Support specifying custom mcp-session-id via McpClientOptions

[E1adi]

Hi team 👋

I'd like to propose adding support for specifying a desired mcp-session-id when initializing an McpClient. This could be exposed as an optional field in McpClientOptions.

Motivation

In some scenarios, it's useful to associate a client session with a pre-defined or externally managed session ID—for example, for debugging, tracing, or coordinating across services. Currently, the session ID appears to be auto-generated internally, which limits flexibility in these cases.

Suggested Change

Extend McpClientOptions to include an optional sessionId (or mcpSessionId) field. If provided, the client should use this value instead of generating a new one.

public sealed class McpClientOptions 
{
  // ...existing fields
  public string? SessionId { get; set; } // optional override for mcp-session-id
}

Benefits

• Enables better traceability across systems
• Facilitates integration with external session management tools
• Adds flexibility without breaking existing behavior

Let me know if this sounds reasonable or if there's a preferred way to approach it. Happy to help with a PR if needed!

Thanks 🙏

[mikekistler]

I like ths proposal. I think this fits well with the "logical session" notion described in this dicsussion on the spec repo. But I think it might be best to wait to see how SEP-1359 and SEP-1364 work out to make sure we don't get out of step with where the spec may be going.

[eiriktsarpalis]

In places where session id's are being used (i.e. in HTTP transports), the id's are being generated by the server and not by the client. As transports work today, I'm not sure how we could support such a client-side setting.

[mikekistler]

I see. I did not read carefully enough. I agree that we don't want clients making up session ids. However, I think we do want clients to be able to specify an "existing" session id for a request, possibly saved from a previous instance of the client.

[eiriktsarpalis]

Possibly, although I would imagine this would need to be defined in the context of a resumption API, which would need to be separate from McpClientFactory.CreateAsync.

[stephentoub]

@halter73, this was addressed by #1029, right?