feat: Plugin dependency/companion MCP server declaration in plugin.json

[ericchansen]

Problem

When building a Copilot CLI plugin, there's no way to declare dependencies on external MCP servers that the plugin's skills need. A plugin can register its own MCP servers via .mcp.json, but it can't express "this plugin works best when WorkIQ / Fabric Power BI / Azure DevOps MCP servers are also available."

This creates a fragmented experience where users install a plugin but then need to manually configure companion MCP servers in ~/.copilot/mcp-config.json to get the full value.

Context

I maintain MSX-MCP, a plugin with 29 MCP tools and 15 skills for Microsoft Sales data. Several of our skills would benefit from M365 data (emails, meetings, Teams messages) via the WorkIQ MCP server, and we already bundle the Fabric Power BI MCP server in our .mcp.json.

A related project (MCAPS-IQ) solves this by putting everything in .vscode/mcp.json -- but that's a VS Code workspace config, not a portable plugin. Users who want the same experience in Copilot CLI have to manually configure each companion server.

Current state

What plugins CAN do:

• Register their own MCP servers in .mcp.json (stdio or http)
• Bundle skills and agents

What plugins CANNOT do:

• Declare optional/required companion MCP servers (e.g., "also install WorkIQ")
• Declare dependencies on other plugins
• Suggest user-level mcp-config.json additions at install time

Additional complexity: config format divergence

The CLI and VS Code use different MCP config formats, making it harder to share configurations:

Surface	File	Root Key	Type Values
Copilot CLI (user-level)	~/.copilot/mcp-config.json	mcpServers	local, http
Copilot CLI (plugin)	.mcp.json	mcpServers	stdio, http
VS Code	.vscode/mcp.json	servers	stdio, http

A plugin author can't provide one config that works for both CLI and VS Code.

Possible solutions

1. companionServers in plugin.json -- optional array of MCP server configs that get merged into the user's mcp-config.json at copilot plugin install time (with user confirmation)

2. dependencies in plugin.json -- declare other plugins or npm packages that should be installed alongside (similar to npm peerDependencies)

3. Post-install hook -- let plugins run a setup script that can suggest or configure companion servers

4. Unified MCP config format -- converge CLI and VS Code on a single schema so configurations are portable

Impact

This affects any plugin that needs to compose multiple MCP servers into a coherent experience -- not just MSX tools. AI agent plugins that combine domain-specific servers (CRM, analytics) with general-purpose ones (calendar, email, file storage) all face this packaging gap.