FastMCP - Build MCP Servers in Python

FastMCP is a Python framework for building Model Context Protocol (MCP) servers that expose tools, resources, and prompts to Large Language Models like Claude. This skill provides production-tested patterns, error prevention, and deployment strategies for building robust MCP servers.

Quick Start

Installation

pip install fastmcp
# or
uv pip install fastmcp

Minimal Server

from fastmcp import FastMCP

# MUST be at module level for FastMCP Cloud
mcp = FastMCP("My Server")

@mcp.tool()
async def hello(name: str) -> str:
    """Say hello to someone."""
    return f"Hello, {name}!"

if __name__ == "__main__":
    mcp.run()

Run it:

# Local development
python server.py

# With FastMCP CLI
fastmcp dev server.py

# HTTP mode
python server.py --transport http --port 8000

What's New in v2.14.x (December 2025)

v2.14.2 (December 31, 2024)

• MCP SDK pinned to <2.x for compatibility
• Supabase provider gains auth_route parameter
• Bug fixes: outputSchema $ref resolution, OAuth Proxy validation, OpenAPI 3.1 support

v2.14.1: Sampling with Tools (SEP-1577)

• ctx.sample() now accepts tools for agentic workflows
• AnthropicSamplingHandler promoted from experimental
• ctx.sample_step() for single LLM call returning SampleStep
• Python 3.13 support added

v2.14.0: Background Tasks (SEP-1686)

• Protocol-native background tasks for long-running operations
• Add task=True to async decorators; progress tracking without blocking
• MCP 2025-11-25 specification support
• SEP-1699: SSE polling and event resumability
• SEP-1330: Multi-select enum elicitation schemas
• SEP-1034: Default values for elicitation schemas

⚠️ Breaking Changes (v2.14.0):

• BearerAuthProvider module removed (use JWTVerifier or OAuthProxy)
• Context.get_http_request() method removed
• fastmcp.Image top-level import removed (use from fastmcp.utilities import Image)
• enable_docket, enable_tasks settings removed (always enabled)
• run_streamable_http_async(), sse_app(), streamable_http_app(), run_sse_async() methods removed
• dependencies parameter removed from decorators
• output_schema=False support eliminated
• FASTMCP_SERVER_ environment variable prefix deprecated

Known Compatibility:

• MCP SDK pinned to <2.x (v2.14.2+)

What's New in v3.0.0 (Beta - January 2026)

⚠️ MAJOR BREAKING CHANGES - FastMCP 3.0 is a complete architectural refactor.

Provider Architecture

All components now sourced via Providers:

• FileSystemProvider - Discover decorated functions from directories with hot-reload
• SkillsProvider - Expose agent skill files as MCP resources
• OpenAPIProvider - Auto-generate from OpenAPI specs
• ProxyProvider - Proxy to remote MCP servers

from fastmcp import FastMCP
from fastmcp.providers import FileSystemProvider

mcp = FastMCP("server")
mcp.add_provider(FileSystemProvider(path="./tools", reload=True))

Transforms (Component Middleware)

Modify components without changing source code:

• Namespace, rename, filter by version
• ResourcesAsTools - Expose resources as tools
• PromptsAsTools - Expose prompts as tools

from fastmcp.transforms import Namespace, VersionFilter

mcp.add_transform(Namespace(prefix="api"))
mcp.add_transform(VersionFilter(min_version="2.0"))

Component Versioning

@mcp.tool(version="2.0")
async def fetch_data(query: str) -> dict:
    # Clients see highest version by default
    # Can request specific version
    return {"data": [...]}

Session-Scoped State

@mcp.tool()
async def set_preference(key: str, value: str, ctx: Context) -> dict:
    await ctx.set_state(key, value)  # Persists across session
    return {"saved": True}

@mcp.tool()
async def get_preference(key: str, ctx: Context) -> dict:
    value = await ctx.get_state(key, default=None)
    return {"value": value}

Other Features

• --reload flag for auto-restart during development
• Automatic threadpool dispatch for sync functions
• Tool timeouts
• OpenTelemetry tracing
• Component authorization: @tool(auth=require_scopes("admin"))

Migration Guide

Pin to v2 if not ready:

# requirements.txt
fastmcp<3

For most servers, updating the import is all you need:

# v2.x and v3.0 compatible
from fastmcp import FastMCP

mcp = FastMCP("server")
# ... rest of code works the same

See: Official Migration Guide

---

Core Concepts

Tools

Functions LLMs can call. Best practices: Clear names, comprehensive docstrings (LLMs read these!), strong type hints (Pydantic validates), structured returns, error handling.

@mcp.tool()
async def async_tool(url: str) -> dict:  # Use async for I/O
    async with httpx.AsyncClient() as client:
        return (await client.get(url)).json()

Resources

Expose data to LLMs. URI schemes: data://, file://, resource://, info://, api://, or custom.

@mcp.resource("user://{user_id}/profile")  # Template with parameters
async def get_user(user_id: str) -> dict:  # CRITICAL: param names must match
    return await fetch_user_from_db(user_id)

Prompts

Pre-configured prompts with parameters.

@mcp.prompt("analyze")
def analyze_prompt(topic: str) -> str:
    return f"Analyze {topic} considering: state, challenges, opportunities, recommendations."

Context Features

Inject Context parameter (with type hint!) for advanced features:

Elicitation (User Input):

from fastmcp import Context

@mcp.tool()
async def confirm_action(action: str, context: Context) -> dict:
    confirmed = await context.request_elicitation(prompt=f"Confirm {action}?", response_type=str)
    return {"status": "completed" if confirmed.lower() == "yes" else "cancelled"}

Progress Tracking:

@mcp.tool()
async def batch_import(file_path: str, context: Context) -> dict:
    data = await read_file(file_path)
    for i, item in enumerate(data):
        await context.report_progress(i + 1, len(data), f"Importing {i + 1}/{len(data)}")
        await import_item(item)
    return {"imported": len(data)}

Sampling (LLM calls from tools):

@mcp.tool()
async def enhance_text(text: str, context: Context) -> str:
    response = await context.request_sampling(
        messages=[{"role": "user", "content": f"Enhance: {text}"}],
        temperature=0.7
    )
    return response["content"]

Background Tasks (v2.14.0+)

Long-running operations that report progress without blocking clients. Uses Docket task scheduler (always enabled in v2.14.0+).

Basic Usage:

@mcp.tool(task=True)  # Enable background task mode
async def analyze_large_dataset(dataset_id: str, context: Context) -> dict:
    """Analyze large dataset with progress tracking."""
    data = await fetch_dataset(dataset_id)

    for i, chunk in enumerate(data.chunks):
        # Report progress to client
        await context.report_progress(
            current=i + 1,
            total=len(data.chunks),
            message=f"Processing chunk {i + 1}/{len(data.chunks)}"
        )
        await process_chunk(chunk)

    return {"status": "complete", "records_processed": len(data)}

Task States: pending → running → completed / failed / cancelled

When to Use:

• Operations taking >30 seconds (LLM timeout risk)
• Batch processing with per-item status updates
• Operations that may need user input mid-execution
• Long-running API calls or data processing

Known Limitation (v2.14.x):

• statusMessage from ctx.report_progress() is not forwarded to clients during background task polling (GitHub Issue #2904)
• Progress messages appear in server logs but not in client UI
• Workaround: Use official MCP SDK (mcp>=1.10.0) instead of FastMCP for now
• Status: Fix pending in PR #2906

Important: Tasks execute through Docket scheduler. Cannot execute tasks through proxies (will raise error).

Sampling with Tools (v2.14.1+)

Servers can pass tools to ctx.sample() for agentic workflows where the LLM can call tools during sampling.

Agentic Sampling:

from fastmcp import Context
from fastmcp.sampling import AnthropicSamplingHandler

# Configure sampling handler
mcp = FastMCP("Agent Server")
mcp.add_sampling_handler(AnthropicSamplingHandler(api_key=os.getenv("ANTHROPIC_API_KEY")))

@mcp.tool()
async def research_topic(topic: str, context: Context) -> dict:
    """Research a topic using agentic sampling with tools."""

    # Define tools available during sampling
    research_tools = [
        {
            "name": "search_web",
            "description": "Search the web for information",
            "inputSchema": {"type": "object", "properties": {"query": {"type": "string"}}}
        },
        {
            "name": "fetch_url",
            "description": "Fetch content from a URL",
            "inputSchema": {"type": "object", "properties": {"url": {"type": "string"}}}
        }
    ]

    # Sample with tools - LLM can call these tools during reasoning
    result = await context.sample(
        messages=[{"role": "user", "content": f"Research: {topic}"}],
        tools=research_tools,
        max_tokens=4096
    )

    return {"research": result.content, "tools_used": result.tool_calls}

Single-Step Sampling:

@mcp.tool()
async def get_single_response(prompt: str, context: Context) -> dict:
    """Get a single LLM response without tool loop."""

    # sample_step() returns SampleStep for inspection
    step = await context.sample_step(
        messages=[{"role": "user", "content": prompt}],
        temperature=0.7
    )

    return {
        "content": step.content,
        "model": step.model,
        "stop_reason": step.stop_reason
    }

Sampling Handlers:

• AnthropicSamplingHandler - For Claude models (v2.14.1+)
• OpenAISamplingHandler - For GPT models

Known Limitation: ctx.sample() works when client connects to a single server but fails with "Sampling not supported" error when multiple servers are configured in client. Tools without sampling work fine. (Community-sourced finding)

Storage Backends

Built on py-key-value-aio for OAuth tokens, response caching, persistent state.

Available Backends:

• Memory (default): Ephemeral, fast, dev-only
• Disk: Persistent, encrypted with FernetEncryptionWrapper, platform-aware (Mac/Windows default)
• Redis: Distributed, production, multi-instance
• Others: DynamoDB, MongoDB, Elasticsearch, Memcached, RocksDB, Valkey

Basic Usage:

from key_value.stores import DiskStore, RedisStore
from key_value.encryption import FernetEncryptionWrapper
from cryptography.fernet import Fernet

# Disk (persistent, single instance)
mcp = FastMCP("Server", storage=DiskStore(path="/app/data/storage"))

# Redis (distributed, production)
mcp = FastMCP("Server", storage=RedisStore(
    host=os.getenv("REDIS_HOST"), password=os.getenv("REDIS_PASSWORD")
))

# Encrypted storage (recommended)
mcp = FastMCP("Server", storage=FernetEncryptionWrapper(
    key_value=DiskStore(path="/app/data"),
    fernet=Fernet(os.getenv("STORAGE_ENCRYPTION_KEY"))
))

Platform Defaults: Mac/Windows use Disk, Linux uses Memory. Override with storage parameter.

Server Lifespans

⚠️ Breaking Change in v2.13.0: Lifespan behavior changed from per-session to per-server-instance.

Initialize/cleanup resources once per server (NOT per session) - critical for DB connections, API clients.

from contextlib import asynccontextmanager
from dataclasses import dataclass

@dataclass
class AppContext:
    db: Database
    api_client: httpx.AsyncClient

@asynccontextmanager
async def app_lifespan(server: FastMCP):
    """Runs ONCE per server instance."""
    db = await Database.connect(os.getenv("DATABASE_URL"))
    api_client = httpx.AsyncClient(base_url=os.getenv("API_BASE_URL"), timeout=30.0)

    try:
        yield AppContext(db=db, api_client=api_client)
    finally:
        await db.disconnect()
        await api_client.aclose()

mcp = FastMCP("Server", lifespan=app_lifespan)

# Access in tools
@mcp.tool()
async def query_db(sql: str, context: Context) -> list:
    app_ctx = context.fastmcp_context.lifespan_context
    return await app_ctx.db.query(sql)

ASGI Integration (FastAPI/Starlette):

mcp = FastMCP("Server", lifespan=mcp_lifespan)
app = FastAPI(lifespan=mcp.lifespan)  # ✅ MUST pass lifespan!

State Management:

context.fastmcp_context.set_state(key, value)  # Store
context.fastmcp_context.get_state(key, default=None)  # Retrieve

Middleware System

8 Built-in Types: TimingMiddleware, ResponseCachingMiddleware, LoggingMiddleware, RateLimitingMiddleware, ErrorHandlingMiddleware, ToolInjectionMiddleware, PromptToolMiddleware, ResourceToolMiddleware

Execution Order (order matters!):

Request Flow:
  → ErrorHandlingMiddleware (catches errors)
    → TimingMiddleware (starts timer)
      → LoggingMiddleware (logs request)
        → RateLimitingMiddleware (checks rate limit)
          → ResponseCachingMiddleware (checks cache)
            → Tool/Resource Handler

Basic Usage:

from fastmcp.middleware import ErrorHandlingMiddleware, TimingMiddleware, LoggingMiddleware

mcp.add_middleware(ErrorHandlingMiddleware())  # First: catch errors
mcp.add_middleware(TimingMiddleware())         # Second: time requests
mcp.add_middleware(LoggingMiddleware(level="INFO"))
mcp.add_middleware(RateLimitingMiddleware(max_requests=100, window_seconds=60))
mcp.add_middleware(ResponseCachingMiddleware(ttl_seconds=300, storage=RedisStore()))

Custom Middleware:

from fastmcp.middleware import BaseMiddleware

class AccessControlMiddleware(BaseMiddleware):
    async def on_call_tool(self, tool_name, arguments, context):
        user = context.fastmcp_context.get_state("user_id")
        if user not in self.allowed_users:
            raise PermissionError(f"User not authorized")
        return await self.next(tool_name, arguments, context)

Hook Hierarchy: on_message (all) → on_request/on_notification → on_call_tool/on_read_resource/on_get_prompt → on_list_* (list operations)

Server Composition

Two Strategies:

1. import_server() - Static snapshot: One-time copy at import, changes don't propagate, fast (no runtime delegation). Use for: Finalized component bundles.

2. mount() - Dynamic link: Live runtime link, changes immediately visible, runtime delegation (slower). Use for: Modular runtime composition.

Basic Usage:

# Import (static)
main_server.import_server(api_server)  # One-time copy

# Mount (dynamic)
main_server.mount(api_server, prefix="api")  # Tools: api.fetch_data
main_server.mount(db_server, prefix="db")    # Resources: resource://db/path

Tag Filtering:

@api_server.tool(tags=["public"])
def public_api(): pass

main_server.import_server(api_server, include_tags=["public"])  # Only public
main_server.mount(api_server, prefix="api", exclude_tags=["admin"])  # No admin

Resource Prefix Formats:

• Path (default since v2.4.0): resource://prefix/path
• Protocol (legacy): prefix+resource://path

main_server.mount(subserver, prefix="api", resource_prefix_format="path")

OAuth & Authentication

4 Authentication Patterns:

1. Token Validation (JWTVerifier): Validate external tokens
2. External Identity Providers (RemoteAuthProvider): OAuth 2.0/OIDC with DCR
3. OAuth Proxy (OAuthProxy): Bridge to providers without DCR (GitHub, Google, Azure, AWS, Discord, Facebook)
4. Full OAuth (OAuthProvider): Complete authorization server

Pattern 1: Token Validation

from fastmcp.auth import JWTVerifier

auth = JWTVerifier(issuer="https://auth.example.com", audience="my-server",
                   public_key=os.getenv("JWT_PUBLIC_KEY"))
mcp = FastMCP("Server", auth=auth)

Pattern 3: OAuth Proxy (Production)

from fastmcp.auth import OAuthProxy
from key_value.stores import RedisStore
from key_value.encryption import FernetEncryptionWrapper
from cryptography.fernet import Fernet

auth = OAuthProxy(
    jwt_signing_key=os.environ["JWT_SIGNING_KEY"],
    client_storage=FernetEncryptionWrapper(
        key_value=RedisStore(host=os.getenv("REDIS_HOST"), password=os.getenv("REDIS_PASSWORD")),
        fernet=Fernet(os.environ["STORAGE_ENCRYPTION_KEY"])
    ),
    upstream_authorization_endpoint="https://github.com/login/oauth/authorize",
    upstream_token_endpoint="https://github.com/login/oauth/access_token",
    upstream_client_id=os.getenv("GITHUB_CLIENT_ID"),
    upstream_client_secret=os.getenv("GITHUB_CLIENT_SECRET"),
    enable_consent_screen=True  # CRITICAL: Prevents confused deputy attacks
)
mcp = FastMCP("GitHub Auth", auth=auth)

OAuth Proxy Features: Token factory pattern (issues own JWTs), consent screens (prevents bypass), PKCE support, RFC 7662 token introspection

Supported Providers: GitHub, Google, Azure, AWS Cognito, Discord, Facebook, WorkOS, AuthKit, Descope, Scalekit, OCI (v2.13.1)

Supabase Provider (v2.14.2+):

from fastmcp.auth import SupabaseProvider

auth = SupabaseProvider(
    auth_route="/custom-auth",  # Custom auth route (new in v2.14.2)
    # ... other config
)

Icons, API Integration, Cloud Deployment

Icons: Add to servers, tools, resources, prompts. Use Icon(url, size), data URIs via Icon.from_file() or Image.to_data_uri() (v2.13.1).

API Integration (3 Patterns):

1. Manual: httpx.AsyncClient with base_url/headers/timeout
2. OpenAPI Auto-Gen: FastMCP.from_openapi(spec, client, route_maps) - GET→Resources/Templates, POST/PUT/DELETE→Tools
3. FastAPI Conversion: FastMCP.from_fastapi(app, httpx_client_kwargs)

Cloud Deployment Critical Requirements:

1. ❗ Module-level server named mcp, server, or app
2. PyPI dependencies only in requirements.txt
3. Public GitHub repo (or accessible)
4. Environment variables for config

# ✅ CORRECT: Module-level export
mcp = FastMCP("server")  # At module level!

# ❌ WRONG: Function-wrapped
def create_server():
    return FastMCP("server")  # Too late for cloud!

Deployment: https://fastmcp.cloud → Sign in → Create Project → Select repo → Deploy

Client Config (Claude Desktop):

{"mcpServers": {"my-server": {"url": "https://project.fastmcp.app/mcp", "transport": "http"}}}

30 Common Errors (With Solutions)

Error 1: Missing Server Object

Error: RuntimeError: No server object found at module level Cause: Server not exported at module level (FastMCP Cloud requirement) Solution: mcp = FastMCP("server") at module level, not inside functions

Error 2: Async/Await Confusion

Error: RuntimeError: no running event loop, TypeError: object coroutine can't be used in 'await' Cause: Mixing sync/async incorrectly Solution: Use async def for tools with await, sync def for non-async code

Error 3: Context Not Injected

Error: TypeError: missing 1 required positional argument: 'context' Cause: Missing Context type annotation Solution: async def tool(context: Context) - type hint required!

Error 4: Resource URI Syntax

Error: ValueError: Invalid resource URI: missing scheme Cause: Resource URI missing scheme prefix Solution: Use @mcp.resource("data://config") not @mcp.resource("config")

Error 5: Resource Template Parameter Mismatch

Error: TypeError: get_user() missing 1 required positional argument Cause: Function parameter names don't match URI template Solution: @mcp.resource("user://{user_id}/profile") → def get_user(user_id: str) - names must match exactly

---

Error 6: Pydantic Validation Error

Error: ValidationError: value is not a valid integer Cause: Type hints don't match provided data Solution: Use Pydantic models: class Params(BaseModel): query: str = Field(min_length=1)

Error 7: Transport/Protocol Mismatch

Error: ConnectionError: Server using different transport Cause: Client and server using incompatible transports Solution: Match transports - stdio: mcp.run() + {"command": "python", "args": ["server.py"]}, HTTP: mcp.run(transport="http", port=8000) + {"url": "http://localhost:8000/mcp", "transport": "http"}

HTTP Timeout Issue (Fixed in v2.14.3):

• HTTP transport was defaulting to 5-second timeout instead of MCP's 30-second default (GitHub Issue #2845)
• Tools taking >5 seconds would fail silently in v2.14.2 and earlier
• Solution: Upgrade to fastmcp>=2.14.3 (timeout now respects MCP's 30s default)

Error 8: Import Errors (Editable Package)

Error: ModuleNotFoundError: No module named 'my_package' Cause: Package not properly installed Solution: pip install -e . or use absolute imports or export PYTHONPATH="/path/to/project"

Error 9: Deprecation Warnings

Error: DeprecationWarning: 'mcp.settings' is deprecated Cause: Using old FastMCP v1 API Solution: Use os.getenv("API_KEY") instead of mcp.settings.get("API_KEY")

Error 10: Port Already in Use

Error: OSError: [Errno 48] Address already in use Cause: Port 8000 already occupied Solution: Use different port --port 8001 or kill process lsof -ti:8000 | xargs kill -9

Error 11: Schema Generation Failures

Error: TypeError: Object of type 'ndarray' is not JSON serializable Cause: Unsupported type hints (NumPy arrays, custom classes) Solution: Return JSON-compatible types: list[float] or convert: {"values": np_array.tolist()}

Custom Classes Not Supported (Community-sourced): FastMCP supports all Pydantic-compatible types, but custom classes must be converted to dictionaries or Pydantic models for tool returns:

# ❌ NOT SUPPORTED
class MyCustomClass:
    def __init__(self, value: str):
        self.value = value

@mcp.tool()
async def get_custom() -> MyCustomClass:
    return MyCustomClass("test")  # Serialization error

# ✅ SUPPORTED - Use dict or Pydantic
@mcp.tool()
async def get_custom() -> dict[str, str]:
    obj = MyCustomClass("test")
    return {"value": obj.value}

# OR use Pydantic BaseModel
from pydantic import BaseModel
class MyModel(BaseModel):
    value: str

@mcp.tool()
async def get_model() -> MyModel:
    return MyModel(value="test")  # Works!

OutputSchema $ref Resolution (Fixed in v2.14.2):

• Root-level $ref in outputSchema wasn't being dereferenced (GitHub Issue #2720)
• Caused MCP spec non-compliance and client compatibility issues
• Solution: Upgrade to fastmcp>=2.14.2 (auto-dereferences $ref)

Error 12: JSON Serialization

Error: TypeError: Object of type 'datetime' is not JSON serializable Cause: Returning non-JSON-serializable objects Solution: Convert: datetime.now().isoformat(), bytes: .decode('utf-8')

Error 13: Circular Import Errors

Error: ImportError: cannot import name 'X' from partially initialized module Cause: Circular dependency (common in cloud deployment) Solution: Use direct imports in __init__.py: from .api_client import APIClient or lazy imports in functions

Error 14: Python Version Compatibility

Error: DeprecationWarning: datetime.utcnow() is deprecated Cause: Using deprecated Python 3.12+ methods Solution: Use datetime.now(timezone.utc) instead of datetime.utcnow()

Error 15: Import-Time Execution

Error: RuntimeError: Event loop is closed Cause: Creating async resources at module import time Solution: Use lazy initialization - create connection class with async connect() method, call when needed in tools

---

Error 16: Storage Backend Not Configured

Error: RuntimeError: OAuth tokens lost on restart, ValueError: Cache not persisting Cause: Using default memory storage in production without persistence Solution: Use encrypted DiskStore (single instance) or RedisStore (multi-instance) with FernetEncryptionWrapper

Error 17: Lifespan Not Passed to ASGI App

Error: RuntimeError: Database connection never initialized, Warning: MCP lifespan hooks not running Cause: FastMCP with FastAPI/Starlette without passing lifespan (v2.13.0 requirement) Solution: app = FastAPI(lifespan=mcp.lifespan) - MUST pass lifespan!

Error 18: Middleware Execution Order Error

Error: RuntimeError: Rate limit not checked before caching Cause: Incorrect middleware ordering (order matters!) Solution: ErrorHandling → Timing → Logging → RateLimiting → ResponseCaching (this order)

Error 19: Circular Middleware Dependencies

Error: RecursionError: maximum recursion depth exceeded Cause: Middleware not calling self.next() or calling incorrectly Solution: Always call result = await self.next(tool_name, arguments, context) in middleware hooks

Error 20: Import vs Mount Confusion

Error: RuntimeError: Subserver changes not reflected, ValueError: Unexpected tool namespacing Cause: Using import_server() when mount() was needed (or vice versa) Solution: import_server() for static bundles (one-time copy), mount() for dynamic composition (live link)

Error 21: Resource Prefix Format Mismatch

Error: ValueError: Resource not found: resource://api/users Cause: Using wrong resource prefix format Solution: Path format (default v2.4.0+): resource://prefix/path, Protocol (legacy): prefix+resource://path - set with resource_prefix_format="path"

Error 22: OAuth Proxy Without Consent Screen

Error: SecurityWarning: Authorization bypass possible Cause: OAuth Proxy without consent screen (security vulnerability) Solution: Always set enable_consent_screen=True - prevents confused deputy attacks (CRITICAL)

Error 23: Missing JWT Signing Key in Production

Error: ValueError: JWT signing key required for OAuth Proxy Cause: OAuth Proxy missing jwt_signing_key Solution: Generate: secrets.token_urlsafe(32), store in FASTMCP_JWT_SIGNING_KEY env var, pass to OAuthProxy(jwt_signing_key=...)

Error 24: Icon Data URI Format Error

Error: ValueError: Invalid data URI format Cause: Incorrectly formatted data URI for icons Solution: Use Icon.from_file("/path/icon.png", size="medium") or Image.to_data_uri() (v2.13.1) - don't manually format

Error 25: Lifespan Behavior Change (v2.13.0)

Error: Warning: Lifespan runs per-server, not per-session Cause: Expecting v2.12 behavior (per-session) in v2.13.0+ (per-server) Solution: v2.13.0+ lifespans run ONCE per server, not per session - use middleware for per-session logic

Error 26: BearerAuthProvider Removed (v2.14.0)

Error: ImportError: cannot import name 'BearerAuthProvider' from 'fastmcp.auth' Cause: BearerAuthProvider module removed in v2.14.0 Solution: Use JWTVerifier for token validation or OAuthProxy for full OAuth flows:

# Before (v2.13.x)
from fastmcp.auth import BearerAuthProvider

# After (v2.14.0+)
from fastmcp.auth import JWTVerifier
auth = JWTVerifier(issuer="...", audience="...", public_key="...")

Error 27: Context.get_http_request() Removed (v2.14.0)

Error: AttributeError: 'Context' object has no attribute 'get_http_request' Cause: Context.get_http_request() method removed in v2.14.0 Solution: Access request info through middleware or use InitializeResult exposed to middleware

Error 28: Image Import Path Changed (v2.14.0)

Error: ImportError: cannot import name 'Image' from 'fastmcp' Cause: fastmcp.Image top-level import removed in v2.14.0 Solution: Use new import path:

# Before (v2.13.x)
from fastmcp import Image

# After (v2.14.0+)
from fastmcp.utilities import Image

Error 29: FastAPI Mount Path Doubling

Error: Client can't connect to /mcp endpoint, gets 404 Source: GitHub Issue #2961 Cause: Mounting FastMCP at /mcp creates endpoint at /mcp/mcp due to path prefix duplication Solution: Mount at root / or adjust client config

# ❌ WRONG - Creates /mcp/mcp endpoint
from fastapi import FastAPI
from fastmcp import FastMCP

mcp = FastMCP("server")
app = FastAPI(lifespan=mcp.lifespan)
app.mount("/mcp", mcp)  # Endpoint becomes /mcp/mcp

# ✅ CORRECT - Mount at root
app.mount("/", mcp)  # Endpoint is /mcp

# ✅ OR adjust client config
# In claude_desktop_config.json:
{"url": "http://localhost:8000/mcp/mcp", "transport": "http"}

Critical: Must also pass lifespan=mcp.lifespan to FastAPI (see Error #17).

Error 30: Background Tasks Fail with "No Active Context" (ASGI Mount)

Error: RuntimeError: No active context found Source: GitHub Issue #2877 Cause: ContextVar propagation issue when FastMCP mounted in FastAPI/Starlette with background tasks (task=True) Solution: Upgrade to fastmcp>=2.14.3

# In v2.14.2 and earlier - FAILS
from fastapi import FastAPI
from fastmcp import FastMCP, Context

mcp = FastMCP("server")
app = FastAPI(lifespan=mcp.lifespan)

@mcp.tool(task=True)
async def sample(name: str, ctx: Context) -> dict:
    # RuntimeError: No active context found
    await ctx.report_progress(1, 1, "Processing")
    return {"status": "OK"}

app.mount("/", mcp)

# ✅ FIXED in v2.14.3
# pip install fastmcp>=2.14.3

Note: Related to Error #17 (Lifespan Not Passed to ASGI App).

---

Production Patterns, Testing, CLI

4 Production Patterns:

1. Utils Module: Single utils.py with Config class, format_success/error helpers
2. Connection Pooling: Singleton httpx.AsyncClient with get_client() class method
3. Retry with Backoff: retry_with_backoff(func, max_retries=3, initial_delay=1.0, exponential_base=2.0)
4. Time-Based Caching: TimeBasedCache(ttl=300) with .get() and .set() methods

Testing:

• Unit: pytest + create_test_client(test_server) + await client.call_tool()
• Integration: Client("server.py") + list_tools() + call_tool() + list_resources()

CLI Commands:

fastmcp dev server.py                # Run with inspector
fastmcp install server.py             # Install to Claude Desktop
FASTMCP_LOG_LEVEL=DEBUG fastmcp dev  # Debug logging

Best Practices: Factory pattern with module-level export, environment config with validation, comprehensive docstrings (LLMs read these!), health check resources

Project Structure:

• Simple: server.py, requirements.txt, .env, README.md
• Production: src/ (server.py, utils.py, tools/, resources/, prompts/), tests/, pyproject.toml

---

References & Summary

Official: https://github.com/jlowin/fastmcp, https://fastmcp.cloud, https://modelcontextprotocol.io, Context7: /jlowin/fastmcp Related Skills: openai-api, claude-api, cloudflare-worker-base, typescript-mcp Package Versions: fastmcp>=2.14.2 (PyPI), Python>=3.10 (3.13 supported in v2.14.1+), httpx, pydantic, py-key-value-aio, cryptography Last Updated: 2026-01-21

17 Key Takeaways:

1. Module-level server export (FastMCP Cloud)
2. Persistent storage (Disk/Redis) for OAuth/caching
3. Server lifespans for resource management
4. Middleware order: errors → timing → logging → rate limiting → caching
5. Composition: import_server() (static) vs mount() (dynamic)
6. OAuth security: consent screens + encrypted storage + JWT signing
7. Async/await properly (don't block event loop)
8. Structured error handling
9. Avoid circular imports
10. Test locally (fastmcp dev)
11. Environment variables (never hardcode secrets)
12. Comprehensive docstrings (LLMs read!)
13. Production patterns (utils, pooling, retry, caching)
14. OpenAPI auto-generation
15. Health checks + monitoring
16. Background tasks for long-running operations (task=True)
17. Sampling with tools for agentic workflows (ctx.sample(tools=[...]))

Production Readiness: Encrypted storage, 4 auth patterns, 8 middleware types, modular composition, OAuth security (consent screens, PKCE, RFC 7662), response caching, connection pooling, timing middleware, background tasks, agentic sampling, FastAPI/Starlette mounting, v3.0 provider architecture

Prevents 30+ errors. 90-95% token savings.