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Attaching SandCastle to the CesiumJS MCP

 I've been playing wit ionosphere maps and Cesium's MCP controller this week. I wanted to try some quick experiments, but quickly realized I hadn't learned how to hooik connect the Cesium Sandcastle to the their MCP as a client. The method Codex Sol 5.6 and I finally devised and successfully used  is documented below the video.

Using that method, I was able to set up the following ionosphere fly-through as seen by a Starlink satellite. More on that soon.


Videos



Attach the Cesium MCP Camera Bridge to a Sandcastle Map

1. Confirm the camera MCP is running

The camera MCP browser bridge listens at:

ws://localhost:3002/mcp/ws

Check its health from PowerShell:

Invoke-RestMethod http://localhost:3002/mcp/health

Before attachment, the response should be healthy but may show "connected": false.

2. Open the map in the Sandcastle editor

If the map URL uses standalone.html, replace that portion with index.html while preserving the complete #c=... fragment: 

https://sandcastle.cesium.com/index.html#c=...

This opens the same shared map in the editable Sandcastle interface.

3. Expose the local viewer

Find the complete viewer construction statement in the Sandcastle source:

var viewer = new Cesium.Viewer("cesiumContainer", {
  // Viewer options...
});

Immediately after its closing });, add:

globalThis.cesiumViewer = viewer;

Do not insert this line inside the viewer options object. Click Run in Sandcastle.

4. Select the preview's JavaScript context

Sandcastle runs the map in a preview frame. DevTools can display log messages from that frame while its Console prompt still targets the parent page.

  1. Open Chrome DevTools and select Console.
  2. Open the execution-context dropdown, usually initially labeled top.
  3. Select the Sandcastle preview context. Depending on the Sandcastle version, it may be named bucket.html, Sandcastle, or something similar.
  4. Test the selected context:
typeof Cesium

The correct context returns:

"object"

Now verify the exposed viewer:

globalThis.cesiumViewer?.camera

It should return the Cesium camera object.

In a browser frame, globalThis.cesiumViewer and window.cesiumViewer refer to the same property. The insertion client expects window.cesiumViewer.

5. Insert the browser bridge

Open web-insertion-client.js from the repository, copy its entire contents, and paste it into the DevTools Console while the preview context remains selected.

Press Enter. The Console should report:

Cesium MCP bridge connected

Verify the WebSocket:

window.cesiumMcpBridge?.readyState

A value of 1 means the bridge is connected.

The MCP health response should now show:

{
  "connected": true
}

Disconnect or reconnect

Disconnect the browser bridge with:

window.cesiumMcpBridge?.close()

Re-running web-insertion-client.js closes an existing bridge before creating a new one.

The camera MCP accepts only one browser bridge at a time. Close any bridge attached to another Cesium tab before connecting this one.

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