diff --git a/backend/app/server.py b/backend/app/server.py
index a1f09ede..ca1e63de 100644
--- a/backend/app/server.py
+++ b/backend/app/server.py
@@ -2,6 +2,7 @@
 import json
 from pathlib import Path
 from typing import Annotated, AsyncIterator, Optional, Sequence
+from uuid import uuid4
 from fastapi.exceptions import RequestValidationError
 
 import orjson
@@ -10,7 +11,8 @@
 from gizmo_agent import agent, ingest_runnable
 from langserve.callbacks import AsyncEventAggregatorCallback
 from langchain.pydantic_v1 import ValidationError
-from langchain.schema.messages import AnyMessage
+from langchain.schema.messages import AnyMessage, FunctionMessage
+from langchain.schema.output import ChatGeneration
 from langchain.schema.runnable import RunnableConfig
 from langserve import add_routes
 from langserve.server import _get_base_run_id_as_str, _unpack_input
@@ -116,6 +118,11 @@ async def consume_astream() -> None:
             try:
                 async for chunk in agent.astream(input_, config):
                     await streamer.send_stream.send(chunk)
+                    # hack: function messages aren't generated by chat model
+                    # so the callback handler doesn't know about them
+                    message = chunk["messages"][-1]
+                    if isinstance(message, FunctionMessage):
+                        streamer.output[uuid4()] = ChatGeneration(message=message)
             except Exception as e:
                 await streamer.send_stream.send(e)
             finally:
diff --git a/backend/app/stream.py b/backend/app/stream.py
index 0cda436d..96af0af6 100644
--- a/backend/app/stream.py
+++ b/backend/app/stream.py
@@ -69,6 +69,8 @@ def on_llm_new_token(
 
 
 def map_chunk_to_msg(chunk: BaseMessageChunk) -> BaseMessage:
+    if not isinstance(chunk, BaseMessageChunk):
+        return chunk
     args = {k: v for k, v in chunk.__dict__.items() if k != "type"}
     if isinstance(chunk, HumanMessageChunk):
         return HumanMessage(**args)