Extension Development in Nimbalyst

Nimbalyst provides MCP tools for building, installing, and hot-reloading extensions directly from within the running app. This enables rapid iteration on extension development without manually running build commands.

The recommended in-app scaffold flow uses File > New Extension Project, Developer > New Extension Project, or the Extension Developer Kit's /new-extension command. Enable Extension Dev Tools in Settings > Advanced before using the build/install workflow.

Available MCP Tools

Build Extension

mcp__nimbalyst-extension-dev__extension_build

Runs npm run build (vite build) on an extension project.

Parameters:

• path: Absolute path to the extension project root (directory containing package.json and manifest.json)

Returns: Build output including any errors or warnings.

Install Extension

mcp__nimbalyst-extension-dev__extension_install

Installs a built extension into the running Nimbalyst instance. The extension must be built first.

Parameters:

• path: Absolute path to the extension project root

Returns: Installation status and any validation warnings.

Hot Reload Extension

mcp__nimbalyst-extension-dev__extension_reload

Rebuilds and reinstalls an extension in one step. This is the most common tool for iterating on extension development.

Parameters:

• extensionId: The extension ID from manifest.json (e.g., "com.nimbalyst.my-extension")
• path: Absolute path to the extension project root

Returns: Build and reload status.

Uninstall Extension

mcp__nimbalyst-extension-dev__extension_uninstall

Removes an installed extension from the running Nimbalyst instance.

Parameters:

• extensionId: The extension ID to uninstall

Get Extension Status

mcp__nimbalyst-extension-dev__extension_get_status

Gets the current status of an installed extension, including whether it loaded successfully and what it contributes.

Parameters:

• extensionId: The extension ID to query

Restart Nimbalyst

mcp__nimbalyst-extension-dev__restart_nimbalyst

Restarts the entire Nimbalyst application. Only use when explicitly requested by the user.

Testing Tools

Run Playwright Tests

mcp__nimbalyst-extension-dev__extension_test_run

Execute Playwright scripts against the running Nimbalyst instance via CDP. Supports inline scripts or .spec.ts file paths.

Parameters:

• script: Inline Playwright code (runs inside an async test with page connected to Nimbalyst)
• testFile: Absolute path to a .spec.ts file
• timeout: Max execution time in ms (default: 30000)

Provide either script or testFile, not both.

Open File for Testing

mcp__nimbalyst-extension-dev__extension_test_open_file

Opens a file in Nimbalyst and waits for the editor to mount. Use before running tests that need a specific file open.

Parameters:

• filePath: Absolute path to the file to open
• waitForExtension: Extension ID to wait for (waits until the extension's editor container renders)
• timeout: Max wait time in ms (default: 5000)

Test AI Tool Handler

mcp__nimbalyst-extension-dev__extension_test_ai_tool

Execute an extension's AI tool handler directly and return the result. Bypasses full MCP routing.

Parameters:

• extensionId: The extension ID
• toolName: Tool name without extension prefix (e.g., "get_elements")
• args: Arguments object to pass to the handler
• filePath: For editor-scoped tools: file path for context

Development Workflow

Initial Setup

1. Create extension project with /new-extension <path> <name> [file-patterns]
2. Implement the extension code
3. Build with extension_build
4. Install with extension_install
5. Test in Nimbalyst

Iteration Loop with Testing

1. Make code changes
2. Run extension_reload to rebuild and reinstall
3. Run extension_get_status to verify it loaded
4. Run extension_test_open_file to open a sample file
5. Run extension_test_run with Playwright scripts to verify behavior
6. Use capture_editor_screenshot for visual verification
7. Check get_renderer_debug_logs for runtime errors
8. Fix issues and repeat from step 1

Inline test example:

extension_test_run({
  script: `
    const editor = page.locator('[data-extension-id="com.nimbalyst.my-ext"]');
    await expect(editor).toBeVisible();
    await editor.locator('.save-btn').click();
    await expect(editor.locator('.status')).toHaveText('Saved');
  `
})

Test file example:

extension_test_run({ testFile: "/path/to/extension/tests/basics.spec.ts" })

Debugging

1. Check extension status with extension_get_status
2. Read logs with mcp__nimbalyst-extension-dev__get_main_process_logs (filter by component: "EXTENSION")
3. Check renderer logs with mcp__nimbalyst-extension-dev__get_renderer_debug_logs

Important Notes

• Always build before install - extension_install requires a pre-built extension
• Use extension_reload for iteration - It combines build + reinstall in one step
• Check manifest validation - Build and install tools validate manifest.json and report warnings
• Extensions load on app start - For full reload, user may need to restart Nimbalyst
• Never restart without permission - Only use restart_nimbalyst when the user explicitly asks

Common Issues

"Extension not found" after install

• Check that the extension ID in manifest.json matches what you're querying
• Verify the build output exists in dist/index.js

Build fails

• Check that npm install was run in the extension directory
• Verify vite.config.ts is properly configured
• Check for TypeScript errors in the output

Extension doesn't appear

• Use extension_get_status to check if it's loaded
• Check the release channel - some extensions require "alpha" channel
• Verify the extension is enabled in Settings > Extensions