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fix: langfuse v4 SDK compatibility and pass user message as trace input

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This commit is contained in:
Roberto Musso
2026-03-23 00:23:59 +01:00
parent 0d5fa3e569
commit 0b491b3643
11 changed files with 330 additions and 190 deletions
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207
services/chat/app/tracing.py
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@@ -1,137 +1,156 @@
"""Langfuse tracing & prompt management for the Chat Service.
"""Langfuse tracing & prompt management for the Chat Service (v4 SDK).
Provides:
- ``langfuse`` — singleton Langfuse client (lazy, no-op when keys are missing)
- ``create_trace()`` — start a new trace for a chat request
- ``get_langfuse_callback()`` — LangChain callback handler for a trace/span
- ``init_langfuse()`` — initialise the singleton client at startup
- ``trace_span()`` — context manager that creates a trace + span
- ``get_langfuse_callback()`` — LangChain callback handler (auto-inherits trace)
- ``get_prompt()`` — fetch a managed prompt from Langfuse by name
- ``flush()`` — ensure all events are sent before shutdown
- ``flush()`` / ``shutdown()`` — lifecycle management
All functions gracefully degrade to no-ops when Langfuse is not configured,
so the service works identically with or without observability keys.
Requires ``langfuse >= 3.0.0`` (v4 / "Fast Preview" SDK).
"""
from __future__ import annotations
import logging
from contextlib import contextmanager
from typing import Any
from shared.config import settings
logger = logging.getLogger(__name__)
# ── Lazy singleton ───────────────────────────────────────────────────────
# ── State ────────────────────────────────────────────────────────────────
_langfuse_client: Any | None = None
_langfuse_disabled: bool = False
_initialised: bool = False
_disabled: bool = False
def _is_configured() -> bool:
return bool(settings.LANGFUSE_SECRET_KEY and settings.LANGFUSE_PUBLIC_KEY)
def _get_langfuse() -> Any | None:
"""Return the Langfuse client singleton, or None if not configured."""
global _langfuse_client, _langfuse_disabled
def init_langfuse() -> None:
"""Initialise the Langfuse singleton. Call once at startup."""
global _initialised, _disabled
if _langfuse_disabled:
return None
if _langfuse_client is not None:
return _langfuse_client
if _initialised or _disabled:
return
if not _is_configured():
_langfuse_disabled = True
_disabled = True
logger.info("tracing: Langfuse keys not set — tracing disabled")
return None
return
try:
from langfuse import Langfuse
_langfuse_client = Langfuse(
Langfuse(
secret_key=settings.LANGFUSE_SECRET_KEY,
public_key=settings.LANGFUSE_PUBLIC_KEY,
host=settings.LANGFUSE_HOST,
)
_initialised = True
logger.info("tracing: Langfuse client initialised (host=%s)", settings.LANGFUSE_HOST)
return _langfuse_client
except Exception as exc:
_langfuse_disabled = True
_disabled = True
logger.warning("tracing: failed to initialise Langfuse: %s", exc)
def _get_client() -> Any | None:
"""Return the singleton Langfuse client, or *None* if disabled."""
if _disabled:
return None
if not _initialised:
init_langfuse()
if _disabled:
return None
try:
from langfuse import get_client
return get_client()
except Exception:
return None
# ── Trace lifecycle ──────────────────────────────────────────────────────
# ── Null span (no-op when Langfuse is disabled) ─────────────────────────
def create_trace(
class _NullSpan:
"""Drop-in replacement when Langfuse is disabled."""
def update(self, **_: Any) -> None: ...
def set_trace_io(self, **_: Any) -> None: ...
def score_trace(self, **_: Any) -> None: ...
# ── Trace context manager ───────────────────────────────────────────────
@contextmanager
def trace_span(
*,
name: str,
user_id: str,
session_id: str | None = None,
trace_id: str | None = None,
input: Any = None,
metadata: dict[str, Any] | None = None,
tags: list[str] | None = None,
) -> Any | None:
"""Create a Langfuse trace. Returns the trace object, or None if disabled."""
lf = _get_langfuse()
):
"""Context manager that creates a Langfuse trace/span.
Yields the span object (or a ``_NullSpan`` if Langfuse is disabled).
A ``CallbackHandler`` created inside this block auto-inherits the trace
context, so there is no need to pass trace IDs manually.
"""
lf = _get_client()
if lf is None:
return None
yield _NullSpan()
return
try:
return lf.trace(
id=trace_id,
from langfuse import Langfuse, propagate_attributes
trace_ctx: dict[str, str] = {}
if trace_id is not None:
trace_ctx["trace_id"] = Langfuse.create_trace_id(seed=trace_id)
with lf.start_as_current_observation(
as_type="span",
name=name,
user_id=user_id,
session_id=session_id,
input=input,
metadata=metadata or {},
tags=tags or [],
)
**({"trace_context": trace_ctx} if trace_ctx else {}),
) as span:
with propagate_attributes(
user_id=user_id,
session_id=session_id,
tags=tags or [],
):
yield span
except Exception as exc:
logger.warning("tracing: create_trace failed: %s", exc)
return None
logger.warning("tracing: trace_span(%s) failed: %s", name, exc)
yield _NullSpan()
# ── LangChain callback handler ──────────────────────────────────────────
def get_langfuse_callback(
*,
trace_id: str | None = None,
trace: Any | None = None,
span_name: str | None = None,
update_parent: bool = True,
) -> Any | None:
"""Return a ``CallbackHandler`` wired to an existing trace.
def get_langfuse_callback() -> Any | None:
"""Return a LangChain ``CallbackHandler`` that auto-inherits the current trace.
This handler is passed to LangChain's ``ainvoke`` / ``astream`` as a
callback so every LLM generation and tool call is automatically
captured as a nested span inside the trace.
If both *trace* and *trace_id* are given, *trace* takes precedence.
Returns None when Langfuse is disabled.
Must be called inside a ``trace_span()`` block for proper linking.
Returns *None* when Langfuse is disabled.
"""
lf = _get_langfuse()
if lf is None:
if _disabled and not _initialised:
return None
try:
from langfuse.callback import CallbackHandler
kwargs: dict[str, Any] = {
"secret_key": settings.LANGFUSE_SECRET_KEY,
"public_key": settings.LANGFUSE_PUBLIC_KEY,
"host": settings.LANGFUSE_HOST,
"update_parent": update_parent,
}
if trace is not None:
kwargs["trace_id"] = trace.id
elif trace_id is not None:
kwargs["trace_id"] = trace_id
if span_name:
kwargs["root_span"] = span_name
return CallbackHandler(**kwargs)
from langfuse.langchain import CallbackHandler
return CallbackHandler()
except Exception as exc:
logger.warning("tracing: get_langfuse_callback failed: %s", exc)
return None
@@ -152,21 +171,8 @@ def get_prompt(
Returns the compiled prompt string, or *fallback* if the prompt is not
found or Langfuse is disabled.
Parameters
----------
name : str
Prompt name as registered in Langfuse.
version : int, optional
Pin to a specific version; omit for the latest production version.
label : str, optional
Fetch by label (e.g. ``"production"``, ``"staging"``).
fallback : str, optional
Value returned when the prompt cannot be fetched.
cache_ttl_seconds : int
How long to cache the prompt locally (default 5 min).
"""
lf = _get_langfuse()
lf = _get_client()
if lf is None:
return fallback
@@ -187,20 +193,15 @@ def get_prompt(
def link_prompt_to_trace(
trace: Any,
span: Any,
prompt_name: str,
*,
version: int | None = None,
label: str | None = None,
) -> None:
"""Attach a Langfuse prompt reference to a trace/generation.
Call this *after* creating a generation on the trace to associate the
prompt that was used. The prompt object is fetched and linked so
Langfuse can display prompt→trace associations in the dashboard.
"""
lf = _get_langfuse()
if lf is None or trace is None:
"""Attach prompt metadata to a span/trace."""
lf = _get_client()
if lf is None or isinstance(span, _NullSpan):
return
try:
@@ -210,7 +211,7 @@ def link_prompt_to_trace(
if label is not None:
kwargs["label"] = label
prompt = lf.get_prompt(**kwargs)
trace.update(metadata={"prompt": {"name": prompt_name, "version": prompt.version}})
span.update(metadata={"prompt": {"name": prompt_name, "version": prompt.version}})
except Exception as exc:
logger.warning("tracing: link_prompt_to_trace(%s) failed: %s", prompt_name, exc)
@@ -226,12 +227,12 @@ def score_trace(
comment: str | None = None,
) -> None:
"""Post a score to a trace (e.g. user feedback, latency, quality)."""
lf = _get_langfuse()
lf = _get_client()
if lf is None:
return
try:
lf.score(trace_id=trace_id, name=name, value=value, comment=comment)
lf.create_score(trace_id=trace_id, name=name, value=value, comment=comment)
except Exception as exc:
logger.warning("tracing: score_trace failed: %s", exc)
@@ -240,22 +241,24 @@ def score_trace(
def flush() -> None:
"""Flush pending Langfuse events. Call this on service shutdown."""
if _langfuse_client is not None:
"""Flush pending Langfuse events."""
lf = _get_client()
if lf is not None:
try:
_langfuse_client.flush()
lf.flush()
except Exception as exc:
logger.warning("tracing: flush failed: %s", exc)
def shutdown() -> None:
"""Flush and close the Langfuse client."""
global _langfuse_client, _langfuse_disabled
if _langfuse_client is not None:
global _initialised, _disabled
lf = _get_client()
if lf is not None:
try:
_langfuse_client.flush()
_langfuse_client.shutdown()
lf.flush()
lf.shutdown()
except Exception as exc:
logger.warning("tracing: shutdown failed: %s", exc)
_langfuse_client = None
_langfuse_disabled = False
_initialised = False
_disabled = False