diff --git a/.github/scripts/release-checksums.sh b/.github/scripts/release-checksums.sh
new file mode 100755
index 0000000..7bc68f3
--- /dev/null
+++ b/.github/scripts/release-checksums.sh
@@ -0,0 +1,75 @@
+#!/bin/sh
+set -eu
+
+if [ "$#" -ne 2 ]; then
+  echo "usage: release-checksums.sh VERSION DIST_DIRECTORY" >&2
+  exit 64
+fi
+
+VERSION="$1"
+DIST_DIRECTORY="$2"
+
+case "$VERSION" in
+  ""|[!0-9A-Za-z]*|*[!0-9A-Za-z.-]*|*..*|*.)
+    echo "release checksums: invalid version: $VERSION" >&2
+    exit 64
+    ;;
+esac
+if [ "${#VERSION}" -gt 64 ]; then
+  echo "release checksums: version is too long" >&2
+  exit 64
+fi
+if [ ! -d "$DIST_DIRECTORY" ]; then
+  echo "release checksums: dist directory does not exist: $DIST_DIRECTORY" >&2
+  exit 66
+fi
+
+DIST_DIRECTORY="$(CDPATH='' cd -- "$DIST_DIRECTORY" && pwd -P)"
+set -- \
+  "Headless-${VERSION}-macos.zip" \
+  "headless-${VERSION}-linux-amd64.tar.gz" \
+  "headless-${VERSION}-linux-arm64.tar.gz"
+
+for asset in "$@"; do
+  path="$DIST_DIRECTORY/$asset"
+  if [ ! -f "$path" ] || [ -L "$path" ] || [ ! -s "$path" ]; then
+    echo "release checksums: missing or unsafe release asset: $asset" >&2
+    exit 66
+  fi
+done
+
+checksum_files() {
+  if command -v sha256sum >/dev/null 2>&1; then
+    sha256sum "$@"
+  elif command -v shasum >/dev/null 2>&1; then
+    shasum -a 256 "$@"
+  else
+    echo "release checksums: sha256sum or shasum is required" >&2
+    exit 69
+  fi
+}
+
+verify_manifest() {
+  if command -v sha256sum >/dev/null 2>&1; then
+    sha256sum -c SHA256SUMS
+  else
+    shasum -a 256 -c SHA256SUMS
+  fi
+}
+
+MANIFEST="$DIST_DIRECTORY/SHA256SUMS"
+TEMP_MANIFEST="$(mktemp "$DIST_DIRECTORY/.SHA256SUMS.XXXXXX")"
+trap '[ -z "$TEMP_MANIFEST" ] || rm -f "$TEMP_MANIFEST"' EXIT INT TERM
+
+(
+  cd "$DIST_DIRECTORY"
+  checksum_files "$@"
+) > "$TEMP_MANIFEST"
+chmod 0644 "$TEMP_MANIFEST"
+mv -f "$TEMP_MANIFEST" "$MANIFEST"
+TEMP_MANIFEST=""
+
+(
+  cd "$DIST_DIRECTORY"
+  verify_manifest
+)
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 3d57ba5..ff1654e 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -89,10 +89,13 @@ jobs:
     permissions:
       contents: write
     steps:
+      - uses: actions/checkout@v7
       - uses: actions/download-artifact@v8
         with:
           path: dist
           merge-multiple: true
+      - name: Generate and verify checksums
+        run: ./.github/scripts/release-checksums.sh "${{ needs.version.outputs.version }}" dist
       - name: Create GitHub Release
         uses: softprops/action-gh-release@v3
         with:
@@ -115,4 +118,5 @@ jobs:
             dist/Headless-${{ needs.version.outputs.version }}-macos.zip
             dist/headless-${{ needs.version.outputs.version }}-linux-amd64.tar.gz
             dist/headless-${{ needs.version.outputs.version }}-linux-arm64.tar.gz
+            dist/SHA256SUMS
           fail_on_unmatched_files: true
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3a6f257..23e9574 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,8 @@ Cutting that release is tracked in
 
 ### Added
 
+- Tagged releases now include a verified `SHA256SUMS` manifest covering every
+  downloadable macOS and Linux package.
 - A single cross-engine conformance scenario now runs against real WKWebView
   and Chromium hosts, locking shared response shapes and declared capability
   errors without mirrored platform assertions.
diff --git a/README.md b/README.md
index 870c5eb..f8827e6 100644
--- a/README.md
+++ b/README.md
@@ -210,9 +210,17 @@ git tag v1.0.0
 git push origin v1.0.0
 ```
 
-Assets: macOS `Headless.app` zip, Linux amd64/arm64 tarballs. See the Actions
-`Release` workflow and the release notes on each tag for install caveats
-(Gatekeeper; Linux Chromium/FFmpeg).
+Assets: macOS `Headless.app` zip, Linux amd64/arm64 tarballs, and
+`SHA256SUMS`. Download the manifest beside the selected package and verify it
+before installing:
+
+```sh
+sha256sum --ignore-missing -c SHA256SUMS              # Linux
+shasum -a 256 --ignore-missing -c SHA256SUMS          # macOS
+```
+
+See the Actions `Release` workflow and the release notes on each tag for
+install caveats (Gatekeeper; Linux Chromium/FFmpeg).
 
 ## Tests
 
diff --git a/apps/headless/Tests/release-checksums.sh b/apps/headless/Tests/release-checksums.sh
new file mode 100755
index 0000000..80005bd
--- /dev/null
+++ b/apps/headless/Tests/release-checksums.sh
@@ -0,0 +1,60 @@
+#!/bin/sh
+set -eu
+
+cd "$(dirname "$0")/.."
+SCRIPT="../../.github/scripts/release-checksums.sh"
+TEST_ROOT="$(mktemp -d "${TMPDIR:-/tmp}/headless-release-checksums.XXXXXX")"
+trap 'rm -rf "$TEST_ROOT"' EXIT INT TERM
+
+VERSION="1.2.3-rc.1"
+MACOS="Headless-${VERSION}-macos.zip"
+AMD64="headless-${VERSION}-linux-amd64.tar.gz"
+ARM64="headless-${VERSION}-linux-arm64.tar.gz"
+
+printf 'macOS package\n' > "$TEST_ROOT/$MACOS"
+printf 'Linux amd64 package\n' > "$TEST_ROOT/$AMD64"
+printf 'Linux arm64 package\n' > "$TEST_ROOT/$ARM64"
+
+"$SCRIPT" "$VERSION" "$TEST_ROOT" >/dev/null
+test "$(wc -l < "$TEST_ROOT/SHA256SUMS" | tr -d ' ')" -eq 3
+if [ "$(uname -s)" = "Darwin" ]; then
+  test "$(stat -f %Lp "$TEST_ROOT/SHA256SUMS")" = "644"
+else
+  test "$(stat -c %a "$TEST_ROOT/SHA256SUMS")" = "644"
+fi
+awk '{print $2}' "$TEST_ROOT/SHA256SUMS" > "$TEST_ROOT/names"
+printf '%s\n' "$MACOS" "$AMD64" "$ARM64" > "$TEST_ROOT/expected-names"
+cmp -s "$TEST_ROOT/names" "$TEST_ROOT/expected-names"
+
+if command -v sha256sum >/dev/null 2>&1; then
+  (cd "$TEST_ROOT" && sha256sum -c SHA256SUMS >/dev/null)
+else
+  (cd "$TEST_ROOT" && shasum -a 256 -c SHA256SUMS >/dev/null)
+fi
+
+cp "$TEST_ROOT/SHA256SUMS" "$TEST_ROOT/original-manifest"
+"$SCRIPT" "$VERSION" "$TEST_ROOT" >/dev/null
+cmp -s "$TEST_ROOT/SHA256SUMS" "$TEST_ROOT/original-manifest"
+
+rm "$TEST_ROOT/$ARM64"
+if "$SCRIPT" "$VERSION" "$TEST_ROOT" >/dev/null 2>&1; then
+  echo "release checksum test: missing assets must fail" >&2
+  exit 1
+fi
+cmp -s "$TEST_ROOT/SHA256SUMS" "$TEST_ROOT/original-manifest"
+
+ln -s "$AMD64" "$TEST_ROOT/$ARM64"
+if "$SCRIPT" "$VERSION" "$TEST_ROOT" >/dev/null 2>&1; then
+  echo "release checksum test: symlink assets must fail" >&2
+  exit 1
+fi
+
+for invalid_version in '' '../1.2.3' '.1.2.3' '1.2.3/' '1..2' '1.2.3.' \
+  'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'; do
+  if "$SCRIPT" "$invalid_version" "$TEST_ROOT" >/dev/null 2>&1; then
+    echo "release checksum test: invalid version accepted: $invalid_version" >&2
+    exit 1
+  fi
+done
+
+echo "Release checksum tests passed"
diff --git a/apps/headless/test.sh b/apps/headless/test.sh
index 65ea996..dd41443 100755
--- a/apps/headless/test.sh
+++ b/apps/headless/test.sh
@@ -2,6 +2,8 @@
 set -euo pipefail
 cd "$(dirname "$0")"
 
+./Tests/release-checksums.sh
+
 for tool in swift swiftc; do
   if ! command -v "$tool" >/dev/null 2>&1; then
     if [[ "$(uname -s)" == "Darwin" ]]; then
diff --git a/docs/roadmap/improvements-backlog.md b/docs/roadmap/improvements-backlog.md
index 766d6f2..e5e19ee 100644
--- a/docs/roadmap/improvements-backlog.md
+++ b/docs/roadmap/improvements-backlog.md
@@ -368,8 +368,10 @@ Owner-decided scope: package managers, no hosted service.
   `HP/ChromiumRuntime.swift:167-184`).
 - **E3.** ([#41](https://github.com/LockInTime/headless/issues/41)) Publish the Docker `production` image to GHCR on tag (release notes
   currently say "not published"); this is also the interim Windows story.
-- **E4.** ([#42](https://github.com/LockInTime/headless/issues/42)) `SHA256SUMS` (+ consider cosign) for all release assets — the QA
-  evidence bundle already ships sums; releases don't.
+- **E4.** ([#42](https://github.com/LockInTime/headless/issues/42)) ~~`SHA256SUMS` for all release assets — the QA evidence bundle already ships
+  sums; releases don't.~~ **Done:** the publish job requires all three named
+  regular package files, generates `SHA256SUMS` atomically, verifies it, and
+  attaches the manifest to the release. Cosign remains optional future work.
 - **E5.** ([#43](https://github.com/LockInTime/headless/issues/43)) npm wrapper package (binary download shim) for `npx` reach.
 - **E6.** ([#44](https://github.com/LockInTime/headless/issues/44)) Version unification + `headless --version` + CHANGELOG + release
   automation (architecture §12). `package.json` says 0.0.0, tags say 1.0.x,