diff --git a/plans/agent-briefings/sx-gate-loop.md b/plans/agent-briefings/sx-gate-loop.md
index 1649a08e..4afc8933 100644
--- a/plans/agent-briefings/sx-gate-loop.md
+++ b/plans/agent-briefings/sx-gate-loop.md
@@ -52,7 +52,8 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
       is K16 [W8], still OPEN — not a W14 pin target until its fix lands
 - [x] crit-2 [W1] — signal-return kont pinned NON-VACUOUSLY (side-effect
       sentinel across two tests; a plain assert would inherit the vacuity)
-- [ ] C1/C1b [W3] — HTTP-mode concurrency fixes, pin
+- [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
 
 ### B. Runner/production env unification
@@ -80,6 +81,16 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 
 ## Progress log (newest first)
 
+- 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
+  `sx_server.exe` (no shared process touched) and asserts three things: an
+  `(error N "Malformed command line: ...")` response is emitted, the
+  follow-up epoch still evaluates (process survived), and no `Fatal error`
+  escapes / exit is clean. Cases: C1 unterminated list (exact review repro),
+  C1 plain-garbage line, C1b non-ASCII byte (`café`), plus a well-formed
+  control session. 4/4 green. The script is deliberately structured to grow
+  into section E's fuzz suite (C3–C7). Test-only.
 - 2026-07-04 — **crit-2 non-vacuous pin (item A.5)**. The original bug's
   signature — handler value becomes the WHOLE program result, discarding
   every outer frame *including the covering test's own assert* — means a
diff --git a/scripts/test-protocol-gate.sh b/scripts/test-protocol-gate.sh
new file mode 100755
index 00000000..9ac7355b
--- /dev/null
+++ b/scripts/test-protocol-gate.sh
@@ -0,0 +1,89 @@
+#!/bin/bash
+# test-protocol-gate.sh — W14 pins for the epoch/command-channel protocol.
+#
+# Pins C1/C1b (review, plans/sx-review/hosts.md): a malformed or non-ASCII
+# line on the top-level command channel used to raise an uncaught
+# Sx_types.Parse_error and KILL the whole sx_server process (the shared
+# channel used by bridges and conformance runners). Fixed in dc7aa709:
+# the server now answers `(error N "Malformed command line: ...")` and
+# keeps serving.
+#
+# Each case spawns its OWN timeout-bounded sx_server.exe subprocess —
+# no shared/sibling process is ever touched. Designed to grow into the
+# W14 section-E protocol fuzz suite (C3-C7).
+#
+# Usage: bash scripts/test-protocol-gate.sh
+# Exit:  0 = all pins green; 1 = a pin failed (fix regressed).
+set -uo pipefail
+
+cd "$(dirname "$0")/.."
+SERVER=hosts/ocaml/_build/default/bin/sx_server.exe
+
+if [[ ! -x "$SERVER" ]]; then
+  echo "SKIP: $SERVER not built (run sx_build target=ocaml first)" >&2
+  exit 2
+fi
+
+pass=0
+fail=0
+
+# run_case NAME INPUT EXPECT_SENTINEL
+#   Feeds INPUT to a fresh server. Asserts:
+#   1. an (error ... "Malformed command line: ...") response is emitted
+#   2. the follow-up epoch still evaluates (EXPECT_SENTINEL in output)
+#   3. the process exits cleanly (no Fatal error, exit 0 on stdin EOF)
+run_case() {
+  local name="$1" input="$2" sentinel="$3"
+  local out rc
+  out=$(printf '%b' "$input" | timeout 60 "$SERVER" 2>&1)
+  rc=$?
+  local ok=1
+  if ! grep -q 'Malformed command line' <<<"$out"; then
+    echo "FAIL: $name — no malformed-line error response"; ok=0
+  fi
+  if ! grep -q "^${sentinel}\$" <<<"$out"; then
+    echo "FAIL: $name — follow-up epoch did not run (process died?)"; ok=0
+  fi
+  if grep -q 'Fatal error' <<<"$out"; then
+    echo "FAIL: $name — Fatal error escaped to the top level"; ok=0
+  fi
+  if [[ $rc -ne 0 ]]; then
+    echo "FAIL: $name — nonzero exit ($rc)"; ok=0
+  fi
+  if [[ $ok -eq 1 ]]; then
+    echo "PASS: $name"
+    pass=$((pass+1))
+  else
+    echo "  --- output ---"; sed 's/^/  /' <<<"$out"; echo "  --------------"
+    fail=$((fail+1))
+  fi
+}
+
+# C1: unterminated list on the command channel (exact review repro)
+run_case "C1 unterminated list survives" \
+  '(epoch 2)\n(eval "(+ 1 2"\n(epoch 3)\n(eval "99")\n' \
+  '99'
+
+# C1: plain-garbage line (second C1 repro shape)
+run_case "C1 garbage line survives" \
+  '(epoch 1)\nnot an s-expr ]]] {{{\n(epoch 2)\n(eval "42")\n' \
+  '42'
+
+# C1b: non-ASCII byte on the command channel (exact review repro; \xc3\xa9 = é)
+run_case "C1b non-ASCII line survives" \
+  '(epoch 1)\n(eval (quote caf\xc3\xa9))\n(epoch 2)\n(eval "99")\n' \
+  '99'
+
+# Control: a well-formed session still works end to end
+ctrl=$(printf '(epoch 1)\n(eval "(+ 40 2)")\n' | timeout 60 "$SERVER" 2>&1)
+if grep -q '^42$' <<<"$ctrl"; then
+  echo "PASS: control well-formed session"
+  pass=$((pass+1))
+else
+  echo "FAIL: control well-formed session"; sed 's/^/  /' <<<"$ctrl"
+  fail=$((fail+1))
+fi
+
+echo
+echo "protocol-gate: $pass passed, $fail failed"
+[[ $fail -eq 0 ]]