diff --git a/.github/workflows/mypy_primer.yml b/.github/workflows/mypy_primer.yml
new file mode 100644
index 000000000000..08e4b117d3c8
--- /dev/null
+++ b/.github/workflows/mypy_primer.yml
@@ -0,0 +1,99 @@
+name: Run mypy_primer
+
+on:
+  # Only run on PR, since we diff against main
+  pull_request:
+    paths:
+      - "**/*.pyi"
+      - ".github/workflows/mypy_primer.yml"
+      - ".github/workflows/mypy_primer_comment.yml"
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  mypy_primer:
+    name: Run
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        shard-index: [0]  # e.g. change this to [0, 1, 2] and --num-shards below to 3
+      fail-fast: false
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          path: numpy_to_test
+          fetch-depth: 0
+      - uses: actions/setup-python@0b93645e9fea7318ecaed2b359559ac225c90a2b # v5.3.0
+        with:
+          python-version: "3.12"
+      - name: Install dependencies
+        run: pip install git+https://github.com/hauntsaninja/mypy_primer.git
+      - name: Run mypy_primer
+        shell: bash
+        run: |
+          cd numpy_to_test
+          MYPY_VERSION=$(grep mypy== requirements/test_requirements.txt | sed -n 's/mypy==\([^;]*\).*/\1/p')
+
+          echo "new commit"
+          git checkout $GITHUB_SHA
+          git rev-list --format=%s --max-count=1 HEAD
+
+          MERGE_BASE=$(git merge-base $GITHUB_SHA origin/$GITHUB_BASE_REF)
+          git worktree add ../numpy_base $MERGE_BASE
+          cd ../numpy_base
+
+          echo "base commit"
+          git rev-list --format=%s --max-count=1 HEAD
+
+          echo ''
+          cd ..
+          # fail action if exit code isn't zero or one
+          # TODO: note that we don't build numpy, so if a project attempts to use the
+          # numpy mypy plugin, we may see some issues involving version skew.
+          (
+            mypy_primer \
+            --new v${MYPY_VERSION} --old v${MYPY_VERSION} \
+            --known-dependency-selector numpy \
+            --old-prepend-path numpy_base --new-prepend-path numpy_to_test \
+            --num-shards 1 --shard-index ${{ matrix.shard-index }} \
+            --debug \
+            --output concise \
+            | tee diff_${{ matrix.shard-index }}.txt
+          ) || [ $? -eq 1 ]
+      - if: ${{ matrix.shard-index == 0 }}
+        name: Save PR number
+        run: |
+          echo ${{ github.event.pull_request.number }} | tee pr_number.txt
+      - name: Upload mypy_primer diff + PR number
+        uses: actions/upload-artifact@6f51ac03b9356f520e9adb1b1b7802705f340c2b # v4.5.0
+        if: ${{ matrix.shard-index == 0 }}
+        with:
+          name: mypy_primer_diffs-${{ matrix.shard-index }}
+          path: |
+            diff_${{ matrix.shard-index }}.txt
+            pr_number.txt
+      - name: Upload mypy_primer diff
+        uses: actions/upload-artifact@6f51ac03b9356f520e9adb1b1b7802705f340c2b # v4.5.0
+        if: ${{ matrix.shard-index != 0 }}
+        with:
+          name: mypy_primer_diffs-${{ matrix.shard-index }}
+          path: diff_${{ matrix.shard-index }}.txt
+
+  join_artifacts:
+    name: Join artifacts
+    runs-on: ubuntu-latest
+    needs: [mypy_primer]
+    permissions:
+      contents: read
+    steps:
+      - name: Merge artifacts
+        uses: actions/upload-artifact/merge@6f51ac03b9356f520e9adb1b1b7802705f340c2b # v4.5.0
+        with:
+          name: mypy_primer_diffs
+          pattern: mypy_primer_diffs-*
+          delete-merged: true
diff --git a/.github/workflows/mypy_primer_comment.yml b/.github/workflows/mypy_primer_comment.yml
new file mode 100644
index 000000000000..e484589ad446
--- /dev/null
+++ b/.github/workflows/mypy_primer_comment.yml
@@ -0,0 +1,96 @@
+name: Comment with mypy_primer diff
+
+on:
+  workflow_run:
+    workflows:
+      - Run mypy_primer
+    types:
+      - completed
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  comment:
+    name: Comment PR from mypy_primer
+    runs-on: ubuntu-latest
+    if: ${{ github.event.workflow_run.conclusion == 'success' }}
+    steps:
+      - name: Hide old comments
+        uses: kanga333/comment-hider@c12bb20b48aeb8fc098e35967de8d4f8018fffdf  # v0.4.0
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          issue_number: ${{ steps.post-comment.outputs.result }}
+
+      - name: Download diffs
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea  # v7.0.1
+        with:
+          script: |
+            const fs = require('fs');
+            const artifacts = await github.rest.actions.listWorkflowRunArtifacts({
+               owner: context.repo.owner,
+               repo: context.repo.repo,
+               run_id: ${{ github.event.workflow_run.id }},
+            });
+            const [matchArtifact] = artifacts.data.artifacts.filter((artifact) =>
+              artifact.name == "mypy_primer_diffs");
+
+            const download = await github.rest.actions.downloadArtifact({
+               owner: context.repo.owner,
+               repo: context.repo.repo,
+               artifact_id: matchArtifact.id,
+               archive_format: "zip",
+            });
+            fs.writeFileSync("diff.zip", Buffer.from(download.data));
+
+      - run: unzip diff.zip
+      - run: |
+          cat diff_*.txt | tee fulldiff.txt
+
+      - name: Post comment
+        id: post-comment
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea  # v7.0.1
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            const MAX_CHARACTERS = 50000
+            const MAX_CHARACTERS_PER_PROJECT = MAX_CHARACTERS / 3
+
+            const fs = require('fs')
+            let data = fs.readFileSync('fulldiff.txt', { encoding: 'utf8' })
+
+            function truncateIfNeeded(original, maxLength) {
+              if (original.length <= maxLength) {
+                return original
+              }
+              let truncated = original.substring(0, maxLength)
+              // further, remove last line that might be truncated
+              truncated = truncated.substring(0, truncated.lastIndexOf('\n'))
+              let lines_truncated = original.split('\n').length - truncated.split('\n').length
+              return `${truncated}\n\n... (truncated ${lines_truncated} lines) ...`
+            }
+
+            const projects = data.split('\n\n')
+            // don't let one project dominate
+            data = projects.map(project => truncateIfNeeded(project, MAX_CHARACTERS_PER_PROJECT)).join('\n\n')
+            // posting comment fails if too long, so truncate
+            data = truncateIfNeeded(data, MAX_CHARACTERS)
+
+            console.log("Diff from mypy_primer:")
+            console.log(data)
+
+            let body
+            if (data.trim()) {
+              body = 'Diff from [mypy_primer](https://github.com/hauntsaninja/mypy_primer), '
+              body += 'showing the effect of this PR on type check results on a corpus of open source code:\n```diff\n'
+              body += data + '```'
+              const prNumber = parseInt(fs.readFileSync("pr_number.txt", { encoding: "utf8" }))
+              await github.rest.issues.createComment({
+                issue_number: prNumber,
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                body
+              })
+            }
+            return prNumber