feat: Add protocol_version parameter to ClientSession

[goingforstudying-ctrl]

Summary

Fixes #2307 - Adds the ability to specify a custom protocol version in ClientSession.initialize().

Problem

ClientSession.initialize() hardcodes protocolVersion=types.LATEST_PROTOCOL_VERSION with no way for the caller to override it. When connecting to an MCP server that requires a specific protocol version (e.g., Snowflake's managed MCP server requires "2025-06-18"), the only workaround is to mutate the module-level constant before calling initialize().

Solution

Added an optional protocol_version parameter to ClientSession.init():

def __init__(
    self,
    ...
    protocol_version: str | None = None,  # NEW — defaults to LATEST_PROTOCOL_VERSION
    *,
    ...
) -> None:
    self._protocol_version = protocol_version or types.LATEST_PROTOCOL_VERSION

Then use it in initialize():

protocolVersion=self._protocol_version,

Changes

File	Change	Reason
session.py	Added protocol_version parameter	Allow custom protocol version
session.py	Store as _protocol_version instance var	Use in initialize()
session.py	Update initialize() to use self._protocol_version	Use custom version
test_session.py	Added test_client_session_custom_protocol_version	Verify custom version works
test_session.py	Added test_client_session_default_protocol_version	Verify backwards compatibility

Testing

• Added test for custom protocol version
• Added test for default protocol version (backwards compatibility)
• Both tests pass

Backwards Compatibility

Fully backwards compatible - existing callers get the current default (LATEST_PROTOCOL_VERSION) when protocol_version is not specified.

Closes #2307

[tsingh2k15]

Tested the logic of this PR against mcp==1.26.0 by applying the changes manually (couldn't install the branch directly). The fix works correctly against Snowflake's managed MCP server; protocol_version="2025-06-18" passed to ClientSession resolves the handshake failure. Backwards compatibility confirmed, existing callers with no protocol_version argument behave identically to before.

#2307 (comment)

[goingforstudying-ctrl]

Hi @jspahrsummers, just checking in on this PR. Please let me know if any changes are needed. Happy to address any feedback!

[goingforstudying-ctrl]

Hi @maintainer, just checking in on this PR. Please let me know if any changes are needed. Happy to address any feedback!

[goingforstudying-ctrl]

Hi @maintainer, just checking in on this PR. Please let me know if any changes are needed. Happy to address any feedback!

[goingforstudying-ctrl]

Hi @jspahrsummers and MCP maintainers,

Just checking in on this PR — it's been about a month since opening and three days since my last ping. @tsingh2k15 kindly tested the changes against Snowflake's managed MCP server and confirmed the fix works correctly with backwards compatibility.

Is there anything I can do to help move this forward? Happy to rebase, add more tests, or make any adjustments needed.

Thanks for your time!

[goingforstudying-ctrl]

Hi @jspahrsummers and MCP maintainers! 👋 Just checking in on this PR — it's been about 6 weeks since opening. @tsingh2k15 kindly tested the changes against Snowflake's managed MCP server and confirmed they work correctly. The CI is all green. Could someone please review when you have a moment? This would help users who need to specify a protocol version. Thanks!

[goingforstudying-ctrl]

Hi maintainers, just checking in on this PR. Please let me know if any changes are needed. Happy to address any feedback!

[goingforstudying-ctrl]

Hi @tsingh2k15 and MCP maintainers! 👋

Just following up on this PR — it's been about 6 weeks since the last activity. The protocol_version parameter addition for ClientSession would be helpful for users needing version negotiation.

I'm happy to make any adjustments needed, rebase against the latest main, or add additional tests. Please let me know if there's anything I can do to help move this forward!

Thanks for maintaining the Python SDK! 🙏

[goingforstudying-ctrl]

Closing this PR as part of weekly cleanup.

Reason: Superseded by newer PR #2458 (feat: JSONRPCDispatcher) created on 2026-04-16T16:48:28Z.

If this was closed in error, please feel free to reopen.