[mcp] Add MCP tool to print out the component tree of the currently open React App (facebook#33305)

## Summary

This tool leverages DevTools to get the component tree from the
currently open React App. This gives realtime information to agents
about the state of the app.

## How did you test this change?

Tested integration with Claude Desktop

Author: jorge-cab

.eslintrc.js

@@ -496,6 +496,7 @@ module.exports = {
         'packages/react-devtools-shared/src/devtools/views/**/*.js',
         'packages/react-devtools-shared/src/hook.js',
         'packages/react-devtools-shared/src/backend/console.js',
+        'packages/react-devtools-shared/src/backend/fiber/renderer.js',
         'packages/react-devtools-shared/src/backend/shared/DevToolsComponentStackFrame.js',
         'packages/react-devtools-shared/src/frontend/utils/withPermissionsCheck.js',
       ],
@@ -504,6 +505,7 @@ module.exports = {
         __IS_FIREFOX__: 'readonly',
         __IS_EDGE__: 'readonly',
         __IS_NATIVE__: 'readonly',
+        __IS_INTERNAL_MCP_BUILD__: 'readonly',
         __IS_INTERNAL_VERSION__: 'readonly',
         chrome: 'readonly',
       },

compiler/packages/react-mcp-server/src/index.ts

@@ -21,6 +21,7 @@ import {queryAlgolia} from './utils/algolia';
 import assertExhaustive from './utils/assertExhaustive';
 import {convert} from 'html-to-text';
 import {measurePerformance} from './tools/runtimePerf';
+import {parseReactComponentTree} from './tools/componentTree';
 
 function calculateMean(values: number[]): string {
   return values.length > 0
@@ -366,6 +367,45 @@ ${calculateMean(results.renderTime)}
   },
 );
 
+server.tool(
+  'parse-react-component-tree',
+  `
+  This tool gets the component tree of a React App.
+  passing in a url will attempt to connect to the browser and get the current state of the component tree. If no url is passed in,
+  the default url will be used (http://localhost:3000).
+
+  <requirements>
+  - The url should be a full url with the protocol (http:// or https://) and the domain name (e.g. localhost:3000).
+  - Also the user should be running a Chrome browser running on debug mode on port 9222. If you receive an error message, advise the user to run
+  the following comand in the terminal:
+  MacOS: "/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --remote-debugging-port=9222 --user-data-dir=/tmp/chrome"
+  Windows: "chrome.exe --remote-debugging-port=9222 --user-data-dir=C:\temp\chrome"
+  </requirements>
+  `,
+  {
+    url: z.string().optional().default('http://localhost:3000'),
+  },
+  async ({url}) => {
+    try {
+      const componentTree = await parseReactComponentTree(url);
+
+      return {
+        content: [
+          {
+            type: 'text' as const,
+            text: componentTree,
+          },
+        ],
+      };
+    } catch (err) {
+      return {
+        isError: true,
+        content: [{type: 'text' as const, text: `Error: ${err.stack}`}],
+      };
+    }
+  },
+);
+
 server.prompt('review-react-code', () => ({
   messages: [
     {

compiler/packages/react-mcp-server/src/tools/componentTree.ts

@@ -0,0 +1,38 @@
+import puppeteer from 'puppeteer';
+
+export async function parseReactComponentTree(url: string): Promise<string> {
+  try {
+    const browser = await puppeteer.connect({
+      browserURL: 'http://127.0.0.1:9222',
+      defaultViewport: null,
+    });
+
+    const pages = await browser.pages();
+
+    let localhostPage = null;
+    for (const page of pages) {
+      const pageUrl = await page.url();
+
+      if (pageUrl.startsWith(url)) {
+        localhostPage = page;
+        break;
+      }
+    }
+
+    if (localhostPage) {
+      const componentTree = await localhostPage.evaluate(() => {
+        return (window as any).__REACT_DEVTOOLS_GLOBAL_HOOK__.rendererInterfaces
+          .get(1)
+          .__internal_only_getComponentTree();
+      });
+
+      return componentTree;
+    } else {
+      throw new Error(
+        `Could not open the page at ${url}. Is your server running?`,
+      );
+    }
+  } catch (error) {
+    throw new Error('Failed extract component tree' + error);
+  }
+}

packages/react-devtools-core/webpack.backend.js

@@ -72,6 +72,7 @@ module.exports = {
       __IS_CHROME__: false,
       __IS_EDGE__: false,
       __IS_NATIVE__: true,
+      __IS_INTERNAL_MCP_BUILD__: false,
       'process.env.DEVTOOLS_PACKAGE': `"react-devtools-core"`,
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,
       'process.env.GITHUB_URL': `"${GITHUB_URL}"`,

packages/react-devtools-core/webpack.standalone.js

@@ -91,6 +91,7 @@ module.exports = {
       __IS_FIREFOX__: false,
       __IS_CHROME__: false,
       __IS_EDGE__: false,
+      __IS_INTERNAL_MCP_BUILD__: false,
       'process.env.DEVTOOLS_PACKAGE': `"react-devtools-core"`,
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,
       'process.env.EDITOR_URL': EDITOR_URL != null ? `"${EDITOR_URL}"` : null,

packages/react-devtools-extensions/webpack.backend.js

@@ -78,6 +78,7 @@ module.exports = {
       __IS_FIREFOX__: IS_FIREFOX,
       __IS_EDGE__: IS_EDGE,
       __IS_NATIVE__: false,
+      __IS_INTERNAL_MCP_BUILD__: false,
     }),
     new Webpack.SourceMapDevToolPlugin({
       filename: '[file].map',

packages/react-devtools-extensions/webpack.config.js

@@ -33,6 +33,8 @@ const IS_FIREFOX = process.env.IS_FIREFOX === 'true';
 const IS_EDGE = process.env.IS_EDGE === 'true';
 const IS_INTERNAL_VERSION = process.env.FEATURE_FLAG_TARGET === 'extension-fb';
 
+const IS_INTERNAL_MCP_BUILD = process.env.IS_INTERNAL_MCP_BUILD === 'true';
+
 const featureFlagTarget = process.env.FEATURE_FLAG_TARGET || 'extension-oss';
 
 const babelOptions = {
@@ -113,6 +115,7 @@ module.exports = {
       __IS_FIREFOX__: IS_FIREFOX,
       __IS_EDGE__: IS_EDGE,
       __IS_NATIVE__: false,
+      __IS_INTERNAL_MCP_BUILD__: IS_INTERNAL_MCP_BUILD,
       __IS_INTERNAL_VERSION__: IS_INTERNAL_VERSION,
       'process.env.DEVTOOLS_PACKAGE': `"react-devtools-extensions"`,
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,

packages/react-devtools-fusebox/webpack.config.frontend.js

@@ -86,6 +86,7 @@ module.exports = {
       __IS_CHROME__: false,
       __IS_FIREFOX__: false,
       __IS_EDGE__: false,
+      __IS_INTERNAL_MCP_BUILD__: false,
       'process.env.DEVTOOLS_PACKAGE': `"react-devtools-fusebox"`,
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,
       'process.env.EDITOR_URL': EDITOR_URL != null ? `"${EDITOR_URL}"` : null,

packages/react-devtools-inline/webpack.config.js

@@ -78,6 +78,7 @@ module.exports = {
       __IS_FIREFOX__: false,
       __IS_EDGE__: false,
       __IS_NATIVE__: false,
+      __IS_INTERNAL_MCP_BUILD__: false,
       'process.env.DEVTOOLS_PACKAGE': `"react-devtools-inline"`,
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,
       'process.env.EDITOR_URL': EDITOR_URL != null ? `"${EDITOR_URL}"` : null,

packages/react-devtools-shared/src/backend/fiber/renderer.js

@@ -5859,6 +5859,86 @@ export function attach(
     return unresolvedSource;
   }
 
+  type InternalMcpFunctions = {
+    __internal_only_getComponentTree?: Function,
+  };
+
+  const internalMcpFunctions: InternalMcpFunctions = {};
+  if (__IS_INTERNAL_MCP_BUILD__) {
+    // eslint-disable-next-line no-inner-declarations
+    function __internal_only_getComponentTree(): string {
+      let treeString = '';
+
+      function buildTreeString(
+        instance: DevToolsInstance,
+        prefix: string = '',
+        isLastChild: boolean = true,
+      ): void {
+        if (!instance) return;
+
+        const name =
+          (instance.kind !== VIRTUAL_INSTANCE
+            ? getDisplayNameForFiber(instance.data)
+            : instance.data.name) || 'Unknown';
+
+        const id = instance.id !== undefined ? instance.id : 'unknown';
+
+        if (name !== 'createRoot()') {
+          treeString +=
+            prefix +
+            (isLastChild ? '└── ' : '├── ') +
+            name +
+            ' (id: ' +
+            id +
+            ')\n';
+        }
+
+        const childPrefix = prefix + (isLastChild ? '    ' : '│   ');
+
+        let childCount = 0;
+        let tempChild = instance.firstChild;
+        while (tempChild !== null) {
+          childCount++;
+          tempChild = tempChild.nextSibling;
+        }
+
+        let child = instance.firstChild;
+        let currentChildIndex = 0;
+
+        while (child !== null) {
+          currentChildIndex++;
+          const isLastSibling = currentChildIndex === childCount;
+          buildTreeString(child, childPrefix, isLastSibling);
+          child = child.nextSibling;
+        }
+      }
+
+      const rootInstances: Array<DevToolsInstance> = [];
+      idToDevToolsInstanceMap.forEach(instance => {
+        if (instance.parent === null || instance.parent.parent === null) {
+          rootInstances.push(instance);
+        }
+      });
+
+      if (rootInstances.length > 0) {
+        for (let i = 0; i < rootInstances.length; i++) {
+          const isLast = i === rootInstances.length - 1;
+          buildTreeString(rootInstances[i], '', isLast);
+          if (!isLast) {
+            treeString += '\n';
+          }
+        }
+      } else {
+        treeString = 'No component tree found.';
+      }
+
+      return treeString;
+    }
+
+    internalMcpFunctions.__internal_only_getComponentTree =
+      __internal_only_getComponentTree;
+  }
+
   return {
     cleanup,
     clearErrorsAndWarnings,
@@ -5898,5 +5978,6 @@ export function attach(
     storeAsGlobal,
     updateComponentFilters,
     getEnvironmentNames,
+    ...internalMcpFunctions,
   };
 }