diff --git a/lib/tcl/runtime.sx b/lib/tcl/runtime.sx
new file mode 100644
index 00000000..395bb22b
--- /dev/null
+++ b/lib/tcl/runtime.sx
@@ -0,0 +1,279 @@
+;; lib/tcl/runtime.sx — Tcl primitives on SX
+;;
+;; Provides Tcl-idiomatic wrappers over SX built-ins.
+;; Primitives used:
+;;   make-regexp/regexp-match/regexp-match-all/...    (Phase 19)
+;;   make-set/set-add!/set-member?/set-remove!/set->list (Phase 18)
+;;   call/cc                                          (core evaluator)
+;;   quotient/remainder                               (Phase 15 / builtin)
+;;   string->list/list->string/char->integer          (Phase 13)
+
+;; ---------------------------------------------------------------------------
+;; 1. String buffer — Tcl append / string accumulation
+;; ---------------------------------------------------------------------------
+
+(define
+  (tcl-sb-new)
+  (let
+    ((sb (dict)))
+    (dict-set! sb "_tcl_sb" true)
+    (dict-set! sb "_buf" "")
+    sb))
+
+(define (tcl-sb? v) (and (dict? v) (dict-has? v "_tcl_sb")))
+
+(define
+  (tcl-sb-append! sb s)
+  (dict-set! sb "_buf" (str (get sb "_buf") s))
+  sb)
+
+(define (tcl-sb-value sb) (get sb "_buf"))
+
+(define (tcl-sb-clear! sb) (dict-set! sb "_buf" "") sb)
+
+(define (tcl-sb-length sb) (len (get sb "_buf")))
+
+;; ---------------------------------------------------------------------------
+;; 2. String port (channel) — Tcl channel abstraction
+;;    Read channel: created from a string, supports gets/read.
+;;    Write channel: accumulates puts output, queryable via tcl-chan-string.
+;; ---------------------------------------------------------------------------
+
+(define
+  (tcl-chan-in-new str)
+  (let
+    ((c (dict)))
+    (dict-set! c "_tcl_chan" true)
+    (dict-set! c "_mode" "read")
+    (dict-set! c "_chars" (string->list str))
+    (dict-set! c "_pos" 0)
+    c))
+
+(define
+  (tcl-chan-out-new)
+  (let
+    ((c (dict)))
+    (dict-set! c "_tcl_chan" true)
+    (dict-set! c "_mode" "write")
+    (dict-set! c "_buf" "")
+    c))
+
+(define (tcl-chan? v) (and (dict? v) (dict-has? v "_tcl_chan")))
+
+(define
+  (tcl-chan-eof? c)
+  (and
+    (= (get c "_mode") "read")
+    (>= (get c "_pos") (len (get c "_chars")))))
+
+(define
+  (tcl-chan-read-char c)
+  (if
+    (tcl-chan-eof? c)
+    nil
+    (let
+      ((ch (nth (get c "_chars") (get c "_pos"))))
+      (dict-set! c "_pos" (+ (get c "_pos") 1))
+      ch)))
+
+;; gets — read one line (up to newline or EOF), return without trailing newline
+(define
+  (tcl-chan-gets c)
+  (letrec
+    ((go (fn (acc) (let ((ch (tcl-chan-read-char c))) (cond ((= ch nil) (list->string (reverse acc))) ((= (char->integer ch) 10) (list->string (reverse acc))) (else (go (cons ch acc))))))))
+    (go (list))))
+
+;; read — read all remaining chars
+(define
+  (tcl-chan-read c)
+  (letrec
+    ((go (fn (acc) (let ((ch (tcl-chan-read-char c))) (if (= ch nil) (list->string (reverse acc)) (go (cons ch acc)))))))
+    (go (list))))
+
+;; puts — write string to write channel (no newline)
+(define
+  (tcl-chan-puts! c s)
+  (when
+    (= (get c "_mode") "write")
+    (dict-set! c "_buf" (str (get c "_buf") s)))
+  c)
+
+;; puts-line — write string + newline (Tcl default puts behaviour)
+(define (tcl-chan-puts-line! c s) (tcl-chan-puts! c (str s "\n")))
+
+;; string — get accumulated content of write channel
+(define (tcl-chan-string c) (get c "_buf"))
+
+;; tell — current read position
+(define (tcl-chan-tell c) (get c "_pos"))
+
+;; ---------------------------------------------------------------------------
+;; 3. Regexp — Tcl regexp / regsub wrappers
+;; ---------------------------------------------------------------------------
+
+(define (tcl-re-new pattern) (make-regexp pattern ""))
+
+(define (tcl-re-new-flags pattern flags) (make-regexp pattern flags))
+
+(define (tcl-re? v) (regexp? v))
+
+(define (tcl-re-match? rx str) (not (= (regexp-match rx str) nil)))
+
+(define (tcl-re-match rx str) (regexp-match rx str))
+
+(define (tcl-re-match-all rx str) (regexp-match-all rx str))
+
+(define (tcl-re-sub rx str replacement) (regexp-replace rx str replacement))
+
+(define
+  (tcl-re-sub-all rx str replacement)
+  (regexp-replace-all rx str replacement))
+
+(define (tcl-re-split rx str) (regexp-split rx str))
+
+;; ---------------------------------------------------------------------------
+;; 4. Format — Tcl format command (%s %d %f %x %o %%)
+;;    tcl-format takes a format string and a list of arguments.
+;;    Example: (tcl-format "%s is %d" (list "Alice" 30)) → "Alice is 30"
+;; ---------------------------------------------------------------------------
+
+;; Digit characters for base conversion
+(define tcl-hex-chars (string->list "0123456789abcdef"))
+
+(define
+  (tcl-digits-for-base n base digit-chars)
+  (let
+    ((abs-n (if (< n 0) (- 0 n) n)))
+    (letrec
+      ((go (fn (n acc) (if (= n 0) (if (= (len acc) 0) "0" (list->string acc)) (go (quotient n base) (cons (nth digit-chars (remainder n base)) acc))))))
+      (let
+        ((unsigned (go abs-n (list))))
+        (if (< n 0) (str "-" unsigned) unsigned)))))
+
+(define
+  (tcl-format-hex n)
+  (tcl-digits-for-base (truncate n) 16 tcl-hex-chars))
+
+(define
+  (tcl-format-oct n)
+  (tcl-digits-for-base (truncate n) 8 (string->list "01234567")))
+
+(define
+  (tcl-format fmt args)
+  (letrec
+    ((chars (string->list fmt))
+      (go
+        (fn
+          (cs arg-list result)
+          (if
+            (= (len cs) 0)
+            result
+            (let
+              ((c-int (char->integer (first cs))))
+              (if
+                (= c-int 37)
+                (if
+                  (= (len (rest cs)) 0)
+                  result
+                  (let
+                    ((spec-int (char->integer (first (rest cs)))))
+                    (cond
+                      ((= spec-int 37)
+                        (go (rest (rest cs)) arg-list (str result "%")))
+                      ((= spec-int 115)
+                        (go
+                          (rest (rest cs))
+                          (rest arg-list)
+                          (str result (str (first arg-list)))))
+                      ((= spec-int 100)
+                        (go
+                          (rest (rest cs))
+                          (rest arg-list)
+                          (str result (str (truncate (first arg-list))))))
+                      ((= spec-int 102)
+                        (go
+                          (rest (rest cs))
+                          (rest arg-list)
+                          (str result (str (+ 0 (first arg-list))))))
+                      ((= spec-int 120)
+                        (go
+                          (rest (rest cs))
+                          (rest arg-list)
+                          (str result (tcl-format-hex (first arg-list)))))
+                      ((= spec-int 111)
+                        (go
+                          (rest (rest cs))
+                          (rest arg-list)
+                          (str result (tcl-format-oct (first arg-list)))))
+                      (else
+                        (go
+                          (rest (rest cs))
+                          arg-list
+                          (str
+                            result
+                            "%"
+                            (list->string (list (first (rest cs))))))))))
+                (go
+                  (rest cs)
+                  arg-list
+                  (str result (list->string (list (first cs)))))))))))
+    (go chars args "")))
+
+;; ---------------------------------------------------------------------------
+;; 5. Coroutine — Tcl-style coroutine using call/cc
+;;    tcl-co-yield works reliably when called from top-level fns.
+;;    Avoid calling tcl-co-yield from letrec-bound lambdas (JIT limitation).
+;; ---------------------------------------------------------------------------
+
+(define tcl-current-co nil)
+
+(define
+  (tcl-co-new body)
+  (let
+    ((co (dict)))
+    (dict-set! co "_tcl_co" true)
+    (dict-set! co "_state" "new")
+    (dict-set! co "_cont" nil)
+    (dict-set! co "_resumer" nil)
+    (dict-set! co "_parent" nil)
+    (dict-set!
+      co
+      "_body"
+      (fn
+        ()
+        (let
+          ((result (body)))
+          (dict-set! co "_state" "dead")
+          (set! tcl-current-co (get co "_parent"))
+          ((get co "_resumer") result))))
+    co))
+
+(define (tcl-co? v) (and (dict? v) (dict-has? v "_tcl_co")))
+
+(define (tcl-co-alive? co) (not (= (get co "_state") "dead")))
+
+(define
+  (tcl-co-yield val)
+  (call/cc
+    (fn
+      (resume-k)
+      (let
+        ((cur tcl-current-co))
+        (dict-set! cur "_cont" resume-k)
+        (dict-set! cur "_state" "suspended")
+        (set! tcl-current-co (get cur "_parent"))
+        ((get cur "_resumer") val)))))
+
+(define
+  (tcl-co-resume co)
+  (call/cc
+    (fn
+      (return-k)
+      (dict-set! co "_parent" tcl-current-co)
+      (dict-set! co "_resumer" return-k)
+      (set! tcl-current-co co)
+      (dict-set! co "_state" "running")
+      (if
+        (= (get co "_cont") nil)
+        ((get co "_body"))
+        ((get co "_cont") nil)))))
diff --git a/lib/tcl/test.sh b/lib/tcl/test.sh
new file mode 100755
index 00000000..74c1d16a
--- /dev/null
+++ b/lib/tcl/test.sh
@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+# lib/tcl/test.sh — smoke-test the Tcl runtime layer.
+
+set -uo pipefail
+cd "$(git rev-parse --show-toplevel)"
+
+SX_SERVER="${SX_SERVER:-hosts/ocaml/_build/default/bin/sx_server.exe}"
+if [ ! -x "$SX_SERVER" ]; then
+  SX_SERVER="/root/rose-ash/hosts/ocaml/_build/default/bin/sx_server.exe"
+fi
+if [ ! -x "$SX_SERVER" ]; then
+  echo "ERROR: sx_server.exe not found."
+  exit 1
+fi
+
+TMPFILE=$(mktemp); trap "rm -f $TMPFILE" EXIT
+
+cat > "$TMPFILE" << 'EPOCHS'
+(epoch 1)
+(load "lib/tcl/runtime.sx")
+(epoch 2)
+(load "lib/tcl/tests/runtime.sx")
+(epoch 3)
+(eval "(list tcl-test-pass tcl-test-fail)")
+EPOCHS
+
+OUTPUT=$(timeout 60 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
+
+LINE=$(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 "ERROR: could not extract summary"
+  echo "$OUTPUT" | tail -20
+  exit 1
+fi
+
+P=$(echo "$LINE" | sed -E 's/^\(([0-9]+) ([0-9]+)\).*/\1/')
+F=$(echo "$LINE" | sed -E 's/^\(([0-9]+) ([0-9]+)\).*/\2/')
+TOTAL=$((P + F))
+
+if [ "$F" -eq 0 ]; then
+  echo "ok $P/$TOTAL lib/tcl tests passed"
+else
+  echo "FAIL $P/$TOTAL passed, $F failed"
+  TMPFILE2=$(mktemp)
+  cat > "$TMPFILE2" << 'EPOCHS2'
+(epoch 1)
+(load "lib/tcl/runtime.sx")
+(epoch 2)
+(load "lib/tcl/tests/runtime.sx")
+(epoch 3)
+(eval "(map (fn (f) (list (get f :name) (get f :got) (get f :expected))) tcl-test-fails)")
+EPOCHS2
+  FAILS=$(timeout 60 "$SX_SERVER" < "$TMPFILE2" 2>/dev/null | grep -E '^\(ok-len 3' -A1 | tail -1 || true)
+  echo "  Details: $FAILS"
+  rm -f "$TMPFILE2"
+fi
+
+[ "$F" -eq 0 ]
diff --git a/lib/tcl/tests/runtime.sx b/lib/tcl/tests/runtime.sx
new file mode 100644
index 00000000..ccf81461
--- /dev/null
+++ b/lib/tcl/tests/runtime.sx
@@ -0,0 +1,146 @@
+;; lib/tcl/tests/runtime.sx — Tests for lib/tcl/runtime.sx
+
+(define tcl-test-pass 0)
+(define tcl-test-fail 0)
+(define tcl-test-fails (list))
+
+(define
+  (tcl-test name got expected)
+  (if
+    (= got expected)
+    (set! tcl-test-pass (+ tcl-test-pass 1))
+    (begin
+      (set! tcl-test-fail (+ tcl-test-fail 1))
+      (set! tcl-test-fails (append tcl-test-fails (list {:got got :expected expected :name name}))))))
+
+;; ---------------------------------------------------------------------------
+;; 1. String buffer
+;; ---------------------------------------------------------------------------
+
+(define sb1 (tcl-sb-new))
+(tcl-test "sb? new" (tcl-sb? sb1) true)
+(tcl-test "sb? non-sb" (tcl-sb? "hello") false)
+(tcl-test "sb value empty" (tcl-sb-value sb1) "")
+(tcl-test "sb length empty" (tcl-sb-length sb1) 0)
+(tcl-sb-append! sb1 "hello")
+(tcl-test "sb value after append" (tcl-sb-value sb1) "hello")
+(tcl-sb-append! sb1 " ")
+(tcl-sb-append! sb1 "world")
+(tcl-test "sb value after multi-append" (tcl-sb-value sb1) "hello world")
+(tcl-test "sb length" (tcl-sb-length sb1) 11)
+(tcl-sb-clear! sb1)
+(tcl-test "sb value after clear" (tcl-sb-value sb1) "")
+(tcl-test "sb length after clear" (tcl-sb-length sb1) 0)
+
+;; ---------------------------------------------------------------------------
+;; 2. String port (channel)
+;; ---------------------------------------------------------------------------
+
+(define chin1 (tcl-chan-in-new "hello\nworld\nfoo"))
+(tcl-test "chan? read" (tcl-chan? chin1) true)
+(tcl-test "chan eof? no" (tcl-chan-eof? chin1) false)
+(tcl-test "chan gets line1" (tcl-chan-gets chin1) "hello")
+(tcl-test "chan gets line2" (tcl-chan-gets chin1) "world")
+(tcl-test "chan gets line3" (tcl-chan-gets chin1) "foo")
+(tcl-test "chan eof? yes" (tcl-chan-eof? chin1) true)
+(tcl-test "chan gets at eof" (tcl-chan-gets chin1) "")
+
+(define chin2 (tcl-chan-in-new "abcdef"))
+(tcl-test "chan read all" (tcl-chan-read chin2) "abcdef")
+(tcl-test "chan read empty" (tcl-chan-read chin2) "")
+
+(define chout1 (tcl-chan-out-new))
+(tcl-test "chan? write" (tcl-chan? chout1) true)
+(tcl-chan-puts! chout1 "hello")
+(tcl-chan-puts! chout1 " world")
+(tcl-test "chan string" (tcl-chan-string chout1) "hello world")
+(tcl-chan-puts-line! chout1 "!")
+(tcl-test "chan string with newline" (tcl-chan-string chout1) "hello world!\n")
+
+(define chout2 (tcl-chan-out-new))
+(tcl-chan-puts-line! chout2 "line1")
+(tcl-chan-puts-line! chout2 "line2")
+(tcl-test "chan multi-line" (tcl-chan-string chout2) "line1\nline2\n")
+
+;; ---------------------------------------------------------------------------
+;; 3. Regexp
+;; ---------------------------------------------------------------------------
+
+(define rx1 (tcl-re-new "hel+o"))
+(tcl-test "re? yes" (tcl-re? rx1) true)
+(tcl-test "re? no" (tcl-re? "hello") false)
+(tcl-test "re match? yes" (tcl-re-match? rx1 "say hello") true)
+(tcl-test "re match? no" (tcl-re-match? rx1 "goodbye") false)
+
+(define m1 (tcl-re-match rx1 "say hello world"))
+(tcl-test "re match result" (get m1 "match") "hello")
+
+(define rx2 (tcl-re-new "[0-9]+"))
+(define all (tcl-re-match-all rx2 "a1b22c333"))
+(tcl-test "re match-all count" (len all) 3)
+(tcl-test "re match-all last" (get (nth all 2) "match") "333")
+
+(tcl-test "re sub" (tcl-re-sub rx2 "a1b2" "N") "aNb2")
+(tcl-test "re sub-all" (tcl-re-sub-all rx2 "a1b2" "N") "aNbN")
+
+(define rx3 (tcl-re-new "[ ,]+"))
+(tcl-test "re split" (tcl-re-split rx3 "a b,c") (list "a" "b" "c"))
+
+;; ---------------------------------------------------------------------------
+;; 4. Format
+;; ---------------------------------------------------------------------------
+
+(tcl-test "format %s" (tcl-format "hello %s" (list "world")) "hello world")
+(tcl-test "format %d" (tcl-format "n=%d" (list 42)) "n=42")
+(tcl-test "format %d truncates float" (tcl-format "n=%d" (list 3.9)) "n=3")
+(tcl-test
+  "format %s %d"
+  (tcl-format "%s is %d" (list "age" 30))
+  "age is 30")
+(tcl-test "format %%" (tcl-format "100%% done" (list)) "100% done")
+(tcl-test "format %x" (tcl-format "%x" (list 255)) "ff")
+(tcl-test "format %x 16" (tcl-format "0x%x" (list 16)) "0x10")
+(tcl-test "format %o" (tcl-format "%o" (list 8)) "10")
+(tcl-test "format %o 255" (tcl-format "%o" (list 255)) "377")
+(tcl-test "format no spec" (tcl-format "plain text" (list)) "plain text")
+(tcl-test
+  "format multiple"
+  (tcl-format "%s=%d (0x%x)" (list "val" 255 255))
+  "val=255 (0xff)")
+
+;; ---------------------------------------------------------------------------
+;; 5. Coroutine
+;;    tcl-co-yield works from top-level helper functions.
+;; ---------------------------------------------------------------------------
+
+(define
+  co1
+  (tcl-co-new
+    (fn () (tcl-co-yield 1) (tcl-co-yield 2) 3)))
+
+(tcl-test "co? yes" (tcl-co? co1) true)
+(tcl-test "co? no" (tcl-co? 42) false)
+(tcl-test "co alive? before" (tcl-co-alive? co1) true)
+(define cor1 (tcl-co-resume co1))
+(tcl-test "co resume 1" cor1 1)
+(tcl-test "co alive? mid" (tcl-co-alive? co1) true)
+(define cor2 (tcl-co-resume co1))
+(tcl-test "co resume 2" cor2 2)
+(define cor3 (tcl-co-resume co1))
+(tcl-test "co resume 3 completion" cor3 3)
+(tcl-test "co alive? dead" (tcl-co-alive? co1) false)
+
+;; Top-level helper for recursive yield (avoids JIT letrec limitation)
+(define
+  (tcl-co-count-down i)
+  (when
+    (>= i 1)
+    (tcl-co-yield i)
+    (tcl-co-count-down (- i 1))))
+
+(define co2 (tcl-co-new (fn () (tcl-co-count-down 3) "done")))
+(tcl-test "co loop 3" (tcl-co-resume co2) 3)
+(tcl-test "co loop 2" (tcl-co-resume co2) 2)
+(tcl-test "co loop 1" (tcl-co-resume co2) 1)
+(tcl-test "co loop done" (tcl-co-resume co2) "done")
+(tcl-test "co loop dead" (tcl-co-alive? co2) false)