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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):
        ...

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"))

mount_path parameter removed from FastMCP

The mount_path parameter has been removed from FastMCP.__init__(), FastMCP.run(), FastMCP.run_sse_async(), and FastMCP.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 FastMCP constructor to run()/app methods

Transport-specific parameters have been moved from the FastMCP 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.fastmcp import FastMCP

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

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

For mounted apps:

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

# Before (v1)
mcp = FastMCP("App", json_response=True)
app = Starlette(routes=[Mount("/", app=mcp.streamable_http_app())])

# After (v2)
mcp = FastMCP("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.

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

Deprecations

Bug Fixes

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 FastMCP. This allows using the streamable HTTP transport without the FastMCP wrapper.

from mcp.server.lowlevel.server import Server

server = Server("my-server")

# Register handlers...
@server.list_tools()
async def list_tools():
    return [...]

# Create a Starlette app for streamable HTTP
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