Option to send additional data (in addition to params) when sending request/message/notification

[E1adi]

Feature Request: Support for Custom Parameters in MCP Client Requests

Summary

Enable the inclusion of arbitrary custom parameters in MCP client requests, beyond those required by the invoked action.

Use Case

In many scenarios, it's useful to pass additional contextual data—such as tenant IDs, trace tokens, feature flags, or other metadata—alongside the standard payload. These parameters can enhance server-side processing, logging, routing, and decision-making.

Proposal

Introduce a mechanism to attach custom key-value pairs to requests. These should be transmitted to the server and accessible during request handling.
Benefits

• 🧩 Adds flexibility for diverse service architectures
• 📈 Improves observability and traceability
• 🔄 Future-proofs the protocol for evolving need

[PederHP]

It is possible to do this currently by using the low-level interfaces and handlers. I can see how adding convenience to make this easier would be useful.

One thing to be aware of is that many servers and clients do a strict validation of payload schemas and simply fail if you provide additional fields. However, using the meta field is allowed by these. So, if anything, it might be best to make using the meta fields more convenient.

Example:

csharp-sdk/src/ModelContextProtocol.Core/Protocol/RequestParams.cs

Line 26 in 70960e1

public JsonObject? Meta { get; set; }

[E1adi]

@PederHP, appreciate the response.
I did notice the Meta property in the RequestParams type, but currently, some actions in the public interface don’t accept a RequestParams object - so there’s no way to pass Meta parameters down to the low-level interface.
While I agree that using the low-level interface is technically an option, it introduces a fair bit of complexity for something that, in my opinion, would be relatively simple to implement and could offer significant value

[PederHP]

@PederHP, appreciate the response.
I did notice the Meta property in the RequestParams type, but currently, some actions in the public interface don’t accept a RequestParams object - so there’s no way to pass Meta parameters down to the low-level interface.
While I agree that using the low-level interface is technically an option, it introduces a fair bit of complexity for something that, in my opinion, would be relatively simple to implement and could offer significant value

That's a good point, and a WithMetadata method could be an option.