158 support resource links

[dstibbe]

Support resource links in tool result content #158

• Refactored PromptMessageContent to ContentBlock
• Added the ResourceLink as a ContentBlock type

Motivation and Context

The new spec introduced a new ContentBlock type: ResourceLink. This type however hasn't been implemented yet.
Noticed that there was actually no ContentBlock type at all in the core as the latest schema reference specified. Instead there is a PromptMessageContent.

How Has This Been Tested?

Added unit tests. Ran all tests.
Also used the build in a standalone project:

• calling a tool returning a ResourceLink via de client directly
• called a tool using Jetbrains Koog framework

Breaking Changes

Added the following:

• ContentBlock
• CreateMessageResultContent, and
• SamplingMessageContent

However, I kept the two intrefaces PromptMessageContent and PromptMessageContentMultimodal purely for backwards compatibility. They are deprecated however.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [ x ] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• [ x ] Documentation update

Checklist

• [ x ] I have read the MCP Documentation
• [ x ] My code follows the repository's style guidelines
• [ x ] New and existing tests pass locally
• [ x ] I have added appropriate error handling - none needed
• [ x ] I have added or updated documentation as needed

Additional context

[Copilot]

Pull Request Overview

This PR adds support for ResourceLink ContentBlock type to align with the MCP specification, refactoring the existing PromptMessageContent type hierarchy to use the more structured ContentBlock interface.

• Introduces ResourceLink as a new ContentBlock type for linking to external resources
• Refactors type hierarchy from PromptMessageContent to ContentBlock with specialized content types
• Maintains backwards compatibility by deprecating old interfaces while keeping them functional

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types.kt	Adds ResourceLink class and ContentBlock interface hierarchy
kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types.util.kt	Updates polymorphic serializers for new ContentBlock types
kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/TypesTest.kt	Adds tests for ResourceLink and updates existing tests
kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/TypesUtilTest.kt	Updates serialization tests for ContentBlock types
kotlin-sdk-core/api/kotlin-sdk-core.api	API documentation for new types and interfaces
CONTRIBUTING.md	Minor update to build instructions

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}