Use of Instruction

[sonomirco]

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• Protocol Specification
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• Server Implementation
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Your Question

Quick question: Is instruction as an option from the server the correct way to pass a system prompt to the client, including all the necessary information for the tools to function?
Which current clients are able to consume this information?

Right now I am using a prompt stored as a resource that can be passed to the client, but from what I read it seems instruction is the way to go, but I don't think it is supported.

[PederHP]

VS Code is one client that supports this. I think fast-agents does too. In general, you cannot guarantee that a client will use it, and when it does, it might not place it as a system message, for security reasons. Don't think of it as passing a system prompt to the client, think of it as providing context for how to use the server (could be more than tools, could also be about resources, sampling, elicitation). How the client makes use of that context is implementation-specific.

If you control the client - which is sometimes the case when MCP is used as architectural glue, you can use a resource as system prompt. I have used this technique on several projects. Where a 1st party MCP client host fetches the system message from a well-known resource on a 1st party MCP server.

If you want an arbitrary client to use a system prompt provided by you that's generally not supported (as instructions are likely to be appended and even bounded instruction-wise to the extent possible) by MCP - which makes sense right - if you're using Copilot or Claude.ai you don't want every single MCP server you use to have the power to override your system prompt.

[sonomirco]

Thanks @PederHP
I started using resources as a way to 'inject' into an arbitrary client; in my case, Claude codes the context on how to use the server. In any case, it will be a handy method, as you suggested, to pass the information to a control client or to easily send the prompt to the instruction and consume it from there.