feat(langfuse): add langfuse handler to callback for langchain integration

Author: NicholasGoh

backend/api/core/agent/orchestration.py

@@ -10,6 +10,7 @@
 from langgraph.prebuilt import ToolNode, tools_condition
 
 from api.core.agent.prompts import SYSTEM_PROMPT
+from api.core.dependencies import LangfuseHandlerDep
 
 
 class State(MessagesState):
@@ -70,7 +71,8 @@ def get_graph(
     return graph_factory(worker_node, tools, checkpointer, name)
 
 
-def get_config():
+def get_config(langfuse_handler: LangfuseHandlerDep):
     return dict(
         configurable=dict(thread_id="1"),
+        callbacks=[langfuse_handler],
     )

backend/api/routers/llms.py

@@ -8,7 +8,7 @@
 from starlette.responses import Response
 
 from api.core.agent.orchestration import get_config, get_graph
-from api.core.dependencies import LLMDep, setup_graph
+from api.core.dependencies import LangfuseHandlerDep, LLMDep, setup_graph
 from api.core.logs import print, uvicorn
 
 router = APIRouter(tags=["chat"])
@@ -26,13 +26,17 @@ async def completions(query: str, llm: LLMDep) -> Response:
 
 
 @router.get("/chat/agent")
-async def agent(query: str, llm: LLMDep) -> Response:
+async def agent(
+    query: str,
+    llm: LLMDep,
+    langfuse_handler: LangfuseHandlerDep,
+) -> Response:
     """Stream LangGraph completions as Server-Sent Events (SSE).
 
     This endpoint streams LangGraph-generated events in real-time, allowing the client
     to receive responses as they are processed, useful for agent-based workflows.
     """
-    return EventSourceResponse(stream_graph(query, llm))
+    return EventSourceResponse(stream_graph(query, llm, langfuse_handler))
 
 
 async def stream_completions(
@@ -57,14 +61,15 @@ async def checkpointer_setup(pool):
 async def stream_graph(
     query: str,
     llm: LLMDep,
+    langfuse_handler: LangfuseHandlerDep,
 ) -> AsyncGenerator[dict[str, str], None]:
     async with setup_graph() as resource:
         graph = get_graph(
             llm,
             tools=resource.tools,
             checkpointer=resource.checkpointer,
         )
-        config = get_config()
+        config = get_config(langfuse_handler)
         events = dict(messages=[HumanMessage(content=query)])
 
         async for event in graph.astream_events(events, config, version="v2"):