Sentry not logging traces for FastAPI's StreamingResponse endpoint

[Macok]

Environment

SaaS (https://sentry.io/)

Steps to Reproduce

I initialised sentry_sdk with

sentry_sdk.init(
        dsn=os.environ['SENTRY_DSN'],
        environment=os.getenv("SENTRY_ENV", "production"),
        send_default_pii=True,
        traces_sample_rate=1.0,
    )

In the Trace Explorer, I can see traces from all endpoints of my app, but not below one:

@app.post("/generate_summary")
def generate_summary(params: GenerateSummaryParams,
                     user: str = Depends(get_current_user)):
    conversation = fetch_conversation(user, params.conversation_id)
    return StreamingResponse(do_generate_summary(conversation, params.doc_ids, user),
                             media_type="text/event-stream")

I tried to manually add transaction inside do_generate_summary() generator like below:

    with sentry_sdk.start_transaction(name="my_transaction"):
        with sentry_sdk.start_span(name="my_span"):
            ...

With above code, something even more strange happens. Now, I can see transactions created by sentry_sdk for my /generate_summary endpoint, but my_transaction transaction and my_span span are missing!

Expected Result

Traces for FastAPI endpoints returning a StreamingResponse should be sent to Sentry.

Actual Result

Traces for FastAPI endpoints returning a StreamingResponse are not being sent to Sentry.

Version

sentry-sdk==2.42.1
fastapi==0.119.1
Python 3.12.12

[linear]

ENG-5765

[getsantry]

Assigning to @getsentry/support for routing ⏲️

[linear]

PY-1928

[alexander-alderman-webb]

Hi @Macok,

Thank for your report. The last comment you made is really useful. Based on the remark, it sounds like you are able to get transactions for endpoints returning StreamingResponse, but not together with your do_generate_summary() function.

With above code, something even more strange happens. Now, I can see transactions created by sentry_sdk for my /generate_summary endpoint, but my_transaction transaction and my_span span are missing!

You could be reaching size limits on the ingestion. To confirm can you add debug=True when initializing Sentry, like so

sentry_sdk.init(
      dsn=os.environ['SENTRY_DSN'],
      environment=os.getenv("SENTRY_ENV", "production"),
      send_default_pii=True,
      traces_sample_rate=1.0,
+     debug=True,
  )

Then you should see a message like the following if the serialized transaction is too large:

ERROR: Unexpected status code: 400 (body: b'{"detail":"envelope exceeded size limits for type \'event\' (https://develop.sentry.dev/sdk/envelopes/#size-limits)"}')

[Macok]

That was it! Thanks for quick help!

Inside do_generate_summary, I'm streaming response from OpenAI chunk by chunk, and streaming the chunks to my frontend:

    response = openai.chat.completions.create(
        model=model,
        messages=messages,
        stream=True
    )
    for chunk in response:
        ...
        yield f'event: CHUNK\ndata: {chunk.choices[0].delta.content}'

Apparently, this creates hundreds of spans within Sentry transaction, leading to envelope exceeded size limits error.

Can you suggest some solution?
Maybe there's a way to have my chunk processing logic grouped as a single span which doesn't create any child spans?
Something like:

with sentry_sdk.create_span(name='chunk_processing', allow_child_spans=False):
    # chunk processing logic showed above goes here