diff --git a/hosts/ocaml/bin/dune b/hosts/ocaml/bin/dune
index b94d477f..1d28418b 100644
--- a/hosts/ocaml/bin/dune
+++ b/hosts/ocaml/bin/dune
@@ -5,3 +5,7 @@
 (executable
  (name mcp_tree)
  (libraries sx unix yojson str))
+
+(executable
+ (name test_cst)
+ (libraries sx))
diff --git a/hosts/ocaml/bin/test_cst.ml b/hosts/ocaml/bin/test_cst.ml
new file mode 100644
index 00000000..d875515b
--- /dev/null
+++ b/hosts/ocaml/bin/test_cst.ml
@@ -0,0 +1,91 @@
+let () =
+  let test_sources = [
+    "(define foo 42)";
+    ";; comment\n(define bar 1)\n\n;; another\n(define baz 2)\n";
+    "(define my-fn\n  (fn (x)\n    ;; check nil\n    (if (nil? x) 0 x)))";
+    "(list 1 2 3)";
+    "{:key \"value\" :num 42}";
+    "'(a b c)";
+    "`(a ,b ,@c)";
+    "(define x \"hello\\nworld\")";
+    ";; top\n;; multi-line\n(define a 1)\n";
+    "";
+    "  \n  ";
+    "(a)\n(b)\n(c)";
+    "(a ;; inline\n  b)";
+  ] in
+  let pass = ref 0 in
+  let fail = ref 0 in
+  List.iter (fun src ->
+    let cst = Sx_parser.parse_all_cst src in
+    let roundtrip = Sx_cst.cst_file_to_source cst.nodes cst.trailing_trivia in
+    if roundtrip = src then begin
+      incr pass;
+      Printf.printf "PASS: %S\n" (if String.length src > 40 then String.sub src 0 40 ^ "..." else src)
+    end else begin
+      incr fail;
+      Printf.printf "FAIL: %S\n  expected: %S\n  got:      %S\n"
+        (if String.length src > 40 then String.sub src 0 40 ^ "..." else src)
+        src roundtrip
+    end
+  ) test_sources;
+  (* Also test CST→AST matches AST parser *)
+  let ast_tests = [
+    "(define foo 42)";
+    "(list 1 2 3)";
+    "{:key \"value\"}";
+    ";; comment\n(define bar 1)";
+  ] in
+  Printf.printf "\nCST→AST equivalence:\n";
+  List.iter (fun src ->
+    let ast_direct = Sx_parser.parse_all src in
+    let cst = Sx_parser.parse_all_cst src in
+    let ast_via_cst = List.map Sx_cst.cst_to_ast cst.nodes in
+    let s1 = String.concat " " (List.map Sx_types.inspect ast_direct) in
+    let s2 = String.concat " " (List.map Sx_types.inspect ast_via_cst) in
+    if s1 = s2 then begin
+      incr pass;
+      Printf.printf "PASS: %S\n" src
+    end else begin
+      incr fail;
+      Printf.printf "FAIL: %S\n  AST:     %s\n  CST→AST: %s\n" src s1 s2
+    end
+  ) ast_tests;
+  (* Test real .sx files from the codebase *)
+  Printf.printf "\nReal file round-trips:\n";
+  let test_file path =
+    try
+      let src = In_channel.with_open_text path In_channel.input_all in
+      let cst = Sx_parser.parse_all_cst src in
+      let roundtrip = Sx_cst.cst_file_to_source cst.nodes cst.trailing_trivia in
+      if roundtrip = src then begin
+        incr pass;
+        Printf.printf "PASS: %s (%d bytes)\n" path (String.length src)
+      end else begin
+        incr fail;
+        (* Find first difference *)
+        let len = min (String.length src) (String.length roundtrip) in
+        let diff_pos = ref len in
+        for i = 0 to len - 1 do
+          if src.[i] <> roundtrip.[i] && !diff_pos = len then diff_pos := i
+        done;
+        Printf.printf "FAIL: %s (diff at byte %d, src=%d rt=%d)\n" path !diff_pos (String.length src) (String.length roundtrip)
+      end
+    with e ->
+      incr fail;
+      Printf.printf "ERROR: %s — %s\n" path (Printexc.to_string e)
+  in
+  let spec_dir = try Sys.getenv "SX_SPEC_DIR" with Not_found -> "spec" in
+  let project_dir = try Sys.getenv "SX_PROJECT_DIR" with Not_found -> "." in
+  List.iter test_file [
+    spec_dir ^ "/evaluator.sx";
+    spec_dir ^ "/parser.sx";
+    spec_dir ^ "/primitives.sx";
+    spec_dir ^ "/render.sx";
+    project_dir ^ "/lib/tree-tools.sx";
+    project_dir ^ "/web/engine.sx";
+    project_dir ^ "/web/io.sx";
+  ];
+
+  Printf.printf "\n%d/%d passed\n" !pass (!pass + !fail);
+  if !fail > 0 then exit 1
diff --git a/hosts/ocaml/lib/sx_cst.ml b/hosts/ocaml/lib/sx_cst.ml
new file mode 100644
index 00000000..b73f4594
--- /dev/null
+++ b/hosts/ocaml/lib/sx_cst.ml
@@ -0,0 +1,146 @@
+(** Concrete Syntax Tree for SX — lossless source representation.
+
+    Every piece of source text is preserved: whitespace, comments,
+    delimiters, raw token text.  The CST supports two projections:
+    - [cst_to_source]: reconstruct the exact original source
+    - [cst_to_ast]: strip trivia, produce [Sx_types.value] for evaluation
+
+    Trivia attaches to nodes (leading on every node, trailing on
+    containers before the close delimiter).  No separate comment map. *)
+
+open Sx_types
+
+(** {1 Types} *)
+
+type trivia =
+  | Whitespace of string      (** Runs of spaces, tabs, newlines *)
+  | LineComment of string     (** ";;" through end of line, including the ";" chars *)
+
+type span = {
+  start_offset : int;
+  end_offset   : int;
+}
+
+type cst_node =
+  | CstAtom of {
+      leading_trivia : trivia list;
+      token : string;              (** Raw source text of the token *)
+      value : value;               (** Parsed semantic value *)
+      span  : span;
+    }
+  | CstList of {
+      leading_trivia : trivia list;
+      open_delim  : char;          (** '(' or '[' *)
+      children    : cst_node list;
+      close_delim : char;          (** ')' or ']' *)
+      trailing_trivia : trivia list;  (** Trivia between last child and close delim *)
+      span : span;
+    }
+  | CstDict of {
+      leading_trivia : trivia list;
+      children : cst_node list;    (** Alternating key/value atoms *)
+      trailing_trivia : trivia list;
+      span : span;
+    }
+
+
+(** {1 CST → Source (lossless reconstruction)} *)
+
+let trivia_to_string ts =
+  let buf = Buffer.create 64 in
+  List.iter (function
+    | Whitespace s -> Buffer.add_string buf s
+    | LineComment s -> Buffer.add_string buf s
+  ) ts;
+  Buffer.contents buf
+
+let rec cst_to_source node =
+  match node with
+  | CstAtom { leading_trivia; token; _ } ->
+    trivia_to_string leading_trivia ^ token
+  | CstList { leading_trivia; open_delim; children; close_delim; trailing_trivia; _ } ->
+    let buf = Buffer.create 256 in
+    Buffer.add_string buf (trivia_to_string leading_trivia);
+    Buffer.add_char buf open_delim;
+    List.iter (fun c -> Buffer.add_string buf (cst_to_source c)) children;
+    Buffer.add_string buf (trivia_to_string trailing_trivia);
+    Buffer.add_char buf close_delim;
+    Buffer.contents buf
+  | CstDict { leading_trivia; children; trailing_trivia; _ } ->
+    let buf = Buffer.create 256 in
+    Buffer.add_string buf (trivia_to_string leading_trivia);
+    Buffer.add_char buf '{';
+    List.iter (fun c -> Buffer.add_string buf (cst_to_source c)) children;
+    Buffer.add_string buf (trivia_to_string trailing_trivia);
+    Buffer.add_char buf '}';
+    Buffer.contents buf
+
+let cst_to_source_file nodes =
+  String.concat "" (List.map cst_to_source nodes)
+
+(** Reconstruct source from a parsed file (nodes + trailing trivia). *)
+let cst_file_to_source nodes trailing =
+  cst_to_source_file nodes ^ trivia_to_string trailing
+
+
+(** {1 CST → AST (strip trivia for evaluation)} *)
+
+let rec cst_to_ast = function
+  | CstAtom { value; _ } -> value
+  | CstList { children; _ } ->
+    List (List.map cst_to_ast children)
+  | CstDict { children; _ } ->
+    let d = make_dict () in
+    let rec pairs = function
+      | k :: v :: rest ->
+        let key_str = match cst_to_ast k with
+          | Keyword k -> k | String k -> k | Symbol k -> k | _ -> ""
+        in
+        dict_set d key_str (cst_to_ast v);
+        pairs rest
+      | _ -> ()
+    in
+    pairs children;
+    Dict d
+
+
+(** {1 CST editing — apply AST-level edits back to the CST} *)
+
+(** Replace the CST node at [path] with [new_source], preserving the
+    original node's leading trivia.  [new_source] is parsed as CST so
+    any comments in it are preserved. *)
+let apply_edit path new_cst_nodes original_cst_nodes =
+  let rec go nodes idx_path =
+    match idx_path with
+    | [] -> nodes  (* shouldn't happen *)
+    | [target] ->
+      List.mapi (fun i node ->
+        if i = target then
+          match new_cst_nodes with
+          | [replacement] ->
+            (* Preserve original leading trivia *)
+            let orig_trivia = match node with
+              | CstAtom { leading_trivia; _ } -> leading_trivia
+              | CstList { leading_trivia; _ } -> leading_trivia
+              | CstDict { leading_trivia; _ } -> leading_trivia
+            in
+            (match replacement with
+             | CstAtom r -> CstAtom { r with leading_trivia = orig_trivia }
+             | CstList r -> CstList { r with leading_trivia = orig_trivia }
+             | CstDict r -> CstDict { r with leading_trivia = orig_trivia })
+          | _ -> node  (* multi-node replacement: use as-is *)
+        else node
+      ) nodes
+    | target :: rest ->
+      List.mapi (fun i node ->
+        if i = target then
+          match node with
+          | CstList r ->
+            CstList { r with children = go r.children rest }
+          | CstDict r ->
+            CstDict { r with children = go r.children rest }
+          | _ -> node
+        else node
+      ) nodes
+  in
+  go original_cst_nodes path
diff --git a/hosts/ocaml/lib/sx_parser.ml b/hosts/ocaml/lib/sx_parser.ml
index 9be3f71d..f508dda9 100644
--- a/hosts/ocaml/lib/sx_parser.ml
+++ b/hosts/ocaml/lib/sx_parser.ml
@@ -290,3 +290,232 @@ let parse_file ?(comments=false) path =
   let src = really_input_string ic n in
   close_in ic;
   parse_all ~comments src
+
+
+(* ================================================================== *)
+(* CST parser — lossless concrete syntax tree                         *)
+(* ================================================================== *)
+
+open Sx_cst
+
+(** Collect leading trivia (whitespace + comments) from current position. *)
+let collect_trivia s =
+  let items = ref [] in
+  let rec go () =
+    if at_end s then ()
+    else match s.src.[s.pos] with
+    | ' ' | '\t' | '\n' | '\r' ->
+      let start = s.pos in
+      while s.pos < s.len && (let c = s.src.[s.pos] in c = ' ' || c = '\t' || c = '\n' || c = '\r') do
+        advance s
+      done;
+      items := Whitespace (String.sub s.src start (s.pos - start)) :: !items;
+      go ()
+    | ';' ->
+      let start = s.pos in
+      while s.pos < s.len && s.src.[s.pos] <> '\n' do advance s done;
+      let text = String.sub s.src start (s.pos - start) in
+      if s.pos < s.len then advance s;
+      (* Include the newline in the comment trivia *)
+      let text = if s.pos > 0 && s.pos <= s.len && s.src.[s.pos - 1] = '\n'
+        then text ^ "\n" else text in
+      items := LineComment text :: !items;
+      go ()
+    | _ -> ()
+  in
+  go ();
+  List.rev !items
+
+(** Read a single CST value — dispatches on first non-trivia char. *)
+let rec read_cst s : cst_node =
+  let trivia = collect_trivia s in
+  if at_end s then
+    raise (Parse_error "Unexpected end of input");
+  let start = s.pos in
+  match s.src.[s.pos] with
+  | '(' -> read_cst_list s trivia start '(' ')'
+  | '[' -> read_cst_list s trivia start '[' ']'
+  | '{' -> read_cst_dict s trivia start
+  | '\'' ->
+    (* Quote sugar: 'x → (quote x) — emit as raw token *)
+    advance s;
+    let inner = read_cst s in
+    let end_pos = s.pos in
+    let token = String.sub s.src start (end_pos - start) in
+    let value = List [Symbol "quote"; cst_to_ast inner] in
+    CstAtom { leading_trivia = trivia; token; value; span = { start_offset = start; end_offset = end_pos } }
+  | '`' ->
+    advance s;
+    let inner = read_cst s in
+    let end_pos = s.pos in
+    let token = String.sub s.src start (end_pos - start) in
+    let value = List [Symbol "quasiquote"; cst_to_ast inner] in
+    CstAtom { leading_trivia = trivia; token; value; span = { start_offset = start; end_offset = end_pos } }
+  | ',' ->
+    advance s;
+    let splice = s.pos < s.len && s.src.[s.pos] = '@' in
+    if splice then advance s;
+    let inner = read_cst s in
+    let end_pos = s.pos in
+    let token = String.sub s.src start (end_pos - start) in
+    let sym = if splice then "splice-unquote" else "unquote" in
+    let value = List [Symbol sym; cst_to_ast inner] in
+    CstAtom { leading_trivia = trivia; token; value; span = { start_offset = start; end_offset = end_pos } }
+  | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = ';' ->
+    (* Datum comment: #; discards next expression *)
+    advance s; advance s;
+    let _discarded = read_cst s in
+    (* Read the real value after the datum comment — attach trivia from #; *)
+    let next = read_cst s in
+    let combined_trivia = trivia @ (match next with
+      | CstAtom r -> r.leading_trivia
+      | CstList r -> r.leading_trivia
+      | CstDict r -> r.leading_trivia) in
+    (match next with
+     | CstAtom r -> CstAtom { r with leading_trivia = combined_trivia }
+     | CstList r -> CstList { r with leading_trivia = combined_trivia }
+     | CstDict r -> CstDict { r with leading_trivia = combined_trivia })
+  | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = '\'' ->
+    advance s; advance s;
+    let inner = read_cst s in
+    let end_pos = s.pos in
+    let token = String.sub s.src start (end_pos - start) in
+    let value = List [Symbol "quote"; cst_to_ast inner] in
+    CstAtom { leading_trivia = trivia; token; value; span = { start_offset = start; end_offset = end_pos } }
+  | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = '|' ->
+    (* Raw string: #|...|  *)
+    advance s; advance s;
+    let buf = Buffer.create 64 in
+    let rec go () =
+      if at_end s then raise (Parse_error "Unterminated raw string");
+      let c = s.src.[s.pos] in
+      advance s;
+      if c = '|' then ()
+      else begin Buffer.add_char buf c; go () end
+    in
+    go ();
+    let end_pos = s.pos in
+    let token = String.sub s.src start (end_pos - start) in
+    CstAtom { leading_trivia = trivia; token; value = String (Buffer.contents buf);
+              span = { start_offset = start; end_offset = end_pos } }
+  | '"' ->
+    let value = String (read_string s) in
+    let end_pos = s.pos in
+    let token = String.sub s.src start (end_pos - start) in
+    CstAtom { leading_trivia = trivia; token; value;
+              span = { start_offset = start; end_offset = end_pos } }
+  | _ ->
+    let sym = read_symbol s in
+    if sym = "" then begin
+      let line = ref 1 and col = ref 1 in
+      for i = 0 to s.pos - 1 do
+        if s.src.[i] = '\n' then (incr line; col := 1) else incr col
+      done;
+      raise (Parse_error (Printf.sprintf "Unexpected char: %c at line %d col %d (pos %d)"
+        s.src.[s.pos] !line !col s.pos))
+    end;
+    let end_pos = s.pos in
+    let token = String.sub s.src start (end_pos - start) in
+    let value = match sym with
+      | "true" -> Bool true
+      | "false" -> Bool false
+      | "nil" -> Nil
+      | _ when sym.[0] = ':' -> Keyword (String.sub sym 1 (String.length sym - 1))
+      | _ -> match try_number sym with Some n -> n | None -> Symbol sym
+    in
+    CstAtom { leading_trivia = trivia; token; value;
+              span = { start_offset = start; end_offset = end_pos } }
+
+and read_cst_list s trivia start open_c close_c =
+  advance s; (* skip open delim *)
+  let children = ref [] in
+  let rec go () =
+    let child_trivia = collect_trivia s in
+    if at_end s then raise (Parse_error "Unterminated list");
+    if s.src.[s.pos] = close_c then begin
+      advance s;
+      let end_pos = s.pos in
+      CstList { leading_trivia = trivia; open_delim = open_c;
+                children = List.rev !children; close_delim = close_c;
+                trailing_trivia = child_trivia;
+                span = { start_offset = start; end_offset = end_pos } }
+    end else begin
+      (* Push collected trivia onto the next child *)
+      let child_start = s.pos in
+      let child = read_cst_inner s in
+      let child_with_trivia = match child with
+        | CstAtom r -> CstAtom { r with leading_trivia = child_trivia @ r.leading_trivia }
+        | CstList r -> CstList { r with leading_trivia = child_trivia @ r.leading_trivia }
+        | CstDict r -> CstDict { r with leading_trivia = child_trivia @ r.leading_trivia }
+      in
+      ignore child_start;
+      children := child_with_trivia :: !children;
+      go ()
+    end
+  in
+  go ()
+
+and read_cst_dict s trivia start =
+  advance s; (* skip { *)
+  let children = ref [] in
+  let rec go () =
+    let child_trivia = collect_trivia s in
+    if at_end s then raise (Parse_error "Unterminated dict");
+    if s.src.[s.pos] = '}' then begin
+      advance s;
+      let end_pos = s.pos in
+      CstDict { leading_trivia = trivia; children = List.rev !children;
+                trailing_trivia = child_trivia;
+                span = { start_offset = start; end_offset = end_pos } }
+    end else begin
+      let child = read_cst_inner s in
+      let child_with_trivia = match child with
+        | CstAtom r -> CstAtom { r with leading_trivia = child_trivia @ r.leading_trivia }
+        | CstList r -> CstList { r with leading_trivia = child_trivia @ r.leading_trivia }
+        | CstDict r -> CstDict { r with leading_trivia = child_trivia @ r.leading_trivia }
+      in
+      children := child_with_trivia :: !children;
+      go ()
+    end
+  in
+  go ()
+
+(** Inner read — no trivia collection (caller handles it). *)
+and read_cst_inner s : cst_node =
+  read_cst s
+
+(** Parse result: list of CST nodes + any trailing trivia after the last node. *)
+type cst_file = {
+  nodes : cst_node list;
+  trailing_trivia : trivia list;
+}
+
+(** Parse a string into a list of CST nodes. *)
+let parse_all_cst src =
+  let s = make_state src in
+  let results = ref [] in
+  let rec go () =
+    let trivia = collect_trivia s in
+    if at_end s then
+      { nodes = List.rev !results; trailing_trivia = trivia }
+    else begin
+      let node = read_cst_inner s in
+      (* Prepend collected trivia to this node *)
+      let node_with_trivia = match node with
+        | CstAtom r -> CstAtom { r with leading_trivia = trivia @ r.leading_trivia }
+        | CstList r -> CstList { r with leading_trivia = trivia @ r.leading_trivia }
+        | CstDict r -> CstDict { r with leading_trivia = trivia @ r.leading_trivia }
+      in
+      results := node_with_trivia :: !results;
+      go ()
+    end
+  in
+  go ()
+
+(** Parse a file into a list of CST nodes. *)
+let parse_file_cst path =
+  let ic = open_in path in
+  let n = in_channel_length ic in
+  let src = really_input_string ic n in
+  close_in ic;
+  parse_all_cst src