Adds error-handling requirements to #2663 specify that -32003 is the correct error for servers to return when a server supports the Tasks extension and a client that does not declare the extension capability attempts to call tasks/get, tasks/update, or tasks/cancel - with an additional carveout for servers supporting both the extension and the 2025-11-25 version of tasks.

The backwards-compatibility section has been converted into a matrix for readability.

Motivation and Context

Currently, the Tasks extension specification does not declare a specific error for servers to return if a client invokes a Task-related method without supporting the extension. It does declare that servers must return -32003 (Missing Required Client Capability) if they require a client to support receiving CreateTaskResult, but this does not imply any behavior regarding other task methods, like tasks/update.

The current spec is ambiguous on this point, and both -32601 and -32003 are reasonable errors to return in this scenario, but we will require -32003 as it more accurately reflects the server's state. -32601 would imply that it does not support Tasks at all, while -32003 clarifies that the server does, but the client does not.

How Has This Been Tested?

N/A

Breaking Changes

N/A

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

Identified in modelcontextprotocol/conformance#262 (comment)