SEP-1686: Tasks

[LucaButBoring]

This PR implements the required changes for modelcontextprotocol/modelcontextprotocol#1686, which adds task augmentation to requests.

Motivation and Context

The current MCP specification supports tool calls that execute a request and eventually receive a response, and tool calls can be passed a progress token to integrate with MCP’s progress-tracking functionality, enabling host applications to receive status updates for a tool call via notifications. However, there is no way for a client to explicitly request the status of a tool call, resulting in states where it is possible for a tool call to have been dropped on the server, and it is unknown if a response or a notification may ever arrive. Similarly, there is no way for a client to explicitly retrieve the result of a tool call after it has completed — if the result was dropped, clients must call the tool again, which is undesirable for tools expected to take minutes or more. This is particularly relevant for MCP servers abstracting existing workflow-based APIs, such as AWS Step Functions, Workflows for Google Cloud, or APIs representing CI/CD pipelines, among other applications.

This proposal (and implementation) solves this by introducing the concept of Tasks, which are pending work objects that can augment any other request. Clients generate a task ID and augment their request with it — that task ID is both a reference to the request and an idempotency token. If the server accepts the task augmentation request, clients can then poll for the status and eventual result of the task with the tasks/get and tasks/result operations.

How Has This Been Tested?

Unit tests and updated sHTTP example.

Breaking Changes

None.

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