Responses.retrieve is not instrumented, so background responses lose output, usage and cost

Summary

Responses.create and Responses.parse are instrumented, but Responses.retrieve is not. For background responses that is the call that carries the actual result, so the generation in the trace never receives its output, token counts or cost.

Why this path matters

With background=True the API returns immediately with status="queued" and no usage. The result is collected later, either with client.responses.retrieve(response_id) or by resuming the stream with client.responses.stream(response_id=...).

The second one is easy to miss: stream() has two branches. Creating a new response goes through partial(self.create, ..., stream=True) and is traced correctly. Resuming an existing one goes through self.retrieve(response_id=..., stream=True) and is not.

Reproduction

No API key needed — this shows which method each path resolves to:

import openai, langfuse.openai
from openai.resources.responses import Responses

calls = []
Responses.create = lambda self, **kw: calls.append(("create", kw.get("stream"))) or iter(())
Responses.retrieve = lambda self, **kw: calls.append(("retrieve", kw.get("stream"))) or iter(())

c = openai.OpenAI(api_key="sk-test", base_url="http://localhost:9")

with c.responses.stream(model="gpt-5.6", input="hi"):   # new response
    pass
with c.responses.stream(response_id="resp_123"):        # resumed response
    pass

print(calls)  # [('create', True), ('retrieve', True)]

And which of those the SDK patches:

import openai
from openai.resources.responses import Responses
before = {m: getattr(Responses, m) for m in ("create", "parse", "retrieve")}
import langfuse.openai
for m in before:
    print(m, before[m] is not getattr(Responses, m))

# create   True
# parse    True
# retrieve False

grep -c background langfuse/openai.py returns 0, and tests/unit/test_openai.py covers neither retrieve nor .stream(.

Impact

A background response produces a generation with an input and a model, and then nothing else: no output, no usage_details, no cost. On a dashboard that reads as a call that was free, which is worse than a call that is missing — the trace looks complete.

Proposed fix

Add Responses.retrieve / AsyncResponses.retrieve to OPENAI_METHODS_V1. retrieve() carries no prompt of its own, so the response id can serve as the generation input; model, output and usage are already extracted from the response object, and the existing streaming extraction handles stream=True.

One open question for maintainers: retrieving a response that was already traced at creation time (non-background usage) would then record its usage a second time. Options are to instrument every retrieve, or only the stream=True path, or to skip usage when the response was not queued. Happy to follow whichever you prefer — PR attached implements the straightforward version.

Version info

• langfuse: main (4.x)
• openai: 2.29.0
• Python: 3.14