diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index ebbebc88..09cb406c 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -48,6 +48,13 @@ jobs:
             python scripts/check_release_provenance.py --allow-unknown
           fi
 
+      - name: Verify vendored provenance against upstream
+        # Real releases only: clone the upstreams at the recorded commits and
+        # prove the vendored extern/NNS-core and tools/NNS actually came from
+        # them. Dry runs may carry placeholder provenance, so skip them.
+        if: github.event_name == 'push' || inputs.publish == 'pypi'
+        run: python scripts/verify_release_provenance.py
+
   build_wheels:
     needs: provenance-check
     name: Wheels on ${{ matrix.os }}
diff --git a/pyproject.toml b/pyproject.toml
index 7efb2a63..1d71613e 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -107,6 +107,7 @@ select = ["E", "F", "I", "B", "UP", "N", "RUF", "TID"]
 "scripts/install_local_r_nns.py" = ["TID251"]
 "scripts/run_live_r_parity_for_changed_api.py" = ["TID251"]
 "scripts/inspect_r_api_update.py" = ["TID251"]
+"scripts/verify_release_provenance.py" = ["TID251"]
 
 [tool.mypy]
 python_version = "3.11"
diff --git a/scripts/verify_release_provenance.py b/scripts/verify_release_provenance.py
new file mode 100644
index 00000000..26a3a145
--- /dev/null
+++ b/scripts/verify_release_provenance.py
@@ -0,0 +1,182 @@
+#!/usr/bin/env python3
+"""Verify that the recorded provenance matches the vendored bytes.
+
+`scripts/check_release_provenance.py` only confirms the manifest is *filled in*
+(no ``unknown`` placeholders) and that the tag matches the version. This script
+goes further: it clones the upstream repositories at the recorded commits and
+proves that the vendored snapshots actually came from them, so a hand-vendored
+tree with a stale ``r_commit`` / ``core_commit`` cannot slip into a release.
+
+Checks (all run against ``sync/nns_source.json``):
+
+* core (hard): every top-level object under ``extern/NNS-core`` exists in
+  ``core_repo`` at ``core_commit``. NNS-core is vendored by a plain copy, so its
+  git objects appear verbatim upstream.
+* R src tree (hard, offline): the recorded ``r_src_tree_hash`` equals the git
+  tree hash of the vendored ``tools/NNS/src``.
+* R tarball (hard when present): if ``r_repo`` commits the vendored tarball at
+  ``r_commit``, its blob must equal the vendored tarball blob. The built R
+  package normalizes ``R/`` sources, so the committed tarball is the reliable
+  cross-repo anchor; if the upstream does not commit one it is reported as a
+  skipped note rather than a failure.
+
+Requires network access (shallow-clones the upstreams) and git. Intended for the
+release workflow's provenance gate, on real releases only.
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+import subprocess
+import tempfile
+from pathlib import Path
+
+PLACEHOLDERS = {"", "unknown", None}
+
+
+def _git(args: list[str], cwd: Path | None = None) -> tuple[int, str]:
+    proc = subprocess.run(
+        ["git", *args],
+        cwd=cwd,
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    return proc.returncode, proc.stdout.strip()
+
+
+def _object_hash(repo: Path, spec: str) -> str | None:
+    code, out = _git(["rev-parse", "--verify", "--quiet", spec], cwd=repo)
+    return out if code == 0 and out else None
+
+
+def _object_exists(repo: Path, obj: str) -> bool:
+    code, _ = _git(["cat-file", "-e", obj], cwd=repo)
+    return code == 0
+
+
+def _shallow_fetch(url: str, sha: str, dest: Path) -> str | None:
+    """Fetch a single commit into a fresh repo at ``dest``; return error or None."""
+    dest.mkdir(parents=True, exist_ok=True)
+    if _git(["init", "-q"], cwd=dest)[0] != 0:
+        return f"git init failed in {dest}"
+    _git(["remote", "add", "origin", url], cwd=dest)
+    code, out = _git(["fetch", "--depth", "1", "origin", sha], cwd=dest)
+    if code != 0:
+        return f"could not fetch {sha} from {url}: {out}"
+    return None
+
+
+def check_offline(manifest: dict[str, object], repo_root: Path) -> list[str]:
+    problems: list[str] = []
+    r_path = str(manifest.get("vendored_r_path") or "tools/NNS")
+    recorded = manifest.get("r_src_tree_hash")
+    actual = _object_hash(repo_root, f"HEAD:{r_path}/src")
+    if actual is None:
+        problems.append(f"could not read vendored {r_path}/src tree from HEAD")
+    elif actual != recorded:
+        problems.append(
+            f"r_src_tree_hash {recorded!r} does not match vendored {r_path}/src "
+            f"tree {actual!r}"
+        )
+    return problems
+
+
+def check_core(manifest: dict[str, object], repo_root: Path, workdir: Path) -> list[str]:
+    core_repo = str(manifest.get("core_repo") or "")
+    core_commit = manifest.get("core_commit")
+    core_path = str(manifest.get("vendored_core_path") or "extern/NNS-core")
+    if core_commit in PLACEHOLDERS or not core_repo:
+        return [f"core_commit/core_repo unset; cannot verify ({core_repo}@{core_commit})"]
+
+    dest = workdir / "core"
+    err = _shallow_fetch(f"https://github.com/{core_repo}.git", str(core_commit), dest)
+    if err:
+        return [err]
+
+    code, listing = _git(["ls-tree", f"HEAD:{core_path}"], cwd=repo_root)
+    if code != 0 or not listing:
+        return [f"could not list vendored {core_path} in HEAD"]
+
+    problems: list[str] = []
+    for line in listing.splitlines():
+        meta, _, name = line.partition("\t")
+        obj = meta.split()[2]
+        if not _object_exists(dest, obj):
+            problems.append(
+                f"vendored {core_path}/{name} ({obj}) is not present in "
+                f"{core_repo}@{core_commit}"
+            )
+    return problems
+
+
+def check_r_tarball(manifest: dict[str, object], repo_root: Path, workdir: Path) -> list[str]:
+    r_repo = str(manifest.get("r_repo") or "")
+    r_commit = manifest.get("r_commit")
+    tarball = str(manifest.get("vendored_r_tarball") or "")
+    if r_commit in PLACEHOLDERS or not r_repo or not tarball:
+        return [f"r_commit/r_repo/tarball unset; cannot verify ({r_repo}@{r_commit})"]
+
+    vendored_blob = _object_hash(repo_root, f"HEAD:{tarball}")
+    if vendored_blob is None:
+        return [f"vendored tarball {tarball} not found in HEAD"]
+
+    dest = workdir / "r"
+    err = _shallow_fetch(f"https://github.com/{r_repo}.git", str(r_commit), dest)
+    if err:
+        return [err]
+
+    base = Path(tarball).name
+    upstream_blob = _object_hash(dest, f"{r_commit}:{base}")
+    if upstream_blob is None:
+        print(
+            f"  note: {r_repo}@{r_commit} does not commit {base}; "
+            "skipping tarball blob cross-check"
+        )
+        return []
+    if upstream_blob != vendored_blob:
+        return [
+            f"vendored {tarball} ({vendored_blob}) does not match "
+            f"{r_repo}@{r_commit}:{base} ({upstream_blob})"
+        ]
+    return []
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--manifest", type=Path, default=Path("sync/nns_source.json"))
+    parser.add_argument(
+        "--repo-root",
+        type=Path,
+        default=Path.cwd(),
+        help="checkout whose HEAD holds the vendored snapshots (default: cwd)",
+    )
+    args = parser.parse_args()
+
+    manifest = json.loads(args.manifest.read_text(encoding="utf-8"))
+    repo_root = args.repo_root.resolve()
+
+    problems: list[str] = []
+    problems.extend(check_offline(manifest, repo_root))
+    with tempfile.TemporaryDirectory() as tmp:
+        workdir = Path(tmp)
+        problems.extend(check_core(manifest, repo_root, workdir))
+        problems.extend(check_r_tarball(manifest, repo_root, workdir))
+
+    if problems:
+        print("Vendored provenance verification FAILED:")
+        for problem in problems:
+            print(f"  - {problem}")
+        return 1
+
+    print(
+        "Vendored provenance OK: extern/NNS-core matches "
+        f"{manifest.get('core_repo')}@{manifest.get('core_commit')} and "
+        f"tools/NNS matches {manifest.get('r_repo')}@{manifest.get('r_commit')}."
+    )
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/tests/tools/test_verify_provenance_offline.py b/tests/tools/test_verify_provenance_offline.py
new file mode 100644
index 00000000..95d60381
--- /dev/null
+++ b/tests/tools/test_verify_provenance_offline.py
@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+import importlib.util
+import json
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+SCRIPT = REPO_ROOT / "scripts" / "verify_release_provenance.py"
+MANIFEST = REPO_ROOT / "sync" / "nns_source.json"
+
+
+def _load_module() -> object:
+    spec = importlib.util.spec_from_file_location("verify_release_provenance", SCRIPT)
+    assert spec is not None and spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+def test_recorded_r_src_tree_hash_matches_vendored_tree() -> None:
+    """The committed r_src_tree_hash must match the vendored tools/NNS/src tree.
+
+    This is the offline half of the release provenance verifier; it catches any
+    edit to the vendored R snapshot that forgets to update the manifest.
+    """
+    module = _load_module()
+    manifest = json.loads(MANIFEST.read_text(encoding="utf-8"))
+    problems = module.check_offline(manifest, REPO_ROOT)  # type: ignore[attr-defined]
+    assert problems == [], problems