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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

Before (v1):

from mcp.types import Content, ResourceReference

After (v2):

from mcp.types import ContentBlock, ResourceTemplateReference

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.

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 ClientSession.read_resource(), subscribe_resource(), and unsubscribe_resource() methods now accept both str and AnyUrl for backwards compatibility.

Deprecations

New Features

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