diff --git a/tests/test-check-rtk-prefix.sh b/tests/test-check-rtk-prefix.sh
index 580bc4f..fa18022 100644
--- a/tests/test-check-rtk-prefix.sh
+++ b/tests/test-check-rtk-prefix.sh
@@ -4,9 +4,16 @@ set -euo pipefail
 # Tests for scripts/check-rtk-prefix.sh — ADR-0023 clause 1.
 #
 # Case 1 is the one that earns the rest: it runs the gate against the corpus as
-# it stood on `main` BEFORE the #113 sweep, and asserts it fails there. A gate
-# that only passes on the already-fixed tree proves nothing about whether it
-# would have caught the drift it was written for.
+# it stood BEFORE the #113 sweep, and asserts it fails there. A gate that only
+# passes on the already-fixed tree proves nothing about whether it would have
+# caught the drift it was written for.
+#
+# Pinned to a commit SHA, not the `main` ref: `main` is the moving integration
+# branch, and the #113 fix (ed8c99e) landed back on it, so querying live
+# `main` for the "pre-sweep" corpus stopped reproducing the drift the moment
+# that fix merged — the case silently started asserting the opposite of what
+# it says. 598a7c3 is the last commit where gitea-issues/SKILL.md still had
+# the bare `git remote get-url origin` call ed8c99e fixed.
 #
 # Everything after that is synthetic. The false-NEGATIVE cases (a bare command
 # the gate must catch) and the false-POSITIVE cases (correct content the gate
@@ -48,12 +55,15 @@ expect() {
   fi
 }
 
-echo "--- the pre-#113 corpus on main trips the gate ---"
+echo "--- the pre-#113 corpus trips the gate ---"
 
-# Derived from git, not hardcoded: the point is "the drift this gate exists for",
-# and a hardcoded path list goes stale the moment a file is renamed.
-if ! git -C "$REPO_ROOT" rev-parse --verify -q main >/dev/null; then
-  echo "SKIP: no local 'main' ref — the historical-corpus case cannot be reconstructed"
+# The commit set is derived from git, not hardcoded, at this fixed SHA: the
+# point is "the drift this gate exists for", and a hardcoded path list goes
+# stale the moment a file is renamed. The SHA itself is deliberately hardcoded
+# — see the header comment for why it must NOT be a moving ref like `main`.
+PRE_SWEEP_REF="598a7c326a4bbfa4a39007eb95d944c4150686e8"
+if ! git -C "$REPO_ROOT" rev-parse --verify -q "$PRE_SWEEP_REF" >/dev/null; then
+  echo "SKIP: pre-sweep commit $PRE_SWEEP_REF not reachable locally (shallow clone?) — the historical-corpus case cannot be reconstructed"
   exit 77
 fi
 
@@ -67,16 +77,16 @@ while IFS= read -r PRE_REL; do
   PRE_FILES+=("$PRE_REL")
   PRE_PATHS+=("$PRE/$PRE_REL")
 done < <(
-  git -C "$REPO_ROOT" ls-tree -r --name-only main -- 'plugins' \
+  git -C "$REPO_ROOT" ls-tree -r --name-only "$PRE_SWEEP_REF" -- 'plugins' \
     | grep -E '^plugins/[^/]+/\.apm/(skills/.*\.md|agents/.*\.agent\.md)$' \
     | grep -v '/README\.md$'
 )
 if [[ ${#PRE_FILES[@]} -eq 0 ]]; then
-  fail "no plugin skill/agent files found on main — the historical case checked nothing"
+  fail "no plugin skill/agent files found at $PRE_SWEEP_REF — the historical case checked nothing"
 else
   for f in ${PRE_FILES[@]+"${PRE_FILES[@]}"}; do
     mkdir -p "$PRE/$(dirname "$f")"
-    git -C "$REPO_ROOT" show "main:$f" > "$PRE/$f"
+    git -C "$REPO_ROOT" show "$PRE_SWEEP_REF:$f" > "$PRE/$f"
   done
   if bash "$SCRIPT" ${PRE_PATHS[@]+"${PRE_PATHS[@]}"} > "$RUN_TMP/pre.out" 2>&1; then
     fail "the pre-sweep corpus passed — the gate would not have caught the #113 drift"