Fix Strix quick gate fallback check matching and count logic

scripts/ci/strix_quick_gate.sh

@@ -837,14 +837,16 @@ PY
 	fi
 
 	local changed_files_output
-	if ! changed_files_output="$(git diff --name-only "$base_sha...$head_sha" --)"; then
+	if ! changed_files_output="$(git diff --name-only "$base_sha...$head_sha" -- 2>/dev/null)"; then
 		if [ "${GITHUB_EVENT_NAME:-}" = "workflow_dispatch" ] && pull_request_metadata_env_present; then
-			if changed_files_output="$(git diff --name-only "$base_sha" "$head_sha" --)"; then
+			if changed_files_output="$(git diff --name-only "$base_sha" "$head_sha" -- 2>/dev/null)"; then
 				echo "Using explicit base/head diff for workflow_dispatch PR-scope Strix evidence." >&2
 			else
 				echo "ERROR: pull request changed file list could not be read; failing closed." >&2
 				return 2
 			fi
+		elif changed_files_output="$(git diff --name-only "$base_sha..$head_sha" -- 2>/dev/null)"; then
+			echo "INFO: Unable to compute PR merge base; falling back to direct base/head diff for changed file enumeration." >&2
 		else
 			if pull_request_head_blob_required; then
 				echo "ERROR: pull request changed file list could not be read; failing closed." >&2
@@ -2957,10 +2959,15 @@ run_current_target_scan() {
 	if [ "$fallback_tried" -eq 0 ]; then
 		local fallback_config_name
 		fallback_config_name="$(fallback_models_config_name_for_model "$PRIMARY_MODEL")"
-		if [ "${#FALLBACK_MODELS[@]}" -eq 0 ]; then
+		local configured_fallback_count=0
+		for candidate_raw in "${FALLBACK_MODELS[@]}"; do
+			candidate="$(normalize_model "$candidate_raw")"
+			[ -n "$candidate" ] && configured_fallback_count=$((configured_fallback_count + 1))
+		done
+		if [ "$configured_fallback_count" -eq 0 ]; then
 			echo "ERROR: No fallback models configured ($fallback_config_name is empty). Configure distinct models." >&2
 		else
-			echo "ERROR: All configured fallback models are the same as the primary model '$PRIMARY_MODEL'. Configure distinct models in $fallback_config_name." >&2
+			echo "ERROR: All configured fallback models are the same as the primary model" >&2
 		fi
 		return 1
 	fi

scripts/ci/test_strix_quick_gate.sh

@@ -3093,6 +3093,100 @@ EOF
 	rm -rf "$tmp_dir"
 }
 
+run_pull_request_target_shallow_head_merge_base_fallback_case() {
+	local tmp_dir
+	tmp_dir="$(mktemp -d)"
+	local bin_dir="$tmp_dir/bin"
+	local origin_repo_dir="$tmp_dir/origin"
+	local repo_root_dir="$tmp_dir/repo"
+	mkdir -p "$bin_dir" "$origin_repo_dir" "$repo_root_dir/scripts/ci"
+
+	cp "$GATE_SCRIPT" "$repo_root_dir/scripts/ci/strix_quick_gate.sh"
+	cp "$REPO_ROOT/scripts/ci/strix_model_utils.sh" "$repo_root_dir/scripts/ci/strix_model_utils.sh"
+	chmod +x "$repo_root_dir/scripts/ci/strix_quick_gate.sh"
+
+	local fake_strix="$bin_dir/strix"
+	local output_log="$tmp_dir/output.log"
+	local strix_llm_file="$tmp_dir/strix_llm.txt"
+	local llm_api_key_file="$tmp_dir/llm_api_key.txt"
+
+	cat >"$fake_strix" <<'EOF'
+#!/usr/bin/env bash
+set -euo pipefail
+echo "scan ok"
+exit 0
+EOF
+	chmod +x "$fake_strix"
+	printf '%s' 'gemini/test-model' >"$strix_llm_file"
+	printf '%s' 'dummy' >"$llm_api_key_file"
+
+	(
+		cd "$origin_repo_dir"
+		git init -q
+		git config user.name 'Strix Test'
+		git config user.email 'strix-test@example.invalid'
+		mkdir -p src
+		printf '%s\n' 'BASE_CONTENT' >src/app.py
+		git add .
+		git commit -qm 'base commit'
+		printf '%s\n' 'MID_CONTENT' >src/app.py
+		git add .
+		git commit -qm 'mid commit'
+		printf '%s\n' 'HEAD_CONTENT' >src/app.py
+		git add .
+		git commit -qm 'head commit'
+	)
+	local base_sha
+	base_sha="$(git -C "$origin_repo_dir" rev-list --max-parents=0 HEAD)"
+	local head_sha
+	head_sha="$(git -C "$origin_repo_dir" rev-parse HEAD)"
+
+	(
+		cd "$repo_root_dir"
+		git init -q
+		git config user.name 'Strix Test'
+		git config user.email 'strix-test@example.invalid'
+		git remote add origin "$origin_repo_dir"
+		git fetch -q --depth=1 origin "$base_sha"
+		git checkout -q FETCH_HEAD
+		git fetch -q --depth=1 origin "$head_sha"
+	)
+
+	set +e
+	(
+		cd "$repo_root_dir"
+		git diff --name-only "$base_sha...$head_sha" -- >/dev/null 2>&1
+	)
+	local merge_base_diff_rc=$?
+	set -e
+	if [ "$merge_base_diff_rc" -eq 0 ]; then
+		record_failure "case=pull-request-target-shallow-head expected base...head diff to fail"
+	fi
+
+	set +e
+	(
+		cd "$repo_root_dir"
+		env -u GITHUB_EVENT_PATH -u STRIX_TEST_CHANGED_FILES_OVERRIDE \
+			PATH="$bin_dir:$PATH" \
+			STRIX_INPUT_FILE_ROOT="$tmp_dir" \
+			GITHUB_EVENT_NAME="pull_request_target" \
+			PR_BASE_SHA="$base_sha" \
+			PR_HEAD_SHA="$head_sha" \
+			STRIX_LLM_FILE="$strix_llm_file" \
+			LLM_API_KEY_FILE="$llm_api_key_file" \
+			STRIX_TARGET_PATH="." \
+			STRIX_REPORTS_DIR="$repo_root_dir/strix_runs" \
+			bash "./scripts/ci/strix_quick_gate.sh" >"$output_log" 2>&1
+	)
+	local rc=$?
+	set -e
+
+	assert_equals "0" "$rc" "case=pull-request-target-shallow-head exit code"
+	assert_file_contains "$output_log" "falling back to direct base/head diff" "case=pull-request-target-shallow-head output"
+
+	rm -rf "$tmp_dir"
+}
+
 run_pull_request_target_aborts_on_pr_head_blob_failure_case() {
 	local case_name="$1"
 	local changed_file="$2"
@@ -4429,6 +4523,8 @@ run_pull_request_target_head_scope_case \
 
 run_pull_request_target_plaintext_runner_token_fails_closed_case
 
+run_pull_request_target_shallow_head_merge_base_fallback_case
+
 run_pull_request_target_rejects_unsafe_changed_path_case \
 	"pull-request-target-parent-directory-changed-path-fails-closed" \
 	"../outside.py"