modelcontextprotocol--python-sdk/docs/migration.md

Migration Guide: v1 to v2

This guide covers the breaking changes introduced in v2 of the MCP Python SDK and how to update your code.

Overview

Version 2 of the MCP Python SDK introduces several breaking changes to improve the API, align with the MCP specification, and provide better type safety.

Breaking Changes

streamablehttp_client removed

The deprecated streamablehttp_client function has been removed. Use streamable_http_client instead.

Before (v1):

from mcp.client.streamable_http import streamablehttp_client

async with streamablehttp_client(
    url="http://localhost:8000/mcp",
    headers={"Authorization": "Bearer token"},
    timeout=30,
    sse_read_timeout=300,
    auth=my_auth,
) as (read_stream, write_stream, get_session_id):
    ...

After (v2):

import httpx
from mcp.client.streamable_http import streamable_http_client

# Configure headers, timeout, and auth on the httpx.AsyncClient
http_client = httpx.AsyncClient(
    headers={"Authorization": "Bearer token"},
    timeout=httpx.Timeout(30, read=300),
    auth=my_auth,
)

async with http_client:
    async with streamable_http_client(
        url="http://localhost:8000/mcp",
        http_client=http_client,
    ) as (read_stream, write_stream):
        ...

get_session_id callback removed from streamable_http_client

The get_session_id callback (third element of the returned tuple) has been removed from streamable_http_client. The function now returns a 2-tuple (read_stream, write_stream) instead of a 3-tuple.

If you need to capture the session ID (e.g., for session resumption testing), you can use httpx event hooks to capture it from the response headers:

Before (v1):

from mcp.client.streamable_http import streamable_http_client

async with streamable_http_client(url) as (read_stream, write_stream, get_session_id):
    async with ClientSession(read_stream, write_stream) as session:
        await session.initialize()
        session_id = get_session_id()  # Get session ID via callback

After (v2):

import httpx
from mcp.client.streamable_http import streamable_http_client

# Option 1: Simply ignore if you don't need the session ID
async with streamable_http_client(url) as (read_stream, write_stream):
    async with ClientSession(read_stream, write_stream) as session:
        await session.initialize()

# Option 2: Capture session ID via httpx event hooks if needed
captured_session_ids: list[str] = []

async def capture_session_id(response: httpx.Response) -> None:
    session_id = response.headers.get("mcp-session-id")
    if session_id:
        captured_session_ids.append(session_id)

http_client = httpx.AsyncClient(
    event_hooks={"response": [capture_session_id]},
    follow_redirects=True,
)

async with http_client:
    async with streamable_http_client(url, http_client=http_client) as (read_stream, write_stream):
        async with ClientSession(read_stream, write_stream) as session:
            await session.initialize()
            session_id = captured_session_ids[0] if captured_session_ids else None

StreamableHTTPTransport parameters removed

The headers, timeout, sse_read_timeout, and auth parameters have been removed from StreamableHTTPTransport. Configure these on the httpx.AsyncClient instead (see example above).

Removed type aliases and classes

The following deprecated type aliases and classes have been removed from mcp.types:

Removed	Replacement
Content	ContentBlock
ResourceReference	ResourceTemplateReference
Cursor	Use str directly
MethodT	Internal TypeVar, not intended for public use
RequestParamsT	Internal TypeVar, not intended for public use
NotificationParamsT	Internal TypeVar, not intended for public use

Before (v1):

from mcp.types import Content, ResourceReference, Cursor

After (v2):

from mcp.types import ContentBlock, ResourceTemplateReference
# Use `str` instead of `Cursor` for pagination cursors

args parameter removed from ClientSessionGroup.call_tool()

The deprecated args parameter has been removed from ClientSessionGroup.call_tool(). Use arguments instead.

Before (v1):

result = await session_group.call_tool("my_tool", args={"key": "value"})

After (v2):

result = await session_group.call_tool("my_tool", arguments={"key": "value"})

cursor parameter removed from ClientSession list methods

The deprecated cursor parameter has been removed from the following ClientSession methods:

• list_resources()
• list_resource_templates()
• list_prompts()
• list_tools()

Use params=PaginatedRequestParams(cursor=...) instead.

Before (v1):

result = await session.list_resources(cursor="next_page_token")
result = await session.list_tools(cursor="next_page_token")

After (v2):

from mcp.types import PaginatedRequestParams

result = await session.list_resources(params=PaginatedRequestParams(cursor="next_page_token"))
result = await session.list_tools(params=PaginatedRequestParams(cursor="next_page_token"))

ClientSession.get_server_capabilities() replaced by initialize_result property

ClientSession now stores the full InitializeResult via an initialize_result property. This provides access to server_info, capabilities, instructions, and the negotiated protocol_version through a single property. The get_server_capabilities() method has been removed.

Before (v1):

capabilities = session.get_server_capabilities()
# server_info, instructions, protocol_version were not stored — had to capture initialize() return value

After (v2):

result = session.initialize_result
if result is not None:
    capabilities = result.capabilities
    server_info = result.server_info
    instructions = result.instructions
    version = result.protocol_version

The high-level Client.initialize_result returns the same InitializeResult but is non-nullable — initialization is guaranteed inside the context manager, so no None check is needed. This replaces v1's Client.server_capabilities; use client.initialize_result.capabilities instead.

McpError renamed to MCPError

The McpError exception class has been renamed to MCPError for consistent naming with the MCP acronym style used throughout the SDK.

Before (v1):

from mcp.shared.exceptions import McpError

try:
    result = await session.call_tool("my_tool")
except McpError as e:
    print(f"Error: {e.error.message}")

After (v2):

from mcp.shared.exceptions import MCPError

try:
    result = await session.call_tool("my_tool")
except MCPError as e:
    print(f"Error: {e.message}")

MCPError is also exported from the top-level mcp package:

from mcp import MCPError

FastMCP renamed to MCPServer

The FastMCP class has been renamed to MCPServer to better reflect its role as the main server class in the SDK. This is a simple rename with no functional changes to the class itself.

Before (v1):

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("Demo")

After (v2):

from mcp.server.mcpserver import MCPServer

mcp = MCPServer("Demo")

mount_path parameter removed from MCPServer

The mount_path parameter has been removed from MCPServer.__init__(), MCPServer.run(), MCPServer.run_sse_async(), and MCPServer.sse_app(). It was also removed from the Settings class.

This parameter was redundant because the SSE transport already handles sub-path mounting via ASGI's standard root_path mechanism. When using Starlette's Mount("/path", app=mcp.sse_app()), Starlette automatically sets root_path in the ASGI scope, and the SseServerTransport uses this to construct the correct message endpoint path.

Transport-specific parameters moved from MCPServer constructor to run()/app methods

Transport-specific parameters have been moved from the MCPServer constructor to the run(), sse_app(), and streamable_http_app() methods. This provides better separation of concerns - the constructor now only handles server identity and authentication, while transport configuration is passed when starting the server.

Parameters moved:

• host, port - HTTP server binding
• sse_path, message_path - SSE transport paths
• streamable_http_path - StreamableHTTP endpoint path
• json_response, stateless_http - StreamableHTTP behavior
• event_store, retry_interval - StreamableHTTP event handling
• transport_security - DNS rebinding protection

Before (v1):

from mcp.server.fastmcp import FastMCP

# Transport params in constructor
mcp = FastMCP("Demo", json_response=True, stateless_http=True)
mcp.run(transport="streamable-http")

# Or for SSE
mcp = FastMCP("Server", host="0.0.0.0", port=9000, sse_path="/events")
mcp.run(transport="sse")

After (v2):

from mcp.server.mcpserver import MCPServer

# Transport params passed to run()
mcp = MCPServer("Demo")
mcp.run(transport="streamable-http", json_response=True, stateless_http=True)

# Or for SSE
mcp = MCPServer("Server")
mcp.run(transport="sse", host="0.0.0.0", port=9000, sse_path="/events")

For mounted apps:

When mounting in a Starlette app, pass transport params to the app methods:

# Before (v1)
from mcp.server.fastmcp import FastMCP

mcp = FastMCP("App", json_response=True)
app = Starlette(routes=[Mount("/", app=mcp.streamable_http_app())])

# After (v2)
from mcp.server.mcpserver import MCPServer

mcp = MCPServer("App")
app = Starlette(routes=[Mount("/", app=mcp.streamable_http_app(json_response=True))])

Note: DNS rebinding protection is automatically enabled when host is 127.0.0.1, localhost, or ::1. This now happens in sse_app() and streamable_http_app() instead of the constructor.

MCPServer.get_context() removed

MCPServer.get_context() has been removed. Context is now injected by the framework and passed explicitly — there is no ambient ContextVar to read from.

If you were calling get_context() from inside a tool/resource/prompt: use the ctx: Context parameter injection instead.

Before (v1):

@mcp.tool()
async def my_tool(x: int) -> str:
    ctx = mcp.get_context()
    await ctx.info("Processing...")
    return str(x)

After (v2):

@mcp.tool()
async def my_tool(x: int, ctx: Context) -> str:
    await ctx.info("Processing...")
    return str(x)

MCPServer.call_tool(), read_resource(), get_prompt() now accept a context parameter

MCPServer.call_tool(), MCPServer.read_resource(), and MCPServer.get_prompt() now accept an optional context: Context | None = None parameter. The framework passes this automatically during normal request handling. If you call these methods directly and omit context, a Context with no active request is constructed for you — tools that don't use ctx work normally, but any attempt to use ctx.session, ctx.request_id, etc. will raise.

The internal layers (ToolManager.call_tool, Tool.run, Prompt.render, ResourceTemplate.create_resource, etc.) now require context as a positional argument.

Replace RootModel by union types with TypeAdapter validation

The following union types are no longer RootModel subclasses:

• ClientRequest
• ServerRequest
• ClientNotification
• ServerNotification
• ClientResult
• ServerResult
• JSONRPCMessage

This means you can no longer access .root on these types or use model_validate() directly on them. Instead, use the provided TypeAdapter instances for validation.

Before (v1):

from mcp.types import ClientRequest, ServerNotification

# Using RootModel.model_validate()
request = ClientRequest.model_validate(data)
actual_request = request.root  # Accessing the wrapped value

notification = ServerNotification.model_validate(data)
actual_notification = notification.root

After (v2):

from mcp.types import client_request_adapter, server_notification_adapter

# Using TypeAdapter.validate_python()
request = client_request_adapter.validate_python(data)
# No .root access needed - request is the actual type

notification = server_notification_adapter.validate_python(data)
# No .root access needed - notification is the actual type

Available adapters:

Union Type	Adapter
ClientRequest	client_request_adapter
ServerRequest	server_request_adapter
ClientNotification	client_notification_adapter
ServerNotification	server_notification_adapter
ClientResult	client_result_adapter
ServerResult	server_result_adapter
JSONRPCMessage	jsonrpc_message_adapter

All adapters are exported from mcp.types.

RequestParams.Meta replaced with RequestParamsMeta TypedDict

The nested RequestParams.Meta Pydantic model class has been replaced with a top-level RequestParamsMeta TypedDict. This affects the ctx.meta field in request handlers and any code that imports or references this type.

Key changes:

• RequestParams.Meta (Pydantic model) → RequestParamsMeta (TypedDict)
• Attribute access (meta.progress_token) → Dictionary access (meta.get("progress_token"))
• progress_token field changed from ProgressToken | None = None to NotRequired[ProgressToken]

In request context handlers:

# Before (v1)
@server.call_tool()
async def handle_tool(name: str, arguments: dict) -> list[TextContent]:
    ctx = server.request_context
    if ctx.meta and ctx.meta.progress_token:
        await ctx.session.send_progress_notification(ctx.meta.progress_token, 0.5, 100)

# After (v2)
async def handle_call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
    if ctx.meta and "progress_token" in ctx.meta:
        await ctx.session.send_progress_notification(ctx.meta["progress_token"], 0.5, 100)
    ...

server = Server("my-server", on_call_tool=handle_call_tool)

RequestContext type parameters simplified

The RequestContext class has been split to separate shared fields from server-specific fields. The shared RequestContext now only takes 1 type parameter (the session type) instead of 3.

RequestContext changes:

• Type parameters reduced from RequestContext[SessionT, LifespanContextT, RequestT] to RequestContext[SessionT]
• Server-specific fields (lifespan_context, experimental, request, close_sse_stream, close_standalone_sse_stream) moved to new ServerRequestContext class in mcp.server.context

Before (v1):

from mcp.client.session import ClientSession
from mcp.shared.context import RequestContext, LifespanContextT, RequestT

# RequestContext with 3 type parameters
ctx: RequestContext[ClientSession, LifespanContextT, RequestT]

After (v2):

from mcp.client.context import ClientRequestContext
from mcp.server.context import ServerRequestContext, LifespanContextT, RequestT

# For client-side context (sampling, elicitation, list_roots callbacks)
ctx: ClientRequestContext

# For server-specific context with lifespan and request types
server_ctx: ServerRequestContext[LifespanContextT, RequestT]

ProgressContext and progress() context manager removed

The mcp.shared.progress module (ProgressContext, Progress, and the progress() context manager) has been removed. This module had no real-world adoption — all users send progress notifications via Context.report_progress() or session.send_progress_notification() directly.

Before:

from mcp.shared.progress import progress

with progress(ctx, total=100) as p:
    await p.progress(25)

After — use Context.report_progress() (recommended):

@server.tool()
async def my_tool(x: int, ctx: Context) -> str:
    await ctx.report_progress(25, 100)
    return "done"

After — use session.send_progress_notification() (low-level):

await session.send_progress_notification(
    progress_token=progress_token,
    progress=25,
    total=100,
)

Resource URI type changed from AnyUrl to str

The uri field on resource-related types now uses str instead of Pydantic's AnyUrl. This aligns with the MCP specification schema which defines URIs as plain strings (uri: string) without strict URL validation. This change allows relative paths like users/me that were previously rejected.

Before (v1):

from pydantic import AnyUrl
from mcp.types import Resource

# Required wrapping in AnyUrl
resource = Resource(name="test", uri=AnyUrl("users/me"))  # Would fail validation

After (v2):

from mcp.types import Resource

# Plain strings accepted
resource = Resource(name="test", uri="users/me")  # Works
resource = Resource(name="test", uri="custom://scheme")  # Works
resource = Resource(name="test", uri="https://example.com")  # Works

If your code passes AnyUrl objects to URI fields, convert them to strings:

# If you have an AnyUrl from elsewhere
uri = str(my_any_url)  # Convert to string

Affected types:

• Resource.uri
• ReadResourceRequestParams.uri
• ResourceContents.uri (and subclasses TextResourceContents, BlobResourceContents)
• SubscribeRequestParams.uri
• UnsubscribeRequestParams.uri
• ResourceUpdatedNotificationParams.uri

The Client and ClientSession methods read_resource(), subscribe_resource(), and unsubscribe_resource() now only accept str for the uri parameter. If you were passing AnyUrl objects, convert them to strings:

# Before (v1)
from pydantic import AnyUrl

await client.read_resource(AnyUrl("test://resource"))

# After (v2)
await client.read_resource("test://resource")
# Or if you have an AnyUrl from elsewhere:
await client.read_resource(str(my_any_url))

Resource templates: matching behavior changes

Resource template matching has been rewritten with RFC 6570 support. Several behaviors have changed:

Path-safety checks applied by default. Extracted parameter values containing .. as a path component, a null byte, or looking like an absolute path (/etc/passwd, C:\Windows) now cause the read to fail — the client receives an "Unknown resource" error and template iteration stops, so a strict template's rejection does not fall through to a later permissive template. This is checked on the decoded value, so ..%2Fetc, %2E%2E, and %00 are caught too. Note that .. is only flagged as a standalone path component, so values like v1.0..v2.0 or HEAD~3..HEAD are unaffected.

If a parameter legitimately needs to receive absolute paths or traversal sequences, exempt it:

from mcp.server.mcpserver import ResourceSecurity

@mcp.resource(
    "inspect://file/{+target}",
    security=ResourceSecurity(exempt_params={"target"}),
)
def inspect_file(target: str) -> str: ...

Template literals and structural delimiters match exactly. The previous matcher built a regex without escaping, so . matched any character and simple {var} swallowed ?, #, &, and ,. Now data://v1.0/{id} no longer matches data://v1X0/42, and api://{id} no longer matches api://foo?x=1 — use api://{id}{?x} or api://{+id} if you need to capture a query tail.

Template syntax errors surface at decoration time. Unclosed braces, duplicate variable names, and unsupported syntax raise InvalidUriTemplate when the decorator runs rather than re.error on first match.

Static URIs with Context-only handlers now error. A non-template URI paired with a handler that takes only a Context parameter previously registered but was silently unreachable (the resource could never be read). This now raises ValueError at decoration time. Context injection for static resources is planned; until then, use a template with at least one variable or access context through other means.

See Resources for the full template syntax, security configuration, and filesystem safety utilities.

Lowlevel Server: constructor parameters are now keyword-only

All parameters after name are now keyword-only. If you were passing version or other parameters positionally, use keyword arguments instead:

# Before (v1)
server = Server("my-server", "1.0")

# After (v2)
server = Server("my-server", version="1.0")

Lowlevel Server: type parameter reduced from 2 to 1

The Server class previously had two type parameters: Server[LifespanResultT, RequestT]. The RequestT parameter has been removed — handlers now receive typed params directly rather than a generic request type.

# Before (v1)
from typing import Any

from mcp.server.lowlevel.server import Server

server: Server[dict[str, Any], Any] = Server(...)

# After (v2)
from typing import Any

from mcp.server import Server

server: Server[dict[str, Any]] = Server(...)

Lowlevel Server: request_handlers and notification_handlers attributes removed

The public server.request_handlers and server.notification_handlers dictionaries have been removed. Handler registration is now done exclusively through constructor on_* keyword arguments. There is no public API to register handlers after construction.

# Before (v1) — direct dict access
from mcp.types import ListToolsRequest

if ListToolsRequest in server.request_handlers:
    ...

# After (v2) — no public access to handler dicts
# Use the on_* constructor params to register handlers
server = Server("my-server", on_list_tools=handle_list_tools)

Lowlevel Server: decorator-based handlers replaced with constructor on_* params

The lowlevel Server class no longer uses decorator methods for handler registration. Instead, handlers are passed as on_* keyword arguments to the constructor.

Before (v1):

from mcp.server.lowlevel.server import Server

server = Server("my-server")

@server.list_tools()
async def handle_list_tools():
    return [types.Tool(name="my_tool", description="A tool", inputSchema={})]

@server.call_tool()
async def handle_call_tool(name: str, arguments: dict):
    return [types.TextContent(type="text", text=f"Called {name}")]

After (v2):

from mcp.server import Server, ServerRequestContext
from mcp.types import (
    CallToolRequestParams,
    CallToolResult,
    ListToolsResult,
    PaginatedRequestParams,
    TextContent,
    Tool,
)

async def handle_list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
    return ListToolsResult(tools=[Tool(name="my_tool", description="A tool", input_schema={})])


async def handle_call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
    return CallToolResult(
        content=[TextContent(type="text", text=f"Called {params.name}")],
        is_error=False,
    )

server = Server("my-server", on_list_tools=handle_list_tools, on_call_tool=handle_call_tool)

Key differences:

• Handlers receive (ctx, params) instead of the full request object or unpacked arguments. ctx is a ServerRequestContext with session, lifespan_context, and experimental fields (plus request_id, meta, etc. for request handlers). params is the typed request params object.
• Handlers return the full result type (e.g. ListToolsResult) rather than unwrapped values (e.g. list[Tool]).
• The automatic jsonschema input/output validation that the old call_tool() decorator performed has been removed. There is no built-in replacement — if you relied on schema validation in the lowlevel server, you will need to validate inputs yourself in your handler.

Notification handlers:

from mcp.server import Server, ServerRequestContext
from mcp.types import ProgressNotificationParams


async def handle_progress(ctx: ServerRequestContext, params: ProgressNotificationParams) -> None:
    print(f"Progress: {params.progress}/{params.total}")

server = Server("my-server", on_progress=handle_progress)

Lowlevel Server: automatic return value wrapping removed

The old decorator-based handlers performed significant automatic wrapping of return values. This magic has been removed — handlers now return fully constructed result types. If you want these conveniences, use MCPServer (previously FastMCP) instead of the lowlevel Server.

call_tool() — structured output wrapping removed:

The old decorator accepted several return types and auto-wrapped them into CallToolResult:

# Before (v1) — returning a dict auto-wrapped into structured_content + JSON TextContent
@server.call_tool()
async def handle(name: str, arguments: dict) -> dict:
    return {"temperature": 22.5, "city": "London"}

# Before (v1) — returning a list auto-wrapped into CallToolResult.content
@server.call_tool()
async def handle(name: str, arguments: dict) -> list[TextContent]:
    return [TextContent(type="text", text="Done")]

# After (v2) — construct the full result yourself
import json

async def handle(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
    data = {"temperature": 22.5, "city": "London"}
    return CallToolResult(
        content=[TextContent(type="text", text=json.dumps(data, indent=2))],
        structured_content=data,
    )

Note: params.arguments can be None (the old decorator defaulted it to {}). Use params.arguments or {} to preserve the old behavior.

read_resource() — content type wrapping removed:

The old decorator auto-wrapped str into TextResourceContents and bytes into BlobResourceContents (with base64 encoding), and applied a default mime type of text/plain:

# Before (v1) — str/bytes auto-wrapped with mime type defaulting
@server.read_resource()
async def handle(uri: str) -> str:
    return "file contents"

@server.read_resource()
async def handle(uri: str) -> bytes:
    return b"\x89PNG..."

# After (v2) — construct TextResourceContents or BlobResourceContents yourself
import base64

async def handle_read(ctx: ServerRequestContext, params: ReadResourceRequestParams) -> ReadResourceResult:
    # Text content
    return ReadResourceResult(
        contents=[TextResourceContents(uri=str(params.uri), text="file contents", mime_type="text/plain")]
    )

async def handle_read(ctx: ServerRequestContext, params: ReadResourceRequestParams) -> ReadResourceResult:
    # Binary content — you must base64-encode it yourself
    return ReadResourceResult(
        contents=[BlobResourceContents(
            uri=str(params.uri),
            blob=base64.b64encode(b"\x89PNG...").decode("utf-8"),
            mime_type="image/png",
        )]
    )

list_tools(), list_resources(), list_prompts() — list wrapping removed:

The old decorators accepted bare lists and wrapped them into the result type:

# Before (v1)
@server.list_tools()
async def handle() -> list[Tool]:
    return [Tool(name="my_tool", ...)]

# After (v2)
async def handle(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
    return ListToolsResult(tools=[Tool(name="my_tool", ...)])

Using MCPServer instead:

If you prefer the convenience of automatic wrapping, use MCPServer which still provides these features through its @mcp.tool(), @mcp.resource(), and @mcp.prompt() decorators. The lowlevel Server is intentionally minimal — it provides no magic and gives you full control over the MCP protocol types.

Lowlevel Server: request_context property removed

The server.request_context property has been removed. Request context is now passed directly to handlers as the first argument (ctx). The request_ctx module-level contextvar has been removed entirely.

Before (v1):

from mcp.server.lowlevel.server import request_ctx

@server.call_tool()
async def handle_call_tool(name: str, arguments: dict):
    ctx = server.request_context  # or request_ctx.get()
    await ctx.session.send_log_message(level="info", data="Processing...")
    return [types.TextContent(type="text", text="Done")]

After (v2):

from mcp.server import ServerRequestContext
from mcp.types import CallToolRequestParams, CallToolResult, TextContent


async def handle_call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
    await ctx.session.send_log_message(level="info", data="Processing...")
    return CallToolResult(
        content=[TextContent(type="text", text="Done")],
        is_error=False,
    )

RequestContext: request-specific fields are now optional

The RequestContext class now uses optional fields for request-specific data (request_id, meta, etc.) so it can be used for both request and notification handlers. In notification handlers, these fields are None.

from mcp.server import ServerRequestContext

# request_id, meta, etc. are available in request handlers
# but None in notification handlers

Experimental: task handler decorators removed

The experimental decorator methods on ExperimentalHandlers (@server.experimental.list_tasks(), @server.experimental.get_task(), etc.) have been removed.

Default task handlers are still registered automatically via server.experimental.enable_tasks(). Custom handlers can be passed as on_* kwargs to override specific defaults.

Before (v1):

server = Server("my-server")
server.experimental.enable_tasks()

@server.experimental.get_task()
async def custom_get_task(request: GetTaskRequest) -> GetTaskResult:
    ...

After (v2):

from mcp.server import Server, ServerRequestContext
from mcp.types import GetTaskRequestParams, GetTaskResult


async def custom_get_task(ctx: ServerRequestContext, params: GetTaskRequestParams) -> GetTaskResult:
    ...


server = Server("my-server")
server.experimental.enable_tasks(on_get_task=custom_get_task)

Deprecations

Bug Fixes

Lowlevel Server: subscribe capability now correctly reported

Previously, the lowlevel Server hardcoded subscribe=False in resource capabilities even when a subscribe_resource() handler was registered. The subscribe capability is now dynamically set to True when an on_subscribe_resource handler is provided. Clients that previously didn't see subscribe: true in capabilities will now see it when a handler is registered, which may change client behavior.

Extra fields no longer allowed on top-level MCP types

MCP protocol types no longer accept arbitrary extra fields at the top level. This matches the MCP specification which only allows extra fields within _meta objects, not on the types themselves.

# This will now raise a validation error
from mcp.types import CallToolRequestParams

params = CallToolRequestParams(
    name="my_tool",
    arguments={},
    unknown_field="value",  # ValidationError: extra fields not permitted
)

# Extra fields are still allowed in _meta
params = CallToolRequestParams(
    name="my_tool",
    arguments={},
    _meta={"progressToken": "tok", "customField": "value"},  # OK
)

New Features

streamable_http_app() available on lowlevel Server

The streamable_http_app() method is now available directly on the lowlevel Server class, not just MCPServer. This allows using the streamable HTTP transport without the MCPServer wrapper.

from mcp.server import Server, ServerRequestContext
from mcp.types import ListToolsResult, PaginatedRequestParams


async def handle_list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
    return ListToolsResult(tools=[...])


server = Server("my-server", on_list_tools=handle_list_tools)

app = server.streamable_http_app(
    streamable_http_path="/mcp",
    json_response=False,
    stateless_http=False,
)

The lowlevel Server also now exposes a session_manager property to access the StreamableHTTPSessionManager after calling streamable_http_app().

Need Help?

If you encounter issues during migration:

1. Check the API Reference for updated method signatures
2. Review the examples for updated usage patterns
3. Open an issue on GitHub if you find a bug or need further assistance