OpenTelemetry - Fastmcp

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:

bash

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

Then run your server with tracing enabled:

bash

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

Or configure via environment variables:

bash

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.

<Card title="OpenTelemetry Python Documentation" icon="book" href="https://opentelemetry.io/docs/languages/python/"> Learn more about the OpenTelemetry Python SDK, auto-instrumentation, and available exporters. </Card>

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:

Span Name	Description
`tools/call {name}`	Tool execution (e.g., `tools/call get_weather`)
`resources/read`	Resource read (URI in `mcp.resource.uri` attribute, not span name)
`prompts/get {name}`	Prompt render (e.g., `prompts/get greeting`)

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, 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:

python

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. </Tip>

Local Development

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

bash

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

# Or download from GitHub releases

Run it alongside your server:

bash

# 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:

bash

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:

python

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

Where custom spans help most

Custom spans are most useful around work that is expensive or hard to debug:

External calls such as databases, vector stores, HTTP APIs, or queue operations
Multi-step tool logic where one stage dominates latency
Prompt or resource generation that fans out to other systems
Sampling calls made from inside a tool via ctx.sample(...)

Avoid wrapping every small helper function or simple in-memory transformation. That usually adds noise without making traces easier to interpret.

Recommended naming and attributes

Use {tool_name}.{operation} or {resource_name}.{operation} for child spans such as search.fetch, search.rank, or docs.render
Add attributes that explain workload shape, such as counts, sizes, cache hits, or IDs
Do not record secrets, prompts with sensitive user data, or raw tokens as span attributes
Let exceptions propagate unless you have a specific recovery path; FastMCP's server spans already mark failures and record exceptions

Instrumenting tools, prompts, and resources

python

from fastmcp import FastMCP
from fastmcp.telemetry import get_tracer

mcp = FastMCP("my-server")

@mcp.tool
async def search(query: str) -> str:
    tracer = get_tracer()

    with tracer.start_as_current_span("search.fetch") as span:
        span.set_attribute("search.query_length", len(query))
        results = await fetch_results(query)
        span.set_attribute("search.result_count", len(results))

    with tracer.start_as_current_span("search.rank"):
        ranked = rank_results(results)

    return format_results(ranked)

@mcp.prompt
async def summarize_prompt(topic: str) -> str:
    tracer = get_tracer()
    with tracer.start_as_current_span("summarize_prompt.render") as span:
        span.set_attribute("prompt.topic_length", len(topic))
        return f"Summarize the latest updates about {topic}."

@mcp.resource("docs://{slug}")
async def docs_resource(slug: str) -> str:
    tracer = get_tracer()
    with tracer.start_as_current_span("docs_resource.load") as span:
        span.set_attribute("docs.slug", slug)
        return await load_doc(slug)

Sampling calls inside tools

If your tool uses ctx.sample(...), keep the LLM work nested under the tool span so traces show both application logic and model latency together.

For providers with their own OTEL integrations, prefer enabling that instrumentation rather than manually creating a span around every model call. For example, if you use Google GenAI, logfire.instrument_google_genai() will emit child spans with token and request metadata under the active FastMCP tool span.

Exporter choices

For local debugging, ConsoleSpanExporter or otel-desktop-viewer gives quick feedback with minimal setup
For shared environments, use OTLP exporters to backends like Logfire, Jaeger, Tempo, Datadog, or New Relic
If traces are too noisy, tune sampling in your OpenTelemetry SDK instead of removing FastMCP instrumentation

Error Handling

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

python

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

# The span will have:
# - status = ERROR with exception message as description
# - error.type = "tool_error" (or exception class name for non-tool errors)
# - exception event with stack trace

Attributes Reference

<Warning> **Migrating from v3.1 or earlier:** The `rpc.system`, `rpc.service`, and `rpc.method` span attributes were removed in favor of the [MCP semantic conventions](https://opentelemetry.io/docs/specs/semconv/gen-ai/mcp/) listed below. If you have dashboards or alerts keyed on those `rpc.*` attributes, update them to use `mcp.method.name` and the `fastmcp.*` attributes instead. </Warning>

MCP Semantic Conventions

FastMCP implements the OpenTelemetry MCP semantic conventions:

Attribute	Description
`mcp.method.name`	The MCP method being called (`tools/call`, `resources/read`, `prompts/get`)
`mcp.session.id`	Session identifier for the MCP connection
`mcp.resource.uri`	The resource URI (for resource operations)
`gen_ai.tool.name`	Tool name (on `tools/call` spans)
`gen_ai.prompt.name`	Prompt name (on `prompts/get` spans)
`error.type`	Error classification (`tool_error` for ToolError, otherwise exception class name)

Auth Attributes

Standard identity attributes:

Attribute	Description
`enduser.id`	Client ID from access token (when authenticated)
`enduser.scope`	Space-separated OAuth scopes (when authenticated)

FastMCP Custom Attributes

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

Attribute	Description
`fastmcp.server.name`	Server name
`fastmcp.component.type`	`tool`, `resource`, `prompt`, or `resource_template`
`fastmcp.component.key`	Full component identifier (e.g., `tool:greet`)
`fastmcp.provider.type`	Provider class (`LocalProvider`, `FastMCPProvider`, `ProxyProvider`)

Provider-specific attributes for delegation context:

Attribute	Description
`fastmcp.delegate.original_name`	Original tool/prompt name before namespacing
`fastmcp.delegate.original_uri`	Original resource URI before namespacing
`fastmcp.proxy.backend_name`	Remote server tool/prompt name
`fastmcp.proxy.backend_uri`	Remote server resource URI

Testing with Telemetry

For testing, use the in-memory exporter:

python

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)