors/lezer-decodal/decodal.grammar` changes.
 
diff --git a/packages/decodal-language-tools/.gitignore b/packages/decodal-language-tools/.gitignore
new file mode 100644
index 0000000..4b2f020
--- /dev/null
+++ b/packages/decodal-language-tools/.gitignore
@@ -0,0 +1 @@
+# wasm-pack output is committed for the browser language tools package.
diff --git a/packages/decodal-language-tools/README.md b/packages/decodal-language-tools/README.md
new file mode 100644
index 0000000..edb0e34
--- /dev/null
+++ b/packages/decodal-language-tools/README.md
@@ -0,0 +1,39 @@
+# decodal-language-tools
+
+Source-level language tooling for Decodal.
+
+This package is generated from the Rust crate in `crates/decodal-language-tools` with `wasm-pack`.
+It currently exposes a comment-preserving formatter that is shared by the official playground and future editor/LSP tooling.
+
+Use `decodal-codemirror` separately when you need CodeMirror 6 language support.
+Use `decodal-wasm` separately when you need to evaluate Decodal in a browser.
+
+## Install
+
+```sh
+npm install decodal-language-tools
+```
+
+JSR:
+
+```sh
+deno add jsr:@hare/decodal-language-tools
+```
+
+## Usage
+
+```js
+import init, { formatSource } from 'decodal-language-tools';
+
+await init();
+
+const result = JSON.parse(formatSource('Server={# host\nhost=String default "localhost";};'));
+
+if (result.ok) {
+  console.log(result.source);
+} else {
+  console.error(result.error);
+}
+```
+
+The exported functions return JSON strings so callers can handle diagnostics without depending on Rust data structures.
diff --git a/packages/decodal-language-tools/decodal_language_tools.d.ts b/packages/decodal-language-tools/decodal_language_tools.d.ts
new file mode 100644
index 0000000..a9c99e0
--- /dev/null
+++ b/packages/decodal-language-tools/decodal_language_tools.d.ts
@@ -0,0 +1,38 @@
+/* tslint:disable */
+/* eslint-disable */
+
+export function formatSource(source: string): string;
+
+export type InitInput = RequestInfo | URL | Response | BufferSource | WebAssembly.Module;
+
+export interface InitOutput {
+    readonly memory: WebAssembly.Memory;
+    readonly formatSource: (a: number, b: number) => [number, number];
+    readonly __wbindgen_externrefs: WebAssembly.Table;
+    readonly __wbindgen_malloc: (a: number, b: number) => number;
+    readonly __wbindgen_realloc: (a: number, b: number, c: number, d: number) => number;
+    readonly __wbindgen_free: (a: number, b: number, c: number) => void;
+    readonly __wbindgen_start: () => void;
+}
+
+export type SyncInitInput = BufferSource | WebAssembly.Module;
+
+/**
+ * Instantiates the given `module`, which can either be bytes or
+ * a precompiled `WebAssembly.Module`.
+ *
+ * @param {{ module: SyncInitInput }} module - Passing `SyncInitInput` directly is deprecated.
+ *
+ * @returns {InitOutput}
+ */
+export function initSync(module: { module: SyncInitInput } | SyncInitInput): InitOutput;
+
+/**
+ * If `module_or_path` is {RequestInfo} or {URL}, makes a request and
+ * for everything else, calls `WebAssembly.instantiate` directly.
+ *
+ * @param {{ module_or_path: InitInput | Promise<InitInput> }} module_or_path - Passing `InitInput` directly is deprecated.
+ *
+ * @returns {Promise<InitOutput>}
+ */
+export default function __wbg_init (module_or_path?: { module_or_path: InitInput | Promise<InitInput> } | InitInput | Promise<InitInput>): Promise<InitOutput>;
diff --git a/packages/decodal-language-tools/decodal_language_tools.js b/packages/decodal-language-tools/decodal_language_tools.js
new file mode 100644
index 0000000..47a21f1
--- /dev/null
+++ b/packages/decodal-language-tools/decodal_language_tools.js
@@ -0,0 +1,209 @@
+/* @ts-self-types="./decodal_language_tools.d.ts" */
+
+/**
+ * @param {string} source
+ * @returns {string}
+ */
+export function formatSource(source) {
+    let deferred2_0;
+    let deferred2_1;
+    try {
+        const ptr0 = passStringToWasm0(source, wasm.__wbindgen_malloc, wasm.__wbindgen_realloc);
+        const len0 = WASM_VECTOR_LEN;
+        const ret = wasm.formatSource(ptr0, len0);
+        deferred2_0 = ret[0];
+        deferred2_1 = ret[1];
+        return getStringFromWasm0(ret[0], ret[1]);
+    } finally {
+        wasm.__wbindgen_free(deferred2_0, deferred2_1, 1);
+    }
+}
+function __wbg_get_imports() {
+    const import0 = {
+        __proto__: null,
+        __wbindgen_init_externref_table: function() {
+            const table = wasm.__wbindgen_externrefs;
+            const offset = table.grow(4);
+            table.set(0, undefined);
+            table.set(offset + 0, undefined);
+            table.set(offset + 1, null);
+            table.set(offset + 2, true);
+            table.set(offset + 3, false);
+        },
+    };
+    return {
+        __proto__: null,
+        "./decodal_language_tools_bg.js": import0,
+    };
+}
+
+function getStringFromWasm0(ptr, len) {
+    return decodeText(ptr >>> 0, len);
+}
+
+let cachedUint8ArrayMemory0 = null;
+function getUint8ArrayMemory0() {
+    if (cachedUint8ArrayMemory0 === null || cachedUint8ArrayMemory0.byteLength === 0) {
+        cachedUint8ArrayMemory0 = new Uint8Array(wasm.memory.buffer);
+    }
+    return cachedUint8ArrayMemory0;
+}
+
+function passStringToWasm0(arg, malloc, realloc) {
+    if (realloc === undefined) {
+        const buf = cachedTextEncoder.encode(arg);
+        const ptr = malloc(buf.length, 1) >>> 0;
+        getUint8ArrayMemory0().subarray(ptr, ptr + buf.length).set(buf);
+        WASM_VECTOR_LEN = buf.length;
+        return ptr;
+    }
+
+    let len = arg.length;
+    let ptr = malloc(len, 1) >>> 0;
+
+    const mem = getUint8ArrayMemory0();
+
+    let offset = 0;
+
+    for (; offset < len; offset++) {
+        const code = arg.charCodeAt(offset);
+        if (code > 0x7F) break;
+        mem[ptr + offset] = code;
+    }
+    if (offset !== len) {
+        if (offset !== 0) {
+            arg = arg.slice(offset);
+        }
+        ptr = realloc(ptr, len, len = offset + arg.length * 3, 1) >>> 0;
+        const view = getUint8ArrayMemory0().subarray(ptr + offset, ptr + len);
+        const ret = cachedTextEncoder.encodeInto(arg, view);
+
+        offset += ret.written;
+        ptr = realloc(ptr, len, offset, 1) >>> 0;
+    }
+
+    WASM_VECTOR_LEN = offset;
+    return ptr;
+}
+
+let cachedTextDecoder = new TextDecoder('utf-8', { ignoreBOM: true, fatal: true });
+cachedTextDecoder.decode();
+const MAX_SAFARI_DECODE_BYTES = 2146435072;
+let numBytesDecoded = 0;
+function decodeText(ptr, len) {
+    numBytesDecoded += len;
+    if (numBytesDecoded >= MAX_SAFARI_DECODE_BYTES) {
+        cachedTextDecoder = new TextDecoder('utf-8', { ignoreBOM: true, fatal: true });
+        cachedTextDecoder.decode();
+        numBytesDecoded = len;
+    }
+    return cachedTextDecoder.decode(getUint8ArrayMemory0().subarray(ptr, ptr + len));
+}
+
+const cachedTextEncoder = new TextEncoder();
+
+if (!('encodeInto' in cachedTextEncoder)) {
+    cachedTextEncoder.encodeInto = function (arg, view) {
+        const buf = cachedTextEncoder.encode(arg);
+        view.set(buf);
+        return {
+            read: arg.length,
+            written: buf.length
+        };
+    };
+}
+
+let WASM_VECTOR_LEN = 0;
+
+let wasmModule, wasmInstance, wasm;
+function __wbg_finalize_init(instance, module) {
+    wasmInstance = instance;
+    wasm = instance.exports;
+    wasmModule = module;
+    cachedUint8ArrayMemory0 = null;
+    wasm.__wbindgen_start();
+    return wasm;
+}
+
+async function __wbg_load(module, imports) {
+    if (typeof Response === 'function' && module instanceof Response) {
+        if (typeof WebAssembly.instantiateStreaming === 'function') {
+            try {
+                return await WebAssembly.instantiateStreaming(module, imports);
+            } catch (e) {
+                const validResponse = module.ok && expectedResponseType(module.type);
+
+                if (validResponse && module.headers.get('Content-Type') !== 'application/wasm') {
+                    console.warn("`WebAssembly.instantiateStreaming` failed because your server does not serve Wasm with `application/wasm` MIME type. Falling back to `WebAssembly.instantiate` which is slower. Original error:\n", e);
+
+                } else { throw e; }
+            }
+        }
+
+        const bytes = await module.arrayBuffer();
+        return await WebAssembly.instantiate(bytes, imports);
+    } else {
+        const instance = await WebAssembly.instantiate(module, imports);
+
+        if (instance instanceof WebAssembly.Instance) {
+            return { instance, module };
+        } else {
+            return instance;
+        }
+    }
+
+    function expectedResponseType(type) {
+        switch (type) {
+            case 'basic': case 'cors': case 'default': return true;
+        }
+        return false;
+    }
+}
+
+function initSync(module) {
+    if (wasm !== undefined) return wasm;
+
+
+    if (module !== undefined) {
+        if (Object.getPrototypeOf(module) === Object.prototype) {
+            ({module} = module)
+        } else {
+            console.warn('using deprecated parameters for `initSync()`; pass a single object instead')
+        }
+    }
+
+    const imports = __wbg_get_imports();
+    if (!(module instanceof WebAssembly.Module)) {
+        module = new WebAssembly.Module(module);
+    }
+    const instance = new WebAssembly.Instance(module, imports);
+    return __wbg_finalize_init(instance, module);
+}
+
+async function __wbg_init(module_or_path) {
+    if (wasm !== undefined) return wasm;
+
+
+    if (module_or_path !== undefined) {
+        if (Object.getPrototypeOf(module_or_path) === Object.prototype) {
+            ({module_or_path} = module_or_path)
+        } else {
+            console.warn('using deprecated parameters for the initialization function; pass a single object instead')
+        }
+    }
+
+    if (module_or_path === undefined) {
+        module_or_path = new URL('decodal_language_tools_bg.wasm', import.meta.url);
+    }
+    const imports = __wbg_get_imports();
+
+    if (typeof module_or_path === 'string' || (typeof Request === 'function' && module_or_path instanceof Request) || (typeof URL === 'function' && module_or_path instanceof URL)) {
+        module_or_path = fetch(module_or_path);
+    }
+
+    const { instance, module } = await __wbg_load(await module_or_path, imports);
+
+    return __wbg_finalize_init(instance, module);
+}
+
+export { initSync, __wbg_init as default };
diff --git a/packages/decodal-language-tools/decodal_language_tools_bg.wasm b/packages/decodal-language-tools/decodal_language_tools_bg.wasm
new file mode 100644
index 0000000..a8446a4
Binary files /dev/null and b/packages/decodal-language-tools/decodal_language_tools_bg.wasm differ
diff --git a/packages/decodal-language-tools/decodal_language_tools_bg.wasm.d.ts b/packages/decodal-language-tools/decodal_language_tools_bg.wasm.d.ts
new file mode 100644
index 0000000..e2142cd
--- /dev/null
+++ b/packages/decodal-language-tools/decodal_language_tools_bg.wasm.d.ts
@@ -0,0 +1,9 @@
+/* tslint:disable */
+/* eslint-disable */
+export const memory: WebAssembly.Memory;
+export const formatSource: (a: number, b: number) => [number, number];
+export const __wbindgen_externrefs: WebAssembly.Table;
+export const __wbindgen_malloc: (a: number, b: number) => number;
+export const __wbindgen_realloc: (a: number, b: number, c: number, d: number) => number;
+export const __wbindgen_free: (a: number, b: number, c: number) => void;
+export const __wbindgen_start: () => void;
diff --git a/packages/decodal-language-tools/jsr.json b/packages/decodal-language-tools/jsr.json
new file mode 100644
index 0000000..bc89ddf
--- /dev/null
+++ b/packages/decodal-language-tools/jsr.json
@@ -0,0 +1,15 @@
+{
+  "name": "@hare/decodal-language-tools",
+  "version": "0.1.2",
+  "license": "MIT",
+  "exports": "./decodal_language_tools.js",
+  "publish": {
+    "include": [
+      "README.md",
+      "decodal_language_tools.js",
+      "decodal_language_tools.d.ts",
+      "decodal_language_tools_bg.wasm",
+      "package.json"
+    ]
+  }
+}
diff --git a/packages/decodal-language-tools/package.json b/packages/decodal-language-tools/package.json
new file mode 100644
index 0000000..1711d5d
--- /dev/null
+++ b/packages/decodal-language-tools/package.json
@@ -0,0 +1,27 @@
+{
+  "name": "decodal-language-tools",
+  "type": "module",
+  "description": "Source-level language tooling for Decodal.",
+  "version": "0.1.2",
+  "license": "MIT OR Apache-2.0",
+  "repository": {
+    "type": "git",
+    "url": "https://gitea.hareworks.net/Hare/Decodal"
+  },
+  "files": [
+    "decodal_language_tools_bg.wasm",
+    "decodal_language_tools.js",
+    "decodal_language_tools.d.ts"
+  ],
+  "main": "decodal_language_tools.js",
+  "types": "decodal_language_tools.d.ts",
+  "sideEffects": [
+    "./snippets/*"
+  ],
+  "keywords": [
+    "decodal",
+    "formatter",
+    "lsp",
+    "tree-sitter"
+  ]
+}
diff --git a/site/decodal-site/package-lock.json b/site/decodal-site/package-lock.json
index 6abbea4..835852b 100644
--- a/site/decodal-site/package-lock.json
+++ b/site/decodal-site/package-lock.json
@@ -16,6 +16,7 @@
         "astro": "^4.16.18",
         "codemirror": "^6.0.2",
         "decodal-codemirror": "file:../../packages/decodal-codemirror",
+        "decodal-language-tools": "file:../../packages/decodal-language-tools",
         "decodal-wasm": "file:../../packages/decodal-wasm",
         "marked": "^12.0.2"
       },
@@ -33,6 +34,10 @@
         "@lezer/lr": "^1.4.10"
       }
     },
+    "../../packages/decodal-language-tools": {
+      "version": "0.1.2",
+      "license": "MIT OR Apache-2.0"
+    },
     "../../packages/decodal-wasm": {
       "version": "0.1.2",
       "license": "MIT OR Apache-2.0"
@@ -3255,6 +3260,10 @@
       "resolved": "../../packages/decodal-codemirror",
       "link": true
     },
+    "node_modules/decodal-language-tools": {
+      "resolved": "../../packages/decodal-language-tools",
+      "link": true
+    },
     "node_modules/decodal-wasm": {
       "resolved": "../../packages/decodal-wasm",
       "link": true
diff --git a/site/decodal-site/package.json b/site/decodal-site/package.json
index ac74f41..798f9d4 100644
--- a/site/decodal-site/package.json
+++ b/site/decodal-site/package.json
@@ -9,7 +9,9 @@
     "preview": "astro preview --host 0.0.0.0",
     "build:wasm": "wasm-pack build ../../crates/decodal-wasm --target web --out-dir ../../packages/decodal-wasm --release && node scripts/prepare-wasm-package.mjs",
     "deploy": "npm run build && node scripts/deploy-pages.mjs",
-    "deploy:wasm": "npm run build:wasm && npm run deploy"
+    "deploy:wasm": "npm run build:wasm && npm run deploy",
+    "build:tools": "wasm-pack build ../../crates/decodal-language-tools --target web --out-dir ../../packages/decodal-language-tools --release && node scripts/prepare-language-tools-package.mjs",
+    "build:packages": "npm run build:wasm && npm run build:tools"
   },
   "dependencies": {
     "@astrojs/check": "^0.9.4",
@@ -20,6 +22,7 @@
     "astro": "^4.16.18",
     "codemirror": "^6.0.2",
     "decodal-codemirror": "file:../../packages/decodal-codemirror",
+    "decodal-language-tools": "file:../../packages/decodal-language-tools",
     "decodal-wasm": "file:../../packages/decodal-wasm",
     "marked": "^12.0.2"
   },
diff --git a/site/decodal-site/scripts/prepare-language-tools-package.mjs b/site/decodal-site/scripts/prepare-language-tools-package.mjs
new file mode 100644
index 0000000..2ebceb1
--- /dev/null
+++ b/site/decodal-site/scripts/prepare-language-tools-package.mjs
@@ -0,0 +1,76 @@
+import { writeFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+
+const packageDir = resolve(import.meta.dirname, '../../../packages/decodal-language-tools');
+
+writeFileSync(
+  resolve(packageDir, '.gitignore'),
+  '# wasm-pack output is committed for the browser language tools package.\n',
+);
+
+writeFileSync(
+  resolve(packageDir, 'README.md'),
+  `# decodal-language-tools
+
+Source-level language tooling for Decodal.
+
+This package is generated from the Rust crate in \`crates/decodal-language-tools\` with \`wasm-pack\`.
+It currently exposes a comment-preserving formatter that is shared by the official playground and future editor/LSP tooling.
+
+Use \`decodal-codemirror\` separately when you need CodeMirror 6 language support.
+Use \`decodal-wasm\` separately when you need to evaluate Decodal in a browser.
+
+## Install
+
+\`\`\`sh
+npm install decodal-language-tools
+\`\`\`
+
+JSR:
+
+\`\`\`sh
+deno add jsr:@hare/decodal-language-tools
+\`\`\`
+
+## Usage
+
+\`\`\`js
+import init, { formatSource } from 'decodal-language-tools';
+
+await init();
+
+const result = JSON.parse(formatSource('Server={# host\\nhost=String default "localhost";};'));
+
+if (result.ok) {
+  console.log(result.source);
+} else {
+  console.error(result.error);
+}
+\`\`\`
+
+The exported functions return JSON strings so callers can handle diagnostics without depending on Rust data structures.
+`,
+);
+
+writeFileSync(
+  resolve(packageDir, 'jsr.json'),
+  JSON.stringify(
+    {
+      name: '@hare/decodal-language-tools',
+      version: '0.1.2',
+      license: 'MIT',
+      exports: './decodal_language_tools.js',
+      publish: {
+        include: [
+          'README.md',
+          'decodal_language_tools.js',
+          'decodal_language_tools.d.ts',
+          'decodal_language_tools_bg.wasm',
+          'package.json',
+        ],
+      },
+    },
+    null,
+    2,
+  ) + '\n',
+);
diff --git a/site/decodal-site/src/pages/index.astro b/site/decodal-site/src/pages/index.astro
index bc726d7..9e5009a 100644
--- a/site/decodal-site/src/pages/index.astro
+++ b/site/decodal-site/src/pages/index.astro
@@ -40,6 +40,14 @@ Production = Server & {
         <span>JSR package</span>
         <strong>jsr.io/@hare/decodal-wasm</strong>
       </a>
+      <a href="https://www.npmjs.com/package/decodal-language-tools">
+        <span>Language tools package</span>
+        <strong>npmjs.com/package/decodal-language-tools</strong>
+      </a>
+      <a href="https://jsr.io/@hare/decodal-language-tools@0.1.2">
+        <span>Language tools on JSR</span>
+        <strong>jsr.io/@hare/decodal-language-tools</strong>
+      </a>
       <a href="https://www.npmjs.com/package/decodal-codemirror">
         <span>CodeMirror package</span>
         <strong>npmjs.com/package/decodal-codemirror</strong>
@@ -62,8 +70,8 @@ Production = Server & {
     <article class="home-card">
       <h2>Where it runs</h2>
       <p>
-        Use the Rust crate for runtime embedding, the WASM package in browsers, CodeMirror/Lezer for the Web editor,
-        and Tree-sitter for general editor integration.
+        Use the Rust crate for runtime embedding, the WASM package in browsers, language-tools for formatting,
+        CodeMirror/Lezer for the Web editor, and Tree-sitter for general editor integration.
       </p>
     </article>
     <article class="home-card">
diff --git a/site/decodal-site/src/pages/playground.astro b/site/decodal-site/src/pages/playground.astro
index 09af8ab..1a179a5 100644
--- a/site/decodal-site/src/pages/playground.astro
+++ b/site/decodal-site/src/pages/playground.astro
@@ -15,6 +15,7 @@ import ManualLayout from '../layouts/ManualLayout.astro';
         </label>
         <button id="load-example" type="button">Load</button>
         <p id="status" class="status">Loading WASM...</p>
+        <button id="format" disabled>Format</button>
         <button id="run" disabled>Run</button>
       </div>
     </div>
diff --git a/site/decodal-site/src/scripts/playground.js b/site/decodal-site/src/scripts/playground.js
index 40c3447..a1cc8e1 100644
--- a/site/decodal-site/src/scripts/playground.js
+++ b/site/decodal-site/src/scripts/playground.js
@@ -1,4 +1,5 @@
-import init, { evaluateProject } from 'decodal-wasm';
+import initRuntime, { evaluateProject } from 'decodal-wasm';
+import initTools, { formatSource } from 'decodal-language-tools';
 import { EditorView, basicSetup } from 'codemirror';
 import { keymap } from '@codemirror/view';
 import { decodal } from 'decodal-codemirror';
@@ -10,6 +11,7 @@ const starterProject = playgroundExamples[0];
 const editorHost = document.getElementById('editor');
 const output = document.getElementById('output');
 const run = document.getElementById('run');
+const formatButton = document.getElementById('format');
 const status = document.getElementById('status');
 const fileTree = document.getElementById('file-tree');
 const activeFile = document.getElementById('active-file');
@@ -176,6 +178,20 @@ function execute() {
   output.classList.toggle('error', !result.ok);
 }
 
+function formatActiveFile() {
+  const result = JSON.parse(formatSource(getEditorText()));
+  if (!result.ok) {
+    output.textContent = result.error;
+    output.classList.add('error');
+    return;
+  }
+  setEditorText(result.source);
+  project.files[project.activePath] = result.source;
+  saveProject();
+  output.textContent = 'Formatted.';
+  output.classList.remove('error');
+}
+
 function renderFileTree() {
   const tree = buildTree(Object.keys(project.files).sort());
   fileTree.replaceChildren(renderTreeList(tree.children));
@@ -227,8 +243,9 @@ function compareNodes(a, b) {
 }
 
 try {
-  await init();
+  await Promise.all([initRuntime(), initTools()]);
   run.disabled = false;
+  formatButton.disabled = false;
   status.textContent = '';
   execute();
 } catch (error) {
@@ -236,6 +253,7 @@ try {
 }
 
 run.addEventListener('click', execute);
+formatButton.addEventListener('click', formatActiveFile);
 loadExample.addEventListener('click', () => {
   const example = playgroundExamples.find((item) => item.id === exampleSelect.value);
   if (!example) return;