OpenTelemetry

Native OpenTelemetry instrumentation for distributed tracing.

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

How It Works

FastMCP uses the OpenTelemetry API for instrumentation. This means:

• On by default - Instrumentation is active out of the box, no opt-in required
• 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.

Because FastMCP only depends on the OpenTelemetry API, span creation is a no-op until you configure an SDK and exporter — so being on by default costs nothing until you opt into collection.

Turning Telemetry Off

To disable FastMCP’s instrumentation entirely, set FASTMCP_ENABLE_TELEMETRY=false (or fastmcp.settings.enable_telemetry = False). When disabled, FastMCP creates no spans even if an SDK is configured.

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.

OpenTelemetry Python Documentation

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:

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)
tasks/{operation}	Task management (tasks/get, tasks/update, or tasks/cancel)

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}, and tasks/{operation}).

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]

Background tasks

Background task traces have two parts:

• Task submission and management requests use normal client-to-server context propagation. tasks/get, tasks/update, and tasks/cancel server spans are descendants of the corresponding FastMCP client spans.
• Deferred execution runs in a Docket worker. Docket records its CONSUMER span as a new trace root with a span link to the submission context, rather than making it a child of the submission span. Custom spans created inside the task are children of that worker span.

Span links preserve the causal relationship without forcing worker sampling to inherit the submit trace’s sampling decision. Some tracing backends do not display links prominently, so the worker trace may look disconnected even though the link is present.

Frequent status polling can produce more detail than you need. You can drop those client and server spans with a sampler that checks the span name before delegating to ParentBased:

from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.sampling import (
    ALWAYS_ON,
    Decision,
    ParentBased,
    Sampler,
    SamplingResult,
)


class DropTaskPolls(Sampler):
    def __init__(self):
        self._delegate = ParentBased(ALWAYS_ON)

    def should_sample(self, parent_context, trace_id, name, *args, **kwargs):
        if name in {"tasks/get"}:
            return SamplingResult(Decision.DROP)
        return self._delegate.should_sample(
            parent_context,
            trace_id,
            name,
            *args,
            **kwargs,
        )

    def get_description(self):
        return "DropTaskPolls"


provider = TracerProvider(sampler=DropTaskPolls())
trace.set_tracer_provider(provider)

The name check must happen before ParentBased delegates. If the name-based sampler is nested inside ParentBased, it is not consulted for child spans whose parent was already sampled.

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}!"

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

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

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:

@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

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 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.

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, tasks/get, etc.)
mcp.protocol.version	The negotiated MCP protocol version for the request
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 key, including type and version delimiter (e.g., tool:greet@ or tool:greet@v2)
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:

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)