First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

import os

from fastapi import FastAPI
from fastapi.testclient import TestClient
from opentelemetry import metrics, trace
from opentelemetry.sdk.metrics import MeterProvider
from opentelemetry.sdk.metrics.export import ConsoleMetricExporter, PeriodicExportingMetricReader
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import BatchSpanProcessor, ConsoleSpanExporter

# Another library has already configured providers with its own exporters.
devnull = open(os.devnull, "w")
tracer_provider = TracerProvider()
tracer_provider.add_span_processor(BatchSpanProcessor(ConsoleSpanExporter(out=devnull)))
trace.set_tracer_provider(tracer_provider)
meter_provider = MeterProvider(metric_readers=[PeriodicExportingMetricReader(ConsoleMetricExporter(out=devnull))])
metrics.set_meter_provider(meter_provider)

with TestClient(FastAPI()):
    pass

print(
    "span processors:", len(tracer_provider._active_span_processor._span_processors),
    "metric readers:", len(meter_provider._all_metric_readers),
)

Run with OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318, then again with OTEL_TRACES_EXPORTER=none OTEL_METRICS_EXPORTER=none OTEL_LOGS_EXPORTER=none added:

OTEL_EXPORTER_OTLP_ENDPOINT set                   -> span processors: 2 metric readers: 2
... plus OTEL_{TRACES,METRICS,LOGS}_EXPORTER=none -> span processors: 1 metric readers: 1

Description

What I'm trying to achieve. We run several FastAPI servers per job (NeMo Gym). In each process, a telemetry library (nemo-lens) creates the OpenTelemetry tracer, meter and logger providers and adds its own OTLP exporters to them. That library finds the collector through OTEL_EXPORTER_OTLP_ENDPOINT, so the variable is set in every process.

Since 0.142.0, FastAPI sees the same variable at startup and adds a second exporter to those providers. The docs say this is intended ("Existing exporters continue sending data to their destinations"). For us it means every span is exported twice, and each metric is exported by two readers with separate state. The cumulative histogram _sum series then dips slightly from time to time, Prometheus treats each dip as a counter reset, and rate() and increase() on those series give values that are far too large.

We'd like a way to say, once per process, that export is already handled and FastAPI should not add its own exporters, without touching every FastAPI() call.

Why telemetry={"auto_configure": False} doesn't fully cover it. It works, but only for apps whose constructor we control and remember to change:

• Apps are built in many places: base server classes, plugins that third parties contribute, and helper scripts. One forgotten FastAPI() silently brings the duplicates back.
• Some FastAPI apps in the same environment come from other libraries, so we can't change their constructors.

The part of the stack that knows export is already handled is the telemetry library, which owns the providers. It doesn't create the apps, though.

What already seems to work. Setting OTEL_TRACES_EXPORTER, OTEL_METRICS_EXPORTER and OTEL_LOGS_EXPORTER to none makes _export_endpoint return None, so FastAPI adds nothing (see the output above). test_no_implicit_export in tests/test_telemetry/test_runtime.py covers this. Our telemetry library builds its exporters explicitly and doesn't read these variables, so they would act as a process-wide "export is handled" signal.

Questions.

1. Is OTEL_<SIGNAL>_EXPORTER=none a supported way to turn off automatic export for a whole process? If so, could the "Configure providers and exporters" section of the OpenTelemetry docs say so next to the auto_configure example? I'm happy to open a docs PR.
2. If you'd rather not commit to that, is there another supported way for a library that owns the providers to opt every app in the process out, without access to the app constructors?

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.142.4

Pydantic Version

2.13.5

Python Version

3.13.14

Additional Context

OpenTelemetry SDK 1.45.1.