Remove Server#notify_progress broadcast API

[koic]

Motivation and Context

Server#notify_progress broadcasts progress notifications to all connected clients. Progress notifications are tied to a specific request's progressToken and have no meaningful broadcast use case. Neither the Python SDK nor the TypeScript SDK provides a server-level progress broadcast capability.

The MCP specification requires that progress notifications only reference tokens provided in an active request:

Progress notifications MUST only reference tokens that:

• Were provided in an active request
• Are associated with an in-progress operation

Ref: https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/progress

Broadcasting progress to all clients does not comply with this requirement, since clients that did not originate the request have no matching progressToken.

The introduction of ServerSession for per-connection state (#275) made this removal possible: Progress and ServerContext now use notification_target: (a ServerSession) to route notifications to the originating session only.

Changes

• Removed Server#notify_progress entirely. The method is no longer needed as notification_target in call_tool_with_args now passes session directly (nil when no session is available, in which case Progress#report and ServerContext#notify_log_message are no-ops).
• Removed ServerContext#notify_progress as well. Progress notifications should only be sent via server_context.report_progress, which enforces the correct progressToken from the originating request.
• Added nil guards to Progress#report and ServerContext#notify_log_message for when no session is available.
• Rewrote progress tests to use ServerSession#handle instead of Server#handle, reflecting that progress notifications are always session-scoped.
• Removed 4 tests that called server.notify_progress directly as a public broadcast API.
• Removed notify_progress from the README.md notification methods list.
• Removed the "Server-Side: Direct notify_progress Usage" section from the README.md.

Progress notifications should be sent via server_context.report_progress inside tool handlers, which automatically scopes them to the originating client session.

Breaking Changes

Progress notifications are scoped to a specific request via progressToken per the MCP specification, so the broadcast behavior of Server#notify_progress was a spec violation. This is treated as a bug fix and is made without a deprecation period. This aligns the Ruby SDK with the Python and TypeScript SDKs, neither of which provides a server-level progress broadcast API.

Server#notify_progress and ServerContext#notify_progress are no longer available. Users should use server_context.report_progress inside tool handlers instead, which provides session-scoped delivery with the correct progressToken.

This feature was only recently introduced in mcp 0.9.0: https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.0

So an early release should help limit its impact on adoption.

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