Built-in Middleware

Middleware

Add cross-cutting functionality to your MCP server with middleware that intercepts and modifies requests and responses.

Middleware adds behavior that applies across multiple operations—authentication, logging, rate limiting, or request transformation—without modifying individual tools or resources.

Tip: MCP middleware is a FastMCP-specific concept and is not part of the official MCP protocol specification.

Overview

MCP middleware forms a pipeline around your server's operations. When a request arrives, it flows through each middleware in order—each can inspect, modify, or reject the request before passing it along. After the operation completes, the response flows back through the same middleware in reverse order.

Request → Middleware A → Middleware B → Handler → Middleware B → Middleware A → Response

This bidirectional flow means middleware can:

• Pre-process: Validate authentication, log incoming requests, check rate limits
• Post-process: Transform responses, record timing metrics, handle errors consistently

The key decision point is call_next(context). Calling it continues the chain; not calling it stops processing entirely.

from fastmcp import FastMCP
from fastmcp.server.middleware import Middleware, MiddlewareContext

class LoggingMiddleware(Middleware):
    async def on_message(self, context: MiddlewareContext, call_next):
        print(f"→ {context.method}")
        result = await call_next(context)
        print(f"← {context.method}")
        return result

mcp = FastMCP("MyServer")
mcp.add_middleware(LoggingMiddleware())

Execution Order

Middleware executes in the order added to the server. The first middleware runs first on the way in and last on the way out:

from fastmcp import FastMCP
from fastmcp.server.middleware.error_handling import ErrorHandlingMiddleware
from fastmcp.server.middleware.rate_limiting import RateLimitingMiddleware
from fastmcp.server.middleware.logging import LoggingMiddleware

mcp = FastMCP("MyServer")
mcp.add_middleware(ErrorHandlingMiddleware())   # 1st in, last out
mcp.add_middleware(RateLimitingMiddleware())    # 2nd in, 2nd out
mcp.add_middleware(LoggingMiddleware())         # 3rd in, first out

This ordering matters. Place error handling early so it catches exceptions from all subsequent middleware. Place logging late so it records the actual execution after other middleware has processed the request.

Server Composition

When using mounted servers, middleware behavior follows a clear hierarchy:

• Parent middleware runs for all requests, including those routed to mounted servers
• Mounted server middleware only runs for requests handled by that specific server

from fastmcp import FastMCP
from fastmcp.server.middleware.logging import LoggingMiddleware

parent = FastMCP("Parent")
parent.add_middleware(AuthMiddleware())  # Runs for ALL requests

child = FastMCP("Child")
child.add_middleware(LoggingMiddleware())  # Only runs for child's tools

parent.mount(child, namespace="child")

Requests to child_tool flow through the parent's AuthMiddleware first, then through the child's LoggingMiddleware.

Hooks

Rather than processing every message identically, FastMCP provides specialized hooks at different levels of specificity. Multiple hooks fire for a single request, going from general to specific:

When a client calls a tool, the middleware chain processes on_message first, then on_request, then on_call_tool. This hierarchy lets you target exactly the right scope—use on_message for logging everything, on_request for authentication, and on_call_tool for tool-specific behavior.

Hook Signature

Every hook follows the same pattern:

async def hook_name(self, context: MiddlewareContext, call_next) -> result_type:
    # Pre-processing
    result = await call_next(context)
    # Post-processing
    return result

Parameters:

• context — MiddlewareContext containing request information
• call_next — Async function to continue the middleware chain

Returns: The appropriate result type for the hook (varies by operation).

MiddlewareContext

The context parameter provides access to request details:

Message Hooks

on_message

Called for every MCP message—both requests and notifications.

async def on_message(self, context: MiddlewareContext, call_next):
    result = await call_next(context)
    return result

Use for: Logging, metrics, or any cross-cutting concern that applies to all traffic.

on_request

Called for MCP requests that expect a response.

async def on_request(self, context: MiddlewareContext, call_next):
    result = await call_next(context)
    return result

Use for: Authentication, authorization, request validation.

on_notification

Called for fire-and-forget MCP notifications.

async def on_notification(self, context: MiddlewareContext, call_next):
    await call_next(context)
    # Notifications don't return values

Use for: Event logging, async side effects.

Operation Hooks

on_call_tool

Called when a tool is executed. The context.message contains name (tool name) and arguments (dict).

async def on_call_tool(self, context: MiddlewareContext, call_next):
    tool_name = context.message.name
    args = context.message.arguments
    result = await call_next(context)
    return result

Returns: Tool execution result or raises ToolError.

on_read_resource

Called when a resource is read. The context.message contains uri (resource URI).

async def on_read_resource(self, context: MiddlewareContext, call_next):
    uri = context.message.uri
    result = await call_next(context)
    return result

Returns: Resource content.

on_get_prompt

Called when a prompt is retrieved. The context.message contains name (prompt name) and arguments (dict).

async def on_get_prompt(self, context: MiddlewareContext, call_next):
    prompt_name = context.message.name
    result = await call_next(context)
    return result

Returns: Prompt messages.

on_list_tools

Called when listing available tools. Returns a list of FastMCP Tool objects before MCP conversion.

async def on_list_tools(self, context: MiddlewareContext, call_next):
    tools = await call_next(context)
    # Filter or modify the tool list
    return tools

Returns: list[Tool] — Can be filtered before returning to client.

on_list_resources

Called when listing available resources. Returns FastMCP Resource objects.

async def on_list_resources(self, context: MiddlewareContext, call_next):
    resources = await call_next(context)
    return resources

Returns: list[Resource]

on_list_resource_templates

Called when listing resource templates.

async def on_list_resource_templates(self, context: MiddlewareContext, call_next):
    templates = await call_next(context)
    return templates

Returns: list[ResourceTemplate]

on_list_prompts

Called when listing available prompts.

async def on_list_prompts(self, context: MiddlewareContext, call_next):
    prompts = await call_next(context)
    return prompts

Returns: list[Prompt]

on_initialize

Called when a client connects and initializes the session. This hook cannot modify the initialization response.

from mcp import McpError
from mcp.types import ErrorData

async def on_initialize(self, context: MiddlewareContext, call_next):
    client_info = context.message.params.get("clientInfo", {})
    client_name = client_info.get("name", "unknown")

    # Reject before call_next to send error to client
    if client_name == "blocked-client":
        raise McpError(ErrorData(code=-32000, message="Client not supported"))

    await call_next(context)
    print(f"Client {client_name} initialized")

Returns: None — The initialization response is handled internally by the MCP protocol.

Warning: Raising McpError after call_next() will only log the error, not send it to the client. The response has already been sent. Always reject before call_next().

Raw Handler

For complete control over all messages, override __call__ instead of individual hooks:

from fastmcp.server.middleware import Middleware, MiddlewareContext

class RawMiddleware(Middleware):
    async def __call__(self, context: MiddlewareContext, call_next):
        print(f"Processing: {context.method}")
        result = await call_next(context)
        print(f"Completed: {context.method}")
        return result

This bypasses the hook dispatch system entirely. Use when you need uniform handling regardless of message type.

Session Availability

The MCP session may not be available during certain phases like initialization. Check before accessing session-specific attributes:

async def on_request(self, context: MiddlewareContext, call_next):
    ctx = context.fastmcp_context

    if ctx.request_context:
        # MCP session available
        session_id = ctx.session_id
        request_id = ctx.request_id
    else:
        # Session not yet established (e.g., during initialization)
        # Use HTTP helpers if needed
        from fastmcp.server.dependencies import get_http_headers
        headers = get_http_headers()

    return await call_next(context)

For HTTP-specific data (headers, client IP) when using HTTP transports, see HTTP Requests.

Built-in Middleware

FastMCP includes production-ready middleware for common server concerns.

Logging

from fastmcp.server.middleware.logging import LoggingMiddleware, StructuredLoggingMiddleware

LoggingMiddleware provides human-readable request and response logging. StructuredLoggingMiddleware outputs JSON-formatted logs for aggregation tools like Datadog or Splunk.

from fastmcp import FastMCP
from fastmcp.server.middleware.logging import LoggingMiddleware

mcp = FastMCP("MyServer")
mcp.add_middleware(LoggingMiddleware(
    include_payloads=True,
    max_payload_length=1000
))

Timing

from fastmcp.server.middleware.timing import TimingMiddleware, DetailedTimingMiddleware

TimingMiddleware logs execution duration for all requests. DetailedTimingMiddleware provides per-operation timing with separate tracking for tools, resources, and prompts.

from fastmcp import FastMCP
from fastmcp.server.middleware.timing import TimingMiddleware

mcp = FastMCP("MyServer")
mcp.add_middleware(TimingMiddleware())

Caching

from fastmcp.server.middleware.caching import ResponseCachingMiddleware

Caches tool calls, resource reads, and list operations with TTL-based expiration.

from fastmcp import FastMCP
from fastmcp.server.middleware.caching import ResponseCachingMiddleware

mcp = FastMCP("MyServer")
mcp.add_middleware(ResponseCachingMiddleware())

Each operation type can be configured independently using settings classes:

from fastmcp.server.middleware.caching import (
    ResponseCachingMiddleware,
    CallToolSettings,
    ListToolsSettings,
    ReadResourceSettings
)

mcp.add_middleware(ResponseCachingMiddleware(
    list_tools_settings=ListToolsSettings(ttl=30),
    call_tool_settings=CallToolSettings(included_tools=["expensive_tool"]),
    read_resource_settings=ReadResourceSettings(enabled=False)
))

Each settings class accepts:

• enabled — Enable/disable caching for this operation
• ttl — Time-to-live in seconds
• included_* / excluded_* — Whitelist or blacklist specific items

For persistence or distributed deployments, configure a different storage backend:

from fastmcp.server.middleware.caching import ResponseCachingMiddleware
from key_value.aio.stores.disk import DiskStore

mcp.add_middleware(ResponseCachingMiddleware(
    cache_storage=DiskStore(directory="cache")
))

See Storage Backends for complete options.

Rate Limiting

from fastmcp.server.middleware.rate_limiting import (
    RateLimitingMiddleware,
    SlidingWindowRateLimitingMiddleware
)

RateLimitingMiddleware uses a token bucket algorithm allowing controlled bursts. SlidingWindowRateLimitingMiddleware provides precise time-window rate limiting without burst allowance.

from fastmcp import FastMCP
from fastmcp.server.middleware.rate_limiting import RateLimitingMiddleware

mcp = FastMCP("MyServer")
mcp.add_middleware(RateLimitingMiddleware(
    max_requests_per_second=10.0,
    burst_capacity=20
))

For sliding window rate limiting:

from fastmcp.server.middleware.rate_limiting import SlidingWindowRateLimitingMiddleware

mcp.add_middleware(SlidingWindowRateLimitingMiddleware(
    max_requests=100,
    window_minutes=1
))

Error Handling

from fastmcp.server.middleware.error_handling import ErrorHandlingMiddleware, RetryMiddleware

ErrorHandlingMiddleware provides centralized error logging and transformation. RetryMiddleware automatically retries with exponential backoff for transient failures.

from fastmcp import FastMCP
from fastmcp.server.middleware.error_handling import ErrorHandlingMiddleware

mcp = FastMCP("MyServer")
mcp.add_middleware(ErrorHandlingMiddleware(
    include_traceback=True,
    transform_errors=True,
    error_callback=my_error_callback
))

For automatic retries:

from fastmcp.server.middleware.error_handling import RetryMiddleware

mcp.add_middleware(RetryMiddleware(
    max_retries=3,
    retry_exceptions=(ConnectionError, TimeoutError)
))

Ping

from fastmcp.server.middleware import PingMiddleware

Keeps long-lived connections alive by sending periodic pings.

from fastmcp import FastMCP
from fastmcp.server.middleware import PingMiddleware

mcp = FastMCP("MyServer")
mcp.add_middleware(PingMiddleware(interval_ms=5000))

The ping task starts on the first message and stops automatically when the session ends. Most useful for stateful HTTP connections; has no effect on stateless connections.

Tool Injection

from fastmcp.server.middleware.tool_injection import (
    ToolInjectionMiddleware,
    PromptToolMiddleware,
    ResourceToolMiddleware
)

ToolInjectionMiddleware dynamically injects tools during request processing. PromptToolMiddleware and ResourceToolMiddleware provide compatibility layers for clients that cannot list or access prompts and resources directly—they expose those capabilities as tools.

from fastmcp import FastMCP
from fastmcp.tools import Tool
from fastmcp.server.middleware.tool_injection import ToolInjectionMiddleware

def my_tool_fn(a: int, b: int) -> int:
    return a + b

my_tool = Tool.from_function(fn=my_tool_fn, name="my_tool")
mcp.add_middleware(ToolInjectionMiddleware(tools=[my_tool]))

Response Limiting

from fastmcp.server.middleware.response_limiting import ResponseLimitingMiddleware

Large tool responses can overwhelm LLM context windows or cause memory issues. You can add response-limiting middleware to enforce size constraints on tool outputs.

from fastmcp import FastMCP
from fastmcp.server.middleware.response_limiting import ResponseLimitingMiddleware

mcp = FastMCP("MyServer")

# Limit all tool responses to 500KB
mcp.add_middleware(ResponseLimitingMiddleware(max_size=500_000))

@mcp.tool
def search(query: str) -> str:
    # This could return a very large result
    return "x" * 1_000_000  # 1MB response

# When called, the response will be truncated to ~500KB with:
# "...\n\n[Response truncated due to size limit]"

When a response exceeds the limit, the middleware extracts all text content, joins it together, truncates to fit within the limit, and returns a single TextContent block. For non-text responses, the serialized JSON is used as the text source.

Note: If a tool defines an output_schema, truncated responses will no longer conform to that schema — the client will receive a plain TextContent block instead of the expected structured output. Keep this in mind when setting size limits for tools with structured responses.

# Limit only specific tools
mcp.add_middleware(ResponseLimitingMiddleware(
    max_size=100_000,
    tools=["search", "fetch_data"],
))

Combining Middleware

Order matters. Place middleware that should run first (on the way in) earliest:

from fastmcp import FastMCP
from fastmcp.server.middleware.error_handling import ErrorHandlingMiddleware
from fastmcp.server.middleware.rate_limiting import RateLimitingMiddleware
from fastmcp.server.middleware.timing import TimingMiddleware
from fastmcp.server.middleware.logging import LoggingMiddleware

mcp = FastMCP("Production Server")

mcp.add_middleware(ErrorHandlingMiddleware())   # Catch all errors
mcp.add_middleware(RateLimitingMiddleware(max_requests_per_second=50))
mcp.add_middleware(TimingMiddleware())
mcp.add_middleware(LoggingMiddleware())

@mcp.tool
def my_tool(data: str) -> str:
    return f"Processed: {data}"

Custom Middleware

When the built-in middleware doesn't fit your needs—custom authentication schemes, domain-specific logging, or request transformation—subclass Middleware and override the hooks you need.

from fastmcp import FastMCP
from fastmcp.server.middleware import Middleware, MiddlewareContext

class CustomMiddleware(Middleware):
    async def on_request(self, context: MiddlewareContext, call_next):
        # Pre-processing
        print(f"→ {context.method}")

        result = await call_next(context)

        # Post-processing
        print(f"← {context.method}")
        return result

mcp = FastMCP("MyServer")
mcp.add_middleware(CustomMiddleware())

Override only the hooks relevant to your use case. Unoverridden hooks pass through automatically.

Denying Requests

Raise the appropriate error type to stop processing and return an error to the client.

from fastmcp.server.middleware import Middleware, MiddlewareContext
from fastmcp.exceptions import ToolError

class AuthMiddleware(Middleware):
    async def on_call_tool(self, context: MiddlewareContext, call_next):
        tool_name = context.message.name

        if tool_name in ["delete_all", "admin_config"]:
            raise ToolError("Access denied: requires admin privileges")

        return await call_next(context)

Do not return error values or skip call_next() to indicate errors—raise exceptions for proper error propagation.

Modifying Requests

Change the message before passing it down the chain.

from fastmcp.server.middleware import Middleware, MiddlewareContext

class InputSanitizer(Middleware):
    async def on_call_tool(self, context: MiddlewareContext, call_next):
        if context.message.name == "search":
            # Normalize search query
            query = context.message.arguments.get("query", "")
            context.message.arguments["query"] = query.strip().lower()

        return await call_next(context)

Modifying Responses

Transform results after the handler executes.

from fastmcp.server.middleware import Middleware, MiddlewareContext

class ResponseEnricher(Middleware):
    async def on_call_tool(self, context: MiddlewareContext, call_next):
        result = await call_next(context)

        if context.message.name == "get_data" and result.structured_content:
            result.structured_content["processed_by"] = "enricher"

        return result

For more complex tool transformations, consider Transforms instead.

Filtering Lists

List operations return FastMCP objects that you can filter before they reach the client. When filtering list results, also block execution in the corresponding operation hook to maintain consistency:

from fastmcp.server.middleware import Middleware, MiddlewareContext
from fastmcp.exceptions import ToolError

class PrivateToolFilter(Middleware):
    async def on_list_tools(self, context: MiddlewareContext, call_next):
        tools = await call_next(context)
        return [tool for tool in tools if "private" not in tool.tags]

    async def on_call_tool(self, context: MiddlewareContext, call_next):
        if context.fastmcp_context:
            tool = await context.fastmcp_context.fastmcp.get_tool(context.message.name)
            if "private" in tool.tags:
                raise ToolError("Tool not found")

        return await call_next(context)

Accessing Component Metadata

During execution hooks, component metadata (like tags) isn't directly available. Look up the component through the server:

from fastmcp.server.middleware import Middleware, MiddlewareContext
from fastmcp.exceptions import ToolError

class TagBasedAuth(Middleware):
    async def on_call_tool(self, context: MiddlewareContext, call_next):
        if context.fastmcp_context:
            try:
                tool = await context.fastmcp_context.fastmcp.get_tool(context.message.name)

                if "requires-auth" in tool.tags:
                    # Check authentication here
                    pass

            except Exception:
                pass  # Let execution handle missing tools

        return await call_next(context)

The same pattern works for resources and prompts:

resource = await context.fastmcp_context.fastmcp.get_resource(context.message.uri)
prompt = await context.fastmcp_context.fastmcp.get_prompt(context.message.name)

Storing State

Middleware can store state that tools access later through the FastMCP context.

from fastmcp.server.middleware import Middleware, MiddlewareContext

class UserMiddleware(Middleware):
    async def on_request(self, context: MiddlewareContext, call_next):
        # Extract user from headers (HTTP transport)
        from fastmcp.server.dependencies import get_http_headers
        headers = get_http_headers() or {}
        user_id = headers.get("x-user-id", "anonymous")

        # Store for tools to access
        if context.fastmcp_context:
            context.fastmcp_context.set_state("user_id", user_id)

        return await call_next(context)

Tools retrieve the state:

from fastmcp import FastMCP, Context

mcp = FastMCP("MyServer")

@mcp.tool
def get_user_data(ctx: Context) -> str:
    user_id = ctx.get_state("user_id")
    return f"Data for user: {user_id}"

See Context State Management for details.

Constructor Parameters

Initialize middleware with configuration:

from fastmcp.server.middleware import Middleware, MiddlewareContext

class ConfigurableMiddleware(Middleware):
    def __init__(self, api_key: str, rate_limit: int = 100):
        self.api_key = api_key
        self.rate_limit = rate_limit
        self.request_counts = {}

    async def on_request(self, context: MiddlewareContext, call_next):
        # Use self.api_key, self.rate_limit, etc.
        return await call_next(context)

mcp.add_middleware(ConfigurableMiddleware(
    api_key="secret",
    rate_limit=50
))

Error Handling in Custom Middleware

Wrap call_next() to handle errors from downstream middleware and handlers.

from fastmcp.server.middleware import Middleware, MiddlewareContext

class ErrorLogger(Middleware):
    async def on_request(self, context: MiddlewareContext, call_next):
        try:
            return await call_next(context)
        except Exception as e:
            print(f"Error in {context.method}: {type(e).__name__}: {e}")
            raise  # Re-raise to let error propagate

Catching and not re-raising suppresses the error entirely. Usually you want to log and re-raise.

Complete Example

Authentication middleware checking API keys for specific tools:

from fastmcp import FastMCP
from fastmcp.server.middleware import Middleware, MiddlewareContext
from fastmcp.server.dependencies import get_http_headers
from fastmcp.exceptions import ToolError

class ApiKeyAuth(Middleware):
    def __init__(self, valid_keys: set[str], protected_tools: set[str]):
        self.valid_keys = valid_keys
        self.protected_tools = protected_tools

    async def on_call_tool(self, context: MiddlewareContext, call_next):
        tool_name = context.message.name

        if tool_name not in self.protected_tools:
            return await call_next(context)

        headers = get_http_headers() or {}
        api_key = headers.get("x-api-key")

        if api_key not in self.valid_keys:
            raise ToolError(f"Invalid API key for protected tool: {tool_name}")

        return await call_next(context)

mcp = FastMCP("Secure Server")
mcp.add_middleware(ApiKeyAuth(
    valid_keys={"key-1", "key-2"},
    protected_tools={"delete_user", "admin_panel"}
))

@mcp.tool
def delete_user(user_id: str) -> str:
    return f"Deleted user {user_id}"

@mcp.tool
def get_user(user_id: str) -> str:
    return f"User {user_id}"  # Not protected

Documentation Index

Fetch the complete documentation index at: https://gofastmcp.com/llms.txt Use this file to discover all available pages before exploring further.