diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 405fa26..456025e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -93,6 +93,9 @@ jobs:
       - name: cargo test
         run: cargo test --locked
 
+      - name: Sweep helper regression check
+        run: bash tests/gate03_sweep_test.sh
+
       # TEST-02 (issue #29). The criterion benches in `benches/scan.rs` cover the
       # four shapes that matter — pattern-set compile, one large file, 500 small
       # files, and a pathological line — but nothing ran them, so they could rot
diff --git a/scripts/gate03-sweep.sh b/scripts/gate03-sweep.sh
index a518167..89e0647 100755
--- a/scripts/gate03-sweep.sh
+++ b/scripts/gate03-sweep.sh
@@ -95,8 +95,9 @@ redact_home() {
 }
 
 sweep_one() {
-  out_dir="$1"
-  dir="$2"
+  local out_dir="$1"
+  local dir="$2"
+  local abs_dir slug report status files_scanned findings
 
   if [ ! -d "$dir" ]; then
     echo "WARNING: skipping '$dir' -- not present on this machine" >&2
@@ -150,7 +151,7 @@ print(sum(len(r['matches']) for r in data))
 # Aggregates findings across every per-directory JSON report already written
 # into <output-dir> by pattern id and severity.
 build_summary() {
-  out_dir="$1"
+  local out_dir="$1"
   python3 -c "
 import glob, json, os, sys
 from collections import Counter
@@ -202,6 +203,8 @@ sys.exit(1 if new else 0)
 }
 
 main() {
+  local out_dir dir
+
   if [ "$#" -eq 0 ]; then
     usage
     exit 2
diff --git a/tests/gate03_sweep_test.sh b/tests/gate03_sweep_test.sh
new file mode 100644
index 0000000..1fe2811
--- /dev/null
+++ b/tests/gate03_sweep_test.sh
@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# Run with: bash tests/gate03_sweep_test.sh
+set -euo pipefail
+
+repo_dir="$(cd "$(dirname "$0")/.." && pwd)"
+# Load the real helpers without executing the command-line entry point.
+source <(sed '/^main "\$@"$/d' "${1:-$repo_dir/scripts/gate03-sweep.sh}")
+
+scratch="$(mktemp -d)"
+trap 'rm -rf -- "$scratch"' EXIT
+mkdir -p "$scratch/input" "$scratch/output" "$scratch/empty"
+
+# A deterministic scanner substitute exercises the helpers' JSON processing.
+fake_scanner() {
+  printf '[{"file":"probe.md","matches":[{"line":1,"pattern_id":"PI001","severity":"CRITICAL"}]}]\n'
+  return "$scanner_status"
+}
+BIN=fake_scanner
+require_binary() { :; }
+
+out_dir=outer_out_dir dir=outer_dir abs_dir=outer_abs_dir slug=outer_slug
+report=outer_report status=outer_status files_scanned=outer_files findings=outer_findings
+before="$(declare -p out_dir dir abs_dir slug report status files_scanned findings)"
+
+scanner_status=0
+sweep_one "$scratch/output" "$scratch/input"
+build_summary "$scratch/output"
+[[ "$(declare -p out_dir dir abs_dir slug report status files_scanned findings)" == "$before" ]]
+
+for scanner_status in 0 1 2; do
+  main "$scratch/output" "$scratch/input" "$scratch/missing"
+  [[ "$(declare -p out_dir dir abs_dir slug report status files_scanned findings)" == "$before" ]]
+  [[ "$(wc -l < "$scratch/output/manifest.tsv")" -eq 2 ]]
+  grep -q $'\t1\t1\tswept$' "$scratch/output/manifest.tsv"
+  grep -q $'\t0\t0\tskipped-missing$' "$scratch/output/manifest.tsv"
+  [[ "$(cat "$scratch/output/summary.tsv")" == $'PI001\tCRITICAL\t1' ]]
+done
+
+do_compare "$scratch/output" "$scratch/output"
+if do_compare "$scratch/empty" "$scratch/output" > "$scratch/delta"; then
+  echo 'comparison must return 1 for a new finding' >&2
+  exit 1
+else
+  [[ "$?" -eq 1 ]]
+fi
+[[ "$(cat "$scratch/delta")" == $'probe.md:1\tPI001' ]]
+[[ "$(declare -p out_dir dir abs_dir slug report status files_scanned findings)" == "$before" ]]
+echo 'gate03 sweep helper checks passed'