How It Works

OpenTelemetry

Native OpenTelemetry instrumentation for distributed tracing.

FastMCP includes native OpenTelemetry instrumentation for observability. Traces are automatically generated for tool, prompt, resource, and resource template operations, providing visibility into server behavior, request handling, and provider delegation chains.

How It Works

FastMCP uses the OpenTelemetry API for instrumentation. This means:

• Zero configuration required - Instrumentation is always active
• No overhead when unused - Without an SDK, all operations are no-ops
• Bring your own SDK - You control collection, export, and sampling
• Works with any OTEL backend - Jaeger, Zipkin, Datadog, New Relic, etc.

Enabling Telemetry

The easiest way to export traces is using opentelemetry-instrument, which configures the SDK automatically:

pip install opentelemetry-distro opentelemetry-exporter-otlp
opentelemetry-bootstrap -a install

Then run your server with tracing enabled:

opentelemetry-instrument \
  --service_name my-fastmcp-server \
  --exporter_otlp_endpoint http://localhost:4317 \
  fastmcp run server.py

Or configure via environment variables:

export OTEL_SERVICE_NAME=my-fastmcp-server
export OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4317

opentelemetry-instrument fastmcp run server.py

This works with any OTLP-compatible backend (Jaeger, Zipkin, Grafana Tempo, Datadog, etc.) and requires no changes to your FastMCP code.

Learn more about the OpenTelemetry Python SDK, auto-instrumentation, and available exporters.

Tracing

FastMCP creates spans for all MCP operations, providing end-to-end visibility into request handling.

Server Spans

The server creates spans for each operation using MCP semantic conventions:

For mounted servers, an additional delegate {name} span shows the delegation to the child server.

Client Spans

The FastMCP client creates spans for outgoing requests with the same naming pattern (tools/call {name}, resources/read {uri}, prompts/get {name}).

Span Hierarchy

Spans form a hierarchy showing the request flow. For mounted servers:

tools/call weather_forecast (CLIENT)
  └── tools/call weather_forecast (SERVER, provider=FastMCPProvider)
        └── delegate get_weather (INTERNAL)
              └── tools/call get_weather (SERVER, provider=LocalProvider)

For proxy providers connecting to remote servers:

tools/call remote_search (CLIENT)
  └── tools/call remote_search (SERVER, provider=ProxyProvider)
        └── [remote server spans via trace context propagation]

Programmatic Configuration

For more control, configure the SDK in your Python code before importing FastMCP:

from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import BatchSpanProcessor
from opentelemetry.exporter.otlp.proto.grpc.trace_exporter import OTLPSpanExporter

# Configure the SDK with OTLP exporter
provider = TracerProvider()
processor = BatchSpanProcessor(OTLPSpanExporter(endpoint="http://localhost:4317"))
provider.add_span_processor(processor)
trace.set_tracer_provider(provider)

# Now import and use FastMCP - traces will be exported automatically
from fastmcp import FastMCP

mcp = FastMCP("my-server")

@mcp.tool()
def greet(name: str) -> str:
    return f"Hello, {name}!"

Tip: The SDK must be configured before importing FastMCP to ensure the tracer provider is set when FastMCP initializes.

Local Development

For quick local trace visualization, otel-desktop-viewer is a lightweight single-binary tool:

# macOS
brew install nico-barbas/brew/otel-desktop-viewer

# Or download from GitHub releases

Run it alongside your server:

# Terminal 1: Start the viewer (UI at http://localhost:8000, OTLP on :4317)
otel-desktop-viewer

# Terminal 2: Run your server with tracing
opentelemetry-instrument fastmcp run server.py

For more features, use Jaeger:

docker run -d --name jaeger \
  -p 16686:16686 \
  -p 4317:4317 \
  jaegertracing/all-in-one:latest

Then view traces at http://localhost:16686

Custom Spans

You can add your own spans using the FastMCP tracer:

from fastmcp import FastMCP
from fastmcp.telemetry import get_tracer

mcp = FastMCP("custom-spans")

@mcp.tool()
async def complex_operation(input: str) -> str:
    tracer = get_tracer()

    with tracer.start_as_current_span("parse_input") as span:
        span.set_attribute("input.length", len(input))
        parsed = parse(input)

    with tracer.start_as_current_span("process_data") as span:
        span.set_attribute("data.count", len(parsed))
        result = process(parsed)

    return result

Error Handling

When errors occur, spans are automatically marked with error status and the exception is recorded:

@mcp.tool()
def risky_operation() -> str:
    raise ValueError("Something went wrong")

# The span will have:
# - status = ERROR
# - exception event with stack trace

Attributes Reference

RPC Semantic Conventions

Standard RPC semantic conventions:

MCP Semantic Conventions

FastMCP implements the OpenTelemetry MCP semantic conventions:

Auth Attributes

Standard identity attributes:

FastMCP Custom Attributes

All custom attributes use the fastmcp. prefix for features unique to FastMCP:

Provider-specific attributes for delegation context:

Testing with Telemetry

For testing, use the in-memory exporter:

import pytest
from collections.abc import Generator
from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import SimpleSpanProcessor
from opentelemetry.sdk.trace.export.in_memory_span_exporter import InMemorySpanExporter

from fastmcp import FastMCP

@pytest.fixture
def trace_exporter() -> Generator[InMemorySpanExporter, None, None]:
    exporter = InMemorySpanExporter()
    provider = TracerProvider()
    provider.add_span_processor(SimpleSpanProcessor(exporter))
    original_provider = trace.get_tracer_provider()
    trace.set_tracer_provider(provider)
    yield exporter
    exporter.clear()
    trace.set_tracer_provider(original_provider)

async def test_tool_creates_span(trace_exporter: InMemorySpanExporter) -> None:
    mcp = FastMCP("test")

    @mcp.tool()
    def hello() -> str:
        return "world"

    await mcp.call_tool("hello", {})

    spans = trace_exporter.get_finished_spans()
    assert any(s.name == "tools/call hello" for s in spans)

Documentation Index

Fetch the complete documentation index at: https://gofastmcp.com/llms.txt Use this file to discover all available pages before exploring further.