Fix SSE transport not properly handling HTTP/2 NO_ERROR disconnections

[okoshi-f]

Description

The current SSE transport implementation does not properly handle HTTP/2 connection terminations that occur due to idle timeouts as specified in RFC 9113. When a connection is closed with the HTTP/2 error code NO_ERROR, the client cannot detect that the connection has been lost, as it treats this as a regular error and silently fails without notifying the application. This causes the transport to become non-functional after long periods of inactivity, with no way for the application to detect and recover from these legitimate connection closures.

If a connection containing the error message NO_ERROR is closed, change it to call the connection disconnect handler so that you can detect the connection disconnect. Developers using Client can then decide what to do and implement the necessary measures.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• MCP spec compatibility implementation
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring (no functional changes)
• Performance improvement
• Tests only (no functional changes)
• Other (please describe):

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly

MCP Spec Compliance

• This PR implements a feature defined in the MCP specification
• Link to relevant spec section: Link text
• Implementation follows the specification exactly

Additional Information

Summary by CodeRabbit

Summary by CodeRabbit

• New Features
  • Added the ability to set a handler for connection loss events when using SSE transport, allowing users to respond to HTTP/2 idle timeout disconnections.
• Documentation
  • Updated the SDK documentation to describe how to handle SSE connection loss and implement reconnection logic.
• Tests
  • Introduced new tests to verify correct handling of SSE connection loss events and ensure backward compatibility.

[coderabbitai]

Walkthrough

The changes introduce connection lost detection and handling for the SSE (Server-Sent Events) transport in the MCP-Go SDK. A handler for connection loss events can now be registered, specifically to address HTTP/2 idle timeout disconnections. The documentation and tests are updated to reflect and validate this new behavior.

Changes

Files/Paths	Change Summary
README.md	Updated documentation to mention the new SSE connection lost handler and its use for HTTP/2 idle timeout disconnections.
client/client.go	Added OnConnectionLost method to the Client struct, enabling registration of a connection lost handler if the transport supports it.
client/transport/sse.go	Added onConnectionLost callback and mutex to SSE struct; introduced SetConnectionLostHandler method; updated readSSE to invoke the handler on "NO_ERROR" (HTTP/2 idle timeout) errors.
client/transport/sse_test.go	Added mockReaderWithError for simulating errors; introduced multiple subtests in TestSSE to verify correct invocation of the connection lost handler under various error conditions, especially for "NO_ERROR" scenarios.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Suggested labels

type: enhancement, area: mcp spec

Suggested reviewers

• rwjblue-glean
• ezynda3

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📥 Commits

Reviewing files that changed from the base of the PR and between 24f4205 and eeee551.

📒 Files selected for processing (3)

• client/client.go (1 hunks)
• client/transport/sse.go (3 hunks)
• client/transport/sse_test.go (3 hunks)

🚧 Files skipped from review as they are similar to previous changes (3)

• client/client.go
• client/transport/sse.go
• client/transport/sse_test.go

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