Support install action via standard registry specification #1590

Is your feature request related to a problem? Please describe.
Currently installing MCP servers is cumbersome and requires technical knowledge. Users must work with JSON, and the experience can vary between clients as described in #292.

This results in clients maintaining their own registries, concentrating powers in a few selected servers. We need a standard and simple installation procedure to promote adoption from less technical users. Users should be able to click on a link to install/activate the server.

Describe the solution you'd like
The registry's server.json standard solves half of the problem. To fully resolve the issue, MCP clients should support a standardized installation protocol in the following way

• MCP client declares and supports an /install?server=BASE64_ENCODED_SERVER_SPEC action
• client decodes the server spec and selects compatible transport
• triggers authentication or request required env variables from standard input prompts
• when client already installed the server, the action should simply open/activate the server

This should work for both desktop and web apps as installation links can be crafted in the following manner

• Desktop app: my-mcpclient://install?server=BASE64_ENCODED_SERVER_SPEC
• Web app: https://www.my-mcpclient.com/install?server=BASE64_ENCODED_SERVER_SPEC

It's important to note that BASE64_ENCODED_SERVER_SPEC is NOT the installed server schema, even if a dominant standard emerged. This is the server specification that lists all supported transports and required env variables. This allows clients to pick the most compatible mode to install. More importantly, it allows the link to be publicly sharable (encoded server info with env vars is NOT safe to share publicly)

Optionally, the install action can support a server registry param, e.g. /install?registry-server=https://registry.modelcontextprotocol.io/v0/servers/{server_id}. This approach allows the link to be more compact but depends on centralized registries. The BASE64_ENCODED_SERVER_SPEC is more portable and works for private servers.

Describe alternatives you've considered
I considered passing the encoded installed server json, but this approach suffers from a couple downsides

• links cannot be publicly shared
• requires a standard across all clients

Additional context
N/A