MCP is not concatenating JSON to the context. MCP is providing JSON to the toolchain; except for the names of tools and resources, most of the information in MCP doesn't go to the model at all, the toolchain uses it to connect to tool (resource, etc.) providers and from there it gets information that it can use either in the context for the LLM or in the UI for the user, and the shape that information goes into the context for the model depends on the model and has nothing to do with MCP.

MCP, is just a way for the toolchain to get information about and communicate with external services, the model doesn't (and if this sounds like the title of the article, there is a reason) need to know about it.