bump default negotiated version

[KKonstantinov]

Bump default negotiated version to 2025-06-18

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• 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

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1245

commit: 8d181be

[felixweinberger]

This change would deviate from the spec I think. The Streamable HTTP transport specification explicitly states:

For backwards compatibility, if the server does not receive an MCP-Protocol-Version header, and has no other way to identify the version - for example, by relying on the protocol version negotiated during initialization - the server SHOULD assume protocol version 2025-03-26.

If we want to change this default, the spec would need to be updated first.

[felixweinberger]

Requesting changes as I don't think we want to ship this as is given comment above.