diff --git a/packages/ludic.ui/style_rule.ludic b/packages/ludic.ui/style_rule.ludic
index 8fa4a2e5..f02ef70b 100644
--- a/packages/ludic.ui/style_rule.ludic
+++ b/packages/ludic.ui/style_rule.ludic
@@ -19,7 +19,7 @@ function lss_rule_add(f: UiFile, decls: UiRule, sel: string, err: []string, line
   let r = new UiRule
   r.keys = decls.keys
   r.vals = decls.vals
-  r.text = `{sel}`                     # a copy: the selector's text is given back below
+  r.text = intern(sel)                 # a copy: lss_rule frees sel (`{sel}` was sel itself, and dangled)
   r.at = `{f.path}:{line}`
   r.sel = sel_parse(sel, err)
   if r.sel != null {
diff --git a/tools/ui-preview/README.md b/tools/ui-preview/README.md
index d2089eba..e7af88db 100644
--- a/tools/ui-preview/README.md
+++ b/tools/ui-preview/README.md
@@ -10,7 +10,8 @@ ludic ui-preview [--font DIR]      # the host on stdin/stdout; DIR holds the gam
 ```
 
 **The wire protocol is [protocol-v1.md](protocol-v1.md)**, frozen: every request, every reply and
-every line format. `smoke.txt` is a scripted session with what each reply must contain.
+every line format. `smoke.txt` is a scripted session with what each reply must contain, and `smoke.py` runs it
+(`python3 tools/ui-preview/smoke.py tools/ui-preview/smoke.txt bin/ludic-ui-preview`).
 
 ## What it does
 
diff --git a/tools/ui-preview/smoke.py b/tools/ui-preview/smoke.py
new file mode 100644
index 00000000..bb84b526
--- /dev/null
+++ b/tools/ui-preview/smoke.py
@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+# smoke.py - runs smoke.txt against a ui-preview binary and checks each reply: `> ` lines are requests,
+# `< ` a reply line must contain the text (in order), `<= ` the exact final line, `<! ` no line may
+#   python3 tools/ui-preview/smoke.py tools/ui-preview/smoke.txt bin/ludic-ui-preview
+import subprocess, sys
+reqs, exp = [], []
+for line in open(sys.argv[1]):
+    line = line.rstrip("\n")
+    if line.startswith("> ") or line == ">": reqs.append(line[2:]); exp.append([])
+    elif line.startswith(("< ", "<= ", "<! ")) and reqs:
+        k, _, t = line.partition(" "); exp[-1].append((k, t))
+p = subprocess.run([sys.argv[2]], input="\n".join(reqs) + "\n", capture_output=True, text=True, errors="replace", timeout=60)
+out = p.stdout.splitlines()
+replies, cur = [], []
+for l in out:
+    cur.append(l)
+    if l == "ok" or l.startswith("ok ") or l.startswith("fail"):
+        replies.append(cur); cur = []
+bad = 0
+print(f"{len(reqs)} requests, {len(replies)} replies, exit {p.returncode}")
+for i, r in enumerate(reqs):
+    rep = replies[i] if i < len(replies) else []
+    pos = 0
+    for k, t in exp[i]:
+        ok = True
+        if k == "<":
+            j = next((j for j in range(pos, len(rep)) if t in rep[j]), None)
+            ok = j is not None; pos = j if ok else pos
+        elif k == "<=": ok = bool(rep) and rep[-1] == t
+        elif k == "<!": ok = not any(t in x for x in rep)
+        if not ok:
+            bad += 1; print(f"FAIL request {i+1} `{r[:60]}`: {k} {t!r}\n   reply: {rep[-6:]}")
+print("smoke:", "OK" if bad == 0 else f"{bad} expectation(s) failed")
+if p.stderr.strip(): print("stderr:", p.stderr.strip()[:500])
diff --git a/tools/ui-preview/smoke.txt b/tools/ui-preview/smoke.txt
index 736ab487..972d912c 100644
--- a/tools/ui-preview/smoke.txt
+++ b/tools/ui-preview/smoke.txt
@@ -103,8 +103,8 @@
 > tree
 < node 0
 < "root"
-< "go"
 < "Food"
+< "go"
 <= ok
 
 > box label