Support different transports in Client

[Kludex]

This pull request updates the MCP client codebase to simplify and standardize the transport interface for client-server communication. The main change is that the streamable_http_client and related transports now return only a 2-tuple (read_stream, write_stream), removing the previously returned get_session_id callback. The documentation and all usage examples have been updated accordingly. Additionally, a new Transport protocol is introduced to formalize the transport interface, and the in-memory transport is refactored to support async context management. Minor documentation clarifications and type cleanups are also included.

Key changes:

Transport Interface Simplification

• The streamable_http_client now returns a 2-tuple (read_stream, write_stream) instead of a 3-tuple with get_session_id. All usages in code, examples, and documentation have been updated to match this new interface. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

• The get_session_id callback is now documented as removed, with migration instructions and alternatives using httpx event hooks provided in docs/migration.md.

Transport Protocol and In-Memory Transport Refactor

• Introduced a new Transport protocol in src/mcp/client/_transport.py to standardize the interface for all client transports. [1] [2]

• Refactored InMemoryTransport to implement async context management (__aenter__/__aexit__), and updated its usage and documentation for consistency with the new protocol. [1] [2] [3] [4]

Documentation and Typing Improvements

• Updated README.v2.md, and all relevant example clients to reflect the new transport interface and provide clear, accurate usage patterns. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

• Minor documentation clarifications and type annotation cleanups, including a note in CLAUDE.md about import placement and type signature simplifications. [1] [2]

These changes improve consistency, usability, and maintainability of the MCP client transport layer and its documentation.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[Copilot]

Pull request overview

This PR introduces support for different transports in the Client class, making it more flexible and extensible. The changes enable the Client to work with URL strings, Transport instances, or Server/MCPServer instances, while standardizing the transport interface.

Changes:

• Introduces a Transport protocol that standardizes how client transports work
• Refactors Client class from manual init to dataclass for better maintainability
• Removes the get_session_id callback from streamable_http_client to align with Transport protocol (now returns 2-tuple instead of 3-tuple)
• Updates InMemoryTransport to implement the Transport protocol as an async context manager
• Updates all tests, examples, and documentation to reflect the 2-tuple return signature

Reviewed changes

Copilot reviewed 25 out of 25 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/mcp/client/_transport.py	New file defining the Transport protocol and TransportStreams type
src/mcp/client/_memory.py	Refactored InMemoryTransport to implement Transport protocol via aenter/aexit
src/mcp/client/client.py	Converted Client to dataclass, added support for URL strings and Transport instances
src/mcp/client/streamable_http.py	Removed get_session_id callback, now returns 2-tuple matching Transport protocol
src/mcp/client/session_group.py	Updated to unpack 2-tuple from streamable_http_client
src/mcp/client/__init__.py	Added Transport to public exports
tests/*	Updated all tests to unpack 2-tuple instead of 3-tuple
examples/*	Updated all example clients to use new 2-tuple signature
docs/migration.md	Added migration guide for get_session_id callback removal
README.md, README.v2.md	Updated code examples to reflect new signature
.github/actions/conformance/client.py	Updated conformance tests for new signature
CLAUDE.md	Emphasized rule about imports at top of file

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The comment mentions that "Transports may return additional values (e.g., streamable_http_client returns a 3-tuple)" but this is no longer accurate. The streamable_http_client now returns a 2-tuple (read_stream, write_stream) as indicated by the TransportStreams type. This comment should be updated or removed.

Suggested change

	# Transports may return additional values (e.g., streamable_http_client returns a 3-tuple)
	# We only need the first two elements (read_stream, write_stream)
	# Transports return a 2-tuple (read_stream, write_stream)

[Copilot]

The import path in the docstring example is incorrect. It shows from mcp.client.memory import InMemoryTransport but InMemoryTransport is in the _memory module (with underscore). The import should be either from mcp.client._memory import InMemoryTransport or InMemoryTransport should be exported from the public client module. Since InMemoryTransport is not currently exported in src/mcp/client/__init__.py, users should either use it via the underscore import (which suggests it's internal) or it should be added to the public exports.

Suggested change

	from mcp.client.memory import InMemoryTransport
	from mcp.client._memory import InMemoryTransport

[Copilot]

The _session and _exit_stack fields are initialized to None in the dataclass but are not marked with field(default=None). While this works because None is the default for Optional types, it's better to be explicit in dataclasses, especially since _transport uses field(init=False). Consider using field(default=None, init=False) for consistency and clarity.

Suggested change

	_session: ClientSession | None = None
	_exit_stack: AsyncExitStack | None = None
	_session: ClientSession | None = field(default=None, init=False)
	_exit_stack: AsyncExitStack | None = field(default=None, init=False)

[Copilot]

The unpacking pattern streams = await exit_stack.enter_async_context(self._transport); read_stream, write_stream = streams[0], streams[1] is unnecessarily verbose. Since TransportStreams is defined as a 2-tuple, you can directly unpack: read_stream, write_stream = await exit_stack.enter_async_context(self._transport). This is more Pythonic and clearer.