---
title: OpenTelemetry
sidebarTitle: Telemetry
description: Native OpenTelemetry instrumentation for distributed tracing.
icon: chart-line
---

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](https://opentelemetry.io/docs/specs/semconv/gen-ai/mcp/):

| Span Name | Description |
|-----------|-------------|
| `tools/call {name}` | Tool execution (e.g., `tools/call get_weather`) |
| `resources/read {uri}` | Resource read (e.g., `resources/read config://database`) |
| `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 {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:

```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](https://github.com/CtrlSpice/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](https://www.jaegertracing.io/):

```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
```

## 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
# - exception event with stack trace
```

## Attributes Reference

### RPC Semantic Conventions

Standard [RPC semantic conventions](https://opentelemetry.io/docs/specs/semconv/rpc/rpc-spans/):

| Attribute | Value |
|-----------|-------|
| `rpc.system` | `"mcp"` |
| `rpc.service` | Server name |
| `rpc.method` | MCP protocol method |

### MCP Semantic Conventions

FastMCP implements the [OpenTelemetry MCP semantic conventions](https://opentelemetry.io/docs/specs/semconv/gen-ai/mcp/):

| 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) |

### Auth Attributes

Standard [identity attributes](https://opentelemetry.io/docs/specs/semconv/attributes-registry/enduser/):

| 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)
```
