MCP OAUTH Fastmcp

FastMCP OAuth with Scalekit Provider

Secure your FastMCP server with OAuth 2.1 in just 5 lines of code using Scalekit's built-in provider. This approach handles token validation, scope enforcement, and authentication flows automatically.

FastMCP advantage

Standard MCP OAuth: ~30 lines of middleware code, manual token validation FastMCP with Scalekit provider: ~5 lines of configuration, automatic token handling

Setup workflow

Copy this checklist and track progress:

FastMCP OAuth Setup:
- [ ] Step 1: Register MCP server in Scalekit
- [ ] Step 2: Install FastMCP and dependencies
- [ ] Step 3: Configure Scalekit provider
- [ ] Step 4: Add scope validation to tools
- [ ] Step 5: Test with MCP Inspector

Step 1: Register MCP server

In Scalekit dashboard:

1. Navigate to Dashboard > MCP Servers > Add MCP Server
2. Enter server name (e.g., FastMCP Todo Server)
3. Set Server URL to http://localhost:3002/ (include trailing slash)
4. Define scopes for your tools (e.g., todo:read, todo:write)
5. Click Save and note the resource_id

Critical: Use base URL with trailing slash. FastMCP appends /mcp automatically.

• ✓ Correct: http://localhost:3002/
• ✗ Wrong: http://localhost:3002/mcp

Step 2: Install dependencies

Create project structure:

mkdir fastmcp-server
cd fastmcp-server
python3 -m venv venv
source venv/bin/activate

Create requirements.txt:

fastmcp>=2.13.0.2
python-dotenv>=1.0.0

Install:

pip install -r requirements.txt

Step 3: Configure Scalekit provider

Create .env file with Scalekit credentials:

PORT=3002
SCALEKIT_ENVIRONMENT_URL=https://your-env.scalekit.com
SCALEKIT_CLIENT_ID=your_client_id
SCALEKIT_RESOURCE_ID=res_your_resource_id
MCP_URL=http://localhost:3002/

Get these values from Scalekit Dashboard > Settings and your MCP server configuration.

Initialize FastMCP server with Scalekit provider (server.py):

import os
from dotenv import load_dotenv
from fastmcp import FastMCP
from fastmcp.server.auth.providers.scalekit import ScalekitProvider

load_dotenv()

# 5-line OAuth setup
mcp = FastMCP(
    "Your Server Name",
    stateless_http=True,
    auth=ScalekitProvider(
        environment_url=os.getenv("SCALEKIT_ENVIRONMENT_URL"),
        client_id=os.getenv("SCALEKIT_CLIENT_ID"),
        resource_id=os.getenv("SCALEKIT_RESOURCE_ID"),
        mcp_url=os.getenv("MCP_URL"),
    ),
)

if __name__ == "__main__":
    mcp.run(transport="http", port=int(os.getenv("PORT", "3002")))

That's it! The Scalekit provider handles:

• Token validation on every request
• OAuth flow with MCP clients
• WWW-Authenticate header responses
• Discovery endpoint (/.well-known/oauth-protected-resource)

Step 4: Add scope validation to tools

Use the built-in get_access_token() dependency to validate scopes:

from fastmcp.server.dependencies import AccessToken, get_access_token

def _require_scope(scope: str) -> str | None:
    """Validate request token has required scope."""
    token: AccessToken = get_access_token()
    if scope not in token.scopes:
        return f"Insufficient permissions: `{scope}` scope required."
    return None

@mcp.tool
def create_todo(title: str, description: str = None) -> dict:
    """Create a new todo item. Requires todo:write scope."""
    error = _require_scope("todo:write")
    if error:
        return {"error": error}

    # Your tool implementation
    todo_id = str(uuid.uuid4())
    return {"id": todo_id, "title": title, "description": description}

@mcp.tool
def list_todos() -> dict:
    """List all todos. Requires todo:read scope."""
    error = _require_scope("todo:read")
    if error:
        return {"error": error}

    # Your tool implementation
    return {"todos": [...]}

Pattern: Every tool that requires authorization should call _require_scope() first.

Step 5: Test with MCP Inspector

Run your server:

source venv/bin/activate
python server.py

Launch MCP Inspector:

npx @modelcontextprotocol/inspector@latest

In Inspector:

1. Enter URL: http://localhost:3002/mcp
2. Leave authentication fields empty (uses dynamic client registration)
3. Click Connect
4. Complete OAuth flow when prompted
5. Test tools with scoped tokens

Testing scope enforcement:

• Call create_todo with token that only has todo:read → should fail
• Call list_todos with todo:read scope → should succeed

Complete example: Todo server

import os
import uuid
from dataclasses import dataclass, asdict
from typing import Optional

from dotenv import load_dotenv
from fastmcp import FastMCP
from fastmcp.server.auth.providers.scalekit import ScalekitProvider
from fastmcp.server.dependencies import AccessToken, get_access_token

load_dotenv()

mcp = FastMCP(
    "Todo Server",
    stateless_http=True,
    auth=ScalekitProvider(
        environment_url=os.getenv("SCALEKIT_ENVIRONMENT_URL"),
        client_id=os.getenv("SCALEKIT_CLIENT_ID"),
        resource_id=os.getenv("SCALEKIT_RESOURCE_ID"),
        mcp_url=os.getenv("MCP_URL"),
    ),
)

@dataclass
class TodoItem:
    id: str
    title: str
    description: Optional[str]
    completed: bool = False

    def to_dict(self) -> dict:
        return asdict(self)

_TODO_STORE: dict[str, TodoItem] = {}

def _require_scope(scope: str) -> Optional[str]:
    token: AccessToken = get_access_token()
    if scope not in token.scopes:
        return f"Insufficient permissions: `{scope}` scope required."
    return None

@mcp.tool
def create_todo(title: str, description: Optional[str] = None) -> dict:
    error = _require_scope("todo:write")
    if error:
        return {"error": error}

    todo = TodoItem(id=str(uuid.uuid4()), title=title, description=description)
    _TODO_STORE[todo.id] = todo
    return {"todo": todo.to_dict()}

@mcp.tool
def list_todos(completed: Optional[bool] = None) -> dict:
    error = _require_scope("todo:read")
    if error:
        return {"error": error}

    todos = [
        todo.to_dict()
        for todo in _TODO_STORE.values()
        if completed is None or todo.completed == completed
    ]
    return {"todos": todos}

@mcp.tool
def get_todo(todo_id: str) -> dict:
    error = _require_scope("todo:read")
    if error:
        return {"error": error}

    todo = _TODO_STORE.get(todo_id)
    if todo is None:
        return {"error": f"Todo `{todo_id}` not found."}

    return {"todo": todo.to_dict()}

@mcp.tool
def update_todo(
    todo_id: str,
    title: Optional[str] = None,
    description: Optional[str] = None,
    completed: Optional[bool] = None,
) -> dict:
    error = _require_scope("todo:write")
    if error:
        return {"error": error}

    todo = _TODO_STORE.get(todo_id)
    if todo is None:
        return {"error": f"Todo `{todo_id}` not found."}

    if title is not None:
        todo.title = title
    if description is not None:
        todo.description = description
    if completed is not None:
        todo.completed = completed

    return {"todo": todo.to_dict()}

@mcp.tool
def delete_todo(todo_id: str) -> dict:
    error = _require_scope("todo:write")
    if error:
        return {"error": error}

    todo = _TODO_STORE.pop(todo_id, None)
    if todo is None:
        return {"error": f"Todo `{todo_id}` not found."}

    return {"deleted": todo_id}

if __name__ == "__main__":
    mcp.run(transport="http", port=int(os.getenv("PORT", "3002")))

Environment variable reference

Scope design patterns

Read-only operations: Use *:read scope

• todo:read, data:read, user:read

Write operations: Use *:write scope

• todo:write, data:write, user:write

Admin operations: Use *:admin scope

• system:admin, user:admin

Multiple scopes per tool: Return error if ANY required scope is missing

def _require_scopes(scopes: list[str]) -> str | None:
    token: AccessToken = get_access_token()
    missing = [s for s in scopes if s not in token.scopes]
    if missing:
        return f"Missing scopes: {', '.join(missing)}"
    return None

@mcp.tool
def admin_action() -> dict:
    error = _require_scopes(["todo:write", "admin:access"])
    if error:
        return {"error": error}
    # Implementation

Production deployment

Security checklist

• Store .env in secret manager (AWS Secrets Manager, Vault)
• Use HTTPS for all public endpoints
• Rotate SCALEKIT_CLIENT_SECRET regularly
• Enable rate limiting on MCP endpoints
• Log all authentication failures
• Monitor token validation errors

Environment-specific configuration

Development:

MCP_URL=http://localhost:3002/

Production:

MCP_URL=https://mcp.yourapp.com/

Update Scalekit dashboard with production URL before deploying.

Common issues

Token validation fails:

• Verify SCALEKIT_RESOURCE_ID matches dashboard
• Check MCP_URL has trailing slash
• Ensure environment variables are loaded (load_dotenv())

Discovery endpoint not found:

• Confirm server is running on correct port
• Verify MCP client uses base URL + /mcp path
• Check firewall/network allows connections

Scope errors persist:

• Verify scopes are defined in Scalekit dashboard
• Check scope strings match exactly (case-sensitive)
• Ensure token was issued with required scopes

MCP Inspector connection fails:

• Leave auth fields empty (uses DCR)
• Check browser console for OAuth errors
• Verify server logs show authentication attempt

Extending your server

Adding new tools with scopes

@mcp.tool
def new_operation(param: str) -> dict:
    """Your tool description."""
    error = _require_scope("your:scope")
    if error:
        return {"error": error}

    # Your implementation
    return {"result": "success"}

Multiple scope requirements

@mcp.tool
def sensitive_operation() -> dict:
    """Requires multiple scopes."""
    error = _require_scopes(["data:read", "data:write", "admin:access"])
    if error:
        return {"error": error}

    # Your implementation
    return {"result": "success"}

Optional scope enhancement

@mcp.tool
def flexible_operation() -> dict:
    """Returns different data based on scopes."""
    token: AccessToken = get_access_token()

    # Basic data for all authenticated users
    result = {"basic": "data"}

    # Enhanced data if user has admin scope
    if "admin:access" in token.scopes:
        result["admin"] = "enhanced_data"

    return result

Complete Working Example

The complete FastMCP todo server shown above is available in the Scalekit MCP Auth Demos repository:

GitHub Repository: scalekit-inc/mcp-auth-demos/tree/main/todo-fastmcp

This example demonstrates:

• Full CRUD operations with scope-based authorization
• In-memory todo storage for testing
• OAuth 2.1 integration via FastMCP ScalekitProvider
• Production-ready error handling and logging

Resources

• Full example: GitHub - todo-fastmcp
• Scalekit MCP Auth Demos
• FastMCP docs: fastmcp.dev
• Scalekit docs: docs.scalekit.com/authenticate/mcp/fastmcp-quickstart
• MCP Inspector: npx @modelcontextprotocol/inspector@latest