Add ergonomic Client class for testing MCP servers

[felixweinberger]

Summary

Adds a high-level Client class that provides an ergonomic API for testing MCP servers with in-memory transport.

Motivation and Context

Testing MCP servers currently requires using the verbose create_connected_server_and_client_session function. This PR introduces a simpler API:

from mcp import Client
from mcp.server.fastmcp import FastMCP

mcp = FastMCP("test")

@mcp.tool()
def add(a: int, b: int) -> int:
    return a + b

async with Client(mcp) as client:
    result = await client.call_tool("add", {"a": 1, "b": 2})

Related issues:

• Closes Add ergonomic test client for testing MCP servers #1728

How Has This Been Tested?

• 24 new tests for Client and InMemoryTransport
• Migrated existing tests from create_connected_server_and_client_session to new API
• Full test suite passes

Breaking Changes

• Removed create_connected_server_and_client_session from mcp.shared.memory (v2 breaking change)

Types of changes

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

New API

High-level (recommended):

from mcp import Client

async with Client(server) as client:
    result = await client.call_tool("my_tool", {"arg": "value"})

Low-level (for advanced use cases):

from mcp.client.transports import InMemoryTransport
from mcp.client.session import ClientSession

transport = InMemoryTransport(server)
async with transport.connect() as (read, write):
    async with ClientSession(read, write) as session:
        await session.initialize()
        # Full ClientSession access...

[felixweinberger]

Key files to review:

• src/mcp/client/client.py (new) - unified Client class
• src/mcp/client/transports/memory.py (new) - InMemoryTransport
• src/mcp/client/__init__.py - public exports
• src/mcp/client/transports/__init__.py - transport exports
• src/mcp/shared/memory.py (deleted) - replaced by InMemoryTransport

Remainder is tests being updated to use the new interface.

NOTE: This only implements test support. Refactoring Client to support different transports for the end-user API (streamable HTTP, stdio, SSE, etc.) is coming, marked as TODO in the Client docstring.

[felixweinberger]

Follow-up tasks

• Add transport types to Client (StreamableHttpTransport, SSETransport, StdioTransport) + infer_transport() for auto-detection
• Add lint rule to enforce Google-style docstrings (first line on same line as """) e.g. """Send a ping request."""
• Audit existing unit tests and as much as possible refactor to use this new Client class instead