diff --git a/data_only_viz/tests/test_multi_hmr_worker.py b/data_only_viz/tests/test_multi_hmr_worker.py
new file mode 100644
index 0000000..882a430
--- /dev/null
+++ b/data_only_viz/tests/test_multi_hmr_worker.py
@@ -0,0 +1,53 @@
+"""MultiHMRWorker availability and thread-safety invariants."""
+
+from pathlib import Path
+import re
+
+
+def test_is_available_returns_false_when_checkpoint_missing(tmp_path, monkeypatch):
+    from data_only_viz import multi_hmr_worker
+
+    # All three resources point at non-existent paths
+    monkeypatch.setattr(multi_hmr_worker, "CKPT", tmp_path / "missing-ckpt.pt")
+    monkeypatch.setattr(multi_hmr_worker, "SMPLX_PATH", tmp_path / "missing-smplx")
+    monkeypatch.setattr(multi_hmr_worker, "MULTIHMR_REPO", tmp_path / "missing-repo")
+
+    assert multi_hmr_worker.MultiHMRWorker.is_available() is False
+
+
+def test_is_available_returns_true_when_all_paths_present(tmp_path, monkeypatch):
+    from data_only_viz import multi_hmr_worker
+
+    # Materialize all three paths
+    ckpt = tmp_path / "fake.pt"
+    ckpt.write_bytes(b"")
+    smplx_dir = tmp_path / "smplx"
+    smplx_dir.mkdir()
+    repo_dir = tmp_path / "repo"
+    repo_dir.mkdir()
+
+    monkeypatch.setattr(multi_hmr_worker, "CKPT", ckpt)
+    monkeypatch.setattr(multi_hmr_worker, "SMPLX_PATH", smplx_dir)
+    monkeypatch.setattr(multi_hmr_worker, "MULTIHMR_REPO", repo_dir)
+
+    assert multi_hmr_worker.MultiHMRWorker.is_available() is True
+
+
+def test_state_mutations_are_all_under_lock():
+    """Static grep: every assignment to state.persons_smplx is preceded by `with state.lock()`."""
+    src = (Path(__file__).parent.parent / "multi_hmr_worker.py").read_text()
+    lines = src.splitlines()
+    assign_indices = [
+        i + 1
+        for i, line in enumerate(lines)
+        if re.search(r"\bstate\.persons_smplx\s*=", line)
+    ]
+    assert assign_indices, "expected at least one persons_smplx assignment in multi_hmr_worker.py"
+
+    for lineno in assign_indices:
+        # Walk backwards up to 20 lines, looking for a `with self.state.lock():` context
+        window = lines[max(0, lineno - 20):lineno]
+        assert any("state.lock()" in w for w in window), (
+            f"line {lineno} mutates persons_smplx without a nearby `state.lock()` context:\n"
+            f"{lines[lineno - 1]}"
+        )