In service of compatibility and flexibility, make two changes around structured tool results:

• restore CallToolResult.content to non-optional status, making the introduction of structuredContent purely a additive change.
• relax spec language around output schema validation from MUST to SHOULD.

Motivation and Context

Changes were motivated by backward compatibility considerations.

How Has This Been Tested?

Tracking changes/tests in TypeScript SDK to follow.

Breaking Changes

Spec is still in draft. However, this actually reduces the delta in the draft spec.

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