mcp-observable-notebook-kit-debug

MCP server for debugging Observable Notebook Kit notebooks.

Enables AI assistants to inspect values, view errors, and capture canvas output from notebooks running in a web browser. For a fully working example, see ./example.

Why?

The Observable Desktop is really cool, but it had some limitations. For one, okay, so I paid Anthropic some money for Claude Code, but it's not the right Anthropic integration to be able to configure an API key in the app, so I'm out of luck. And I was trying to get some WebGPU experiments running, but the app didn't have access to a WebGPU context. So one thing led to another, and I wrote a quick MCP server to exfiltrate values from notebooks running in the browser.

Setup

1. Install the package

npm install @rreusser/mcp-observable-notebook-kit-debug

2. Add the Vite plugin

In your vite.config.js:

import { defineConfig } from "vite";
import { observable, config } from "@observablehq/notebook-kit/vite";
import { debugNotebook } from "@rreusser/mcp-observable-notebook-kit-debug";

export default defineConfig({
  ...config(),
  plugins: [debugNotebook(), observable()],
});

3. Run the dev server

vite -c vite.config.js

4. Configure the MCP server

Add to your .mcp.json:

{
  "mcpServers": {
    "Notebook": {
      "command": "mcp-notebook-kit-debug",
      "args": []
    }
  }
}

Note that you might need an absolute path to the mcp-notebook-kit-debug bin, and you might be using opencode or some other tool, so this step could vary a bit.

5. Go!

You can now use an agent like Claude Code to poke and prod at notebooks running in a web browser, inspecting values and even capturing canvas output as images.

MCP Tools

Observable Runtime Tools

These tools interact directly with the Observable runtime's reactive graph.

Tool	Description
ListValues	List all named values in the Observable runtime's reactive graph
GetValue	Get a value from the runtime by name; returns images for Canvas/SVG elements
GetValues	Get multiple values at once (snapshot of runtime state)
GetValueMetadata	Get metadata: state, type, dependencies (inputs), and dependents (outputs)
GetDependencyGraph	Get the dependency graph showing how values depend on each other
SetInputValue	Set an input widget's .value property and trigger reactive updates
RuntimeEval	Evaluate an expression with access to all notebook variables

Browser Tools

These tools interact with the browser/DOM context, outside the Observable runtime.

Tool	Description
BrowserEval	Execute JavaScript in the browser (has DOM access but NOT Observable runtime)
GetElementContent	Get content from a DOM element by CSS selector; captures canvas/SVG as images
MouseClick	Simulate a mouse click at a position or on an element
MouseDrag	Simulate a mouse drag from start to end position
MouseHover	Simulate mouse hover at a position, triggering hover states and tooltips
MouseWheel	Simulate a mouse wheel scroll at a position
SendKeys	Simulate keyboard input to an element

Session Tools

These tools manage notebook connections and debug sessions.

Tool	Description
ListNotebooks	List all connected notebooks (use when multiple notebooks are open)
FocusNotebook	Set a default notebook for subsequent commands (when multiple are connected)
Refresh	Refresh the page and wait for notebook initialization
Navigate	Navigate a notebook to a different URL (e.g., switch from notebook-a to notebook-b)
GetConsoleMessages	View console logs from the current session
GetErrors	Get all errors (DOM-reported and values in rejected state)

Multi-Notebook Support

When multiple notebooks are open in different browser tabs, you can target a specific notebook using the notebook parameter on any tool:

# By path (without .html extension)
notebook: "index"
notebook: "second-notebook"

# By index
notebook: "0"
notebook: "1"

# By URL
notebook: "http://localhost:5173/"

If multiple notebooks are connected and you don't specify which one, the tool will return an error listing the available notebooks.

Use ListNotebooks to see all connected notebooks with their URLs and indices.

Navigating Between Notebooks

The Navigate tool allows you to navigate an open notebook to a different URL, or open a URL in your default browser if no notebooks are connected:

// If no notebooks are connected, opens URL in default browser
Navigate({ url: "http://localhost:5173/index.html" })

// Navigate to a different notebook (relative URL)
Navigate({ url: "/second-notebook.html" })

// Navigate using absolute URL
Navigate({ url: "http://localhost:5173/second-notebook.html" })

// Navigate a specific notebook (when multiple are open)
Navigate({ url: "/second-notebook.html", notebook: "index" })

// Skip waiting for page load (useful if target page doesn't have debug plugin)
Navigate({ url: "/some-page.html", wait_for_completion: false })

Behavior:

• No notebooks connected: Opens the URL in your OS default browser
• One notebook connected: Navigates that notebook's browser window to the new URL
• Multiple notebooks connected:
  • Navigates the focused notebook (if you've used FocusNotebook)
  • Navigates the specified notebook (if notebook parameter provided)
  • Returns an error listing all notebooks (if no focus is set and no notebook specified)

The tool waits for the new page to load and initialize, similar to Refresh, and reports any errors that occur during initialization.

Value States

Values in an Observable notebook can be in one of three states:

• fulfilled: The value has been computed successfully
• pending: The value is still being computed (e.g., async/Promise)
• rejected: The computation threw an error

The GetValue and GetValues tools return state information along with the value or error.

Image Support

GetValue automatically returns images inline for:

• Canvas elements: Captured as PNG
• SVG elements: Rendered to canvas and captured as PNG

No need to save to files - images are returned directly in the MCP response.

Runtime Evaluation

Use RuntimeEval to evaluate expressions in the Observable runtime context with access to all notebook variables. The body must use a return statement.

// Compute derived values from notebook variables
RuntimeEval({ body: "return number * 2 + rangeValue" })

// Filter or transform data
RuntimeEval({ body: "return data.filter(d => d.value > 0)" })

// Multi-statement expressions
RuntimeEval({
  body: `
    const doubled = number * 2;
    const added = doubled + rangeValue;
    return { doubled, added };
  `
})

// Persist the result as a named variable for later retrieval
RuntimeEval({
  name: "myResult",
  body: "return number * 2"
})

Dependencies are auto-detected from the expression. If name is provided, the result persists in the runtime and can be retrieved with GetValue. If name starts with _tmp_, it is automatically deleted after the value resolves.

Setting Input Values

Use SetInputValue to programmatically change the value of interactive input widgets created with Inputs.* (e.g., Inputs.range, Inputs.select, Inputs.text). This sets the widget's .value property and dispatches an input event, triggering reactive updates to dependent values.

// If the notebook has: slider = Inputs.range([0, 100])
SetInputValue({ name: "slider", value: 50 })

// If the notebook has: dropdown = Inputs.select(["A", "B", "C"])
SetInputValue({ name: "dropdown", value: "B" })

Mouse Interaction

Simulate mouse events for testing interactive visualizations:

• MouseClick: Click at coordinates or on an element (supports left/middle/right buttons)
• MouseDrag: Drag from start to end position with configurable duration
• MouseHover: Hover at a position, dispatching mouseenter/mouseover/mousemove events
• MouseWheel: Scroll at a position with deltaX/deltaY

All mouse tools accept an optional selector parameter to target a specific element, with coordinates relative to that element.

Keyboard Interaction

Use SendKeys to simulate keyboard input:

// Type plain text
SendKeys({ keys: "hello world" })

// Use special keys with braces
SendKeys({ keys: "{Enter}" })
SendKeys({ keys: "{Tab}" })
SendKeys({ keys: "{ArrowDown}" })

// Modifier combinations
SendKeys({ keys: "{Ctrl+a}" })  // Select all
SendKeys({ keys: "{Ctrl+c}" })  // Copy
SendKeys({ keys: "{Shift+Tab}" })  // Reverse tab

// Target a specific element
SendKeys({ selector: "#my-input", keys: "typed text{Enter}" })

// Hold modifiers for all keys
SendKeys({ keys: "abc", modifiers: { shiftKey: true } })  // Types "ABC"

Supported special keys: {Enter}, {Tab}, {Escape}, {Esc}, {Backspace}, {Delete}, {Insert}, {Space}, {ArrowUp}, {ArrowDown}, {ArrowLeft}, {ArrowRight}, {Home}, {End}, {PageUp}, {PageDown}, {F1}-{F12}.

License

© 2026 Ricky Reusser. MIT License.