diff --git a/py/src/braintrust/parameters.py b/py/src/braintrust/parameters.py
index 9af61b7f3..5d10d0cd8 100644
--- a/py/src/braintrust/parameters.py
+++ b/py/src/braintrust/parameters.py
@@ -134,12 +134,14 @@ def parameters_to_json_schema(parameters: EvalParameters) -> dict[str, Any]:
 
     for name, schema in parameters.items():
         if isinstance(schema, dict) and schema.get("type") == "prompt":
-            # Prompt parameter
-            result[name] = {
-                "type": "prompt",
-                "default": schema.get("default"),
-                "description": schema.get("description"),
-            }
+            # Prompt parameter - only include optional fields if they have values
+            # to avoid serializing None as null which Zod .optional() rejects
+            param_data: dict[str, Any] = {"type": "prompt"}
+            if schema.get("default") is not None:
+                param_data["default"] = schema["default"]
+            if schema.get("description") is not None:
+                param_data["description"] = schema["description"]
+            result[name] = param_data
         else:
             # Pydantic model
             try:
diff --git a/py/src/braintrust/test_parameters.py b/py/src/braintrust/test_parameters.py
new file mode 100644
index 000000000..fb7dac06a
--- /dev/null
+++ b/py/src/braintrust/test_parameters.py
@@ -0,0 +1,111 @@
+"""Tests for parameters module."""
+
+import json
+import pytest
+from .parameters import parameters_to_json_schema
+
+
+def test_parameters_to_json_schema_omits_null_description():
+    """Test that description field is omitted when not set (not serialized as null).
+
+    This is important because the frontend Zod schema uses .optional() which
+    accepts undefined but rejects null values.
+
+    Regression test for: Remote eval "Not connected" when description is missing
+    """
+    params = {
+        "main": {
+            "type": "prompt",
+            "name": "Main prompt",
+            # description intentionally omitted
+            "default": {
+                "prompt": {
+                    "type": "chat",
+                    "messages": [{"role": "user", "content": "{{input}}"}],
+                },
+                "options": {"model": "gpt-4o"},
+            },
+        }
+    }
+
+    result = parameters_to_json_schema(params)
+
+    # Verify the structure is correct
+    assert "main" in result
+    assert result["main"]["type"] == "prompt"
+    assert "default" in result["main"]
+
+    # Critical: description should NOT be present (not even as None/null)
+    assert "description" not in result["main"], \
+        "description should be omitted when not set, not serialized as null"
+
+    # Verify it serializes to JSON without null description
+    json_str = json.dumps(result)
+    assert '"description": null' not in json_str
+
+
+def test_parameters_to_json_schema_includes_description_when_set():
+    """Test that description field is included when explicitly set."""
+    params = {
+        "main": {
+            "type": "prompt",
+            "name": "Main prompt",
+            "description": "This is the main prompt",
+            "default": {
+                "prompt": {
+                    "type": "chat",
+                    "messages": [{"role": "user", "content": "{{input}}"}],
+                },
+                "options": {"model": "gpt-4o"},
+            },
+        }
+    }
+
+    result = parameters_to_json_schema(params)
+
+    assert result["main"]["description"] == "This is the main prompt"
+
+
+def test_parameters_to_json_schema_omits_null_default():
+    """Test that default field is omitted when not set."""
+    params = {
+        "main": {
+            "type": "prompt",
+            "name": "Main prompt",
+            # default intentionally omitted
+        }
+    }
+
+    result = parameters_to_json_schema(params)
+
+    assert "main" in result
+    assert result["main"]["type"] == "prompt"
+    assert "default" not in result["main"], \
+        "default should be omitted when not set, not serialized as null"
+
+
+def test_parameters_to_json_schema_includes_both_when_set():
+    """Test that both description and default are included when set."""
+    params = {
+        "scoring_prompt": {
+            "type": "prompt",
+            "name": "Scoring Prompt",
+            "description": "The prompt used for scoring",
+            "default": {
+                "prompt": {
+                    "type": "chat",
+                    "messages": [
+                        {"role": "system", "content": "You are a scorer."},
+                        {"role": "user", "content": "Score this: {{input}}"},
+                    ],
+                },
+                "options": {"model": "claude-opus-4-20250514"},
+            },
+        }
+    }
+
+    result = parameters_to_json_schema(params)
+
+    assert result["scoring_prompt"]["type"] == "prompt"
+    assert result["scoring_prompt"]["description"] == "The prompt used for scoring"
+    assert result["scoring_prompt"]["default"]["prompt"]["type"] == "chat"