diff --git a/actions/setup/sh/install_threat_detect_binary.sh b/actions/setup/sh/install_threat_detect_binary.sh
index ff6a03f7e6d..7dc7151e55d 100755
--- a/actions/setup/sh/install_threat_detect_binary.sh
+++ b/actions/setup/sh/install_threat_detect_binary.sh
@@ -1,18 +1,22 @@
 #!/usr/bin/env bash
 set +o histexpand
 
-# Install the threat-detect binary from GitHub Releases with SHA256 checksum verification.
+# Install the threat-detect binary from GitHub Releases with setup-action-pinned SHA256 verification.
 # Used when `features: gh-aw-detection: true` is set in the workflow frontmatter to enable
 # the external threat-detect binary detection path instead of inline engine execution.
 #
-# Usage: install_threat_detect_binary.sh VERSION [--rootless]
+# Usage: install_threat_detect_binary.sh VERSION [--sha256-amd64 DIGEST --sha256-arm64 DIGEST] [--rootless]
 #
 # Arguments:
 #   VERSION    - threat-detect version to install (e.g., v0.2.2) or "latest" to
 #                install the latest release via GitHub's latest-release download endpoint
-#   --rootless - Install to ~/.local/bin without sudo; appends that directory to
-#                $GITHUB_PATH so subsequent steps find the binary.  Use this on
-#                ARC/DinD runners that enforce allowPrivilegeEscalation: false.
+#   --sha256-amd64 - Expected SHA256 digest for the Linux amd64 binary (required
+#                    when VERSION differs from the version pinned in this action)
+#   --sha256-arm64 - Expected SHA256 digest for the Linux arm64 binary (required
+#                    when VERSION differs from the version pinned in this action)
+#   --rootless     - Install to ~/.local/bin without sudo; appends that directory to
+#                    $GITHUB_PATH so subsequent steps find the binary. Use this on
+#                    ARC/DinD runners that enforce allowPrivilegeEscalation: false.
 #
 # Platform support:
 #   - Linux (x64, arm64): Downloads pre-built binary
@@ -24,7 +28,7 @@ set +o histexpand
 #
 # Security features:
 #   - Downloads directly from GitHub releases
-#   - Verifies SHA256 checksum against official checksums.txt
+#   - Verifies SHA256 against setup-action-pinned, architecture-specific digests
 #   - Fails fast if checksum verification fails
 
 set -euo pipefail
@@ -34,28 +38,68 @@ THREAT_DETECT_REPO="github/gh-aw-threat-detection"
 THREAT_DETECT_INSTALL_DIR="/usr/local/bin"
 THREAT_DETECT_INSTALL_NAME="threat-detect"
 MACOS_FAQ_URL="https://github.github.com/gh-aw/reference/faq/#why-are-macos-runners-not-supported"
+PINNED_THREAT_DETECT_VERSION="v0.5.1"
+PINNED_THREAT_DETECT_SHA256_AMD64="1b27989fb52cbdc401e48137e508bea8b915aad4911468fb0fd9c87c2a7cd31b"
+PINNED_THREAT_DETECT_SHA256_ARM64="204ba220229ac3fda80f7603b6e2a816f69e8a5ea2833c0ac534107bbffb827a"
 
-# Parse arguments: treat the first non-flag argument as VERSION, all --<flag> arguments as flags.
+# Parse arguments.
 THREAT_DETECT_VERSION=""
+THREAT_DETECT_SHA256_AMD64=""
+THREAT_DETECT_SHA256_ARM64=""
 ROOTLESS=false
-for arg in "$@"; do
-  case "$arg" in
-    --rootless) ROOTLESS=true ;;
-    --*) echo "WARNING: Unknown flag: $arg" >&2 ;;
+while [ "$#" -gt 0 ]; do
+  case "$1" in
+    --rootless)
+      ROOTLESS=true
+      shift
+      ;;
+    --sha256-amd64|--sha256-arm64)
+      if [ "$#" -lt 2 ]; then
+        echo "ERROR: $1 requires a SHA256 digest" >&2
+        exit 1
+      fi
+      if [ "$1" = "--sha256-amd64" ]; then
+        THREAT_DETECT_SHA256_AMD64="$2"
+      else
+        THREAT_DETECT_SHA256_ARM64="$2"
+      fi
+      shift 2
+      ;;
+    --*)
+      echo "ERROR: Unknown flag: $1" >&2
+      exit 1
+      ;;
     *)
       if [ -z "$THREAT_DETECT_VERSION" ]; then
-        THREAT_DETECT_VERSION="$arg"
+        THREAT_DETECT_VERSION="$1"
+      else
+        echo "ERROR: Unexpected argument: $1" >&2
+        exit 1
       fi
+      shift
       ;;
   esac
 done
 
 if [ -z "$THREAT_DETECT_VERSION" ]; then
   echo "ERROR: threat-detect version is required"
-  echo "Usage: $0 VERSION [--rootless]"
+  echo "Usage: $0 VERSION [--sha256-amd64 DIGEST --sha256-arm64 DIGEST] [--rootless]"
   exit 1
 fi
 
+# Use the digests embedded in this immutable action for the compiler-pinned version.
+if [ "$THREAT_DETECT_VERSION" = "$PINNED_THREAT_DETECT_VERSION" ]; then
+  THREAT_DETECT_SHA256_AMD64="${THREAT_DETECT_SHA256_AMD64:-$PINNED_THREAT_DETECT_SHA256_AMD64}"
+  THREAT_DETECT_SHA256_ARM64="${THREAT_DETECT_SHA256_ARM64:-$PINNED_THREAT_DETECT_SHA256_ARM64}"
+fi
+
+for digest in "$THREAT_DETECT_SHA256_AMD64" "$THREAT_DETECT_SHA256_ARM64"; do
+  if [[ ! "$digest" =~ ^[[:xdigit:]]{64}$ ]]; then
+    echo "ERROR: Valid SHA256 digests are required for both supported architectures" >&2
+    exit 1
+  fi
+done
+
 # In rootless mode, install into the user's home directory instead of /usr/local/bin
 # so that ARC/DinD runners with allowPrivilegeEscalation: false can run without sudo.
 if [ "$ROOTLESS" = "true" ]; then
@@ -110,8 +154,6 @@ if [ "$THREAT_DETECT_VERSION" = "latest" ]; then
 else
   BASE_URL="https://github.com/${THREAT_DETECT_REPO}/releases/download/${THREAT_DETECT_VERSION}"
 fi
-CHECKSUMS_URL="${BASE_URL}/checksums.txt"
-
 # Platform-portable SHA256 function
 sha256_hash() {
   local file="$1"
@@ -129,28 +171,20 @@ sha256_hash() {
 TEMP_DIR=$(mktemp -d)
 trap 'rm -rf "$TEMP_DIR"' EXIT
 
-# Download checksums
-echo "Downloading checksums from \"${CHECKSUMS_URL}\"..."
-curl -fsSL --retry 5 --retry-delay 10 --retry-max-time 180 --retry-all-errors -o "${TEMP_DIR}/checksums.txt" "${CHECKSUMS_URL}"
-
 verify_checksum() {
   local file="$1"
   local fname="$2"
+  local expected_checksum="$3"
+  local actual_checksum
 
   echo "Verifying SHA256 checksum for ${fname}..."
-  EXPECTED_CHECKSUM=$(awk -v fname="${fname}" '$2 == fname {print $1; exit}' "${TEMP_DIR}/checksums.txt" | tr 'A-F' 'a-f')
+  expected_checksum=$(printf '%s' "$expected_checksum" | tr 'A-F' 'a-f')
+  actual_checksum=$(sha256_hash "$file" | tr 'A-F' 'a-f')
 
-  if [ -z "$EXPECTED_CHECKSUM" ]; then
-    echo "ERROR: Could not find checksum for ${fname} in checksums.txt"
-    return 1
-  fi
-
-  ACTUAL_CHECKSUM=$(sha256_hash "$file" | tr 'A-F' 'a-f')
-
-  if [ "$EXPECTED_CHECKSUM" != "$ACTUAL_CHECKSUM" ]; then
+  if [ "$expected_checksum" != "$actual_checksum" ]; then
     echo "ERROR: Checksum verification failed!"
-    echo "  Expected: $EXPECTED_CHECKSUM"
-    echo "  Got:      $ACTUAL_CHECKSUM"
+    echo "  Expected: $expected_checksum"
+    echo "  Got:      $actual_checksum"
     echo "  The downloaded file may be corrupted or tampered with"
     return 1
   fi
@@ -161,9 +195,16 @@ verify_checksum() {
 install_linux_binary() {
   # Determine binary name based on architecture
   local binary_name
+  local expected_checksum
   case "$ARCH" in
-    x86_64|amd64) binary_name="threat-detect-linux-amd64" ;;
-    aarch64|arm64) binary_name="threat-detect-linux-arm64" ;;
+    x86_64|amd64)
+      binary_name="threat-detect-linux-amd64"
+      expected_checksum="$THREAT_DETECT_SHA256_AMD64"
+      ;;
+    aarch64|arm64)
+      binary_name="threat-detect-linux-arm64"
+      expected_checksum="$THREAT_DETECT_SHA256_ARM64"
+      ;;
     *) echo "ERROR: Unsupported Linux architecture: ${ARCH}"; exit 1 ;;
   esac
 
@@ -172,7 +213,7 @@ install_linux_binary() {
   curl -fsSL --retry 5 --retry-delay 10 --retry-max-time 180 --retry-all-errors -o "${TEMP_DIR}/${binary_name}" "${binary_url}"
 
   # Verify checksum
-  verify_checksum "${TEMP_DIR}/${binary_name}" "${binary_name}"
+  verify_checksum "${TEMP_DIR}/${binary_name}" "${binary_name}" "${expected_checksum}"
 
   # Make binary executable and install
   chmod +x "${TEMP_DIR}/${binary_name}"
diff --git a/actions/setup/sh/install_threat_detect_binary_test.sh b/actions/setup/sh/install_threat_detect_binary_test.sh
index 38447933228..f4ff6e0df14 100644
--- a/actions/setup/sh/install_threat_detect_binary_test.sh
+++ b/actions/setup/sh/install_threat_detect_binary_test.sh
@@ -5,11 +5,12 @@ set +o histexpand
 # Run: bash install_threat_detect_binary_test.sh
 #
 # The tests run the real script with a stubbed `uname` (to fake the platform) and a
-# stubbed `curl` (to record the requested asset URL and serve a fake binary plus a
-# matching checksums.txt), so no network access is required.
+# stubbed `curl` (to record the requested asset URL and serve a fake binary), so no
+# network access is required.
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 INSTALL_SCRIPT="${SCRIPT_DIR}/install_threat_detect_binary.sh"
+VERSION_CONSTANTS="${SCRIPT_DIR}/../../../pkg/constants/version_constants.go"
 
 TESTS_PASSED=0
 TESTS_FAILED=0
@@ -17,12 +18,13 @@ TESTS_FAILED=0
 pass() { echo "PASS: $1"; TESTS_PASSED=$((TESTS_PASSED + 1)); }
 fail() { echo "FAIL: $1"; echo "  $2"; TESTS_FAILED=$((TESTS_FAILED + 1)); }
 
-# run_installer OS ARCH VERSION -> runs the installer in an isolated sandbox.
+# run_installer OS ARCH VERSION [CHECKSUM_MODE] -> runs the installer in an isolated sandbox.
 # Sets globals: RUN_OUTPUT (stdout+stderr), RUN_STATUS (exit code), RUN_ASSET (downloaded asset name).
 run_installer() {
   local fake_os="$1"
   local fake_arch="$2"
   local version="$3"
+  local checksum_mode="${4:-valid}"
 
   local sandbox
   sandbox=$(mktemp -d)
@@ -39,7 +41,7 @@ case "\$1" in
 esac
 EOF
 
-  # Stubbed curl records the requested asset and serves a fake binary + checksums.txt.
+  # Stubbed curl records the requested asset and serves a fake binary.
   cat >"${sandbox}/bin/curl" <<'EOF'
 #!/usr/bin/env bash
 out=""
@@ -57,17 +59,10 @@ done
 payload='#!/usr/bin/env bash
 echo "fake threat-detect"'
 
-name="${url##*/}"
 echo "${url}" >>"${SANDBOX_URL_LOG}"
-if [ "$name" = "checksums.txt" ]; then
-  hash=$(printf '%s\n' "$payload" | sha256sum | awk '{print $1}')
-  for asset in threat-detect-linux-amd64 threat-detect-linux-arm64; do
-    echo "${hash}  ${asset}"
-  done >"$out"
-else
-  echo "$name" >>"${SANDBOX_ASSET_LOG}"
-  printf '%s\n' "$payload" >"$out"
-fi
+name="${url##*/}"
+echo "$name" >>"${SANDBOX_ASSET_LOG}"
+printf '%s\n' "$payload" >"$out"
 EOF
 
   chmod +x "${sandbox}/bin/uname" "${sandbox}/bin/curl"
@@ -77,9 +72,22 @@ EOF
   : >"${asset_log}"
   : >"${url_log}"
 
+  local expected_hash
+  expected_hash=$(printf '%s\n' '#!/usr/bin/env bash' 'echo "fake threat-detect"' | sha256sum | awk '{print $1}')
+  local installer_args=("${version}" --rootless)
+  case "$checksum_mode" in
+    valid)
+      installer_args+=(--sha256-amd64 "$expected_hash" --sha256-arm64 "$expected_hash")
+      ;;
+    mismatch)
+      installer_args+=(--sha256-amd64 "$(printf '0%.0s' {1..64})" --sha256-arm64 "$(printf '0%.0s' {1..64})")
+      ;;
+    missing) ;;
+  esac
+
   RUN_OUTPUT=$(cd "${sandbox}" && env PATH="${sandbox}/bin:${PATH}" HOME="${sandbox}" \
     SANDBOX_ASSET_LOG="${asset_log}" SANDBOX_URL_LOG="${url_log}" GITHUB_PATH="" \
-    bash "${INSTALL_SCRIPT}" "${version}" --rootless 2>&1)
+    bash "${INSTALL_SCRIPT}" "${installer_args[@]}" 2>&1)
   RUN_STATUS=$?
   RUN_ASSET=$(cat "${asset_log}")
   RUN_URLS=$(cat "${url_log}")
@@ -149,21 +157,68 @@ assert_failure "Unknown OS is rejected" FreeBSD x86_64 "Unsupported operating sy
 echo "Test 7: unknown Linux architecture fails fast..."
 assert_failure "Unknown Linux architecture is rejected" Linux riscv64 "Unsupported Linux architecture"
 
-# Test 8: latest release assets must be downloaded directly without the GitHub API.
+# Test 8: latest release assets must be downloaded directly without the GitHub API or checksums file.
 echo "Test 8: latest release assets use direct downloads without the GitHub API..."
 run_installer Linux x86_64 latest
 if [ "${RUN_STATUS}" -ne 0 ]; then
   fail "Latest release installer succeeds" "installer exited with ${RUN_STATUS}: ${RUN_OUTPUT}"
-elif ! echo "${RUN_URLS}" | grep -qF "https://github.com/github/gh-aw-threat-detection/releases/latest/download/checksums.txt"; then
-  fail "Latest release installer downloads checksums directly" "expected latest release checksum URL, got: ${RUN_URLS}"
 elif ! echo "${RUN_URLS}" | grep -qF "https://github.com/github/gh-aw-threat-detection/releases/latest/download/threat-detect-linux-amd64"; then
   fail "Latest release installer downloads the binary directly" "expected latest release binary URL, got: ${RUN_URLS}"
+elif echo "${RUN_URLS}" | grep -qF "checksums.txt"; then
+  fail "Latest release installer avoids release-hosted checksums" "unexpected checksum URL: ${RUN_URLS}"
 elif echo "${RUN_URLS}" | grep -qF "api.github.com"; then
   fail "Latest release installer avoids GitHub API" "unexpected API URL: ${RUN_URLS}"
 else
   pass "Latest release installer uses direct downloads without GitHub API"
 fi
 
+# Test 9: checksum mismatches fail closed.
+echo "Test 9: checksum mismatch fails closed..."
+run_installer Linux x86_64 v0.4.0 mismatch
+if [ "${RUN_STATUS}" -eq 0 ]; then
+  fail "Checksum mismatch is rejected" "installer unexpectedly succeeded: ${RUN_OUTPUT}"
+elif ! echo "${RUN_OUTPUT}" | grep -qF "Checksum verification failed"; then
+  fail "Checksum mismatch is rejected" "expected checksum failure in: ${RUN_OUTPUT}"
+else
+  pass "Checksum mismatch is rejected"
+fi
+
+# Test 10: an unpinned version without explicit digests fails before download.
+echo "Test 10: missing digests fail closed before download..."
+run_installer Linux x86_64 v0.4.0 missing
+if [ "${RUN_STATUS}" -eq 0 ]; then
+  fail "Missing digests are rejected" "installer unexpectedly succeeded: ${RUN_OUTPUT}"
+elif ! echo "${RUN_OUTPUT}" | grep -qF "Valid SHA256 digests are required"; then
+  fail "Missing digests are rejected" "expected digest validation failure in: ${RUN_OUTPUT}"
+elif [ -n "${RUN_ASSET}" ]; then
+  fail "Missing digests are rejected before download" "installer attempted a binary download: ${RUN_ASSET}"
+else
+  pass "Missing digests are rejected before download"
+fi
+
+# Test 11: compiled workflows use the embedded digest and fail on mismatch.
+echo "Test 11: pinned version remains secure without explicit digest arguments..."
+run_installer Linux x86_64 v0.5.1 missing
+if [ "${RUN_STATUS}" -eq 0 ]; then
+  fail "Pinned version uses embedded digest" "installer unexpectedly accepted the fake binary"
+elif ! echo "${RUN_OUTPUT}" | grep -qF "Checksum verification failed"; then
+  fail "Pinned version uses embedded digest" "expected checksum failure in: ${RUN_OUTPUT}"
+elif [ "${RUN_ASSET}" != "threat-detect-linux-amd64" ]; then
+  fail "Pinned version uses embedded digest" "expected one architecture-specific download, got: ${RUN_ASSET}"
+else
+  pass "Pinned version uses embedded digest"
+fi
+
+# Test 12: the installer and compiler pin the same threat-detect version.
+echo "Test 12: installer digest version matches the compiler-pinned version..."
+compiler_version=$(sed -n 's/^const DefaultThreatDetectVersion Version = "\(.*\)"/\1/p' "$VERSION_CONSTANTS")
+installer_version=$(sed -n 's/^PINNED_THREAT_DETECT_VERSION="\(.*\)"/\1/p' "$INSTALL_SCRIPT")
+if [ -z "$compiler_version" ] || [ "$installer_version" != "$compiler_version" ]; then
+  fail "Installer and compiler versions match" "compiler=${compiler_version:-missing}, installer=${installer_version:-missing}"
+else
+  pass "Installer and compiler versions match"
+fi
+
 echo
 echo "==============================="
 echo "Tests passed: $TESTS_PASSED"
diff --git a/docs/src/content/docs/reference/threat-detection.md b/docs/src/content/docs/reference/threat-detection.md
index 6de60b9eddc..409cc67b9de 100644
--- a/docs/src/content/docs/reference/threat-detection.md
+++ b/docs/src/content/docs/reference/threat-detection.md
@@ -67,7 +67,9 @@ safe-outputs:
 The `features.gh-aw-detection` flag controls the detection implementation, not
 whether threat detection runs. The external `threat-detect` implementation is
 the default; set `features.gh-aw-detection: false` to select the legacy inline
-engine implementation.
+engine implementation. The external binary's version and per-architecture
+SHA-256 digests are pinned in the immutable setup action, so installation does
+not trust a checksum downloaded alongside the binary.
 
 > [!NOTE]
 > When a workflow explicitly sets `threat-detection: false`, that setting takes precedence over any imported fragments. Imported shared workflows that configure safe outputs without a `threat-detection` key will not re-enable threat detection in the importing workflow.