diff --git a/.github/scripts/tests/test_cua_driver_release_wiring.py b/.github/scripts/tests/test_cua_driver_release_wiring.py
index bbd9ff4777..84e4c16571 100644
--- a/.github/scripts/tests/test_cua_driver_release_wiring.py
+++ b/.github/scripts/tests/test_cua_driver_release_wiring.py
@@ -111,6 +111,7 @@ def test_release_please_owns_driver_and_lume(self) -> None:
         self.assertIn('"component": "cua-driver-rs"', config)
         self.assertIn('"component": "lume"', config)
         self.assertIn("5c625bfb5d1ff62eadeeb3772007f7f66fdcf071", workflow)
+        self.assertIn("validate_release_please_tags.py --target HEAD", workflow)
         self.assertIn('-p cua-driver --precise "$DRIVER_VERSION"', workflow)
         self.assertIn(
             "gh pr list --state open --base main --limit 100 --json number",
diff --git a/.github/scripts/tests/test_validate_release_please_tags.py b/.github/scripts/tests/test_validate_release_please_tags.py
new file mode 100644
index 0000000000..c8fd505b6f
--- /dev/null
+++ b/.github/scripts/tests/test_validate_release_please_tags.py
@@ -0,0 +1,83 @@
+from __future__ import annotations
+
+import importlib.util
+from pathlib import Path
+import subprocess
+
+import pytest
+
+SCRIPT_PATH = Path(__file__).resolve().parents[1] / "validate_release_please_tags.py"
+SPEC = importlib.util.spec_from_file_location("validate_release_please_tags", SCRIPT_PATH)
+assert SPEC is not None and SPEC.loader is not None
+MODULE = importlib.util.module_from_spec(SPEC)
+SPEC.loader.exec_module(MODULE)
+TagValidationError = MODULE.TagValidationError
+package_tag = MODULE.package_tag
+validate_tags = MODULE.validate_tags
+
+
+CONFIG = {
+    "include-component-in-tag": True,
+    "include-v-in-tag": True,
+    "tag-separator": "-",
+    "packages": {
+        "libs/cua-driver": {
+            "component": "cua-driver-rs",
+            "release-type": "simple",
+        }
+    },
+}
+MANIFEST = {"libs/cua-driver": "0.18.0"}
+
+
+def run_git(repo: Path, *args: str) -> str:
+    return subprocess.run(
+        ["git", *args], cwd=repo, check=True, capture_output=True, text=True
+    ).stdout.strip()
+
+
+@pytest.fixture
+def repo(tmp_path: Path) -> Path:
+    run_git(tmp_path, "init", "-b", "main")
+    run_git(tmp_path, "config", "user.name", "Release Test")
+    run_git(tmp_path, "config", "user.email", "release-test@example.com")
+    (tmp_path / "file").write_text("first\n")
+    run_git(tmp_path, "add", "file")
+    run_git(tmp_path, "commit", "-m", "first")
+    return tmp_path
+
+
+def test_package_tag_uses_manifest_tag_options() -> None:
+    assert package_tag(CONFIG, CONFIG["packages"]["libs/cua-driver"], "0.18.0") == (
+        "cua-driver-rs-v0.18.0"
+    )
+
+
+def test_allows_missing_tag_for_newly_merged_release(repo: Path) -> None:
+    assert validate_tags(repo_root=repo, config=CONFIG, manifest=MANIFEST, target="HEAD") == []
+
+
+def test_accepts_manifest_tag_on_target_branch(repo: Path) -> None:
+    run_git(repo, "tag", "cua-driver-rs-v0.18.0")
+    (repo / "file").write_text("second\n")
+    run_git(repo, "commit", "-am", "second")
+
+    assert validate_tags(repo_root=repo, config=CONFIG, manifest=MANIFEST, target="HEAD") == [
+        "cua-driver-rs-v0.18.0"
+    ]
+
+
+def test_rejects_manifest_tag_outside_target_branch(repo: Path) -> None:
+    base = run_git(repo, "rev-parse", "HEAD")
+    run_git(repo, "switch", "-c", "release-metadata")
+    (repo / "metadata").write_text("notes\n")
+    run_git(repo, "add", "metadata")
+    run_git(repo, "commit", "-m", "metadata")
+    run_git(repo, "tag", "cua-driver-rs-v0.18.0")
+    run_git(repo, "switch", "main")
+    (repo / "file").write_text("main\n")
+    run_git(repo, "commit", "-am", "main")
+    assert run_git(repo, "merge-base", "HEAD", "release-metadata") == base
+
+    with pytest.raises(TagValidationError, match="is not an ancestor"):
+        validate_tags(repo_root=repo, config=CONFIG, manifest=MANIFEST, target="HEAD")
diff --git a/.github/scripts/validate_release_please_tags.py b/.github/scripts/validate_release_please_tags.py
new file mode 100644
index 0000000000..23f24783c6
--- /dev/null
+++ b/.github/scripts/validate_release_please_tags.py
@@ -0,0 +1,111 @@
+#!/usr/bin/env python3
+"""Reject Release Please manifest tags that are not on the target branch."""
+
+from __future__ import annotations
+
+import argparse
+import json
+from pathlib import Path
+import subprocess
+from typing import Any, Mapping, Sequence
+
+
+class TagValidationError(RuntimeError):
+    """A released manifest version has an invalid Git tag."""
+
+
+def package_tag(config: Mapping[str, Any], package: Mapping[str, Any], version: str) -> str:
+    def option(name: str, default: Any) -> Any:
+        return package.get(name, config.get(name, default))
+
+    component = package.get("component", package.get("package-name"))
+    if option("include-component-in-tag", False) and not component:
+        raise TagValidationError("component tag requested without a component or package name")
+
+    prefix = (
+        str(component) + str(option("tag-separator", "-"))
+        if option("include-component-in-tag", False)
+        else ""
+    )
+    version_prefix = "v" if option("include-v-in-tag", True) else ""
+    return f"{prefix}{version_prefix}{version}"
+
+
+def git(*args: str, cwd: Path, check: bool = True) -> subprocess.CompletedProcess[str]:
+    return subprocess.run(
+        ["git", *args],
+        cwd=cwd,
+        check=check,
+        capture_output=True,
+        text=True,
+    )
+
+
+def validate_tags(
+    *,
+    repo_root: Path,
+    config: Mapping[str, Any],
+    manifest: Mapping[str, Any],
+    target: str,
+) -> list[str]:
+    packages = config.get("packages")
+    if not isinstance(packages, Mapping):
+        raise TagValidationError("release config must contain a packages object")
+
+    checked: list[str] = []
+    for path, package in packages.items():
+        if path not in manifest:
+            raise TagValidationError(f"release manifest does not contain {path!r}")
+        if not isinstance(package, Mapping):
+            raise TagValidationError(f"release package config for {path!r} must be an object")
+
+        tag = package_tag(config, package, str(manifest[path]))
+        tag_ref = f"refs/tags/{tag}^{{commit}}"
+        if git("rev-parse", "--verify", "--quiet", tag_ref, cwd=repo_root, check=False).returncode:
+            # Immediately after a release PR merges, its new manifest version has
+            # no tag yet. Release Please must be allowed to create that tag.
+            continue
+        if git(
+            "merge-base", "--is-ancestor", tag_ref, target, cwd=repo_root, check=False
+        ).returncode:
+            tag_sha = git("rev-parse", tag_ref, cwd=repo_root).stdout.strip()
+            target_sha = git("rev-parse", target, cwd=repo_root).stdout.strip()
+            raise TagValidationError(
+                f"manifest tag {tag} ({tag_sha}) for {path} is not an ancestor of "
+                f"{target} ({target_sha}); refusing to generate release history"
+            )
+        checked.append(tag)
+    return checked
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--repo-root", type=Path, default=Path.cwd())
+    parser.add_argument("--config", type=Path, default=Path("release-please-config.json"))
+    parser.add_argument("--manifest", type=Path, default=Path(".release-please-manifest.json"))
+    parser.add_argument("--target", default="HEAD")
+    args = parser.parse_args(argv)
+
+    try:
+        repo_root = args.repo_root.resolve()
+        config_path = args.config if args.config.is_absolute() else repo_root / args.config
+        manifest_path = args.manifest if args.manifest.is_absolute() else repo_root / args.manifest
+        checked = validate_tags(
+            repo_root=repo_root,
+            config=json.loads(config_path.read_text()),
+            manifest=json.loads(manifest_path.read_text()),
+            target=args.target,
+        )
+    except (OSError, ValueError, subprocess.CalledProcessError, TagValidationError) as error:
+        print(f"release tag validation error: {error}")
+        return 1
+
+    if checked:
+        print(f"release manifest tags are on {args.target}: {', '.join(checked)}")
+    else:
+        print("release manifest tags do not exist yet; Release Please may create them")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/.github/workflows/release-please.yml b/.github/workflows/release-please.yml
index b32ddda676..06ba8b1d1a 100644
--- a/.github/workflows/release-please.yml
+++ b/.github/workflows/release-please.yml
@@ -61,6 +61,9 @@ jobs:
           echo "::error::Select cua-driver-rs or lume when forcing a bump type."
           exit 1
 
+      - name: Reject manifest tags outside main history
+        run: python3 .github/scripts/validate_release_please_tags.py --target HEAD
+
       - name: Resolve targeted release request
         id: request
         if: >-