diff --git a/.github/workflows/runner_input_validation.yml b/.github/workflows/runner_input_validation.yml
new file mode 100644
index 0000000..7bfaf06
--- /dev/null
+++ b/.github/workflows/runner_input_validation.yml
@@ -0,0 +1,24 @@
+name: Runner input validation
+
+on:
+  pull_request:
+    paths:
+      - action.sh
+      - action.yml
+      - tests/runner_input_validation_test.sh
+      - .github/workflows/runner_input_validation.yml
+  push:
+    branches:
+      - main
+    paths:
+      - action.sh
+      - action.yml
+      - tests/runner_input_validation_test.sh
+      - .github/workflows/runner_input_validation.yml
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: tests/runner_input_validation_test.sh
diff --git a/action.sh b/action.sh
index ea30192..743f804 100755
--- a/action.sh
+++ b/action.sh
@@ -164,6 +164,22 @@ do
   esac
 done
 
+function is_exact_runner_version {
+  [[ "$1" =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]]
+}
+
+function validate_inputs {
+  if [[ "${runner_ver}" != "latest" ]] && ! is_exact_runner_version "${runner_ver}"; then
+    echo "Invalid runner_ver: expected 'latest' or a version in X.Y.Z format" >&2
+    exit 1
+  fi
+
+  if [[ ! "${shutdown_timeout}" =~ ^[0-9]+$ || ${#shutdown_timeout} -gt 5 ]] || (( 10#${shutdown_timeout} > 86400 )); then
+    echo "Invalid shutdown_timeout: expected an integer from 0 through 86400" >&2
+    exit 1
+  fi
+}
+
 function gcloud_auth {
   # NOTE: when --project is specified, it updates the config
   echo ${service_account_key} | gcloud --project  ${project_id} --quiet auth activate-service-account --key-file - &>/dev/null
@@ -179,6 +195,16 @@ function start_vm {
     gcloud_auth
   fi
 
+  if [[ "${actions_preinstalled}" != "true" && "${runner_ver}" == "latest" ]]; then
+    latest_ver=$(curl -sL https://api.github.com/repos/actions/runner/releases/latest | jq -r '.tag_name' | sed -e 's/^v//')
+    if ! is_exact_runner_version "$latest_ver"; then
+      echo "Invalid resolved runner version: expected a version in X.Y.Z format" >&2
+      exit 2
+    fi
+    runner_ver="$latest_ver"
+    echo "✅ runner_ver=latest is specified. v$latest_ver is detected as the latest version."
+  fi
+
   RUNNER_TOKEN=$(curl -S -s -XPOST \
       -H "authorization: Bearer ${token}" \
       https://api.github.com/repos/${GITHUB_REPOSITORY}/actions/runners/registration-token |\
@@ -258,15 +284,6 @@ function start_vm {
     cd /actions-runner
     $startup_script"
   else
-    if [[ "$runner_ver" = "latest" ]]; then
-      latest_ver=$(curl -sL https://api.github.com/repos/actions/runner/releases/latest | jq -r '.tag_name' | sed -e 's/^v//')
-      runner_ver="$latest_ver"
-      echo "✅ runner_ver=latest is specified. v$latest_ver is detected as the latest version."
-      if [[ -z "$latest_ver" || "null" == "$latest_ver" ]]; then
-        echo "❌ could not retrieve the latest version of a runner"
-        exit 2
-      fi
-    fi
     echo "✅ Startup script will install GitHub Actions v$runner_ver"
     if $arm ; then
       startup_script="${startup_prelude}
@@ -352,6 +369,7 @@ function start_vm {
 }
 
 safety_on
+validate_inputs
 case "$command" in
   start)
     start_vm
diff --git a/action.yml b/action.yml
index 83e7811..e8b82e9 100644
--- a/action.yml
+++ b/action.yml
@@ -21,7 +21,7 @@ inputs:
       This key should be created and stored as a secret. Should be JSON key.
     required: false
   runner_ver:
-    description: Version of the GitHub Runner. "latest" will resolve the latest version.
+    description: Version of the GitHub Runner in X.Y.Z format. "latest" will resolve the latest version.
     default: "latest"
     required: true
   vm_name_prefix:
@@ -87,7 +87,7 @@ inputs:
     default: cloud-platform
     required: true
   shutdown_timeout:
-    description: "Shutdown grace period (in seconds)."
+    description: "Shutdown grace period in seconds, from 0 through 86400."
     default: 30
     required: true
   actions_preinstalled:
@@ -117,13 +117,16 @@ runs:
   using: "composite"
   steps:
     - id: gce-github-runner-script
+      env:
+        INPUT_RUNNER_VER: ${{ inputs.runner_ver }}
+        INPUT_SHUTDOWN_TIMEOUT: ${{ inputs.shutdown_timeout }}
       run: >
         ${{ github.action_path }}/action.sh
         --command=start
         --token=${{ inputs.token }}
         --project_id=${{ inputs.project_id }}
         --service_account_key='${{ inputs.service_account_key }}'
-        --runner_ver=${{ inputs.runner_ver }}
+        --runner_ver="${INPUT_RUNNER_VER}"
         --vm_name_prefix=${{ inputs.vm_name_prefix }}
         --machine_zone=${{ inputs.machine_zone }}
         --machine_type=${{ inputs.machine_type }}
@@ -132,7 +135,7 @@ runs:
         --accelerator=${{ inputs.accelerator }}
         --disk_size=${{ inputs.disk_size }}
         --scopes=${{ inputs.scopes }}
-        --shutdown_timeout=${{ inputs.shutdown_timeout }}
+        --shutdown_timeout="${INPUT_SHUTDOWN_TIMEOUT}"
         --runner_service_account=${{ inputs.runner_service_account }}
         --image_project=${{ inputs.image_project }}
         --image=${{ inputs.image }}
diff --git a/tests/runner_input_validation_test.sh b/tests/runner_input_validation_test.sh
new file mode 100755
index 0000000..03c57bd
--- /dev/null
+++ b/tests/runner_input_validation_test.sh
@@ -0,0 +1,111 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+repo_root=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
+tmp_dir=$(mktemp -d)
+trap 'rm -rf "$tmp_dir"' EXIT
+mkdir -p "$tmp_dir/bin"
+
+cat > "$tmp_dir/bin/curl" <<'EOF'
+#!/usr/bin/env bash
+if [[ "$*" == *"/actions/runner/releases/latest"* ]]; then
+  printf '{"tag_name":"%s"}\n' "${MOCK_LATEST_TAG:-v2.321.0}"
+else
+  printf '{"token":"runner-token"}\n'
+fi
+EOF
+
+cat > "$tmp_dir/bin/gcloud" <<'EOF'
+#!/usr/bin/env bash
+printf '%s\n' "$*" >> "${GCLOUD_LOG}"
+if [[ "$*" == *"compute instances describe"* ]]; then
+  printf '{"labels":{"gh_ready":"1"}}\n'
+fi
+EOF
+chmod +x "$tmp_dir/bin/curl" "$tmp_dir/bin/gcloud"
+
+run_action() {
+  local runner_ver=$1
+  local shutdown_timeout=$2
+  local latest_tag=${3:-v2.321.0}
+  : > "$tmp_dir/gcloud.log"
+  set +e
+  output=$(
+    PATH="$tmp_dir/bin:$PATH" \
+    GCLOUD_LOG="$tmp_dir/gcloud.log" \
+    MOCK_LATEST_TAG="$latest_tag" \
+    GITHUB_REPOSITORY=example/repo \
+    GITHUB_REPOSITORY_OWNER=example \
+    GITHUB_RUN_ID=123 \
+    GITHUB_RUN_ATTEMPT=1 \
+    GITHUB_OUTPUT="$tmp_dir/output" \
+    "$repo_root/action.sh" \
+      --command=start \
+      --token=test-token \
+      --project_id= \
+      --service_account_key= \
+      --runner_ver="$runner_ver" \
+      --vm_name_prefix=test-runner \
+      --machine_zone=us-east1-c \
+      --machine_type=n1-standard-1 \
+      --scopes=cloud-platform \
+      --shutdown_timeout="$shutdown_timeout" \
+      --preemptible=false \
+      --ephemeral=false \
+      --no_external_address=false \
+      --actions_preinstalled=false \
+      --arm=false \
+      2>&1
+  )
+  status=$?
+  set -e
+}
+
+assert_rejected_before_gcloud() {
+  local expected_message=$1
+  if [[ $status -eq 0 ]]; then
+    printf 'expected failure, got success\n%s\n' "$output" >&2
+    exit 1
+  fi
+  if [[ "$output" != *"$expected_message"* ]]; then
+    printf 'missing error %q in output:\n%s\n' "$expected_message" "$output" >&2
+    exit 1
+  fi
+  if [[ -s "$tmp_dir/gcloud.log" ]]; then
+    printf 'gcloud ran for rejected input:\n' >&2
+    cat "$tmp_dir/gcloud.log" >&2
+    exit 1
+  fi
+}
+
+run_action '2.321.0; touch /tmp/runner-version-marker' 30
+assert_rejected_before_gcloud 'Invalid runner_ver'
+
+run_action '2.321' 30
+assert_rejected_before_gcloud 'Invalid runner_ver'
+
+run_action '2.321.0' '30; touch /tmp/shutdown-timeout-marker'
+assert_rejected_before_gcloud 'Invalid shutdown_timeout'
+
+run_action '2.321.0' 86401
+assert_rejected_before_gcloud 'Invalid shutdown_timeout'
+
+run_action '2.321.0' 999999999999999999999999
+assert_rejected_before_gcloud 'Invalid shutdown_timeout'
+
+run_action latest 30 'v2.321.0; touch /tmp/resolved-version-marker'
+assert_rejected_before_gcloud 'Invalid resolved runner version'
+
+run_action '2.321.0' 0
+if [[ $status -ne 0 ]]; then
+  printf 'valid explicit version failed:\n%s\n' "$output" >&2
+  exit 1
+fi
+
+run_action latest 86400
+if [[ $status -ne 0 ]]; then
+  printf 'valid latest version failed:\n%s\n' "$output" >&2
+  exit 1
+fi
+
+printf 'runner input validation tests passed\n'