Support a non-SSE JSON-only mode similar to the TypeScript SDKs `enableJsonResponse: true`

[halter73]

The server needs to be configured to support JSON mode (or not). For example in the TypeScript SDK there is enableJsonResponse: true on StreamableHTTPServerTransport. At the same time, the client also has configuration for what it prefers, that's what TransportMode on SseClientTransport in the .NET SDK is for. I have no idea about Python but if the SDK is public, you can search through the source code for similarly named symbols.

Originally posted by @TomasHubelbauer in #449

[TomasHubelbauer]

Thank you for creating an issue for this 🙏

[peter-sidebotham]

Missing support/samples for streamable http with json appears to be a barrier to using MCP services with Azure networking WAF products which lack support for SSE. This includes Azure Frontdoor Service and Application Gateway (although it looks like Application Gateway may have just introduced preview support for it).

[PederHP]

I also think there could be value to changing the surface of the client SDK to align more closely with the TypeScript SDK. Having the Streamable HTTP Transport be hidden behind a mode of the SSE Transport is something I think will confuse many developers coming from other SDKs. The auto-detect convenience is extremely useful, as it's boilerplate everyone needs to write otherwise. But wouldn't it make sense to just make all three transports public and remove mode altogether? That aligns with the surface of the official SDKs more closely - where streamable HTTP is the first-class / entrypoint HTTP transport and not SSE.

Should I make a PR with a suggested refactor and some client samples?

[TomasHubelbauer]

I agree with exposing the streamable HTTP transport. I am one of the people this has confused as I've prototyped my MCP feature in TypeScript using Bun first and then attempted to migrate it over to .NET only to find the SDKs do not align. At first I assumed the streamable HTTP transport wasn't supported by the .NET SDK until I came looking through the source code. I left a comment about this on #356 where streamable HTTP was introduced and reiterating here in support of making this change.