diff --git a/lib/ruby/test.sh b/lib/ruby/test.sh
new file mode 100755
index 00000000..861e1c62
--- /dev/null
+++ b/lib/ruby/test.sh
@@ -0,0 +1,85 @@
+#!/usr/bin/env bash
+# Ruby-on-SX test runner.
+# Usage:
+#   bash lib/ruby/test.sh              # run all tests
+#   bash lib/ruby/test.sh -v           # verbose
+#   bash lib/ruby/test.sh tests/tokenizer.sx   # single file
+
+set -euo pipefail
+cd "$(git rev-parse --show-toplevel)"
+
+SX_SERVER="hosts/ocaml/_build/default/bin/sx_server.exe"
+if [ ! -x "$SX_SERVER" ]; then
+  MAIN_ROOT=$(git worktree list | head -1 | awk '{print $1}')
+  if [ -x "$MAIN_ROOT/$SX_SERVER" ]; then
+    SX_SERVER="$MAIN_ROOT/$SX_SERVER"
+  else
+    echo "ERROR: sx_server.exe not found."
+    exit 1
+  fi
+fi
+
+VERBOSE=""
+FILES=()
+for arg in "$@"; do
+  case "$arg" in
+    -v|--verbose) VERBOSE=1 ;;
+    *) FILES+=("$arg") ;;
+  esac
+done
+
+if [ ${#FILES[@]} -eq 0 ]; then
+  mapfile -t FILES < <(find lib/ruby/tests -maxdepth 2 -name '*.sx' | sort)
+fi
+
+TOTAL_PASS=0
+TOTAL_FAIL=0
+FAILED_FILES=()
+
+for FILE in "${FILES[@]}"; do
+  [ -f "$FILE" ] || { echo "skip $FILE (not found)"; continue; }
+  TMPFILE=$(mktemp)
+  cat > "$TMPFILE" <<EPOCHS
+(epoch 1)
+(load "lib/ruby/tokenizer.sx")
+(epoch 2)
+(load "$FILE")
+(epoch 3)
+(eval "(list rb-test-pass rb-test-fail)")
+EPOCHS
+
+  OUTPUT=$(timeout 60 "$SX_SERVER" < "$TMPFILE" 2>&1 || true)
+  rm -f "$TMPFILE"
+
+  LINE=$(echo "$OUTPUT" | awk '/^\(ok-len 3 / {getline; print; exit}')
+  if [ -z "$LINE" ]; then
+    LINE=$(echo "$OUTPUT" | grep -E '^\(ok 3 \([0-9]+ [0-9]+\)\)' | tail -1 \
+            | sed -E 's/^\(ok 3 //; s/\)$//')
+  fi
+  if [ -z "$LINE" ]; then
+    echo "✗ $FILE: could not extract summary"
+    echo "$OUTPUT" | tail -20
+    TOTAL_FAIL=$((TOTAL_FAIL + 1))
+    FAILED_FILES+=("$FILE")
+    continue
+  fi
+  P=$(echo "$LINE" | sed -E 's/^\(([0-9]+) ([0-9]+)\).*/\1/')
+  F=$(echo "$LINE" | sed -E 's/^\(([0-9]+) ([0-9]+)\).*/\2/')
+  TOTAL_PASS=$((TOTAL_PASS + P))
+  TOTAL_FAIL=$((TOTAL_FAIL + F))
+  if [ "$F" -gt 0 ]; then
+    FAILED_FILES+=("$FILE")
+    printf '✗ %-40s %d/%d\n' "$FILE" "$P" "$((P+F))"
+  elif [ "$VERBOSE" = "1" ]; then
+    printf '✓ %-40s %d passed\n' "$FILE" "$P"
+  fi
+done
+
+TOTAL=$((TOTAL_PASS + TOTAL_FAIL))
+if [ $TOTAL_FAIL -eq 0 ]; then
+  echo "✓ $TOTAL_PASS/$TOTAL ruby-on-sx tests passed"
+else
+  echo "✗ $TOTAL_PASS/$TOTAL passed, $TOTAL_FAIL failed in: ${FAILED_FILES[*]}"
+fi
+
+[ $TOTAL_FAIL -eq 0 ]
diff --git a/lib/ruby/tests/tokenizer.sx b/lib/ruby/tests/tokenizer.sx
new file mode 100644
index 00000000..b1411ec2
--- /dev/null
+++ b/lib/ruby/tests/tokenizer.sx
@@ -0,0 +1,210 @@
+;; Ruby tokenizer tests.
+;; Final value: {:pass N :fail N :fails (list)}
+
+(define rb-deep=?
+  (fn (a b)
+    (cond
+      ((= a b) true)
+      ((and (dict? a) (dict? b))
+        (let ((ak (keys a)) (bk (keys b)))
+          (if (not (= (len ak) (len bk)))
+            false
+            (every?
+              (fn (k) (and (has-key? b k) (rb-deep=? (get a k) (get b k))))
+              ak))))
+      ((and (list? a) (list? b))
+        (if (not (= (len a) (len b)))
+          false
+          (let ((i 0) (ok true))
+            (define rb-de-loop
+              (fn ()
+                (when (and ok (< i (len a)))
+                  (do
+                    (when (not (rb-deep=? (nth a i) (nth b i)))
+                      (set! ok false))
+                    (set! i (+ i 1))
+                    (rb-de-loop)))))
+            (rb-de-loop)
+            ok)))
+      (:else false))))
+
+(define rb-test-pass 0)
+(define rb-test-fail 0)
+(define rb-test-fails (list))
+
+(define rb-test
+  (fn (name actual expected)
+    (if (rb-deep=? actual expected)
+      (set! rb-test-pass (+ rb-test-pass 1))
+      (do
+        (set! rb-test-fail (+ rb-test-fail 1))
+        (append! rb-test-fails {:name name :actual actual :expected expected})))))
+
+;; Helper: tokenize, drop newline+eof, return {:type :value} pairs
+(define rb-toks
+  (fn (src)
+    (map
+      (fn (tok) {:value (get tok "value") :type (get tok "type")})
+      (filter
+        (fn (tok)
+          (let ((ty (get tok "type")))
+            (not (or (= ty "newline") (= ty "eof")))))
+        (rb-tokenize src)))))
+
+;; Helper: get just types
+(define rb-types
+  (fn (src) (map (fn (t) (get t "type")) (rb-toks src))))
+
+;; Helper: get first token type
+(define rb-first-type
+  (fn (src) (get (get (rb-tokenize src) 0) "type")))
+
+(define rb-first-value
+  (fn (src) (get (get (rb-tokenize src) 0) "value")))
+
+;; ── 1. Keywords ────────────────────────���─────────────────────────
+(rb-test "keyword def" (rb-toks "def") (list {:value "def" :type "keyword"}))
+(rb-test "keyword end" (rb-toks "end") (list {:value "end" :type "keyword"}))
+(rb-test "keyword class" (rb-toks "class") (list {:value "class" :type "keyword"}))
+(rb-test "keyword if" (rb-toks "if") (list {:value "if" :type "keyword"}))
+(rb-test "keyword while" (rb-toks "while") (list {:value "while" :type "keyword"}))
+(rb-test "keyword nil" (rb-toks "nil") (list {:value "nil" :type "keyword"}))
+(rb-test "keyword true" (rb-toks "true") (list {:value "true" :type "keyword"}))
+(rb-test "keyword false" (rb-toks "false") (list {:value "false" :type "keyword"}))
+(rb-test "keyword return" (rb-toks "return") (list {:value "return" :type "keyword"}))
+(rb-test "keyword yield" (rb-toks "yield") (list {:value "yield" :type "keyword"}))
+(rb-test "keyword begin" (rb-toks "begin") (list {:value "begin" :type "keyword"}))
+(rb-test "keyword rescue" (rb-toks "rescue") (list {:value "rescue" :type "keyword"}))
+(rb-test "keyword self" (rb-toks "self") (list {:value "self" :type "keyword"}))
+(rb-test "keyword super" (rb-toks "super") (list {:value "super" :type "keyword"}))
+
+;; ── 2. Identifiers ────────────────────────────────────────────────
+(rb-test "ident simple" (rb-toks "foo") (list {:value "foo" :type "ident"}))
+(rb-test "ident underscore" (rb-toks "_foo") (list {:value "_foo" :type "ident"}))
+(rb-test "ident with digit" (rb-toks "foo2") (list {:value "foo2" :type "ident"}))
+(rb-test "ident predicate" (rb-toks "empty?") (list {:value "empty?" :type "ident"}))
+(rb-test "ident bang" (rb-toks "save!") (list {:value "save!" :type "ident"}))
+(rb-test "defined?" (rb-toks "defined?") (list {:value "defined?" :type "keyword"}))
+
+;; ── 3. Constants ──────────────────────────────────────────────────
+(rb-test "const simple" (rb-toks "Foo") (list {:value "Foo" :type "const"}))
+(rb-test "const upcase" (rb-toks "MY_CONST") (list {:value "MY_CONST" :type "const"}))
+(rb-test "const class" (rb-toks "String") (list {:value "String" :type "const"}))
+
+;; ── 4. Sigil variables ───────────────────────────────────────────
+(rb-test "ivar" (rb-toks "@name") (list {:value "@name" :type "ivar"}))
+(rb-test "cvar" (rb-toks "@@count") (list {:value "@@count" :type "cvar"}))
+(rb-test "gvar" (rb-toks "$global") (list {:value "$global" :type "gvar"}))
+
+;; ── 5. Integers ───────────────────────────────────────────────────
+(rb-test "int decimal" (rb-first-value "42") 42)
+(rb-test "int zero" (rb-first-value "0") 0)
+(rb-test "int underscore" (rb-first-value "1_000") 1000)
+(rb-test "int hex" (rb-first-value "0xFF") 255)
+(rb-test "int hex lower" (rb-first-value "0xff") 255)
+(rb-test "int octal" (rb-first-value "0o17") 15)
+(rb-test "int binary" (rb-first-value "0b1010") 10)
+(rb-test "int type" (rb-first-type "42") "int")
+
+;; ── 6. Floats ─────────────────────────────────────────────────────
+(rb-test "float simple" (rb-first-type "3.14") "float")
+(rb-test "float value" (rb-first-value "3.14") "3.14")
+(rb-test "float exp" (rb-first-type "1.5e10") "float")
+(rb-test "float exp value" (rb-first-value "1.5e10") "1.5e10")
+
+;; ── 7. Strings ────────────────────────────────────────────────────
+(rb-test "dq string" (rb-first-value "\"hello\"") "hello")
+(rb-test "dq string type" (rb-first-type "\"hello\"") "string")
+(rb-test "sq string" (rb-first-value "'world'") "world")
+(rb-test "dq escape nl" (rb-first-value "\"a\\nb\"") "a\nb")
+(rb-test "dq escape tab" (rb-first-value "\"a\\tb\"") "a\tb")
+(rb-test "dq escape quote" (rb-first-value "\"a\\\"b\"") "a\"b")
+(rb-test "sq no escape" (rb-first-value "'a\\nb'") "a\\nb")
+(rb-test "sq escape backslash" (rb-first-value "'a\\\\'") "a\\")
+(rb-test "dq interp kept" (rb-first-value "\"#{x}\"") "#{x}")
+
+;; ── 8. Symbols ────────────────────────────────────────────────────
+(rb-test "symbol simple" (rb-first-type ":foo") "symbol")
+(rb-test "symbol value" (rb-first-value ":foo") "foo")
+(rb-test "symbol predicate" (rb-first-value ":empty?") "empty?")
+(rb-test "symbol dq" (rb-first-value ":\"hello world\"") "hello world")
+(rb-test "symbol sq" (rb-first-value ":'hello'") "hello")
+
+;; ── 9. %w and %i literals ────────────────────────────────────────
+(rb-test "%w bracket" (rb-first-type "%w[a b c]") "words")
+(rb-test "%w value" (rb-first-value "%w[a b c]") (list "a" "b" "c"))
+(rb-test "%w paren" (rb-first-value "%w(x y)") (list "x" "y"))
+(rb-test "%i bracket" (rb-first-type "%i[a b]") "isymbols")
+(rb-test "%i value" (rb-first-value "%i[foo bar]") (list "foo" "bar"))
+
+;; ── 10. Punctuation ───────────────────────────────────────────────
+(rb-test "dot" (rb-first-type ".") "dot")
+(rb-test "dotdot" (rb-first-type "..") "dotdot")
+(rb-test "dotdotdot" (rb-first-type "...") "dotdotdot")
+(rb-test "dcolon" (rb-first-type "::") "dcolon")
+(rb-test "comma" (rb-first-type ",") "comma")
+(rb-test "semi" (rb-first-type ";") "semi")
+(rb-test "lparen" (rb-first-type "(") "lparen")
+(rb-test "rparen" (rb-first-type ")") "rparen")
+(rb-test "lbracket" (rb-first-type "[") "lbracket")
+(rb-test "rbracket" (rb-first-type "]") "rbracket")
+(rb-test "lbrace" (rb-first-type "{") "lbrace")
+(rb-test "rbrace" (rb-first-type "}") "rbrace")
+(rb-test "pipe" (rb-first-type "|") "pipe")
+
+;; ── 11. Operators ─────────────────────────────────────────────────
+(rb-test "op plus" (rb-first-value "+") "+")
+(rb-test "op minus" (rb-first-value "-") "-")
+(rb-test "op star" (rb-first-value "*") "*")
+(rb-test "op slash" (rb-first-value "/") "/")
+(rb-test "op eq" (rb-first-value "=") "=")
+(rb-test "op eqeq" (rb-first-value "==") "==")
+(rb-test "op neq" (rb-first-value "!=") "!=")
+(rb-test "op lt" (rb-first-value "<") "<")
+(rb-test "op gt" (rb-first-value ">") ">")
+(rb-test "op lte" (rb-first-value "<=") "<=")
+(rb-test "op gte" (rb-first-value ">=") ">=")
+(rb-test "op spaceship" (rb-first-value "<=>") "<=>")
+(rb-test "op tripleq" (rb-first-value "===") "===")
+(rb-test "op match" (rb-first-value "=~") "=~")
+(rb-test "op nomatch" (rb-first-value "!~") "!~")
+(rb-test "op lshift" (rb-first-value "<<") "<<")
+(rb-test "op rshift" (rb-first-value ">>") ">>")
+(rb-test "op and" (rb-first-value "&&") "&&")
+(rb-test "op or" (rb-first-value "||") "||")
+(rb-test "op power" (rb-first-value "**") "**")
+(rb-test "op plus-eq" (rb-first-value "+=") "+=")
+(rb-test "op minus-eq" (rb-first-value "-=") "-=")
+(rb-test "op arrow" (rb-first-value "->") "->")
+(rb-test "op hash-rocket" (rb-first-value "=>") "=>")
+
+;; ── 12. Comments ──────────────────────────────────────────────────
+(rb-test "comment skipped" (len (rb-toks "# this is a comment")) 0)
+(rb-test "comment mid-line" (rb-types "x = 1 # comment") (list "ident" "op" "int"))
+
+;; ── 13. Multi-token sequences ─────────────────────────────────────
+(rb-test "method call" (rb-types "foo.bar")
+  (list "ident" "dot" "ident"))
+(rb-test "class def" (rb-types "class Foo")
+  (list "keyword" "const"))
+(rb-test "method def" (rb-types "def greet(name)")
+  (list "keyword" "ident" "lparen" "ident" "rparen"))
+(rb-test "assignment" (rb-types "x = 42")
+  (list "ident" "op" "int"))
+(rb-test "block params" (rb-types "|x, y|")
+  (list "pipe" "ident" "comma" "ident" "pipe"))
+(rb-test "scope resolution" (rb-types "Foo::Bar")
+  (list "const" "dcolon" "const"))
+(rb-test "range" (rb-types "1..10")
+  (list "int" "dotdot" "int"))
+(rb-test "exclusive range" (rb-types "1...10")
+  (list "int" "dotdotdot" "int"))
+
+;; ── 14. Line/col tracking ────────────────────────────────────────
+(define rb-tok1 (get (rb-tokenize "hello\nworld") 0))
+(define rb-tok2 (get (rb-tokenize "hello\nworld") 2))
+(rb-test "line track start" (get rb-tok1 "line") 1)
+(rb-test "line track second" (get rb-tok2 "line") 2)
+(rb-test "col track start" (get rb-tok1 "col") 1)
+
+(list rb-test-pass rb-test-fail)
diff --git a/lib/ruby/tokenizer.sx b/lib/ruby/tokenizer.sx
new file mode 100644
index 00000000..c8965201
--- /dev/null
+++ b/lib/ruby/tokenizer.sx
@@ -0,0 +1,549 @@
+;; Ruby tokenizer for Ruby 2.7 subset.
+;; Token: {:type T :value V :line L :col C}
+;;
+;; Types: keyword ident ivar cvar gvar const
+;;        int float string symbol
+;;        op dot dotdot dotdotdot dcolon colon
+;;        lparen rparen lbracket rbracket lbrace rbrace
+;;        comma semi pipe newline words isymbols eof
+
+;; ── Character code table ──────────────────────────────────────────
+(define rb-ord-table
+  (let ((t (dict)) (i 0))
+    (define rb-build-table
+      (fn ()
+        (when (< i 128)
+          (do
+            (dict-set! t (char-from-code i) i)
+            (set! i (+ i 1))
+            (rb-build-table)))))
+    (rb-build-table)
+    t))
+
+(define rb-ord (fn (c) (or (get rb-ord-table c) 0)))
+
+;; ── Character predicates ──────────────────────────────────────────
+(define rb-digit?
+  (fn (c) (and (string? c) (>= (rb-ord c) 48) (<= (rb-ord c) 57))))
+
+(define rb-hex-digit?
+  (fn (c)
+    (and (string? c)
+         (or (and (>= (rb-ord c) 48) (<= (rb-ord c) 57))
+             (and (>= (rb-ord c) 97) (<= (rb-ord c) 102))
+             (and (>= (rb-ord c) 65) (<= (rb-ord c) 70))))))
+
+(define rb-octal-digit?
+  (fn (c) (and (string? c) (>= (rb-ord c) 48) (<= (rb-ord c) 55))))
+
+(define rb-binary-digit? (fn (c) (or (= c "0") (= c "1"))))
+
+(define rb-lower?
+  (fn (c) (and (string? c) (>= (rb-ord c) 97) (<= (rb-ord c) 122))))
+
+(define rb-upper?
+  (fn (c) (and (string? c) (>= (rb-ord c) 65) (<= (rb-ord c) 90))))
+
+(define rb-ident-start?
+  (fn (c) (or (rb-lower? c) (rb-upper? c) (= c "_"))))
+
+(define rb-ident-cont?
+  (fn (c) (or (rb-lower? c) (rb-upper? c) (rb-digit? c) (= c "_"))))
+
+(define rb-space? (fn (c) (or (= c " ") (= c "\t") (= c "\r"))))
+
+;; ── Reserved words ────────────────────────────────────────────────
+(define rb-keywords
+  (list "__ENCODING__" "__LINE__" "__FILE__"
+        "BEGIN" "END"
+        "alias" "and"
+        "begin" "break"
+        "case" "class"
+        "def" "defined?" "do"
+        "else" "elsif" "end" "ensure"
+        "false" "for"
+        "if" "in"
+        "module"
+        "next" "nil" "not"
+        "or"
+        "redo" "rescue" "retry" "return"
+        "self" "super"
+        "then" "true"
+        "undef" "unless" "until"
+        "when" "while"
+        "yield"))
+
+(define rb-keyword? (fn (w) (contains? rb-keywords w)))
+
+;; ── Token constructor ─────────────────────────────────────────────
+(define rb-make-token
+  (fn (type value line col) {:type type :value value :line line :col col}))
+
+;; ── Radix number parser ───────────────────────────────────────────
+(define rb-parse-radix
+  (fn (s radix)
+    (let ((n (len s)) (i 0) (acc 0))
+      (define rb-rad-loop
+        (fn ()
+          (when (< i n)
+            (do
+              (let ((c (substring s i (+ i 1))))
+                (cond
+                  ((and (>= (rb-ord c) 48) (<= (rb-ord c) 57))
+                    (set! acc (+ (* acc radix) (- (rb-ord c) 48))))
+                  ((and (>= (rb-ord c) 97) (<= (rb-ord c) 102))
+                    (set! acc (+ (* acc radix) (+ 10 (- (rb-ord c) 97)))))
+                  ((and (>= (rb-ord c) 65) (<= (rb-ord c) 70))
+                    (set! acc (+ (* acc radix) (+ 10 (- (rb-ord c) 65)))))))
+              (set! i (+ i 1))
+              (rb-rad-loop)))))
+      (rb-rad-loop)
+      acc)))
+
+;; ── Strip underscores from numeric literals ───────────────────────
+(define rb-strip-underscores
+  (fn (s)
+    (let ((n (len s)) (i 0) (parts (list)))
+      (define rb-su-loop
+        (fn ()
+          (when (< i n)
+            (do
+              (let ((c (substring s i (+ i 1))))
+                (when (not (= c "_"))
+                  (append! parts c)))
+              (set! i (+ i 1))
+              (rb-su-loop)))))
+      (rb-su-loop)
+      (join "" parts))))
+
+;; ── Main tokenizer ────────────────────────────────────────────────
+(define rb-tokenize
+  (fn (src)
+    (let ((tokens (list))
+          (pos 0)
+          (line 1)
+          (col 1)
+          (src-len (len src)))
+      (define rb-peek
+        (fn (offset)
+          (if (< (+ pos offset) src-len)
+            (substring src (+ pos offset) (+ pos offset 1))
+            nil)))
+      (define rb-cur (fn () (rb-peek 0)))
+      (define rb-advance!
+        (fn ()
+          (let ((c (rb-cur)))
+            (set! pos (+ pos 1))
+            (if (= c "\n")
+              (do (set! line (+ line 1)) (set! col 1))
+              (set! col (+ col 1))))))
+      (define rb-advance-n!
+        (fn (n)
+          (when (> n 0)
+            (do (rb-advance!) (rb-advance-n! (- n 1))))))
+      (define rb-push!
+        (fn (type value tok-line tok-col)
+          (append! tokens (rb-make-token type value tok-line tok-col))))
+      (define rb-read-while
+        (fn (pred)
+          (let ((start pos))
+            (define rb-rw-loop
+              (fn ()
+                (when (and (< pos src-len) (pred (rb-cur)))
+                  (do (rb-advance!) (rb-rw-loop)))))
+            (rb-rw-loop)
+            (substring src start pos))))
+      (define rb-skip-line-comment!
+        (fn ()
+          (define rb-slc-loop
+            (fn ()
+              (when (and (< pos src-len) (not (= (rb-cur) "\n")))
+                (do (rb-advance!) (rb-slc-loop)))))
+          (rb-slc-loop)))
+      (define rb-read-escape
+        (fn ()
+          (rb-advance!)
+          (let ((c (rb-cur)))
+            (cond
+              ((= c "n") (do (rb-advance!) "\n"))
+              ((= c "t") (do (rb-advance!) "\t"))
+              ((= c "r") (do (rb-advance!) "\r"))
+              ((= c "\\") (do (rb-advance!) "\\"))
+              ((= c "'") (do (rb-advance!) "'"))
+              ((= c "\"") (do (rb-advance!) "\""))
+              ((= c "a") (do (rb-advance!) (char-from-code 7)))
+              ((= c "b") (do (rb-advance!) (char-from-code 8)))
+              ((= c "f") (do (rb-advance!) (char-from-code 12)))
+              ((= c "v") (do (rb-advance!) (char-from-code 11)))
+              ((= c "e") (do (rb-advance!) (char-from-code 27)))
+              ((= c "s") (do (rb-advance!) " "))
+              ((= c "0") (do (rb-advance!) (char-from-code 0)))
+              (:else (do (rb-advance!) (str "\\" c)))))))
+      (define rb-read-sq-string
+        (fn ()
+          (let ((parts (list)))
+            (rb-advance!)
+            (define rb-sq-loop
+              (fn ()
+                (cond
+                  ((>= pos src-len) nil)
+                  ((= (rb-cur) "'") (rb-advance!))
+                  ((and (= (rb-cur) "\\")
+                        (let ((n (rb-peek 1)))
+                          (or (= n "\\") (= n "'"))))
+                    (do
+                      (rb-advance!)
+                      (append! parts (rb-cur))
+                      (rb-advance!)
+                      (rb-sq-loop)))
+                  (:else
+                    (do
+                      (append! parts (rb-cur))
+                      (rb-advance!)
+                      (rb-sq-loop))))))
+            (rb-sq-loop)
+            (join "" parts))))
+      (define rb-read-dq-string
+        (fn ()
+          (let ((parts (list)))
+            (rb-advance!)
+            (define rb-dq-loop
+              (fn ()
+                (cond
+                  ((>= pos src-len) nil)
+                  ((= (rb-cur) "\"") (rb-advance!))
+                  ((= (rb-cur) "\\")
+                    (do
+                      (append! parts (rb-read-escape))
+                      (rb-dq-loop)))
+                  ((and (= (rb-cur) "#") (= (rb-peek 1) "{"))
+                    (do
+                      (append! parts "#{")
+                      (rb-advance-n! 2)
+                      (let ((depth 1))
+                        (define rb-interp-inner
+                          (fn ()
+                            (when (and (< pos src-len) (> depth 0))
+                              (do
+                                (let ((c (rb-cur)))
+                                  (cond
+                                    ((= c "{")
+                                      (do
+                                        (set! depth (+ depth 1))
+                                        (append! parts c)
+                                        (rb-advance!)))
+                                    ((= c "}")
+                                      (do
+                                        (set! depth (- depth 1))
+                                        (when (> depth 0)
+                                          (do (append! parts c) (rb-advance!)))))
+                                    (:else
+                                      (do (append! parts c) (rb-advance!)))))
+                                (rb-interp-inner)))))
+                        (rb-interp-inner))
+                      (when (= (rb-cur) "}")
+                        (do (append! parts "}") (rb-advance!)))
+                      (rb-dq-loop)))
+                  (:else
+                    (do
+                      (append! parts (rb-cur))
+                      (rb-advance!)
+                      (rb-dq-loop))))))
+            (rb-dq-loop)
+            (join "" parts))))
+      (define rb-read-percent-words
+        (fn ()
+          (rb-advance-n! 2)
+          (let ((open-ch (rb-cur)))
+            (let ((close-ch
+                    (cond
+                      ((= open-ch "[") "]")
+                      ((= open-ch "(") ")")
+                      ((= open-ch "{") "}")
+                      ((= open-ch "<") ">")
+                      (:else open-ch))))
+              (rb-advance!)
+              (let ((items (list)))
+                (define rb-pw-skip
+                  (fn ()
+                    (when (and (< pos src-len) (or (rb-space? (rb-cur)) (= (rb-cur) "\n")))
+                      (do (rb-advance!) (rb-pw-skip)))))
+                (define rb-pw-word
+                  (fn (wparts)
+                    (if (or (>= pos src-len)
+                             (rb-space? (rb-cur))
+                             (= (rb-cur) "\n")
+                             (= (rb-cur) close-ch))
+                      (append! items (join "" wparts))
+                      (do
+                        (append! wparts (rb-cur))
+                        (rb-advance!)
+                        (rb-pw-word wparts)))))
+                (define rb-pw-loop
+                  (fn ()
+                    (rb-pw-skip)
+                    (when (and (< pos src-len) (not (= (rb-cur) close-ch)))
+                      (do
+                        (rb-pw-word (list))
+                        (rb-pw-loop)))))
+                (rb-pw-loop)
+                (when (= (rb-cur) close-ch) (rb-advance!))
+                items)))))
+      (define rb-read-ident-word
+        (fn ()
+          (let ((start pos))
+            (rb-read-while rb-ident-cont?)
+            (when (and (= (rb-cur) "?") (not (= (rb-peek 1) "=")))
+              (rb-advance!))
+            (when (and (= (rb-cur) "!") (not (or (= (rb-peek 1) "=") (= (rb-peek 1) "~"))))
+              (rb-advance!))
+            (substring src start pos))))
+      (define rb-read-number!
+        (fn (tok-line tok-col)
+          (let ((start pos))
+            (cond
+              ((and (= (rb-cur) "0") (let ((p (rb-peek 1))) (or (= p "b") (= p "B"))))
+                (do
+                  (rb-advance-n! 2)
+                  (let ((bin-str (rb-read-while rb-binary-digit?)))
+                    (rb-push! "int" (rb-parse-radix bin-str 2) tok-line tok-col))))
+              ((and (= (rb-cur) "0") (let ((p (rb-peek 1))) (or (= p "o") (= p "O"))))
+                (do
+                  (rb-advance-n! 2)
+                  (let ((oct-str (rb-read-while rb-octal-digit?)))
+                    (rb-push! "int" (rb-parse-radix oct-str 8) tok-line tok-col))))
+              ((and (= (rb-cur) "0") (let ((p (rb-peek 1))) (or (= p "x") (= p "X"))))
+                (do
+                  (rb-advance-n! 2)
+                  (let ((hex-str (rb-read-while rb-hex-digit?)))
+                    (rb-push! "int" (rb-parse-radix hex-str 16) tok-line tok-col))))
+              (:else
+                (do
+                  (rb-read-while (fn (c) (or (rb-digit? c) (= c "_"))))
+                  (let ((is-float false))
+                    (when (and (= (rb-cur) ".") (rb-digit? (rb-peek 1)))
+                      (do
+                        (set! is-float true)
+                        (rb-advance!)
+                        (rb-read-while (fn (c) (or (rb-digit? c) (= c "_"))))))
+                    (when (or (= (rb-cur) "e") (= (rb-cur) "E"))
+                      (do
+                        (set! is-float true)
+                        (rb-advance!)
+                        (when (or (= (rb-cur) "+") (= (rb-cur) "-"))
+                          (rb-advance!))
+                        (rb-read-while rb-digit?)))
+                    (let ((num-str (rb-strip-underscores (substring src start pos))))
+                      (if is-float
+                        (rb-push! "float" num-str tok-line tok-col)
+                        (rb-push! "int" (parse-int num-str) tok-line tok-col))))))))))
+      (define rb-read-op!
+        (fn (tok-line tok-col)
+          (let ((c0 (rb-cur)) (c1 (rb-peek 1)) (c2 (rb-peek 2)))
+            (cond
+              ((and (= c0 "<") (= c1 "=") (= c2 ">"))
+                (do (rb-advance-n! 3) (rb-push! "op" "<=>" tok-line tok-col)))
+              ((and (= c0 "=") (= c1 "=") (= c2 "="))
+                (do (rb-advance-n! 3) (rb-push! "op" "===" tok-line tok-col)))
+              ((and (= c0 "*") (= c1 "*") (= c2 "="))
+                (do (rb-advance-n! 3) (rb-push! "op" "**=" tok-line tok-col)))
+              ((and (= c0 "<") (= c1 "<") (= c2 "="))
+                (do (rb-advance-n! 3) (rb-push! "op" "<<=" tok-line tok-col)))
+              ((and (= c0 ">") (= c1 ">") (= c2 "="))
+                (do (rb-advance-n! 3) (rb-push! "op" ">>=" tok-line tok-col)))
+              ((and (= c0 "&") (= c1 "&") (= c2 "="))
+                (do (rb-advance-n! 3) (rb-push! "op" "&&=" tok-line tok-col)))
+              ((and (= c0 "|") (= c1 "|") (= c2 "="))
+                (do (rb-advance-n! 3) (rb-push! "op" "||=" tok-line tok-col)))
+              ((and (= c0 "*") (= c1 "*"))
+                (do (rb-advance-n! 2) (rb-push! "op" "**" tok-line tok-col)))
+              ((and (= c0 "=") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "==" tok-line tok-col)))
+              ((and (= c0 "!") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "!=" tok-line tok-col)))
+              ((and (= c0 "<") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "<=" tok-line tok-col)))
+              ((and (= c0 ">") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" ">=" tok-line tok-col)))
+              ((and (= c0 "=") (= c1 "~"))
+                (do (rb-advance-n! 2) (rb-push! "op" "=~" tok-line tok-col)))
+              ((and (= c0 "!") (= c1 "~"))
+                (do (rb-advance-n! 2) (rb-push! "op" "!~" tok-line tok-col)))
+              ((and (= c0 "<") (= c1 "<"))
+                (do (rb-advance-n! 2) (rb-push! "op" "<<" tok-line tok-col)))
+              ((and (= c0 ">") (= c1 ">"))
+                (do (rb-advance-n! 2) (rb-push! "op" ">>" tok-line tok-col)))
+              ((and (= c0 "&") (= c1 "&"))
+                (do (rb-advance-n! 2) (rb-push! "op" "&&" tok-line tok-col)))
+              ((and (= c0 "|") (= c1 "|"))
+                (do (rb-advance-n! 2) (rb-push! "op" "||" tok-line tok-col)))
+              ((and (= c0 "+") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "+=" tok-line tok-col)))
+              ((and (= c0 "-") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "-=" tok-line tok-col)))
+              ((and (= c0 "*") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "*=" tok-line tok-col)))
+              ((and (= c0 "/") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "/=" tok-line tok-col)))
+              ((and (= c0 "%") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "%=" tok-line tok-col)))
+              ((and (= c0 "&") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "&=" tok-line tok-col)))
+              ((and (= c0 "|") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "|=" tok-line tok-col)))
+              ((and (= c0 "^") (= c1 "="))
+                (do (rb-advance-n! 2) (rb-push! "op" "^=" tok-line tok-col)))
+              ((and (= c0 "-") (= c1 ">"))
+                (do (rb-advance-n! 2) (rb-push! "op" "->" tok-line tok-col)))
+              ((and (= c0 "=") (= c1 ">"))
+                (do (rb-advance-n! 2) (rb-push! "op" "=>" tok-line tok-col)))
+              ((and (= c0 "|") (nil? c1))
+                (do (rb-advance!) (rb-push! "pipe" "|" tok-line tok-col)))
+              ((= c0 "|")
+                (do (rb-advance!) (rb-push! "pipe" "|" tok-line tok-col)))
+              (:else
+                (do (rb-advance!) (rb-push! "op" c0 tok-line tok-col)))))))
+      (define rb-scan!
+        (fn ()
+          (cond
+            ((>= pos src-len) nil)
+            ((rb-space? (rb-cur)) (do (rb-advance!) (rb-scan!)))
+            ((= (rb-cur) "#") (do (rb-skip-line-comment!) (rb-scan!)))
+            ((= (rb-cur) "\n")
+              (do
+                (let ((l line) (c col))
+                  (rb-advance!)
+                  (rb-push! "newline" nil l c))
+                (rb-scan!)))
+            ((rb-digit? (rb-cur))
+              (do
+                (let ((l line) (c col))
+                  (rb-read-number! l c))
+                (rb-scan!)))
+            ((rb-ident-start? (rb-cur))
+              (do
+                (let ((l line) (c col))
+                  (let ((w (rb-read-ident-word)))
+                    (if (rb-keyword? w)
+                      (rb-push! "keyword" w l c)
+                      (if (rb-upper? (substring w 0 1))
+                        (rb-push! "const" w l c)
+                        (rb-push! "ident" w l c)))))
+                (rb-scan!)))
+            ((= (rb-cur) "@")
+              (do
+                (let ((l line) (c col))
+                  (if (= (rb-peek 1) "@")
+                    (do
+                      (rb-advance-n! 2)
+                      (let ((name (rb-read-while rb-ident-cont?)))
+                        (rb-push! "cvar" (str "@@" name) l c)))
+                    (do
+                      (rb-advance!)
+                      (let ((name (rb-read-while rb-ident-cont?)))
+                        (rb-push! "ivar" (str "@" name) l c)))))
+                (rb-scan!)))
+            ((= (rb-cur) "$")
+              (do
+                (let ((l line) (c col))
+                  (rb-advance!)
+                  (let ((name (rb-read-while rb-ident-cont?)))
+                    (rb-push! "gvar" (str "$" name) l c)))
+                (rb-scan!)))
+            ((= (rb-cur) "\"")
+              (do
+                (let ((l line) (c col))
+                  (rb-push! "string" (rb-read-dq-string) l c))
+                (rb-scan!)))
+            ((= (rb-cur) "'")
+              (do
+                (let ((l line) (c col))
+                  (rb-push! "string" (rb-read-sq-string) l c))
+                (rb-scan!)))
+            ((and (= (rb-cur) ":") (= (rb-peek 1) ":"))
+              (do
+                (let ((l line) (c col))
+                  (rb-advance-n! 2)
+                  (rb-push! "dcolon" "::" l c))
+                (rb-scan!)))
+            ((= (rb-cur) ":")
+              (do
+                (let ((l line) (c col))
+                  (rb-advance!)
+                  (cond
+                    ((= (rb-cur) "\"")
+                      (rb-push! "symbol" (rb-read-dq-string) l c))
+                    ((= (rb-cur) "'")
+                      (rb-push! "symbol" (rb-read-sq-string) l c))
+                    ((rb-ident-start? (rb-cur))
+                      (let ((name (rb-read-ident-word)))
+                        (rb-push! "symbol" name l c)))
+                    (:else
+                      (rb-push! "colon" ":" l c))))
+                (rb-scan!)))
+            ((and (= (rb-cur) "%")
+                  (let ((p (rb-peek 1)))
+                    (or (= p "w") (= p "W") (= p "i") (= p "I"))))
+              (do
+                (let ((l line) (c col))
+                  (let ((kind (rb-peek 1)))
+                    (let ((items (rb-read-percent-words)))
+                      (if (or (= kind "i") (= kind "I"))
+                        (rb-push! "isymbols" items l c)
+                        (rb-push! "words" items l c)))))
+                (rb-scan!)))
+            ((= (rb-cur) ".")
+              (do
+                (let ((l line) (c col))
+                  (cond
+                    ((and (= (rb-peek 1) ".") (= (rb-peek 2) "."))
+                      (do (rb-advance-n! 3) (rb-push! "dotdotdot" "..." l c)))
+                    ((= (rb-peek 1) ".")
+                      (do (rb-advance-n! 2) (rb-push! "dotdot" ".." l c)))
+                    (:else
+                      (do (rb-advance!) (rb-push! "dot" "." l c)))))
+                (rb-scan!)))
+            ((= (rb-cur) ",")
+              (do
+                (let ((l line) (c col)) (rb-push! "comma" "," l c) (rb-advance!))
+                (rb-scan!)))
+            ((= (rb-cur) ";")
+              (do
+                (let ((l line) (c col)) (rb-push! "semi" ";" l c) (rb-advance!))
+                (rb-scan!)))
+            ((= (rb-cur) "(")
+              (do
+                (let ((l line) (c col)) (rb-push! "lparen" "(" l c) (rb-advance!))
+                (rb-scan!)))
+            ((= (rb-cur) ")")
+              (do
+                (let ((l line) (c col)) (rb-push! "rparen" ")" l c) (rb-advance!))
+                (rb-scan!)))
+            ((= (rb-cur) "[")
+              (do
+                (let ((l line) (c col)) (rb-push! "lbracket" "[" l c) (rb-advance!))
+                (rb-scan!)))
+            ((= (rb-cur) "]")
+              (do
+                (let ((l line) (c col)) (rb-push! "rbracket" "]" l c) (rb-advance!))
+                (rb-scan!)))
+            ((= (rb-cur) "{")
+              (do
+                (let ((l line) (c col)) (rb-push! "lbrace" "{" l c) (rb-advance!))
+                (rb-scan!)))
+            ((= (rb-cur) "}")
+              (do
+                (let ((l line) (c col)) (rb-push! "rbrace" "}" l c) (rb-advance!))
+                (rb-scan!)))
+            ((or (= (rb-cur) "+") (= (rb-cur) "-") (= (rb-cur) "*")
+                 (= (rb-cur) "/") (= (rb-cur) "%") (= (rb-cur) "=")
+                 (= (rb-cur) "!") (= (rb-cur) "<") (= (rb-cur) ">")
+                 (= (rb-cur) "&") (= (rb-cur) "^") (= (rb-cur) "~")
+                 (= (rb-cur) "|"))
+              (do
+                (let ((l line) (c col)) (rb-read-op! l c))
+                (rb-scan!)))
+            (:else (do (rb-advance!) (rb-scan!))))))
+      (rb-scan!)
+      (rb-push! "eof" nil line col)
+      tokens)))
diff --git a/plans/ruby-on-sx.md b/plans/ruby-on-sx.md
index c10a4035..30d49960 100644
--- a/plans/ruby-on-sx.md
+++ b/plans/ruby-on-sx.md
@@ -51,7 +51,7 @@ Core mapping:
 ## Roadmap
 
 ### Phase 1 — tokenizer + parser
-- [ ] Tokenizer: keywords (`def end class module if unless while until do return yield begin rescue ensure case when then else elsif`), identifiers (lowercase = local/method, `@` = ivar, `@@` = cvar, `$` = global, uppercase = constant), numbers (int, float, `0x` `0o` `0b`, `_` separators), strings (`"…"` interpolation, `'…'` literal, `%w[a b c]`, `%i[a b c]`), symbols `:foo` `:"…"`, operators (`+ - * / % ** == != < > <= >= <=> === =~ !~ << >> & | ^ ~ ! && || and or not`), `:: . , ; ( ) [ ] { } -> => |`, comments `#`
+- [x] Tokenizer: keywords (`def end class module if unless while until do return yield begin rescue ensure case when then else elsif`), identifiers (lowercase = local/method, `@` = ivar, `@@` = cvar, `$` = global, uppercase = constant), numbers (int, float, `0x` `0o` `0b`, `_` separators), strings (`"…"` interpolation, `'…'` literal, `%w[a b c]`, `%i[a b c]`), symbols `:foo` `:"…"`, operators (`+ - * / % ** == != < > <= >= <=> === =~ !~ << >> & | ^ ~ ! && || and or not`), `:: . , ; ( ) [ ] { } -> => |`, comments `#`
 - [ ] Parser: program is sequence of statements separated by newlines or `;`; method def `def name(args) … end`; class `class Foo < Bar … end`; module `module M … end`; block `do |a, b| … end` and `{ |a, b| … }`; call sugar (no parens), `obj.method`, `Mod::Const`; arg shapes (positional, default, splat `*args`, double-splat `**opts`, block `&blk`)
 - [ ] If/while/case expressions (return values), `unless`/`until`, postfix modifiers
 - [ ] Begin/rescue/ensure/retry, raise, raise with class+message
@@ -117,7 +117,7 @@ Core mapping:
 
 _Newest first._
 
-- _(none yet)_
+- 2026-04-25: Phase 1 tokenizer complete — `lib/ruby/tokenizer.sx` + `lib/ruby/tests/tokenizer.sx` (107/107 tests). Keywords, identifiers (@ivar @@cvar $gvar), numbers (dec/hex/octal/binary/float), strings (dq with interpolation kept raw, sq), symbols, %w/%i literals, operators (all compound forms), punctuation, comments, line/col tracking.
 
 ## Blockers