diff --git a/plans/agent-briefings/sx-gate-loop.md b/plans/agent-briefings/sx-gate-loop.md
index 4afc8933..0d69a69b 100644
--- a/plans/agent-briefings/sx-gate-loop.md
+++ b/plans/agent-briefings/sx-gate-loop.md
@@ -54,7 +54,9 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
       sentinel across two tests; a plain assert would inherit the vacuity)
 - [x] C1/C1b [W3] — command-channel crash guards pinned
       (`scripts/test-protocol-gate.sh`, seed for section E's fuzz suite)
-- [ ] S4 [conformance] — housekeeping repro, pin
+- [x] S4 [hosts] — soft error pages not cached (HTTP-mode pin in
+      `scripts/test-protocol-gate.sh`; NB S4 lives in hosts.md, not
+      conformance — "housekeeping" was a mislabel from F-15's tag)
 
 ### B. Runner/production env unification
 - [ ] Audit runner-only bindings (`values`/`call-with-values` F7/K42, JS
@@ -81,6 +83,16 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 
 ## Progress log (newest first)
 
+- 2026-07-04 — **S4 error-page-cache pin (item A.7) — section A COMPLETE**.
+  Extended `scripts/test-protocol-gate.sh` with an HTTP-mode case: fresh
+  `sx_server.exe --http <random-port>` (timeout-bounded, own PID killed at
+  end), GET the same nonexistent path twice, assert BOTH requests re-render
+  (2 `[sx-http]` lines — pre-fix the 2nd was cache-served at 0.0005s) and
+  the `[cache] … error page, not cached` is_err gate line appears. Findings
+  from prototyping: standalone worktree renders ALL docs pages as soft error
+  pages (no content), so a positive "real page IS cached" control is not
+  assertable here — documented in the script; startup takes ~12-15s (poll
+  loop, 40s budget). 5/5 protocol-gate green + 267/0 sx pins. Test-only.
 - 2026-07-04 — **C1/C1b command-channel pins (item A.6)**. These are
   protocol-level, not .sx-suite pins: authored
   `scripts/test-protocol-gate.sh` — each case spawns its OWN timeout-bounded
diff --git a/scripts/test-protocol-gate.sh b/scripts/test-protocol-gate.sh
index 9ac7355b..798fcab8 100755
--- a/scripts/test-protocol-gate.sh
+++ b/scripts/test-protocol-gate.sh
@@ -84,6 +84,62 @@ else
   fail=$((fail+1))
 fi
 
+# ---------------------------------------------------------------------------
+# S4 (review, hosts.md): soft error pages must NOT be stored in the HTTP
+# response cache. Pre-fix, a routing-failure page was cached as HTTP 200 and
+# served byte-identically from cache to every later visitor (cold 2s → warm
+# 0.0005s, ONE render line). Post-fix (dc7aa709), http_render_page returns
+# (html, is_error) and cache insertion is gated on `not is_err` (the skip is
+# logged as "[cache] <path> → error page, not cached").
+#
+# Pin: GET the same nonexistent path twice against a fresh --http server and
+# assert BOTH requests re-render (two [sx-http] render lines) plus the
+# is_err gate line appearing in the log. NB: in a standalone worktree all
+# docs pages render as soft error pages (no content), so a positive
+# "real page IS cached" control is not assertable here.
+# ---------------------------------------------------------------------------
+s4_case() {
+  local port=$((18000 + RANDOM % 2000))
+  local log; log=$(mktemp)
+  timeout 90 "$SERVER" --http "$port" >"$log" 2>&1 &
+  local srv=$!
+  local up=0
+  for _ in $(seq 1 40); do
+    if curl -s -o /dev/null "http://localhost:$port/" 2>/dev/null; then up=1; break; fi
+    sleep 1
+  done
+  if [[ $up -ne 1 ]]; then
+    echo "FAIL: S4 — http server did not come up on :$port"
+    kill "$srv" 2>/dev/null; rm -f "$log"
+    fail=$((fail+1)); return
+  fi
+  local miss="/sx/gate-pin-missing-$$-$RANDOM"
+  curl -s -o /dev/null "http://localhost:$port$miss"
+  curl -s -o /dev/null "http://localhost:$port$miss"
+  sleep 1
+  local renders
+  renders=$(grep -c "sx-http\] $miss " "$log")
+  local ok=1
+  if [[ "$renders" -ne 2 ]]; then
+    echo "FAIL: S4 — expected 2 renders of $miss (not cache-served), got $renders"
+    ok=0
+  fi
+  if ! grep -q 'error page, not cached' "$log"; then
+    echo "FAIL: S4 — is_err cache gate line absent from server log"
+    ok=0
+  fi
+  if [[ $ok -eq 1 ]]; then
+    echo "PASS: S4 soft error page not cached (both GETs re-rendered)"
+    pass=$((pass+1))
+  else
+    echo "  --- log tail ---"; tail -12 "$log" | sed 's/^/  /'; echo "  ---------------"
+    fail=$((fail+1))
+  fi
+  kill "$srv" 2>/dev/null
+  rm -f "$log"
+}
+s4_case
+
 echo
 echo "protocol-gate: $pass passed, $fail failed"
 [[ $fail -eq 0 ]]