diff --git a/.github/workflows/auto-label.yml b/.github/workflows/auto-label.yml
index 393c3bf19..46ff31e6a 100644
--- a/.github/workflows/auto-label.yml
+++ b/.github/workflows/auto-label.yml
@@ -1,7 +1,9 @@
 name: Auto Label
 on:
   pull_request:
-    types: [opened, reopened, synchronized]
+    # `synchronize` is the GitHub activity type for "new commits pushed to the PR".
+    # It is NOT `synchronized` — unknown types are silently ignored, not rejected.
+    types: [opened, reopened, synchronize]
   issues:
     types: [opened, edited]
 permissions:
@@ -18,10 +20,13 @@ jobs:
 
             // --- Label pull requests by changed file paths ---
             if (context.eventName === 'pull_request') {
-              const { data: files } = await github.rest.pulls.listFiles({
+              // Paginate: listFiles returns only the first 30 files by default, so a
+              // large PR would otherwise be labelled from an arbitrary prefix of its diff.
+              const files = await github.paginate(github.rest.pulls.listFiles, {
                 owner: context.repo.owner,
                 repo: context.repo.repo,
-                pull_number: context.payload.pull_request.number
+                pull_number: context.payload.pull_request.number,
+                per_page: 100
               });
 
               for (const file of files) {
diff --git a/tests/unit/test_auto_label_workflow.py b/tests/unit/test_auto_label_workflow.py
new file mode 100644
index 000000000..8730ead7b
--- /dev/null
+++ b/tests/unit/test_auto_label_workflow.py
@@ -0,0 +1,127 @@
+"""Regression tests for the Auto Label workflow.
+
+Both defects these cover were silent: an unknown `pull_request` activity type is
+ignored rather than rejected, and an unpaginated `listFiles` still concludes
+green on a truncated diff. Neither produced a red check, so static assertions
+alone would not have caught them — the pagination test below executes the
+workflow's own embedded script and fails against the previous implementation.
+"""
+
+from __future__ import annotations
+
+import json
+import shutil
+import subprocess
+import textwrap
+from pathlib import Path
+
+import pytest
+import yaml
+
+WORKFLOW_PATH = Path(__file__).resolve().parents[2] / ".github/workflows/auto-label.yml"
+
+
+def _load_workflow() -> dict:
+    assert WORKFLOW_PATH.exists(), "Auto Label workflow should exist"
+    return yaml.safe_load(WORKFLOW_PATH.read_text())
+
+
+def _get_script(workflow: dict) -> str:
+    return workflow["jobs"]["label"]["steps"][0]["with"]["script"]
+
+
+def test_auto_label_workflow_file_is_valid_yaml() -> None:
+    workflow = _load_workflow()
+    assert workflow["name"] == "Auto Label"
+
+
+def test_auto_label_triggers_on_synchronize_not_synchronized() -> None:
+    """`synchronized` is not a GitHub activity type; the real one is `synchronize`.
+
+    GitHub ignores unknown activity types instead of rejecting the workflow, so
+    the typo silently stopped the workflow from ever running on a push to an
+    open PR.
+    """
+    workflow = _load_workflow()
+    # PyYAML parses the YAML 'on' key as Python True.
+    types = workflow[True]["pull_request"]["types"]
+    assert "synchronize" in types
+    assert "synchronized" not in types
+    assert "opened" in types
+    assert "reopened" in types
+
+
+def test_auto_label_paginates_changed_files() -> None:
+    """`pulls.listFiles` defaults to 30 per page; a larger PR must not truncate."""
+    script = _get_script(_load_workflow())
+    assert "github.paginate" in script
+    assert "per_page: 100" in script
+
+
+@pytest.mark.skipif(shutil.which("node") is None, reason="node is required")
+def test_auto_label_labels_files_beyond_the_first_page(tmp_path: Path) -> None:
+    """Execute the workflow's embedded script over a 120-file diff.
+
+    The 120th file is the only `.py` in the diff, so the `python` label can only
+    appear if the listing paginated past the first page. The mocked client
+    raises if the unpaginated `pulls.listFiles` is called at all, which is what
+    makes this fail against the previous implementation rather than pass
+    vacuously.
+    """
+    script = _get_script(_load_workflow())
+    script_path = tmp_path / "auto_label.js"
+    script_path.write_text(script)
+
+    harness = tmp_path / "harness.mjs"
+    harness.write_text(textwrap.dedent("""
+            import fs from 'fs';
+            const script = fs.readFileSync(process.argv[2], 'utf8');
+
+            const files = [];
+            for (let i = 0; i < 119; i++) files.push({ filename: `apps/web/src/x${i}.ts` });
+            files.push({ filename: 'src/late.py' });  // only reachable via pagination
+
+            let added = null;
+            const github = {
+              paginate: async (_fn, opts) => {
+                if (opts.per_page !== 100) throw new Error('per_page not set to 100');
+                return files;
+              },
+              rest: {
+                pulls: {
+                  listFiles: async () => {
+                    throw new Error('unpaginated listFiles called');
+                  },
+                },
+                issues: { addLabels: async (o) => { added = o.labels; } },
+              },
+            };
+            const context = {
+              eventName: 'pull_request',
+              repo: { owner: 'o', repo: 'r' },
+              payload: { pull_request: { number: 1 } },
+            };
+            const core = { warning: () => {} };
+
+            await new Function(
+              'github', 'context', 'core',
+              `return (async () => { ${script} })()`
+            )(github, context, core);
+
+            process.stdout.write(JSON.stringify(added ?? []));
+            """).strip())
+
+    result = subprocess.run(
+        ["node", str(harness), str(script_path)],
+        capture_output=True,
+        text=True,
+        timeout=60,
+    )
+    assert result.returncode == 0, f"harness failed: {result.stderr}"
+
+    labels = json.loads(result.stdout)
+    assert "python" in labels, (
+        "the 120th changed file was not labelled, so the changed-file listing "
+        "was truncated to the first page"
+    )
+    assert "javascript" in labels