diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index d87d1a2..cadd4e0 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -272,3 +272,48 @@ jobs:
           fi
           grep -q "must be a valid JSON array" "$OUTPUT"
           rm -f "$OUTPUT"
+
+      - name: 'Test: warning_days non-integer value'
+        shell: bash
+        run: |
+          OUTPUT=$(mktemp)
+          EXIT=0
+          INPUT_CERTIFICATES='["tests/fixtures/valid.pem"]' \
+          INPUT_WARNING_DAYS=abc \
+            bash validate.sh > "$OUTPUT" 2>&1 || EXIT=$?
+          if [[ "$EXIT" -ne 1 ]]; then
+            echo "::error::Expected exit code 1 for non-integer warning_days, got $EXIT"
+            cat "$OUTPUT"
+            exit 1
+          fi
+          grep -q "positive integer" "$OUTPUT"
+          rm -f "$OUTPUT"
+
+      - name: 'Test: warning_days zero'
+        shell: bash
+        run: |
+          OUTPUT=$(mktemp)
+          EXIT=0
+          INPUT_CERTIFICATES='["tests/fixtures/valid.pem"]' \
+          INPUT_WARNING_DAYS=0 \
+            bash validate.sh > "$OUTPUT" 2>&1 || EXIT=$?
+          if [[ "$EXIT" -ne 1 ]]; then
+            echo "::error::Expected exit code 1 for warning_days=0, got $EXIT"
+            cat "$OUTPUT"
+            exit 1
+          fi
+          grep -q "positive integer" "$OUTPUT"
+          rm -f "$OUTPUT"
+
+      - name: 'Test: GITHUB_STEP_SUMMARY unset'
+        shell: bash
+        run: |
+          EXIT=0
+          ( unset GITHUB_STEP_SUMMARY
+            INPUT_CERTIFICATES='["tests/fixtures/valid.pem"]' \
+            INPUT_WARNING_DAYS=30 \
+              bash validate.sh ) || EXIT=$?
+          if [[ "$EXIT" -ne 0 ]]; then
+            echo "::error::Expected exit code 0 when GITHUB_STEP_SUMMARY is unset, got $EXIT"
+            exit 1
+          fi
diff --git a/README.md b/README.md
index 0fe89ca..5e93666 100644
--- a/README.md
+++ b/README.md
@@ -178,6 +178,8 @@ See the [`examples/`](examples/) directory for ready-to-use workflow files.
 ## Requirements
 
 - Runner must have `openssl` and `jq` installed (both are available by default on `ubuntu-latest` and `macos-latest` GitHub-hosted runners)
+- The action checks for both tools at startup and exits with a clear `::error::` message if either is missing
+- Safe to run locally or with [`act`](https://github.com/nektos/act): when `GITHUB_STEP_SUMMARY` is unset the Step Summary block is silently skipped — all other output and exit codes remain stable
 
 ---
 
diff --git a/validate.sh b/validate.sh
index 7dc55c1..b695fa2 100644
--- a/validate.sh
+++ b/validate.sh
@@ -17,10 +17,19 @@
 
 set -uo pipefail
 
+# -------------------------------------------------------
+# Tool availability check — fail fast with a clear message
+# -------------------------------------------------------
+for tool in openssl jq; do
+  if ! command -v "$tool" &>/dev/null; then
+    echo "::error::Required tool '$tool' is not installed on this runner."
+    exit 1
+  fi
+done
+
 INPUT_CERTIFICATES="${INPUT_CERTIFICATES:?'INPUT_CERTIFICATES environment variable is required'}"
 INPUT_WARNING_DAYS="${INPUT_WARNING_DAYS:-30}"
 INPUT_FAIL_ON_WARN="${INPUT_FAIL_ON_WARN:-false}"
-GITHUB_STEP_SUMMARY="${GITHUB_STEP_SUMMARY:-/dev/null}"
 GITHUB_OUTPUT="${GITHUB_OUTPUT:-/dev/null}"
 
 FAIL_ON_WARN="${INPUT_FAIL_ON_WARN,,}"
@@ -29,8 +38,8 @@ if [[ "$FAIL_ON_WARN" != "true" && "$FAIL_ON_WARN" != "false" ]]; then
   exit 1
 fi
 
-if [[ ! "$INPUT_WARNING_DAYS" =~ ^[0-9]+$ ]]; then
-  echo "::error::INPUT_WARNING_DAYS must be a non-negative integer, got: '$INPUT_WARNING_DAYS'"
+if ! [[ "$INPUT_WARNING_DAYS" =~ ^[0-9]+$ ]] || [[ "$((10#$INPUT_WARNING_DAYS))" -eq 0 ]]; then
+  echo "::error::warning_days must be a positive integer, got: '$INPUT_WARNING_DAYS'"
   exit 1
 fi
 
@@ -260,28 +269,30 @@ echo ""
 # -------------------------------------------------------
 # Write summary to GitHub Step Summary for a nice UI table
 # -------------------------------------------------------
-{
-  echo "## 🔐 Certificate Validation Summary"
-  echo ""
-  echo "| Certificate | Status |"
-  echo "|-------------|--------|"
-  for row in "${SUMMARY_ROWS[@]}"; do
-    echo "$row"
-  done
-  echo ""
-  if [[ $FAIL_COUNT -gt 0 ]]; then
-    echo "> ❌ **$FAIL_COUNT certificate(s) FAILED validation with no valid replacement.**"
-  elif [[ $WARN_COUNT -gt 0 ]]; then
-    echo "> ⚠️ **$WARN_COUNT certificate(s) are nearing expiry with no newer replacement.**"
-  else
-    echo "> ✅ **All certificate(s) are valid (or have valid replacements).**"
-  fi
-  echo ""
-  echo "| Metric | Count |"
-  echo "|--------|-------|"
-  echo "| Failed | $FAIL_COUNT |"
-  echo "| Warnings | $WARN_COUNT |"
-} >> "$GITHUB_STEP_SUMMARY"
+if [[ -n "${GITHUB_STEP_SUMMARY:-}" ]]; then
+  {
+    echo "## 🔐 Certificate Validation Summary"
+    echo ""
+    echo "| Certificate | Status |"
+    echo "|-------------|--------|"
+    for row in "${SUMMARY_ROWS[@]}"; do
+      echo "$row"
+    done
+    echo ""
+    if [[ $FAIL_COUNT -gt 0 ]]; then
+      echo "> ❌ **$FAIL_COUNT certificate(s) FAILED validation with no valid replacement.**"
+    elif [[ $WARN_COUNT -gt 0 ]]; then
+      echo "> ⚠️ **$WARN_COUNT certificate(s) are nearing expiry with no newer replacement.**"
+    else
+      echo "> ✅ **All certificate(s) are valid (or have valid replacements).**"
+    fi
+    echo ""
+    echo "| Metric | Count |"
+    echo "|--------|-------|"
+    echo "| Failed | $FAIL_COUNT |"
+    echo "| Warnings | $WARN_COUNT |"
+  } >> "$GITHUB_STEP_SUMMARY"
+fi
 
 # -------------------------------------------------------
 # Emit outputs for programmatic use