streamable HTTP client: Don't block on standalone SSE Get #634
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When connecting to an MCP server that supports making SSE GET requests client.Connect gets hung up waiting for the first notification from the server. This change modifies the tests to simulate a server that keeps the stand alone SSE connection open. It then adjusts the connection setup to run the standalone SSE connection in the background.
This was referenced Dec 18, 2025
jba
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Jan 11, 2026
mcp: add DisableStandaloneSSE option to control standalone SSE stream Currently, StreamableClientTransport always establishes a standalone SSE connection after initialization to receive server-initiated messages. While this is useful for receiving notifications like ToolListChangedNotification, it causes problems in several real-world scenarios: 1. **Unnecessary resource usage**: Many use cases (for me), especially in scientific computing scenarios, only require simple request-response communication. Clients don't need server-initiated notifications, making the persistent SSE connection wasteful. 2. **Server compatibility issues**: Some third-party MCP servers don't properly handle GET requests for SSE streams. When the client automatically tries to establish the SSE connection, it fails or hangs, blocking the entire connection process. This is particularly problematic when using third-party servers that cannot be modified. This issue is similar to what was discussed in issue #634. 3. **Lack of user control**: The MCP specification states that the standalone SSE stream is optional ("The client MAY issue an HTTP GET"), but the current SDK implementation doesn't provide a way to opt out. This implementation is also inconsistent with other MCP SDKs like github.com/mark3labs/mcp-go, which provides getListeningEnabled control (defaults to false, requires explicit enablement). Fixes #728 Co-authored-by: zhaoxin <zhaoxin@pjlab.org.cn>
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Problem
When connecting to some MCP servers that supports making SSE GET requests, client.Connect gets hung up waiting for the first notification from the server.
More details in issue #633
Solution
This change modifies the tests to simulate a server that keeps the stand alone SSE connection open. It then adjusts the connection setup to run the standalone SSE connection in the background.
Open question
Work was done 2 weeks ago in #604 / #583 to make some errors from starting the stand alone SSE connection be returned as part of client.Connect. I'm not clear on why that is necessary. Clients are not required to accept SSE messages from servers. Stateless clients will want to opt out of even attempting to create the standalone SSE connection.
This PR undoes some of it so may not be the correct solution.
I'll clean up the tests once I know more.