Inconsistency in System Role support: Sampling vs. Prompts

[vinsguru]

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Question Category

• Protocol Specification
• SDK Usage
• Server Implementation
• General Implementation
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Your Question

Hi MCP Team,

The Observation:

• In the Sampling spec there is a dedicated top-level systemPrompt field. This allows the server to explicitly define the persona/rules for the LLM.
• In the Prompts spec (prompts/get), the response returns a list of objects, but the role is strictly limited to user or assistant.

The Problem:

When a server exposes a complex workflow via a Prompt, it often needs to set a specific Instruction Manual or Persona (e.g., "Act as a technical validator"). Since there is no system role in the PromptMessage, we are forced to:

• Prepend instructions as a user message (which can be overridden or ignored more easily by the model).
• Prepend instructions as an assistant message (mimicking memory).

Neither of these is as robust as the system role supported by most modern LLM APIs and the MCP Sampling spec itself.

Questions:

• Is this omission intentional? If so, what is the recommended "best practice" for a server to provide system-level instructions within a Prompt template without polluting the user/assistant message history?
• If it is not intentional, are there plans to align roles with the sampling/createMessage request structure to include a system role or a systemPrompt field?