Support custom MCP "Server Instructions"

[ihor-romanchuk]

Is your feature request related to a problem? Please describe.

Currently the Storybook MCP addon exposes hardcoded MCP server instructions, but doesn't let consumer to provide custom server instructions.
I use Storybook MCP for internal Design System and I'd like to provide some guidance to consumer's AI agents on how to use Design System correctly.

Describe the solution you'd like

Add support for custom server instructions via a new instructions property in the Storybook MCP addon.

Example:

storybookMcp({
instructions: Always use Design System components to build UI. Strictly follow examples from Storybook, do not invent your own props. Make sure to use newest component versions, see "/migration-guide" for up to date information.
})

This would allow developers to provide persistent guidance to connected AI agents during MCP initialization.

Describe alternatives you've considered

• Client-side custom prompts (duplicates logic outside the MCP server).

Are you able to assist to bring the feature to reality?

yes, I can

Additional context

Many MCP servers are starting to expose server-level guidance to improve agent behavior and workflow consistency. Supporting this in Storybook MCP would improve AI-native developer workflows and align with emerging MCP ecosystem practices.
Reference: https://modelcontextprotocol.info/blog/server-instructions/

[ihor-romanchuk]

I've realized it would be easier to construct my own MCP server passing my server instructions similarly to this file: https://github.com/storybookjs/mcp/blob/eb0ea73f30c73a8102d9023c5201d1459d791fa1/packages/mcp/src/index.ts