SseClientTransport compatibility issue with non-compliant MCP SSE services

[loocor]

Describe the bug
The SseClientTransport implementation in the rmcp SDK has compatibility issues that prevent connections to SSE services that don't fully comply with the MCP specification (such as modelscope.cn). Specifically, the SseClientTransport::start_with_client function, after establishing an SSE connection, mandatorily waits for an event named "endpoint". Some services may not send this event or use a different format, causing the connection to never complete.

To Reproduce
Steps to reproduce:

1. Use rmcp SDK's SseClientTransport to connect to modelscope.cn's SSE service
2. Initial HTTP GET request succeeds (returns 200 OK)
3. SseClientTransport waits for the "endpoint" event
4. Since modelscope.cn doesn't send an "endpoint" event as expected, the connection never completes
5. The connection eventually times out or fails

Code example:

let connection_result = SseClientTransport::start("https://mcp.api-inference.modelscope.cn/sse/{id}").await;
// Connection fails because no "endpoint" event is received

Expected behavior
SseClientTransport should be able to adapt to different SSE service implementations, including those that don't fully comply with the MCP specification. Ideally, it should:

1. Provide an option to skip waiting for the "endpoint" event
2. Or use a default endpoint path (like "/messages") after timing out waiting for the "endpoint" event
3. Or implement an automatic fallback mechanism that tries different connection strategies

Logs

2025-05-18T06:35:29.051798Z  WARN mcpmate::http::pool::connection: Failed to trigger connection to server 'fetch': Failed to connect to server: Client error: HTTP status client error (404 Not Found) for url (https://mcp.api-inference.modelscope.cn/sse/{id}/messages/?session_id={session_id})
2025-05-18T06:35:44.587829Z  WARN mcpmate::http::pool::connection: Failed to trigger connection to server 'thinking': Failed to connect to server: Client error: HTTP status client error (404 Not Found) for url (https://mcp.api-inference.modelscope.cn/sse/{id}/messages/?session_id={session_id})

Additional context

1. Through code analysis, we found that the issue is in the rmcp SDK's SseClientTransport::start_with_client function (sdk/crates/rmcp/src/transport/sse_client.rs lines 121-132):

let mut sse_stream = client.get_stream(config.uri.clone(), None, None).await?;
// wait the endpoint event
let endpoint = loop {
    let sse = sse_stream
        .next()
        .await
        .ok_or(SseTransportError::UnexpectedEndOfStream)??;
    let Some("endpoint") = sse.event.as_deref() else {
        continue;
    };
    break sse.data.unwrap_or_default();
};

2. This code mandatorily waits for an event named "endpoint", but some services (like modelscope.cn) may not send this event or use a different format.

3. We tried various solutions, including modifying the URL format and implementing an automatic switching mechanism, but due to limitations in the rmcp SDK's internal implementation, these methods couldn't fully resolve the issue.

4. Suggested solutions:

• Modify the rmcp SDK to add an option to skip waiting for the "endpoint" event
• Or create a custom SseClientTransport implementation specifically for non-standard services
• Or implement a more flexible fallback mechanism in SseClientTransport

[4t145]

Accroding to spec, the server MUST send endpoint event. So you should also ask them to follow the standard.

For rcmp side, we can add a use_endpoint option for SseClientConfig, and skip get endpoint when config.use_endpoint.is_some(), is this looks good to you?

But I also want to point out that it's impossible to maintain this crate if every user of non-standart transport going to ask us to support their case. There is an interface to implement customized by using trait Transport.

[4t145]

You can not say it's a bug and add a bug label, so I removed it.

[loocor]

Accroding to spec, the server MUST send endpoint event. So you should also ask them to follow the standard.

For rcmp side, we can add a use_endpoint option for SseClientConfig, and skip get endpoint when config.use_endpoint.is_some(), is this looks good to you?

Thanks for the swift response! The use_endpoint option for SseClientConfig sounds like a solid plan.

Totally get that servers should send the endpoint event per spec. However, in practice, we're seeing quite a few SSE implementations, particularly older ones, that predate full MCP adoption or don't strictly adhere to it. Given SSE is often a stepping stone, there might be inertia against retrofitting these older services for full compliance.

This flexibility in the SDK would be a big help for integrating with these real-world services and definitely broadens its applicability.

[4t145]

Solved in #187

[loocor]

Solved in #187

Unfortunately, after testing, I found that this patch doesn't resolve the issue: even if we skip waiting for the "endpoint" event from https://mcp.api-inference.modelscope.cn/sse/{id}, there's still a URL mismatch (it becomes something like https://mcp.api-inference.modelscope.cn/sse/{id}/), which results in a 404 error.

[4t145]

It looks like a bug introduced by latest change

[4t145]

@loocor just merged #197

now you are supposed to set use_message_endpoint to Some("https://mcp.api-inference.modelscope.cn/sse/{id}" or Some("sse/{id}")

[loocor]

@loocor just merged #197

now you are supposed to set use_message_endpoint to Some("https://mcp.api-inference.modelscope.cn/sse/{id}" or Some("sse/{id}")

The new fix works well 👍
This issue can be closed now.