Add `logging` support

[dak2]

Motivation and Context

A server can send structured logging messages to the client. https://modelcontextprotocol.io/specification/2025-06-18/server/utilities/logging#logging

Logging was specified in the 2024-11-05 specification, but since it was not supported in ruby-sdk, I implemented it. https://modelcontextprotocol.io/specification/2024-11-05/server/utilities/logging

I also made it possible to output a simple notification message in the examples.

How Has This Been Tested?

• [x] Check existing and added test cases for this case
• [x] Check a sse server of examples can send logging message to the client, for more details, see below

Server

Press Ctrl+C to stop the server
Puma starting in single mode...
* Puma version: 6.6.0 ("Return to Forever")
* Ruby version: ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [arm64-darwin24]
*  Min threads: 0
*  Max threads: 5
*  Environment: development
*          PID: 67186
* Listening on http://127.0.0.1:9393
* Listening on http://[::1]:9393
Use Ctrl-C to stop
[MCP] Request: initialize (id: init-1)
[SSE] INFO 17:44:56.927 - New client initializing session
[MCP] Response: success (id: init-1)
[SSE] INFO 17:44:56.927 - Session created: 4821e46a-b75e-4469-8e69-8b4eaa4554de
::1 - - [02/Nov/2025:17:44:56 +0900] "POST /mcp HTTP/1.1" 200 - 0.0002
[MCP] Request: logging/setLevel (id: 18042833-4f74-4008-beaf-2795fd9bc4b4)
[MCP] Response: success (id: 18042833-4f74-4008-beaf-2795fd9bc4b4)
::1 - - [02/Nov/2025:17:44:56 +0900] "POST /mcp HTTP/1.1" 200 - 0.0001
[SSE] INFO 17:44:56.929 - SSE connection request for session: 4821e46a-b75e-4469-8e69-8b4eaa4554de
[SSE] INFO 17:44:56.929 - SSE stream established
::1 - - [02/Nov/2025:17:44:56 +0900] "GET /mcp HTTP/1.1" 200 - 0.0001
[MCP] Request: tools/call (id: de0de93c-b33d-4747-bb89-1157418bb06d)
[SSE] INFO 17:45:16.081 - Returning notification message: logging support
[SSE] INFO 17:45:16.082 - Response sent via SSE stream
::1 - - [02/Nov/2025:17:45:16 +0900] "POST /mcp HTTP/1.1" 200 - 0.0010

Client

 % ruby examples/streamable_http_client.rb
=== MCP SSE Test Client ===
[CLIENT] INFO 17:44:56.925 - Initializing session...
[CLIENT] INFO 17:44:56.928 - Session initialized: 4821e46a-b75e-4469-8e69-8b4eaa4554de
[CLIENT] INFO 17:44:56.928 - Server info: {"name" => "sse_test_server", "version" => "0.1.0"}
[CLIENT] INFO 17:44:56.928 - Connecting to SSE stream...
[CLIENT] INFO 17:44:56.929 - SSE stream connected successfully

=== Available Actions ===
1. Send custom notification
2. Test echo
3. List tools
0. Exit

Choose an action: 
1
Enter notification message: logging support
Enter delay in seconds (0 for immediate): 
[CLIENT] INFO 17:45:16.082 - SSE data: {"jsonrpc":"2.0","id":"de0de93c-b33d-4747-bb89-1157418bb06d","result":{"content":[{"type":"text","text":"Notification: logging support (timestamp: 2025-11-02T17:45:16+09:00)"}],"isError":false}}
[CLIENT] INFO 17:45:16.082 - SSE data: {"jsonrpc":"2.0","method":"notifications/message","params":{"data":{"details":"Response accepted and sent via SSE"},"level":"info"}}
[CLIENT] INFO 17:45:16.082 - Notification sent successfully

=== Available Actions ===
1. Send custom notification
2. Test echo
3. List tools
0. Exit

Choose an action: 

Breaking Changes

None

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
• [x] I have added or updated documentation as needed

Additional context

[koic]

@dak2 Oops, can you rebase with the latest master to resolve the conflicts?

[dak2]

@koic Thank you for letting me know! The conflict has been resolved. Could you please review it again?

[koic]

Oops, sorry if my understanding of the specification and this implementation is different. As I understand it, with logging/setLevel, only messages at the specified level or higher should be sent from the server to the client as notifications/message. In other words, if the level error is specified, messages at info or warning level should not be notified. I believe "Only sends error level and above" in the "Message Flow" section corresponds to this. https://modelcontextprotocol.io/specification/2025-06-18/server/utilities/logging#message-flow

It seems to me that the current PR is not implemented in this way. Could you check?

[dak2]

@koic Thank you for checking it!

Oops, I missed it! That's true. This implementation doesn't meet a logging specification. I'll implement it soon when I have free time.

[dak2]

@koic

I implemented the logic to determine whether or not to send a notification based on level. Could you please review it again?

https://github.com/modelcontextprotocol/ruby-sdk/compare/ef5049bab808dec719765a771c04e7c897c2a8a3..c9227469f523c930ab98b49567abf4dbd7213abe

If the notification level does not match, it returns nil. According to the specification, there are no restrictions on notifications when the level does not match, and the java-sdk and go-sdk implementations appear to be the same.

[koic]

User-facing documentation should be included in the README.

[dak2]

@koic

User-facing documentation should be included in the README.

I wrote how to use notifications/message in the README.md, but I would like feedback on whether the text is easy to understand.

[koic]

@dak2 FYI, the json_rpc_handler was merged into this ruby-sdk repository in #175.

[dak2]

@koic

Thanks for pointing that out!! I've made a few commits.