diff --git a/py/src/braintrust/prompt_cache/disk_cache.py b/py/src/braintrust/prompt_cache/disk_cache.py
index b74080249..cddc46edb 100644
--- a/py/src/braintrust/prompt_cache/disk_cache.py
+++ b/py/src/braintrust/prompt_cache/disk_cache.py
@@ -128,10 +128,15 @@ def set(self, key: str, value: T) -> None:
                 os.makedirs(self._dir, exist_ok=True)
             file_path = self._get_entry_path(key)
 
+            # Serialize before opening the file: opening it for writing truncates any
+            # existing entry, so a serialization error would otherwise leave an empty
+            # file behind and destroy the previously cached value.
+            if self._serializer is not None:
+                value = self._serializer(value)
+            payload = json.dumps(value).encode("utf-8")
+
             with gzip.open(file_path, "wb") as f:
-                if self._serializer is not None:
-                    value = self._serializer(value)
-                f.write(json.dumps(value).encode("utf-8"))
+                f.write(payload)
 
             self._evict_if_full()
         except Exception as e:
diff --git a/py/src/braintrust/prompt_cache/test_disk_cache.py b/py/src/braintrust/prompt_cache/test_disk_cache.py
index 7f49986ed..618b9b2ab 100644
--- a/py/src/braintrust/prompt_cache/test_disk_cache.py
+++ b/py/src/braintrust/prompt_cache/test_disk_cache.py
@@ -24,6 +24,21 @@ def tearDown(self):
         except Exception:
             pass
 
+    def test_failed_serialization_keeps_existing_entry(self):
+        self.cache.set("key", {"a": 1})
+
+        # A value that json cannot serialize must not clobber the stored entry.
+        self.cache.set("key", {"a": object()})
+
+        assert self.cache.get("key") == {"a": 1}
+
+    def test_failed_serialization_does_not_create_entry(self):
+        self.cache.set("key", {"a": object()})
+
+        with self.assertRaises(KeyError):
+            self.cache.get("key")
+        assert os.listdir(self.cache_dir) == []
+
     def test_keys_with_invalid_paths(self):
         data = {"1": "2"}
         weird_keys = [