Add an optional 'size' field to the Resource type.

[evalstate]

Adds an optional 'size' field to the Resource type. The 'size' represents the raw size in bytes (before encoding or tokenization).

This can be used by the Client/Host Application to pre-emptively estimate Context Window usage and make informed decisions about resource handling before attempting to load or process resources.

Motivation and Context

When working with LLMs, it's crucial to manage context window limitations effectively. This addition allows Host/Client applications to make smarter decisions about resource handling before attempting to process them.

For example:

• UI clients can warn users before attempting to process files that would exceed context limits
• Host applications can implement pre-filtering or chunking strategies based on resource sizes
• Better progress indicators for resource loading and processing
• Applications like Claude Analysis Tool would be able to estimate whether an application log file could be handled within the context window, or needs to be mounted in a Root or summarized first.

How Has This Been Tested?

Schema change only - no runtime implications to test. SDK implementations will need their own testing coverage when incorporating this field.

Breaking Changes

No breaking changes:

• Field is optional in the schema
• Existing implementations will continue to work without modification
• SDK updates can be made in minor version updates

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

This change aligns with common patterns in file system and storage APIs where size information is readily available. It provides valuable metadata for client applications while maintaining backward compatibility.

The size field specifically represents raw byte size to avoid confusion with token counts or other encoded sizes. This distinction is documented in the schema comments to prevent misuse.

[jspahrsummers]

Thanks, this is a great idea! Just one wording nit, but I'm happy with this.

Tagging in @dsp-ant for second approval.

[dsp-ant]

This needs to rebased to be moved into draft

[jspahrsummers]

@dsp-ant This is backwards compatible, so I think it can be additive to the last release, no?