diff --git a/tests/test_cli_run_name.py b/tests/test_cli_run_name.py
new file mode 100644
index 0000000..0259881
--- /dev/null
+++ b/tests/test_cli_run_name.py
@@ -0,0 +1,40 @@
+"""Tests for the --name/-n run-name deduplication logic."""
+
+import re
+from pathlib import Path
+
+import pytest
+
+from EvoScientist.cli import _deduplicate_run_name
+
+
+# =============================================================================
+# _deduplicate_run_name
+# =============================================================================
+
+
+class TestDeduplicateRunName:
+    def test_fresh_name(self, tmp_path: Path):
+        assert _deduplicate_run_name("experiment", tmp_path) == "experiment"
+
+    def test_single_collision(self, tmp_path: Path):
+        (tmp_path / "experiment").mkdir()
+        assert _deduplicate_run_name("experiment", tmp_path) == "experiment_1"
+
+    def test_multiple_collisions(self, tmp_path: Path):
+        (tmp_path / "experiment").mkdir()
+        (tmp_path / "experiment_1").mkdir()
+        (tmp_path / "experiment_2").mkdir()
+        assert _deduplicate_run_name("experiment", tmp_path) == "experiment_3"
+
+    def test_gap_in_sequence(self, tmp_path: Path):
+        """Picks the first available suffix, not the highest + 1."""
+        (tmp_path / "run").mkdir()
+        (tmp_path / "run_2").mkdir()
+        # _1 doesn't exist, so it should pick _1
+        assert _deduplicate_run_name("run", tmp_path) == "run_1"
+
+    def test_does_not_collide_with_files(self, tmp_path: Path):
+        """A regular file (not dir) with the same name still counts as taken."""
+        (tmp_path / "run").write_text("")
+        assert _deduplicate_run_name("run", tmp_path) == "run_1"