Web frameworks

LLM observability for FastAPI and Starlette

One middleware line: each request is a root trace, and the authenticated user in scope["user"] becomes its user id.

Set up FastAPI and Starlette

The snippet is copied from the SDK README, not paraphrased. Keys shown as tk_live_... are the environment key from Settings › Environments.

Python

# FastAPI / Starlette
from evalkit import EvalKitMiddleware
app.add_middleware(EvalKitMiddleware)

Copied from sdk-py/README.md § Web frameworks

Install → first trace

01
Install the SDK
pip install syntropylabs-evalkit or npm install syntropylabs-evalkit.
02
Call init() once
At the top of your entrypoint, with your environment key and a service name.
03
Add EvalKitMiddleware
Each incoming request becomes a root trace with downstream model, database and HTTP calls nested underneath.
04
Send one request and open its trace
The request appears in Traces as a root span; Services shows the endpoint.

Sample data in the product’s own table. Hover, focus or tap a claim to see the columns it points at.

Sample run

Operation	Service	Status	Model	Latency	Tokens	≈Cost	Score
support_agent.turn8f3a1c0d94e2session sess_4b1e	support-api	ok	gpt-4o-mini	1.84 s	3,412	$0.0006	92%auto
refund_agent.turnc21d7e5a30b8session sess_9a02tool loop	refund-worker	ok	claude-sonnet-4	4.31 s	7,905	$0.0389	68%auto
claude-code.turn5be04f7d1a96session sess_f77c	Claude Code	unset	claude-sonnet-4	48.20 s	61,208	$0.19	—
rag.answere9a2b6c4d015error	docs-bot	error	gpt-4o	6.02 s	9,880	$0.0312	41%auto
voice.turn17c8d3f2a4e0session sess_20d1	ivr-agent	ok	gpt-4o-realtime	0.92 s	1,104	$0.0071	—

fig. 1 · the traces table this integration fills · sample data

Taken from the SDK READMEs’ coverage tables. with flag means the signal leaves your machine only when you opt in.

What Syntropylabs shows for FastAPI and Starlette
Signal	Captured	Detail
Each incoming request becomes a root trace	yes	—
Model, database and HTTP calls nested under the request	yes	—
End-user identity from the request	yes	user_id from scope["user"] when the user is authenticated
Request and response headers	yes	credential header values recorded as ***
Your own functions	yes	every function in your app’s source tree, on by default

init() has to run before the middleware is added, as early as possible in the process.
user_id is filled only when the request’s user is authenticated; anything you set yourself wins.

Trace FastAPI and Starlette today

Tracing is free for one project on every plan, coding-agent traces included. Evaluations, datasets and simulation are Pro and up.