Rehydrate remote prompt parameters

py/src/braintrust/parameters.py

@@ -246,6 +246,28 @@ def validate_json_schema(parameters: dict[str, Any], schema: ParametersSchema) -
     return candidate
 
 
+def _rehydrate_remote_parameters(
+    parameters: dict[str, Any],
+    schema: ParametersSchema,
+) -> ValidatedParameters:
+    properties = schema.get("properties")
+    if not isinstance(properties, dict):
+        return parameters
+
+    result: ValidatedParameters = dict(parameters)
+    for name, property_schema in properties.items():
+        if not isinstance(property_schema, dict) or name not in result:
+            continue
+
+        if property_schema.get("x-bt-type") == "prompt":
+            prompt_data = result[name]
+            if not isinstance(prompt_data, dict):
+                raise ValueError(f"Invalid parameter '{name}': prompt value must be an object")
+            result[name] = _create_prompt(name, prompt_data)
+
+    return result
+
+
 def _validate_local_parameters(
     parameters: dict[str, Any],
     parameter_schema: EvalParameters,
@@ -334,7 +356,8 @@ def validate_parameters(
     if isinstance(parameter_schema, RemoteEvalParameters):
         merged = dict(parameter_schema.data)
         merged.update(parameters)
-        return validate_json_schema(merged, parameter_schema.schema)
+        validated = validate_json_schema(merged, parameter_schema.schema)
+        return _rehydrate_remote_parameters(validated, parameter_schema.schema)
 
     return _validate_local_parameters(parameters, parameter_schema)
 

py/src/braintrust/test_parameters.py

@@ -90,7 +90,7 @@ def test_validate_remote_parameters_merges_saved_data_and_runtime_overrides():
     }
 
 
-def test_validate_remote_parameters_keeps_prompt_values_as_dicts():
+def test_validate_remote_parameters_rehydrates_prompt_values():
     parameters = RemoteEvalParameters(
         id="params-123",
         project_id="project-123",
@@ -122,7 +122,61 @@ def test_validate_remote_parameters_keeps_prompt_values_as_dicts():
 
     result = validate_parameters({}, parameters)
 
-    assert isinstance(result["main"], dict)
+    assert hasattr(result["main"], "build")
+    built = result["main"].build(input="test input")
+    assert built["messages"] == [{"role": "user", "content": "test input"}]
+    assert built["model"] == "gpt-5-mini"
+
+
+def test_validate_remote_parameters_allows_prompt_overrides():
+    parameters = RemoteEvalParameters(
+        id="params-123",
+        project_id="project-123",
+        name="Saved parameters",
+        slug="saved-parameters",
+        version="v1",
+        schema={
+            "type": "object",
+            "properties": {
+                "main": {
+                    "type": "object",
+                    "x-bt-type": "prompt",
+                },
+            },
+            "additionalProperties": True,
+        },
+        data={
+            "main": {
+                "prompt": {
+                    "type": "chat",
+                    "messages": [{"role": "user", "content": "{{input}}"}],
+                },
+                "options": {
+                    "model": "gpt-5-mini",
+                },
+            },
+        },
+    )
+
+    result = validate_parameters(
+        {
+            "main": {
+                "prompt": {
+                    "type": "chat",
+                    "messages": [{"role": "user", "content": "override {{input}}"}],
+                },
+                "options": {
+                    "model": "gpt-5-nano",
+                },
+            }
+        },
+        parameters,
+    )
+
+    assert hasattr(result["main"], "build")
+    built = result["main"].build(input="test input")
+    assert built["messages"] == [{"role": "user", "content": "override test input"}]
+    assert built["model"] == "gpt-5-nano"
 
 
 @pytest.mark.skipif(not HAS_PYDANTIC, reason="pydantic not installed")