Clarification needed: Distinction between "hosts" and "clients" in MCP terminology

[privetin]

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• Protocol Specification
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Your Question

I've noticed inconsistent terminology in the MCP documentation regarding how applications like Claude Desktop are referred to:

In clients.mdx, these applications are listed as "clients" in the feature support matrix
In architecture.mdx and introduction documentation, they are referred to as "hosts" that contain MCP clients
In various discussions (e.g., #64 Authentication, #102 Statelessness), both terms are used somewhat interchangeably
Could someone please clarify:

What is the distinction between hosts and clients in MCP?
When should we use each term?
Is Claude Desktop (and similar applications) correctly referred to as a client or a host?
This clarity would help ensure consistent terminology usage across documentation and discussions.

[tadasant]

I've seen @dsp-ant address this in some community interactions:

I think in practice the distinction is rather technical. In the end the host is an application that deals with one or more MCP Servers. It uses clients for it, whereas a client is a connection to an MCP server, and is literally what a stdio_client would be in the Python SDK. It's really just at technicallity, in the end, it's the same application and so people often use it interchangeable to refer to the host application. That's probably fine

Hence we use it as such only in the specs

That was a response to this question by someone else:

terminology question for a saturday evening. the https://spec.modelcontextprotocol.io/specification/2024-11-05/architecture/ page really clearly lays out the difference between Host, Client and Server - but i feel that everywhere else the terms Host and Client are being used interchangeably (which in turn blurs what people think each part is responsible for)... the distinction between Host and Client are cricitcal right? but a lot of the docs seems to be using the word Client when it should be Host...

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More concretely, to answer your questions:

What is the distinction between hosts and clients in MCP?

The host is an application that deals with one or more MCP servers.

When should we use each term?

If you want to be technically precise, host is equivalent to app, and client is the low-level detail that is holding the connection to the MCP server.

If you're talking in a context where the distinction is irrelevant (i.e. just about anywhere but the spec), "client" can be used to cover both.

Is Claude Desktop (and similar applications) correctly referred to as a client or a host?

Precisely speaking, they are hosts. But they can be referred to as clients when the distinction doesn't matter.

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My personal opinion: it's probably not a priority issue to address even if it is worth a change, but I do think it'd be clearer to be consistent across contexts and discussions here. I might go further and suggest that the term "app" be used in more public contexts when referring to standalone GUI's, and then "host" and "client" can be framework or spec-level details when precise language is helpful. Introducing "app" could be used for cases where "it doesn't matter" + is a term less technical people can easily grok.