diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 714b545eeb..85bf6823df 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -294,7 +294,7 @@ jobs:
       - id: forward-roll-baseline
         name: Resolve the published forward-roll baseline
         if: steps.plan.outputs.state_root_compat == 'true'
-        run: node scripts/release-cli-publication.mjs resolve-nightly-predecessor "$GITHUB_OUTPUT"
+        run: node scripts/release-cli-publication.mjs resolve-nightly-predecessor "$GITHUB_OUTPUT" HEAD
 
       - name: Download the forward-roll baseline
         if: steps.plan.outputs.state_root_compat == 'true'
diff --git a/.github/workflows/cli-package-validation.yml b/.github/workflows/cli-package-validation.yml
index a11d653674..cd1d1fd80b 100644
--- a/.github/workflows/cli-package-validation.yml
+++ b/.github/workflows/cli-package-validation.yml
@@ -356,7 +356,7 @@ jobs:
       # is now this job's output.
       - name: Resolve the current npm Nightly as immutable evidence
         id: predecessor
-        run: node scripts/release-cli-publication.mjs resolve-nightly-predecessor "$GITHUB_OUTPUT"
+        run: node scripts/release-cli-publication.mjs resolve-nightly-predecessor "$GITHUB_OUTPUT" HEAD
       # Three runs of one script against one sandbox, not three runners. Two of
       # these transitions are between tarballs that were published and frozen,
       # so nothing in a pull request can change their outcome except the
diff --git a/scripts/release-cli-publication.mjs b/scripts/release-cli-publication.mjs
index 0065ce9540..60200a5178 100644
--- a/scripts/release-cli-publication.mjs
+++ b/scripts/release-cli-publication.mjs
@@ -17,8 +17,9 @@
  * under the License.
  */
 
+import { execFileSync } from 'node:child_process';
 import { appendFileSync, copyFileSync, mkdirSync, readFileSync, writeFileSync } from 'node:fs';
-import { basename, join, resolve } from 'node:path';
+import { basename, dirname, join, resolve } from 'node:path';
 import { fileURLToPath } from 'node:url';
 import { createHash } from 'node:crypto';
 import { CLI_RELEASE_ARTIFACT_LIMITS } from './release-cli-artifact-policy.mjs';
@@ -33,6 +34,7 @@ const REGISTRY_ORIGIN = 'https://registry.npmjs.org';
 const REPOSITORY = 'apache/maka';
 const PUBLICATION_WORKFLOW_PATH = '.github/workflows/npm-publication.yml';
 const REGISTRY_REQUEST_TIMEOUT_MS = 30_000;
+const DEFAULT_REPO_ROOT = dirname(dirname(fileURLToPath(import.meta.url)));
 const RELEASE_RECORD_KEYS = [
   'schemaVersion',
   'packageName',
@@ -218,7 +220,33 @@ export async function fetchRegistryRelease({
   return { ...record, tarballPath, sha256 };
 }
 
-export async function resolveRegistryNightlyPredecessor({ fetchImpl = fetch } = {}) {
+export function assertCommitIsAncestor({
+  commit,
+  head = 'HEAD',
+  repoRoot = DEFAULT_REPO_ROOT,
+  exec = execFileSync,
+}) {
+  if (typeof commit !== 'string' || !/^[0-9a-f]{7,40}$/i.test(commit)) {
+    throw new Error(`Invalid git commit SHA for ancestor check: ${commit}`);
+  }
+  try {
+    exec('git', ['merge-base', '--is-ancestor', commit, head], {
+      cwd: repoRoot,
+      stdio: ['ignore', 'ignore', 'pipe'],
+    });
+  } catch {
+    throw new Error(
+      `Registry Nightly gitHead ${commit} is not an ancestor of ${head}; forward-roll baseline must precede the change under test (#4447)`,
+    );
+  }
+}
+
+export async function resolveRegistryNightlyPredecessor({
+  fetchImpl = fetch,
+  fencedAncestorHead,
+  repoRoot = DEFAULT_REPO_ROOT,
+  exec = execFileSync,
+} = {}) {
   const packageMetadata = await fetchJson(
     fetchImpl,
     `${REGISTRY_ORIGIN}/${PACKAGE_NAME}`,
@@ -237,6 +265,16 @@ export async function resolveRegistryNightlyPredecessor({ fetchImpl = fetch } =
     throw new Error('Registry Nightly identity does not match its dist-tag');
   }
 
+  const gitHead = versionMetadata.gitHead;
+  if (fencedAncestorHead !== undefined) {
+    if (!gitHead) {
+      throw new Error(
+        `Registry Nightly ${version} is missing gitHead metadata required for ancestor fencing (#4447)`,
+      );
+    }
+    assertCommitIsAncestor({ commit: gitHead, head: fencedAncestorHead, repoRoot, exec });
+  }
+
   const tarball = `${PACKAGE_NAME}-${version}.tgz`;
   const tarballUrl = parseRegistryTarballUrl(versionMetadata.dist?.tarball, tarball);
   const integrity = parseSha512Integrity(versionMetadata.dist?.integrity);
@@ -244,6 +282,7 @@ export async function resolveRegistryNightlyPredecessor({ fetchImpl = fetch } =
     version,
     tarballUrl,
     integrity,
+    ...(gitHead ? { gitHead } : {}),
   };
 }
 
@@ -621,13 +660,14 @@ async function main() {
     });
     return;
   }
-  if (command === 'resolve-nightly-predecessor' && args.length === 1) {
-    const [output] = args;
-    const predecessor = await resolveRegistryNightlyPredecessor();
+  if (command === 'resolve-nightly-predecessor' && (args.length === 1 || args.length === 2)) {
+    const [output, fencedAncestorHead] = args;
+    const predecessor = await resolveRegistryNightlyPredecessor({ fencedAncestorHead });
     appendOutputs(output, {
       version: predecessor.version,
       tarball_url: predecessor.tarballUrl,
       integrity: predecessor.integrity,
+      ...(predecessor.gitHead ? { git_head: predecessor.gitHead } : {}),
     });
     return;
   }
diff --git a/scripts/release-cli-publication.test.mjs b/scripts/release-cli-publication.test.mjs
index 28cc5d13fa..084ca4d0d7 100644
--- a/scripts/release-cli-publication.test.mjs
+++ b/scripts/release-cli-publication.test.mjs
@@ -26,6 +26,7 @@ import { join, resolve } from 'node:path';
 import test from 'node:test';
 import { CLI_RELEASE_ARTIFACT_LIMITS } from './release-cli-artifact-policy.mjs';
 import {
+  assertCommitIsAncestor,
   assertRegistryNightlyPredecessor,
   fetchRegistryRelease,
   parseCliNightlyVersion,
@@ -332,6 +333,64 @@ test('a newer Nightly invalidates previously qualified predecessor evidence', as
   );
 });
 
+test('fences the predecessor commit to an ancestor of the change under test (#4447)', async () => {
+  const ancestorCommit = '1111111111111111111111111111111111111111';
+  const nonAncestorCommit = '2222222222222222222222222222222222222222';
+  const fixture = {
+    ...createCandidate('0.2.0-dev.42.20260829', '0.2.0'),
+    gitHead: ancestorCommit,
+  };
+
+  const execStub = (cmd, args) => {
+    assert.equal(cmd, 'git');
+    assert.equal(args[0], 'merge-base');
+    assert.equal(args[1], '--is-ancestor');
+    if (args[2] === ancestorCommit) return;
+    const err = new Error('not an ancestor');
+    err.status = 1;
+    throw err;
+  };
+
+  // 1. Success when gitHead is an ancestor
+  const predecessor = await resolveRegistryNightlyPredecessor({
+    fetchImpl: registryFetch({ fixture }),
+    fencedAncestorHead: 'HEAD',
+    exec: execStub,
+  });
+  assert.equal(predecessor.gitHead, ancestorCommit);
+
+  // 2. Fails loudly when gitHead is NOT an ancestor
+  const nonAncestorFixture = {
+    ...createCandidate('0.2.0-dev.42.20260829', '0.2.0'),
+    gitHead: nonAncestorCommit,
+  };
+  await assert.rejects(
+    resolveRegistryNightlyPredecessor({
+      fetchImpl: registryFetch({ fixture: nonAncestorFixture }),
+      fencedAncestorHead: 'HEAD',
+      exec: execStub,
+    }),
+    /is not an ancestor of HEAD; forward-roll baseline must precede the change under test/,
+  );
+
+  // 3. Fails loudly when gitHead is missing from registry metadata and fence was requested
+  const missingGitHeadFixture = createCandidate('0.2.0-dev.42.20260829', '0.2.0');
+  await assert.rejects(
+    resolveRegistryNightlyPredecessor({
+      fetchImpl: registryFetch({ fixture: missingGitHeadFixture }),
+      fencedAncestorHead: 'HEAD',
+      exec: execStub,
+    }),
+    /is missing gitHead metadata required for ancestor fencing/,
+  );
+
+  // 4. Invalid commit SHA rejected
+  assert.throws(
+    () => assertCommitIsAncestor({ commit: 'not-a-sha', exec: execStub }),
+    /Invalid git commit SHA/,
+  );
+});
+
 test('signature audit must contain Maka provenance for the finalized version', () => {
   const fixture = createPreparedCandidate();
   const verified = {
@@ -616,6 +675,7 @@ function registryFetch({ fixture, bytes = fixture.bytes }) {
         name: 'maka-agent',
         version: fixture.version,
         dist: { tarball: tarballUrl, integrity, shasum },
+        ...(fixture.gitHead ? { gitHead: fixture.gitHead } : {}),
       });
     }
     if (url === 'https://registry.npmjs.org/maka-agent') {