fix(output_keys; minor fixes):

langcorn/server/api.py

@@ -7,6 +7,7 @@
 from fastapi.security.utils import get_authorization_scheme_param
 from langchain.callbacks import get_openai_callback
 from langchain.schema import messages_from_dict, messages_to_dict
+from langchain.chains.base import Chain
 from loguru import logger
 from pydantic import BaseModel
 from uvicorn.importer import import_from_string
@@ -80,17 +81,27 @@ def run(self, prompt: str):
         return r.dict()
 
 
-def derive_fields(language_app) -> (list[str], list[str]):
+def _derive_output(language_app: Chain) -> list[str]:
+    if hasattr(language_app, "output_variables"):
+        return language_app.output_variables
+    elif hasattr(language_app, "output_keys"):
+        return language_app.output_keys
+    elif hasattr(language_app, "output_key"):
+        return language_app.output_key
+    return ["output"]
+
+
+def derive_fields(language_app: Chain) -> (list[str], list[str]):
     if hasattr(language_app, "input_variables"):
-        return language_app.input_variables, language_app.output_variables
+        return language_app.input_variables, _derive_output(language_app)
     elif hasattr(language_app, "prompt"):
         return language_app.prompt.input_variables, [language_app.output_key]
     elif hasattr(language_app, "input_keys"):
-        return language_app.input_keys, [language_app.output_key]
-    return [language_app.input_key], ["output"]
+        return language_app.input_keys, _derive_output(language_app)
+    return [language_app.input_key], _derive_output(language_app)
 
 
-def derive_class(name, fields, add_memory=False):
+def derive_class(name, fields, add_memory=False) -> BaseModel:
     annotations = {f: str for f in fields}
     if add_memory:
         annotations["memory"] = list[dict]