diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 44e0c18..3b56200 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -34,3 +34,25 @@ jobs:
           # quietly stop running (issue #3's test contract).
           CEREMONY_REQUIRE_NPM: 1
         run: bash test/run.sh
+
+  # Exercises the composite actions the way a consumer does — action.yml
+  # resolving, $GITHUB_ACTION_PATH, the relative lib sourcing — which the
+  # test suite, driving the scripts directly, cannot prove (issue #5's
+  # acceptance criterion).
+  action-exercise:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Construct an armed scratch tree in the workspace
+        # This repo carries no VERSION or CHANGELOG.md of its own (until
+        # #11 dogfoods); the guard reads the workspace, so give it a
+        # transient armed tree there. The changelog is scratch-named so a
+        # future real CHANGELOG.md is never shadowed; VERSION cannot be —
+        # the file backend hardcodes it — so if #11 ever adds a real one,
+        # this write must go.
+        run: |
+          printf '0.0.1-dev\n' > VERSION
+          printf '# Changelog\n\n## Unreleased\n\n- Scratch entry.\n' > CHANGELOG.scratch.md
+      - uses: ./actions/changelog-armed
+        with:
+          changelog: CHANGELOG.scratch.md
diff --git a/actions/changelog-armed/action.yml b/actions/changelog-armed/action.yml
new file mode 100644
index 0000000..6a63684
--- /dev/null
+++ b/actions/changelog-armed/action.yml
@@ -0,0 +1,24 @@
+name: Changelog armed
+description: >-
+  Assert the changelog is armed for the tree's version state — the
+  version-keyed guard (box#108, rig#66; the unconditional form was reverted
+  by rig#44 and cast#108). The caller must have checked out its own
+  repository first: the guard reads the consumer's tree at the workspace.
+inputs:
+  version-source:
+    description: Where the tree's version lives ("file" or "package-json")
+    required: false
+    default: file
+  changelog:
+    description: Path to the changelog, relative to the workspace
+    required: false
+    default: CHANGELOG.md
+runs:
+  using: composite
+  steps:
+    - name: changelog armed
+      shell: bash
+      env:
+        CHANGELOG: ${{ inputs.changelog }}
+        VERSION_SOURCE: ${{ inputs.version-source }}
+      run: bash "$GITHUB_ACTION_PATH/changelog-armed.sh"
diff --git a/actions/changelog-armed/changelog-armed.sh b/actions/changelog-armed/changelog-armed.sh
new file mode 100644
index 0000000..e0596a7
--- /dev/null
+++ b/actions/changelog-armed/changelog-armed.sh
@@ -0,0 +1,158 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# changelog-armed.sh [<changelog>] [<version-source>] — assert that the
+# changelog is ARMED: that there is a heading for the next PR's entry to
+# land under, and that it is the right one for the state this tree is in.
+#
+# Ported from box .github/scripts/changelog-armed.sh (box#108, confirmed
+# cross-repo as rig#66) — box is the only repo that carries this guard
+# today. Rig and cast LOST it: the naive form — "always require
+# '## Unreleased' on top" — is false by construction on the ceremony PR's
+# own tree, which legitimately stamps that heading away, so rig#44 and
+# cast#108 both had to revert exactly that. This version-keyed form is the
+# guard rig and cast regain when they adopt ceremony (#13, #15). Anyone
+# tempted to simplify this back to the unconditional form should read
+# those two reverts first.
+#
+# The failure it exists to catch (box#108, rig#66) leaves no trace: the
+# ceremony PR stamps '## Unreleased' into '## X.Y.Z — DATE' by hand, and
+# nothing puts the heading back. A PR authored BEFORE the release wrote its
+# entry under '## Unreleased'; that heading is gone by the time it merges, so
+# git lands the entry under whatever heading now occupies that position — the
+# just-shipped section — CLEANLY, with no conflict. The one signal an author
+# would trust ("git told me to look") is absent exactly when the result is
+# wrong, and the drift is only ever discovered by reading the file.
+#
+# The rule, keyed on the tree's version (a VERSION file or package.json,
+# per version-source), because the two states are genuinely different:
+#
+#   version ends in -dev  ->  the top section MUST be '## Unreleased'
+#   version is bare       ->  the top section may be '## Unreleased' (armed,
+#                             the ceremony's own re-arm) or the stamped
+#                             section for exactly that version — AND the
+#                             section for that version must exist and carry
+#                             prose, because it is the one about to ship
+#
+# The consequence worth stating plainly: a ceremony PR that stamps and forgets
+# to re-arm still passes here — its version is bare, and a bare tree is
+# allowed to be stamped. It goes red the moment the '-dev' bump lands on main,
+# which the release workflow does automatically in the same job as the
+# publish. So the guard does not block the release; it refuses to let main
+# SIT disarmed, which is the window a late PR can fall into.
+#
+# A file of its own (not inlined in action.yml) so
+# test/changelog-armed.test.sh can drive it against constructed trees for
+# both states — the same discipline as the libs it sources.
+
+changelog="${1:-${CHANGELOG:-CHANGELOG.md}}"
+version_source="${2:-${VERSION_SOURCE:-file}}"
+
+# The shared libs travel with this action: a consumer's
+# `uses: heavy-duty/ceremony/actions/changelog-armed@<tag>` downloads this
+# whole repository at that ref, so ../../lib is always present and always at
+# the same ref — no checkout step, no version skew possible.
+here="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
+# shellcheck source=lib/version.sh
+. "$here/../../lib/version.sh"
+# shellcheck source=lib/changelog.sh
+. "$here/../../lib/changelog.sh"
+
+[ -f "$changelog" ] || { echo "changelog-armed: no such file: $changelog" >&2; exit 1; }
+
+# version_read refuses loudly on a missing or empty source; the wrapper line
+# names the guard so a workflow log shows which check refused.
+ver="$(version_read "$version_source")" || {
+  echo "changelog-armed: cannot read the version (version-source: $version_source)" >&2
+  exit 1
+}
+
+# The TOP section: the first '## ' heading in the file. Everything above it is
+# the changelog's own preamble and belongs to no section.
+top="$(grep -m1 '^## ' "$changelog" || true)"
+[ -n "$top" ] || {
+  echo "changelog-armed: $changelog has no '## ' section at all — nothing for a PR entry to land under" >&2
+  exit 1
+}
+
+# '## 0.7.0 — 2026-07-19' -> '0.7.0'. Split on whitespace, the same shape
+# changelog_section matches on, so the two cannot disagree about what a
+# section header is.
+top_ver="$(printf '%s\n' "$top" | awk '{ print $2 }')"
+
+# version_is_dev is the single definition of the -dev special case (#3): an
+# rc is a pre-release, not a dev tree, and keys on the bare rules below.
+if version_is_dev "$ver"; then
+  if [ "$top_ver" != "Unreleased" ]; then
+    cat >&2 <<EOF
+changelog-armed: the version is '$ver' (a development tree) but the top
+  section of $changelog is:
+
+    $top
+
+  A -dev tree MUST carry '## Unreleased' at the top. Without it, a PR that
+  wrote its entry under '## Unreleased' before the release merges CLEANLY into
+  the section above — the one that already shipped — and the changelog quietly
+  misattributes it (box#108, rig#66).
+
+  The fix is to re-arm: add an empty '## Unreleased' immediately above
+  '$top'. The release ceremony is supposed to do this in the same edit that
+  stamps the version.
+EOF
+    exit 1
+  fi
+else
+  # A bare version is the ceremony tree and the merge commit that publishes
+  # it. Both arrangements are legal there: re-armed ('## Unreleased' back on
+  # top, above the section just stamped) or not yet re-armed (the stamped
+  # section still on top). What is NOT legal is a stamped top section naming
+  # some OTHER version — that is a ceremony that stamped the wrong number,
+  # and the release workflow would publish a body that is not this release's.
+  if [ "$top_ver" != "Unreleased" ] && [ "$top_ver" != "$ver" ]; then
+    cat >&2 <<EOF
+changelog-armed: the version is '$ver' but the top section of $changelog is:
+
+    $top
+
+  A bare version means this tree is a release. Its top section must be either
+  '## Unreleased' (re-armed after stamping) or the stamped section for '$ver'
+  itself. A stamped section naming a different version means the ceremony
+  stamped the wrong number, and the published release body would come from
+  the wrong section.
+EOF
+    exit 1
+  fi
+  # The top heading is deliberately left UNCONSTRAINED above — both ceremony
+  # shapes must stay legal, which is the rig#44 / cast#108 lesson and is not
+  # negotiable. That asymmetry leaves a gap of its own, the HALF-ceremony
+  # tree: version bumped to the release, a populated '## Unreleased' still on
+  # top, and no stamped section for the version anywhere. The test above is
+  # false on its first clause, short-circuits, and passes. Nothing else
+  # refuses until the publisher extracts the notes — which happens AFTER the
+  # merge, on main, and publishes a release with an empty body, the worst
+  # place for this to land. So make the same assert one step earlier through
+  # the very extractor the publisher uses — changelog_section (#4) — so the
+  # guard and the publisher cannot disagree about what a section is or when
+  # one counts as empty (rig#67).
+  if [ -z "$(changelog_section "$changelog" "$ver")" ]; then
+    cat >&2 <<EOF
+changelog-armed: the version is '$ver' but $changelog has no non-empty
+  section for '$ver'. The top section is:
+
+    $top
+
+  This is a HALF-DONE ceremony: the version was bumped but its section was
+  never stamped — the stamp is MISSING, not misnumbered. A bare version means
+  this tree is a release, and the section it is about to publish has to exist
+  and have prose in it. Left alone, this passes CI, merges, and only then does
+  the publisher refuse to extract the notes — on main, after the fact, with
+  the release already half-shipped.
+
+  The fix is the ceremony's first edit: stamp '## Unreleased' into
+  '## $ver — DATE', then put an empty '## Unreleased' back above it.
+EOF
+    exit 1
+  fi
+fi
+
+echo "changelog-armed: version '$ver' agrees with the top section ($top_ver)"
diff --git a/test/changelog-armed.test.sh b/test/changelog-armed.test.sh
new file mode 100644
index 0000000..2f9dcac
--- /dev/null
+++ b/test/changelog-armed.test.sh
@@ -0,0 +1,215 @@
+#!/usr/bin/env bash
+# Contract tests for actions/changelog-armed (issue #5). Constructed fixture
+# trees — a dir with a changelog plus a VERSION file or package.json, not
+# git repos — the same discipline as the box suite this guard is ported
+# from. set -u, not -e: failing commands are behavior for the harness to
+# inspect.
+set -u
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+# shellcheck source=test/harness.sh
+. "$ROOT/test/harness.sh"
+
+SCRIPT="$ROOT/actions/changelog-armed/changelog-armed.sh"
+
+TMP="$(mktemp -d)"
+trap 'rm -rf "$TMP"' EXIT
+
+# The guard reads the consumer's tree at its working directory, so every
+# case runs from inside a constructed fixture tree.
+in_tree() {
+  local dir="$1"
+  shift
+  (cd "$TMP/$dir" && bash "$SCRIPT" "$@")
+}
+
+# tree <name> <version> — a fixture tree with a VERSION file; the changelog
+# body arrives on stdin.
+tree() {
+  mkdir -p "$TMP/$1"
+  printf '%s\n' "$2" >"$TMP/$1/VERSION"
+  cat >"$TMP/$1/CHANGELOG.md"
+}
+
+# pkg_tree <name> <version> — the same, package-json backend.
+pkg_tree() {
+  mkdir -p "$TMP/$1"
+  printf '{ "name": "fixture", "version": "%s" }\n' "$2" >"$TMP/$1/package.json"
+  cat >"$TMP/$1/CHANGELOG.md"
+}
+
+# --- the -dev rows: top section MUST be '## Unreleased' ----------------------
+
+tree dev-armed 1.2.4-dev <<'EOF'
+# Changelog
+
+## Unreleased
+
+- Pending entry.
+
+## 1.2.3 — 2026-07-20
+
+- The shipped entry.
+EOF
+check "-dev + Unreleased on top passes" 0 "agrees" in_tree dev-armed
+
+tree dev-stamped 1.2.4-dev <<'EOF'
+# Changelog
+
+## 1.2.3 — 2026-07-20
+
+- The shipped entry.
+EOF
+check "-dev + stamped top fails" 1 "development tree" in_tree dev-stamped
+check "-dev failure names the file" 1 "CHANGELOG.md" in_tree dev-stamped
+check "-dev failure teaches the re-arm fix" 1 "re-arm" in_tree dev-stamped
+
+# --- the bare rows: both ceremony shapes legal, half-ceremonies refused -----
+
+tree bare-armed 1.2.3 <<'EOF'
+# Changelog
+
+## Unreleased
+
+## 1.2.3 — 2026-07-20
+
+- The shipped entry.
+EOF
+check "bare + re-armed tree passes" 0 "agrees" in_tree bare-armed
+
+tree bare-stamped 1.2.3 <<'EOF'
+# Changelog
+
+## 1.2.3 — 2026-07-20
+
+- The shipped entry.
+
+## 1.2.2 — 2026-07-01
+
+- Older entry.
+EOF
+check "bare + own stamped section on top passes" 0 "agrees" in_tree bare-stamped
+
+tree bare-empty-stamp 1.2.3 <<'EOF'
+# Changelog
+
+## 1.2.3 — 2026-07-20
+
+## 1.2.2 — 2026-07-01
+
+- Older entry.
+EOF
+check "bare + own stamped section but EMPTY fails" 1 "no non-empty" \
+  in_tree bare-empty-stamp
+
+tree bare-wrong-stamp 1.2.3 <<'EOF'
+# Changelog
+
+## 9.9.9 — 2026-07-20
+
+- An entry under the wrong number.
+EOF
+check "bare + top section naming another version fails" 1 "stamped the wrong number" \
+  in_tree bare-wrong-stamp
+
+tree bare-half-ceremony 1.2.3 <<'EOF'
+# Changelog
+
+## Unreleased
+
+- Pending entry that was never stamped.
+
+## 1.2.2 — 2026-07-01
+
+- Older entry.
+EOF
+check "bare + no section for the version anywhere fails" 1 "HALF-DONE ceremony" \
+  in_tree bare-half-ceremony
+
+# Whole-version matching: 1.2.3 must not be satisfied by a 1.2.3-rc1 section.
+tree bare-rc-only 1.2.3 <<'EOF'
+# Changelog
+
+## Unreleased
+
+## 1.2.3-rc1 — 2026-07-15
+
+- The candidate's entry.
+EOF
+check "bare: an rc section never satisfies the bare version" 1 "HALF-DONE ceremony" \
+  in_tree bare-rc-only
+
+# An rc is a pre-release, not a dev tree (#3's version_is_dev): it keys on
+# the bare rules, so a stamped rc section of its own is shippable.
+tree rc-stamped 2.0.0-rc1 <<'EOF'
+# Changelog
+
+## Unreleased
+
+## 2.0.0-rc1 — 2026-07-20
+
+- The candidate's entry.
+EOF
+check "rc keys as bare, own stamped section passes" 0 "agrees" in_tree rc-stamped
+
+# --- degenerate trees --------------------------------------------------------
+
+tree no-sections 1.2.3-dev <<'EOF'
+# Changelog
+
+Only preamble prose, no sections.
+EOF
+check "changelog with no '## ' at all fails" 1 "nothing for a PR entry to land under" \
+  in_tree no-sections
+
+mkdir -p "$TMP/no-changelog"
+printf '1.2.3\n' >"$TMP/no-changelog/VERSION"
+check "missing changelog fails" 1 "no such file" in_tree no-changelog
+
+mkdir -p "$TMP/no-version"
+printf '# Changelog\n\n## Unreleased\n' >"$TMP/no-version/CHANGELOG.md"
+check "missing version source fails" 1 "cannot read the version" in_tree no-version
+
+check "unknown version-source refused" 1 "unknown backend" \
+  in_tree dev-armed CHANGELOG.md carrier-pigeon
+
+# --- the package-json backend ------------------------------------------------
+
+pkg_tree pkg-dev-armed 0.2.0-dev <<'EOF'
+# Changelog
+
+## Unreleased
+
+- Pending entry.
+
+## 0.1.0 — 2026-07-20
+
+- The shipped entry.
+EOF
+check "package-json: -dev + armed passes" 0 "agrees" \
+  in_tree pkg-dev-armed CHANGELOG.md package-json
+
+pkg_tree pkg-bare-armed 0.1.0 <<'EOF'
+# Changelog
+
+## Unreleased
+
+## 0.1.0 — 2026-07-20
+
+- The shipped entry.
+EOF
+check "package-json: bare + armed passes" 0 "agrees" \
+  in_tree pkg-bare-armed CHANGELOG.md package-json
+
+# --- the action's wiring: inputs arrive as env vars --------------------------
+
+mkdir -p "$TMP/env-tree"
+printf '1.2.4-dev\n' >"$TMP/env-tree/VERSION"
+printf '# Changelog\n\n## Unreleased\n\n- Pending.\n' >"$TMP/env-tree/NOTES.md"
+# A non-default changelog name proves the env var is honored, not the default.
+env_tree() {
+  (cd "$TMP/env-tree" && CHANGELOG=NOTES.md VERSION_SOURCE=file bash "$SCRIPT")
+}
+check "env vars drive the script the way action.yml does" 0 "agrees" env_tree
+
+summary