diff --git a/.github/workflows/pert_weekly_period_lock_acquisition.yml b/.github/workflows/pert_weekly_period_lock_acquisition.yml
new file mode 100644
index 0000000..445009d
--- /dev/null
+++ b/.github/workflows/pert_weekly_period_lock_acquisition.yml
@@ -0,0 +1,59 @@
+name: PERT Weekly Period Lock Acquisition Preflight
+
+on:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  actions: read
+  artifact-metadata: write
+
+jobs:
+  period-lock-acquisition:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    env:
+      ARTIFACT_NAME: pert-weekly-period-lock-${{ github.run_id }}
+      PYTHONPATH: src
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.2.2
+      - name: Set up Python
+        uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
+        with:
+          python-version: "3.11"
+      - name: Reject unsupported rerun attempts
+        run: test "${GITHUB_RUN_ATTEMPT}" -le 2
+      - name: Build and read back original period lock (attempt 1)
+        if: ${{ github.run_attempt == 1 }}
+        run: |
+          set -euo pipefail
+          mkdir -p "${RUNNER_TEMP}/pert-period-lock/bundle"
+          python scripts/period_lock_acquisition_preflight.py build \
+            --output-dir "${RUNNER_TEMP}/pert-period-lock/bundle" \
+            --run-id "${GITHUB_RUN_ID}" \
+            --artifact-name "${ARTIFACT_NAME}"
+      - name: Upload immutable original period lock bundle (attempt 1)
+        if: ${{ github.run_attempt == 1 }}
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
+        with:
+          name: ${{ env.ARTIFACT_NAME }}
+          path: ${{ runner.temp }}/pert-period-lock/bundle
+          if-no-files-found: error
+          retention-days: 30
+      - name: Download original period lock from this run (attempt 2)
+        if: ${{ github.run_attempt == 2 }}
+        uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v4.3.0
+        with:
+          name: ${{ env.ARTIFACT_NAME }}
+          path: ${{ runner.temp }}/pert-period-lock/downloaded
+          run-id: ${{ github.run_id }}
+          github-token: ${{ github.token }}
+      - name: Exact verify of original period lock (attempt 2)
+        if: ${{ github.run_attempt == 2 }}
+        run: |
+          set -euo pipefail
+          python scripts/period_lock_acquisition_preflight.py verify \
+            --bundle-dir "${RUNNER_TEMP}/pert-period-lock/downloaded" \
+            --run-id "${GITHUB_RUN_ID}" \
+            --artifact-name "${ARTIFACT_NAME}"
diff --git a/docs/weekly_period_lock_acquisition_preflight.md b/docs/weekly_period_lock_acquisition_preflight.md
new file mode 100644
index 0000000..f9d1805
--- /dev/null
+++ b/docs/weekly_period_lock_acquisition_preflight.md
@@ -0,0 +1,28 @@
+# Weekly period-lock acquisition preflight
+
+This isolated workflow is a test-only proof of same-run GitHub Actions artifact
+acquisition. It is not the RSS/source production pipeline and does not publish
+producer output.
+
+The workflow is intentionally `workflow_dispatch`-only. A maintainer must
+select the exact PR head for the initial run; this keeps `actions:read` away
+from untrusted `pull_request` code. The one allowed rerun remains the same
+workflow run and therefore retains the original `github.run_id`.
+
+- Attempt 1 builds one deterministic representative `pert.weekly.period_lock.v1`
+  lock and one immutable input snapshot. The period and snapshot are fixed
+  fixture values; no wall-clock fallback is allowed.
+- The artifact name is `pert-weekly-period-lock-<run_id>`, and the bundle has
+  exactly `period_lock.json`, `input_snapshot.json`, and `bundle_manifest.json`.
+  The manifest binds the name, original run id, source attempt `1`, and each
+  member digest. Retention is 30 days and overwrite is not used.
+- Attempt 2 is only the same run's controlled rerun. It downloads that exact
+  artifact using the same `github.run_id`, then verifies canonical bytes,
+  source run/attempt, artifact name, snapshot identity, member set, and all
+  digests. It never derives a new period or uploads a replacement.
+
+The workflow has no contents write, secrets, or OIDC permission. `actions:read`
+is used for the readback path and `artifact-metadata:write` is retained for the
+artifact upload boundary. This preflight must fail closed if the original-run
+artifact is missing, duplicated, replaced, or cannot be read with the declared
+permissions.
diff --git a/scripts/period_lock_acquisition_preflight.py b/scripts/period_lock_acquisition_preflight.py
new file mode 100644
index 0000000..cf726e4
--- /dev/null
+++ b/scripts/period_lock_acquisition_preflight.py
@@ -0,0 +1,254 @@
+"""Isolated GitHub Actions preflight for same-run weekly lock acquisition.
+
+This is test-only harness code. It uses a fixed representative snapshot and
+never derives a period from the wall clock.
+"""
+
+from __future__ import annotations
+
+import argparse
+import hashlib
+import json
+import re
+from pathlib import Path
+from typing import Any
+
+from political_event_tracking_research.weekly_period_lock import (
+    LOCK_VERSION,
+    PeriodLockError,
+    parse_period_lock,
+    parse_period_lock_bytes,
+    serialize_period_lock,
+)
+
+BUNDLE_VERSION = "pert.weekly.period_lock_acquisition.v1"
+BUNDLE_FILES = ("bundle_manifest.json", "input_snapshot.json", "period_lock.json")
+SNAPSHOT_VERSION = "pert.weekly.input_snapshot.v1"
+_RUN_ID_RE = re.compile(r"^[1-9][0-9]*$")
+
+
+def expected_artifact_name(run_id: str) -> str:
+    _validate_run_id(run_id)
+    return f"pert-weekly-period-lock-{run_id}"
+
+
+def _validate_run_id(run_id: object) -> str:
+    if type(run_id) is not str or not _RUN_ID_RE.fullmatch(run_id):
+        raise ValueError("period_lock_run_invalid")
+    return run_id
+
+
+def _canonical_json(value: object, error_code: str) -> bytes:
+    try:
+        return json.dumps(value, ensure_ascii=False, sort_keys=True, separators=(",", ":"), allow_nan=False).encode("utf-8")
+    except (TypeError, ValueError, UnicodeError, OverflowError, RecursionError):
+        raise ValueError(error_code) from None
+
+
+def _parse_canonical_json(raw: bytes, error_code: str) -> dict[str, Any]:
+    if type(raw) is not bytes:
+        raise ValueError(error_code)
+
+    def pairs(items: list[tuple[str, object]]) -> dict[str, object]:
+        result: dict[str, object] = {}
+        for key, value in items:
+            if key in result:
+                raise ValueError(error_code)
+            result[key] = value
+        return result
+
+    def reject_constant(_: str) -> None:
+        raise ValueError(error_code)
+
+    try:
+        value = json.loads(raw.decode("utf-8"), object_pairs_hook=pairs, parse_constant=reject_constant)
+    except ValueError:
+        raise ValueError(error_code) from None
+    if not isinstance(value, dict) or _canonical_json(value, error_code) != raw:
+        raise ValueError(error_code)
+    return value
+
+
+def _sha256(raw: bytes) -> str:
+    return hashlib.sha256(raw).hexdigest()
+
+
+def _fixture_lock_payload(run_id: str) -> dict[str, object]:
+    return {
+        "lock_version": LOCK_VERSION,
+        "calendar": "utc_iso_week_monday_sunday",
+        "period_start": "2026-07-06",
+        "period_end_exclusive": "2026-07-13",
+        "as_of": "2026-07-12",
+        "workflow_ref": "QuantStrategyLab/PoliticalEventTrackingResearch/.github/workflows/pert_weekly_period_lock_acquisition.yml@refs/heads/main",
+        "source_run_id": run_id,
+        "source_attempt": 1,
+        "producer_ref": "a" * 40,
+        "source_snapshot_id": "pert_weekly_input_snapshot_20260712",
+        "source_snapshot_digest": "b" * 64,
+        "source_provenance": "official_political_event_tracking_research_v1",
+        "source_artifacts": [
+            {"path": "data/live/source_events.csv", "sha256": "c" * 64, "row_count": 11},
+            {"path": "data/live/source_manifest.json", "sha256": "d" * 64, "row_count": 1},
+        ],
+    }
+
+
+def _fixture_snapshot(lock_payload: dict[str, object]) -> dict[str, object]:
+    return {
+        "snapshot_version": SNAPSHOT_VERSION,
+        "source_run_id": lock_payload["source_run_id"],
+        "source_snapshot_id": lock_payload["source_snapshot_id"],
+        "source_snapshot_digest": lock_payload["source_snapshot_digest"],
+        "source_artifacts": lock_payload["source_artifacts"],
+    }
+
+
+def _fixture_manifest(run_id: str, artifact_name: str, lock_bytes: bytes, snapshot_bytes: bytes) -> dict[str, object]:
+    return {
+        "bundle_version": BUNDLE_VERSION,
+        "artifact_name": artifact_name,
+        "source_run_id": run_id,
+        "source_attempt": 1,
+        "lock_version": LOCK_VERSION,
+        "snapshot_version": SNAPSHOT_VERSION,
+        "files": [
+            {"name": "input_snapshot.json", "sha256": _sha256(snapshot_bytes)},
+            {"name": "period_lock.json", "sha256": _sha256(lock_bytes)},
+        ],
+    }
+
+
+def _assert_empty_output_dir(output_dir: Path) -> None:
+    if output_dir.is_symlink() or not output_dir.is_dir():
+        raise ValueError("period_lock_output_dir_invalid")
+    try:
+        if any(output_dir.iterdir()):
+            raise ValueError("period_lock_output_not_empty")
+    except OSError:
+        raise ValueError("period_lock_output_dir_invalid") from None
+
+
+def build_bundle(output_dir: Path, run_id: str, artifact_name: str | None = None) -> dict[str, object]:
+    run_id = _validate_run_id(run_id)
+    expected_name = expected_artifact_name(run_id)
+    if artifact_name is not None and artifact_name != expected_name:
+        raise ValueError("period_lock_artifact_name_mismatch")
+    _assert_empty_output_dir(output_dir)
+
+    lock_payload = _fixture_lock_payload(run_id)
+    lock = parse_period_lock(lock_payload)
+    lock_bytes = serialize_period_lock(lock)
+    snapshot = _fixture_snapshot(lock_payload)
+    snapshot_bytes = _canonical_json(snapshot, "period_lock_snapshot_invalid")
+    manifest = _fixture_manifest(run_id, expected_name, lock_bytes, snapshot_bytes)
+    manifest_bytes = _canonical_json(manifest, "period_lock_manifest_invalid")
+    payloads = {
+        "period_lock.json": lock_bytes,
+        "input_snapshot.json": snapshot_bytes,
+        "bundle_manifest.json": manifest_bytes,
+    }
+    try:
+        for name, raw in payloads.items():
+            with (output_dir / name).open("xb") as handle:
+                handle.write(raw)
+    except (OSError, ValueError):
+        raise ValueError("period_lock_bundle_write_invalid") from None
+    return verify_bundle(output_dir, run_id, expected_name)
+
+
+def _read_exact_bundle(output_dir: Path) -> dict[str, bytes]:
+    if output_dir.is_symlink() or not output_dir.is_dir():
+        raise ValueError("period_lock_bundle_shape_invalid")
+    try:
+        names = {path.name for path in output_dir.iterdir()}
+    except OSError:
+        raise ValueError("period_lock_bundle_shape_invalid") from None
+    if names != set(BUNDLE_FILES):
+        raise ValueError("period_lock_bundle_shape_invalid")
+    result: dict[str, bytes] = {}
+    for name in BUNDLE_FILES:
+        path = output_dir / name
+        try:
+            if path.is_symlink() or not path.is_file():
+                raise ValueError("period_lock_bundle_member_invalid")
+            result[name] = path.read_bytes()
+        except (OSError, ValueError):
+            raise ValueError("period_lock_bundle_member_invalid") from None
+    return result
+
+
+def verify_bundle(output_dir: Path, expected_run_id: str, expected_artifact: str) -> dict[str, object]:
+    expected_run_id = _validate_run_id(expected_run_id)
+    if expected_artifact != expected_artifact_name(expected_run_id):
+        raise ValueError("period_lock_artifact_name_mismatch")
+    payloads = _read_exact_bundle(output_dir)
+    try:
+        lock = parse_period_lock_bytes(payloads["period_lock.json"])
+    except PeriodLockError:
+        raise ValueError("period_lock_invalid") from None
+    if serialize_period_lock(lock) != payloads["period_lock.json"] or lock.source_run_id != expected_run_id or lock.source_attempt != 1:
+        raise ValueError("period_lock_run_mismatch")
+    snapshot = _parse_canonical_json(payloads["input_snapshot.json"], "period_lock_snapshot_invalid")
+    expected_snapshot = _fixture_snapshot(_fixture_lock_payload(expected_run_id))
+    if snapshot != expected_snapshot:
+        raise ValueError("period_lock_snapshot_mismatch")
+    expected_manifest_bytes = _canonical_json(
+        _fixture_manifest(expected_run_id, expected_artifact, payloads["period_lock.json"], payloads["input_snapshot.json"]),
+        "period_lock_manifest_invalid",
+    )
+    if payloads["bundle_manifest.json"] != expected_manifest_bytes:
+        raise ValueError("period_lock_manifest_mismatch")
+    manifest = _parse_canonical_json(payloads["bundle_manifest.json"], "period_lock_manifest_invalid")
+    if set(manifest) != {"bundle_version", "artifact_name", "source_run_id", "source_attempt", "lock_version", "snapshot_version", "files"}:
+        raise ValueError("period_lock_manifest_invalid")
+    if (
+        manifest["bundle_version"] != BUNDLE_VERSION
+        or manifest["artifact_name"] != expected_artifact
+        or manifest["source_run_id"] != expected_run_id
+        or type(manifest["source_attempt"]) is not int
+        or manifest["source_attempt"] != 1
+        or manifest["lock_version"] != LOCK_VERSION
+        or manifest["snapshot_version"] != SNAPSHOT_VERSION
+    ):
+        raise ValueError("period_lock_manifest_mismatch")
+    files = manifest["files"]
+    if not isinstance(files, list) or {item.get("name") for item in files if isinstance(item, dict)} != {"input_snapshot.json", "period_lock.json"}:
+        raise ValueError("period_lock_manifest_files_invalid")
+    for item in files:
+        if not isinstance(item, dict) or set(item) != {"name", "sha256"} or item["name"] not in payloads:
+            raise ValueError("period_lock_manifest_files_invalid")
+        if item["sha256"] != _sha256(payloads[item["name"]]):
+            raise ValueError("period_lock_artifact_digest_mismatch")
+    return {
+        "bundle_version": BUNDLE_VERSION,
+        "artifact_name": expected_artifact,
+        "source_run_id": expected_run_id,
+        "source_attempt": 1,
+        "files": {name: _sha256(raw) for name, raw in sorted(payloads.items())},
+    }
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser()
+    subparsers = parser.add_subparsers(dest="command", required=True)
+    build = subparsers.add_parser("build")
+    build.add_argument("--output-dir", type=Path, required=True)
+    build.add_argument("--run-id", required=True)
+    build.add_argument("--artifact-name")
+    verify = subparsers.add_parser("verify")
+    verify.add_argument("--bundle-dir", type=Path, required=True)
+    verify.add_argument("--run-id", required=True)
+    verify.add_argument("--artifact-name", required=True)
+    args = parser.parse_args()
+    evidence = (
+        build_bundle(args.output_dir, args.run_id, args.artifact_name)
+        if args.command == "build"
+        else verify_bundle(args.bundle_dir, args.run_id, args.artifact_name)
+    )
+    print(json.dumps(evidence, sort_keys=True, separators=(",", ":")))
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/tests/test_period_lock_acquisition_preflight.py b/tests/test_period_lock_acquisition_preflight.py
new file mode 100644
index 0000000..5ac9031
--- /dev/null
+++ b/tests/test_period_lock_acquisition_preflight.py
@@ -0,0 +1,117 @@
+from __future__ import annotations
+
+import json
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parents[1]))
+
+from scripts.period_lock_acquisition_preflight import (  # noqa: E402
+    BUNDLE_FILES,
+    BUNDLE_VERSION,
+    build_bundle,
+    expected_artifact_name,
+    verify_bundle,
+)
+
+
+def test_build_and_verify_test_only_bundle_is_deterministic(tmp_path: Path) -> None:
+    first_dir = tmp_path / "first"
+    second_dir = tmp_path / "second"
+    first_dir.mkdir()
+    second_dir.mkdir()
+
+    build_bundle(first_dir, "29399816773")
+    build_bundle(second_dir, "29399816773")
+
+    assert {path.name for path in first_dir.iterdir()} == set(BUNDLE_FILES)
+    assert {path.name for path in second_dir.iterdir()} == set(BUNDLE_FILES)
+    assert [path.read_bytes() for path in sorted(first_dir.iterdir())] == [
+        path.read_bytes() for path in sorted(second_dir.iterdir())
+    ]
+    assert verify_bundle(first_dir, "29399816773", expected_artifact_name("29399816773"))
+
+
+def test_verify_rejects_wrong_run_or_artifact_name(tmp_path: Path) -> None:
+    build_bundle(tmp_path, "29399816773")
+    with pytest.raises(ValueError, match="period_lock_run_mismatch"):
+        verify_bundle(tmp_path, "29399816774", expected_artifact_name("29399816774"))
+    with pytest.raises(ValueError, match="period_lock_artifact_name_mismatch"):
+        verify_bundle(tmp_path, "29399816773", "wrong-artifact")
+
+
+@pytest.mark.parametrize("mutation", ["lock", "snapshot", "manifest"])
+def test_verify_rejects_tampered_bundle(tmp_path: Path, mutation: str) -> None:
+    build_bundle(tmp_path, "29399816773")
+    target = tmp_path / mutation_to_filename(mutation)
+    target.write_bytes(target.read_bytes() + b" ")
+    with pytest.raises(ValueError):
+        verify_bundle(tmp_path, "29399816773", expected_artifact_name("29399816773"))
+
+
+def test_verify_rejects_missing_multiple_or_wrong_attempt(tmp_path: Path) -> None:
+    missing_dir = tmp_path / "missing"
+    missing_dir.mkdir()
+    build_bundle(missing_dir, "29399816773")
+    (missing_dir / "input_snapshot.json").unlink()
+    with pytest.raises(ValueError, match="period_lock_bundle_shape_invalid"):
+        verify_bundle(missing_dir, "29399816773", expected_artifact_name("29399816773"))
+
+    wrong_attempt_dir = tmp_path / "wrong-attempt"
+    wrong_attempt_dir.mkdir()
+    build_bundle(wrong_attempt_dir, "29399816773")
+    manifest_path = wrong_attempt_dir / "bundle_manifest.json"
+    manifest = json.loads(manifest_path.read_bytes())
+    manifest["source_attempt"] = 2
+    manifest_path.write_bytes(json.dumps(manifest, sort_keys=True, separators=(",", ":")).encode())
+    with pytest.raises(ValueError, match="period_lock_manifest_mismatch"):
+        verify_bundle(wrong_attempt_dir, "29399816773", expected_artifact_name("29399816773"))
+
+    (wrong_attempt_dir / "extra.json").write_text("{}", encoding="utf-8")
+    with pytest.raises(ValueError, match="period_lock_bundle_shape_invalid"):
+        verify_bundle(wrong_attempt_dir, "29399816773", expected_artifact_name("29399816773"))
+
+
+@pytest.mark.parametrize("mutation", ["reorder", "duplicate"])
+def test_verify_rejects_manifest_record_mutation(tmp_path: Path, mutation: str) -> None:
+    build_bundle(tmp_path, "29399816773")
+    manifest_path = tmp_path / "bundle_manifest.json"
+    manifest = json.loads(manifest_path.read_bytes())
+    if mutation == "reorder":
+        manifest["files"] = list(reversed(manifest["files"]))
+    else:
+        manifest["files"].append(manifest["files"][0])
+    manifest_path.write_bytes(json.dumps(manifest, sort_keys=True, separators=(",", ":")).encode())
+    with pytest.raises(ValueError, match="period_lock_manifest_mismatch"):
+        verify_bundle(tmp_path, "29399816773", expected_artifact_name("29399816773"))
+
+
+def test_workflow_isolated_and_minimally_permissioned() -> None:
+    workflow = Path(".github/workflows/pert_weekly_period_lock_acquisition.yml").read_text(encoding="utf-8")
+    assert "actions: read" in workflow
+    assert "contents: read" in workflow
+    assert "artifact-metadata: write" in workflow
+    assert "workflow_dispatch:" in workflow
+    assert "pull_request:" not in workflow
+    assert "contents: write" not in workflow
+    assert "id-token:" not in workflow
+    assert "secrets." not in workflow
+    assert "run_attempt" in workflow
+    assert "run-id: ${{ github.run_id }}" in workflow
+    assert "retention-days: 30" in workflow
+    assert "upload-artifact@" in workflow and "github.run_attempt == 1" in workflow
+    assert "download-artifact@" in workflow and "github.run_attempt == 2" in workflow
+    for action in ("actions/checkout@", "actions/setup-python@", "actions/upload-artifact@", "actions/download-artifact@"):
+        assert action in workflow
+        ref = workflow.split(action, 1)[1].split()[0]
+        assert len(ref) == 40 and all(char in "0123456789abcdef" for char in ref)
+
+
+def mutation_to_filename(mutation: str) -> str:
+    return {
+        "lock": "period_lock.json",
+        "snapshot": "input_snapshot.json",
+        "manifest": "bundle_manifest.json",
+    }[mutation]