diff --git a/__tests__/resolve-payload-fields.test.ts b/__tests__/resolve-payload-fields.test.ts
index 0db6a3c6..350cc200 100644
--- a/__tests__/resolve-payload-fields.test.ts
+++ b/__tests__/resolve-payload-fields.test.ts
@@ -14,6 +14,7 @@ const payload = {
   githubToken: 'ghs_token',
   headHttpUrl: 'https://github.com/acme/repo.git',
   repoUrl: 'https://github.com/acme/other.git',
+  pullRequestNumber: 269,
   owner: 'acme',
   hasCmRepo: true,
   cmRepo: 'cm-repo',
@@ -50,6 +51,7 @@ describe('toStepOutputs', () => {
     expect(toStepOutputs(payload)).toEqual({
       github_token: 'ghs_token',
       url: 'https://github.com/acme/repo.git',
+      pull_request_number: '269',
       has_cm_repo: 'true',
       cm_repository: 'acme/cm-repo',
       cm_repo_ref: 'main',
@@ -58,12 +60,22 @@ describe('toStepOutputs', () => {
     })
   })
 
+  it.each([1.2, -1, 0, Infinity, 'abc'])(
+    'blanks a pull request number that is not a positive integer: %p',
+    pullRequestNumber => {
+      expect(
+        toStepOutputs({ ...payload, pullRequestNumber }).pull_request_number
+      ).toBe('')
+    }
+  )
+
   it('falls back to repoUrl and blanks the cm repo when absent', () => {
     expect(
       toStepOutputs({ repoUrl: 'https://github.com/acme/other.git' })
     ).toEqual({
       github_token: '',
       url: 'https://github.com/acme/other.git',
+      pull_request_number: '',
       has_cm_repo: 'false',
       cm_repository: '',
       cm_repo_ref: '',
diff --git a/action.yml b/action.yml
index a15150f6..341089bd 100644
--- a/action.yml
+++ b/action.yml
@@ -104,7 +104,8 @@ runs:
         git config checkout.defaultRemote origin
         git fetch --shallow-since="6 months ago" origin $'${{ steps.safe-strings.outputs.base_ref }}'
         git remote add upstream $'${{ steps.safe-strings.outputs.url }}'
-        git fetch --shallow-since="6 months ago" upstream $'${{ steps.safe-strings.outputs.head_ref }}' || echo "::warning::Failed to fetch head branch. The branch may have been deleted."
+        # refs/pull/N/head is mirrored on the base repo, so a fork PR head needs no fork access.
+        git fetch --shallow-since="6 months ago" origin $'refs/pull/${{ steps.payload-fields.outputs.pull_request_number }}/head:refs/remotes/upstream/${{ steps.safe-strings.outputs.head_ref }}' || echo "::warning::Failed to fetch head branch. The branch may have been deleted."
         git checkout -b $'upstream/${{ steps.safe-strings.outputs.head_ref}}' $'upstream/${{ steps.safe-strings.outputs.head_ref}}' || true
         git checkout $'${{ steps.safe-strings.outputs.base_ref }}' || true
         git checkout $'${{ steps.safe-strings.outputs.head_ref }}' || true
diff --git a/scripts/resolve-payload-fields.js b/scripts/resolve-payload-fields.js
index 717c7ff5..35edde91 100644
--- a/scripts/resolve-payload-fields.js
+++ b/scripts/resolve-payload-fields.js
@@ -155,9 +155,13 @@ function normalizeForEngine(raw) {
  */
 function toStepOutputs(payload) {
   const hasCmRepo = payload.hasCmRepo === true
+  const prNumber = Number(payload.pullRequestNumber)
+  // A positive integer or '': spliced into the fetch refspec without the safe-strings escaping step.
+  const isPrNumber = Number.isInteger(prNumber) && prNumber > 0
   return {
     github_token: payload.githubToken || '',
     url: payload.headHttpUrl || payload.repoUrl || '',
+    pull_request_number: isPrNumber ? String(prNumber) : '',
     has_cm_repo: String(hasCmRepo),
     cm_repository: hasCmRepo ? `${payload.owner}/${payload.cmRepo}` : '',
     cm_repo_ref: payload.cmRepoRef || '',