diff --git a/skills/strix-ci-setup/SKILL.md b/skills/strix-ci-setup/SKILL.md
index 416c1c665..97859d653 100644
--- a/skills/strix-ci-setup/SKILL.md
+++ b/skills/strix-ci-setup/SKILL.md
@@ -85,7 +85,12 @@ if [ -z "$BASE_BRANCH" ]; then
   BASE_BRANCH="${BASE_BRANCH#origin/}"
 fi
 DIFF_BASE="origin/${BASE_BRANCH:-main}"
-git rev-parse --verify --quiet "$DIFF_BASE" >/dev/null || DIFF_BASE="HEAD~1"
+# Fail loudly rather than silently narrowing scope (e.g. to HEAD~1, which on a
+# multi-commit branch would scan only the last commit and let earlier ones pass).
+if ! git rev-parse --verify --quiet "$DIFF_BASE" >/dev/null; then
+  echo "Cannot resolve diff base '$DIFF_BASE'. Fetch the base branch (git fetch origin <base>) or set --diff-base explicitly." >&2
+  exit 1
+fi
 strix -n -t ./ --scan-mode quick --scope-mode diff --diff-base "$DIFF_BASE" --max-budget 10
 ```
 
diff --git a/skills/strix-fix-findings/SKILL.md b/skills/strix-fix-findings/SKILL.md
index fd00fad32..bb28acb6a 100644
--- a/skills/strix-fix-findings/SKILL.md
+++ b/skills/strix-fix-findings/SKILL.md
@@ -49,7 +49,10 @@ if [ -z "$DIFF_BASE" ]; then
     git rev-parse --verify --quiet "$b" >/dev/null && DIFF_BASE="$b" && break
   done
 fi
-DIFF_BASE="${DIFF_BASE:-HEAD~1}"
+# No silent fallback: a guess like HEAD~1 would cover only the last commit of a
+# multi-commit fix branch. If no base resolves, ask the user for the base branch
+# (or use the focused --instruction verification below, which needs no diff base).
+[ -n "$DIFF_BASE" ] || { echo "Set DIFF_BASE to the branch your fix will merge into." >&2; exit 1; }
 strix -n -t ./ --scan-mode quick --scope-mode diff --diff-base "$DIFF_BASE" --max-budget 5
 
 # Or re-test with the original finding as focus (no diff base needed)