diff --git a/autolens/interop/coolest.py b/autolens/interop/coolest.py
index 227d7d31c..a399fdcd0 100644
--- a/autolens/interop/coolest.py
+++ b/autolens/interop/coolest.py
@@ -71,6 +71,8 @@ def _coolest_modules():
 
 
 def _path_no_ext(file_path: str) -> str:
+    # The coolest JSONSerializer rejects relative paths.
+    file_path = os.path.abspath(file_path)
     if file_path.endswith(".json"):
         return file_path[: -len(".json")]
     return file_path
@@ -245,7 +247,7 @@ def to_coolest(
     )
 
     path_no_ext = _path_no_ext(file_path)
-    os.makedirs(os.path.dirname(os.path.abspath(path_no_ext)), exist_ok=True)
+    os.makedirs(os.path.dirname(path_no_ext), exist_ok=True)
     JSONSerializer(path_no_ext, obj=root, check_external_files=False).dump_simple()
 
     return f"{path_no_ext}.json"
diff --git a/test_autolens/interop/test_coolest.py b/test_autolens/interop/test_coolest.py
index b9aced616..c959aa33f 100644
--- a/test_autolens/interop/test_coolest.py
+++ b/test_autolens/interop/test_coolest.py
@@ -146,6 +146,22 @@ def test__round_trip__nfw_uses_sigma_crit_from_cosmology(tmp_path):
     assert nfw_default.kappa_s == pytest.approx(0.15, rel=1e-3)
 
 
+def test__round_trip__relative_file_path(tmp_path, monkeypatch):
+    # The coolest JSONSerializer rejects relative paths — to_coolest /
+    # from_coolest must absolutize them.
+    monkeypatch.chdir(tmp_path)
+
+    file_path = interop_coolest.to_coolest(
+        galaxies=lens_model_galaxies(), file_path="template"
+    )
+
+    assert file_path == str(tmp_path / "template.json")
+
+    tracer_back = interop_coolest.from_coolest(file_path="template.json")
+
+    assert len(tracer_back.galaxies) == 3
+
+
 def test__from_coolest__missing_point_estimate_raises(tmp_path):
     pytest.importorskip("coolest")
     from coolest.template.lazy import (