Add ability to control and test resource subscription in MCP Inspector

[olaservo]

I copied this from this discussion for more visibility in the repo: https://github.com/orgs/modelcontextprotocol/discussions/187

Feature Request

Add functionality to MCP Inspector for subscribing to a resource.

Exposing a Dynamic Resource

I have an SSE-based MCP that exposes a dynamic resource - finite state machines (called puzzles).

From the MCP Inspector, I can list the resources, and I can use the resource template to enter a puzzleId and get a snapshot of the puzzle. So far so good.

Subscribing to Changes

I want to allow clients to subscribe to to a given puzzle so that they can be updated with a server-sent event when its state changes.

Server-side

Examples are clear.

The mcp-servers/everything example shows how to accept subscribe and unsubscribe requests.
It simulates changes happening and being reported with a setInterval that sends notifications for all the dummy resources ever so often.

Client-side

There is no affordance in the the reference client - the MCP Inspector - for subscribing to a resource.

When a resource is selected, there is a refresh button. It seems like an obvious place for a companion toggle button: Subscribe | Unsubscribe. It would send a subscribe or unsubscribe request to the server.

[cliffhall]

Since I was the one who complained, I figured I'd have a look at fixing it.

Given that the rest of the inspector seems quite complete, I wonder if there was a reason that subscribing to resources was left out of the initial implementation. Could have something to do with the way the inspector connects with the MCP via a proxy server?

@ashwin-ant and @jspahrsummers : I notice that you guys seem to have done the bulk of the work on the Resources tab, where I expect the subscribe to resource feature should go. Is there anything you remember about why this was left out?

[cliffhall]

STATUS UPDATE

Subscribe is working!

• I added the button, sent the subscribe request, handled it on my server.
• Below, I use the perform_action_on_puzzle tool to change the state of a puzzle.
• Immediately after running the tool, the notification appears in the Server Notifications panel below.

Remaining work

• I need to manage a local set of subscribed uris, so that the UI can display a toggle button, being informed by whether the selected resource is in that set.

[cliffhall]

This has now been addressed with #170