diff --git a/.github/workflows/make_prs_for_other_repos.yaml b/.github/workflows/make_prs_for_other_repos.yaml
index db3fd2730..ad927f5ee 100644
--- a/.github/workflows/make_prs_for_other_repos.yaml
+++ b/.github/workflows/make_prs_for_other_repos.yaml
@@ -1,6 +1,6 @@
 name: "Make PRs for Nextstrain projects which depend on Auspice"
 on:
-  pull_request:
+  workflow_dispatch:
 jobs:
   make-pr-on-nextstrain-dot-org: # <job_id>
     runs-on: ubuntu-latest
@@ -12,30 +12,24 @@ jobs:
         uses: actions/checkout@v2
         with:
           repository: nextstrain/nextstrain.org
-      - name: Install Auspice from PRs HEAD commit
-        if: ${{ github.event_name == 'pull_request' }}
-        # Note: $GITHUB_SHA is _not_ the same commit as the HEAD commit on the PR branch
-        # see https://gitpro.ttaallkk.topmunity/t/github-sha-not-the-same-as-the-triggering-commit/18286/2
+      - name: Install Auspice from source ref HEAD
         shell: bash
         run: |
-          AUSPICE_COMMIT=$(cat $GITHUB_EVENT_PATH | jq -r .pull_request.head.sha)
-          echo "auspice_commit=$AUSPICE_COMMIT" >> $GITHUB_ENV
           npm ci
-          npm install nextstrain/auspice#${AUSPICE_COMMIT}
+          npm install nextstrain/auspice#${GITHUB_SHA}
           git add package.json package-lock.json
       - name: Create Pull Request for testing on nextstrain.org repo
-        if: ${{ github.event_name == 'pull_request' }}
         id: cpr
         uses: peter-evans/create-pull-request@v3
         with:
           token: ${{ secrets.NEXTSTRAIN_BOT_PAT }}
-          branch: "auspice-pr-${{ github.event.pull_request.number }}"
-          commit-message: "[testing only] upgrade auspice to ${{ env.auspice_commit }}"
+          branch: "auspice-pr-${{ github.event.workflow_dispatch.ref }}"
+          commit-message: "[testing only] upgrade auspice to ${{ github.sha }}"
           author: 'nextstrain-bot <nextstrain-bot@users.noreply.github.com>'
           committer: 'nextstrain-bot <nextstrain-bot@users.noreply.github.com>'
-          title: '[bot] [DO NOT MERGE] Test auspice PR ${{ github.event.pull_request.number }}'
+          title: '[bot] [DO NOT MERGE] Test auspice ref ${{ github.event.workflow_dispatch.ref }}'
           body: |
-            This PR has been created to test Auspice from [PR ${{ github.event.pull_request.number }}](https://github.com/nextstrain/auspice/pull/${{ github.event.pull_request.number }})
+            This PR has been created to test Auspice from [ref ${{ github.event.workflow_dispatch.ref }}](https://github.com/nextstrain/auspice/pull/${{ github.event.workflow_dispatch.ref }})
 
             This message and corresponding commits were automatically created by a GitHub Action from [nextstrain/auspice](https://github.com/nextstrain/auspice)
           draft: true