diff --git a/.eslintrc.js b/.eslintrc.js
index 036df09251..cb2d737a12 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -201,9 +201,11 @@ module.exports = {
     spyOnDev: true,
     spyOnDevAndProd: true,
     spyOnProd: true,
-    __PROFILE__: true,
-    __UMD__: true,
     __EXPERIMENTAL__: true,
+    __EXTENSION__: true,
+    __PROFILE__: true,
+    __TEST__: true,
+    __UMD__: true,
     __VARIANT__: true,
     gate: true,
     trustedTypes: true,
diff --git a/packages/react-devtools-core/webpack.backend.js b/packages/react-devtools-core/webpack.backend.js
index b0809234ff..f111d7330c 100644
--- a/packages/react-devtools-core/webpack.backend.js
+++ b/packages/react-devtools-core/webpack.backend.js
@@ -4,6 +4,7 @@ const {
   GITHUB_URL,
   getVersionString,
 } = require('react-devtools-extensions/utils');
+const {resolveFeatureFlags} = require('react-devtools-shared/buildUtils');
 
 const NODE_ENV = process.env.NODE_ENV;
 if (!NODE_ENV) {
@@ -39,17 +40,20 @@ module.exports = {
   resolve: {
     alias: {
       react: resolve(builtModulesDir, 'react'),
-      'react-dom': resolve(builtModulesDir, 'react-dom'),
       'react-debug-tools': resolve(builtModulesDir, 'react-debug-tools'),
+      'react-devtools-feature-flags': resolveFeatureFlags('core/backend'),
+      'react-dom': resolve(builtModulesDir, 'react-dom'),
       'react-is': resolve(builtModulesDir, 'react-is'),
       scheduler: resolve(builtModulesDir, 'scheduler'),
     },
   },
   plugins: [
     new DefinePlugin({
-      __DEV__: true,
-      __PROFILE__: false,
+      __DEV__,
       __EXPERIMENTAL__: true,
+      __EXTENSION__: false,
+      __PROFILE__: false,
+      __TEST__: NODE_ENV === 'test',
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,
       'process.env.GITHUB_URL': `"${GITHUB_URL}"`,
     }),
diff --git a/packages/react-devtools-core/webpack.standalone.js b/packages/react-devtools-core/webpack.standalone.js
index 4d00d0f2d3..64556ae658 100644
--- a/packages/react-devtools-core/webpack.standalone.js
+++ b/packages/react-devtools-core/webpack.standalone.js
@@ -4,6 +4,7 @@ const {
   GITHUB_URL,
   getVersionString,
 } = require('react-devtools-extensions/utils');
+const {resolveFeatureFlags} = require('react-devtools-shared/buildUtils');
 
 const NODE_ENV = process.env.NODE_ENV;
 if (!NODE_ENV) {
@@ -33,8 +34,9 @@ module.exports = {
   resolve: {
     alias: {
       react: resolve(builtModulesDir, 'react'),
-      'react-dom': resolve(builtModulesDir, 'react-dom'),
       'react-debug-tools': resolve(builtModulesDir, 'react-debug-tools'),
+      'react-devtools-feature-flags': resolveFeatureFlags('core/standalone'),
+      'react-dom': resolve(builtModulesDir, 'react-dom'),
       'react-is': resolve(builtModulesDir, 'react-is'),
       scheduler: resolve(builtModulesDir, 'scheduler'),
     },
@@ -47,9 +49,11 @@ module.exports = {
   },
   plugins: [
     new DefinePlugin({
-      __DEV__: false,
-      __PROFILE__: false,
+      __DEV__,
       __EXPERIMENTAL__: true,
+      __EXTENSION__: false,
+      __PROFILE__: false,
+      __TEST__: NODE_ENV === 'test',
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,
       'process.env.GITHUB_URL': `"${GITHUB_URL}"`,
       'process.env.NODE_ENV': `"${NODE_ENV}"`,
diff --git a/packages/react-devtools-extensions/webpack.config.js b/packages/react-devtools-extensions/webpack.config.js
index e2720ec240..1a406ccdaf 100644
--- a/packages/react-devtools-extensions/webpack.config.js
+++ b/packages/react-devtools-extensions/webpack.config.js
@@ -3,6 +3,7 @@
 const {resolve} = require('path');
 const {DefinePlugin} = require('webpack');
 const {GITHUB_URL, getVersionString} = require('./utils');
+const {resolveFeatureFlags} = require('react-devtools-shared/buildUtils');
 
 const NODE_ENV = process.env.NODE_ENV;
 if (!NODE_ENV) {
@@ -39,6 +40,7 @@ module.exports = {
     alias: {
       react: resolve(builtModulesDir, 'react'),
       'react-debug-tools': resolve(builtModulesDir, 'react-debug-tools'),
+      'react-devtools-feature-flags': resolveFeatureFlags('extension'),
       'react-dom': resolve(builtModulesDir, 'react-dom'),
       'react-is': resolve(builtModulesDir, 'react-is'),
       scheduler: resolve(builtModulesDir, 'scheduler'),
@@ -49,9 +51,11 @@ module.exports = {
   },
   plugins: [
     new DefinePlugin({
-      __DEV__: false,
-      __PROFILE__: false,
+      __DEV__,
       __EXPERIMENTAL__: true,
+      __EXTENSION__: true,
+      __PROFILE__: false,
+      __TEST__: NODE_ENV === 'test',
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,
       'process.env.GITHUB_URL': `"${GITHUB_URL}"`,
       'process.env.NODE_ENV': `"${NODE_ENV}"`,
diff --git a/packages/react-devtools-inline/webpack.config.js b/packages/react-devtools-inline/webpack.config.js
index 85c7da82ea..af829455c6 100644
--- a/packages/react-devtools-inline/webpack.config.js
+++ b/packages/react-devtools-inline/webpack.config.js
@@ -4,6 +4,7 @@ const {
   GITHUB_URL,
   getVersionString,
 } = require('react-devtools-extensions/utils');
+const {resolveFeatureFlags} = require('react-devtools-shared/buildUtils');
 
 const NODE_ENV = process.env.NODE_ENV;
 if (!NODE_ENV) {
@@ -11,7 +12,7 @@ if (!NODE_ENV) {
   process.exit(1);
 }
 
-const __DEV__ = true; // NODE_ENV === 'development';
+const __DEV__ = NODE_ENV === 'development';
 
 const DEVTOOLS_VERSION = getVersionString();
 
@@ -32,6 +33,7 @@ module.exports = {
     react: 'react',
     // TODO: Once this package is published, remove the external
     // 'react-debug-tools': 'react-debug-tools',
+    'react-devtools-feature-flags': resolveFeatureFlags('inline'),
     'react-dom': 'react-dom',
     'react-is': 'react-is',
     scheduler: 'scheduler',
@@ -42,8 +44,10 @@ module.exports = {
   plugins: [
     new DefinePlugin({
       __DEV__,
-      __PROFILE__: false,
       __EXPERIMENTAL__: true,
+      __EXTENSION__: false,
+      __PROFILE__: false,
+      __TEST__: NODE_ENV === 'test',
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,
       'process.env.GITHUB_URL': `"${GITHUB_URL}"`,
       'process.env.NODE_ENV': `"${NODE_ENV}"`,
diff --git a/packages/react-devtools-shared/buildUtils.js b/packages/react-devtools-shared/buildUtils.js
new file mode 100644
index 0000000000..d5f48d6342
--- /dev/null
+++ b/packages/react-devtools-shared/buildUtils.js
@@ -0,0 +1,33 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ */
+
+const {resolve} = require('path');
+
+function resolveFeatureFlags(target) {
+  let flagsPath;
+  switch (target) {
+    case 'core/backend':
+    case 'core/standalone':
+    case 'inline':
+    case 'shell':
+      flagsPath = 'DevToolsFeatureFlags.default';
+      break;
+    case 'extension':
+      flagsPath = 'DevToolsFeatureFlags.extension';
+      break;
+    default:
+      console.error(`Invalid target "${target}"`);
+      process.exit(1);
+  }
+
+  return resolve(__dirname, 'src/config/', flagsPath);
+}
+
+module.exports = {
+  resolveFeatureFlags,
+};
diff --git a/packages/react-devtools-shared/src/config/DevToolsFeatureFlags.default.js b/packages/react-devtools-shared/src/config/DevToolsFeatureFlags.default.js
new file mode 100644
index 0000000000..9e2d09d630
--- /dev/null
+++ b/packages/react-devtools-shared/src/config/DevToolsFeatureFlags.default.js
@@ -0,0 +1,16 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+/************************************************************************
+ * This file is forked between different DevTools implementations.
+ * It should never be imported directly!
+ * It should always be imported from "react-devtools-feature-flags".
+ ************************************************************************/
+
+// TODO Add feature flags here...
diff --git a/packages/react-devtools-shared/src/config/DevToolsFeatureFlags.extension.js b/packages/react-devtools-shared/src/config/DevToolsFeatureFlags.extension.js
new file mode 100644
index 0000000000..a224621bb7
--- /dev/null
+++ b/packages/react-devtools-shared/src/config/DevToolsFeatureFlags.extension.js
@@ -0,0 +1,29 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+/************************************************************************
+ * This file is forked between different DevTools implementations.
+ * It should never be imported directly!
+ * It should always be imported from "react-devtools-feature-flags".
+ ************************************************************************/
+
+// TODO Add feature flags here...
+
+/************************************************************************
+ * Do not edit the code below.
+ * It ensures this fork exports the same types as the default flags file.
+ ************************************************************************/
+
+import typeof * as FeatureFlagsType from './DevToolsFeatureFlags.default';
+import typeof * as ExportsType from './DevToolsFeatureFlags.extension';
+
+// eslint-disable-next-line no-unused-vars
+type Check<_X, Y: _X, X: Y = _X> = null;
+// eslint-disable-next-line no-unused-expressions
+(null: Check<ExportsType, FeatureFlagsType>);
diff --git a/packages/react-devtools-shared/src/hook.js b/packages/react-devtools-shared/src/hook.js
index f9f65df7fb..62234fdc46 100644
--- a/packages/react-devtools-shared/src/hook.js
+++ b/packages/react-devtools-shared/src/hook.js
@@ -172,7 +172,11 @@ export function installHook(target: any): DevToolsHook | null {
     // In that case, we'll patch later (when the frontend attaches).
     //
     // Don't patch in test environments because we don't want to interfere with Jest's own console overrides.
-    if (process.env.NODE_ENV !== 'test') {
+    //
+    // Note that because this function is inlined, this conditional check must only use static booleans.
+    // Otherwise the extension will throw with an undefined error.
+    // (See comments in the try/catch below for more context on inlining.)
+    if (!__EXTENSION__ && !__TEST__) {
       try {
         const appendComponentStack =
           window.__REACT_DEVTOOLS_APPEND_COMPONENT_STACK__ !== false;
diff --git a/packages/react-devtools-shell/webpack.config.js b/packages/react-devtools-shell/webpack.config.js
index 500fed2fb1..c61e885029 100644
--- a/packages/react-devtools-shell/webpack.config.js
+++ b/packages/react-devtools-shell/webpack.config.js
@@ -4,6 +4,7 @@ const {
   GITHUB_URL,
   getVersionString,
 } = require('react-devtools-extensions/utils');
+const {resolveFeatureFlags} = require('react-devtools-shared/buildUtils');
 
 const NODE_ENV = process.env.NODE_ENV;
 if (!NODE_ENV) {
@@ -33,8 +34,9 @@ const config = {
   resolve: {
     alias: {
       react: resolve(builtModulesDir, 'react'),
-      'react-dom': resolve(builtModulesDir, 'react-dom'),
       'react-debug-tools': resolve(builtModulesDir, 'react-debug-tools'),
+      'react-devtools-feature-flags': resolveFeatureFlags('shell'),
+      'react-dom': resolve(builtModulesDir, 'react-dom'),
       'react-is': resolve(builtModulesDir, 'react-is'),
       scheduler: resolve(builtModulesDir, 'scheduler'),
     },
@@ -45,8 +47,10 @@ const config = {
   plugins: [
     new DefinePlugin({
       __DEV__,
-      __PROFILE__: false,
       __EXPERIMENTAL__: true,
+      __EXTENSION__: false,
+      __PROFILE__: false,
+      __TEST__: NODE_ENV === 'test',
       'process.env.GITHUB_URL': `"${GITHUB_URL}"`,
       'process.env.DEVTOOLS_VERSION': `"${DEVTOOLS_VERSION}"`,
     }),
diff --git a/scripts/flow/createFlowConfigs.js b/scripts/flow/createFlowConfigs.js
index ff7bf1b9c6..62a54d1112 100644
--- a/scripts/flow/createFlowConfigs.js
+++ b/scripts/flow/createFlowConfigs.js
@@ -53,6 +53,7 @@ module.name_mapper='ReactServerStreamConfig$$' -> 'forks/ReactServerStreamConfig
 module.name_mapper='ReactServerFormatConfig$$' -> 'forks/ReactServerFormatConfig.${serverRenderer}'
 module.name_mapper='ReactFlightServerConfig$$' -> 'forks/ReactFlightServerConfig.${serverRenderer}'
 module.name_mapper='ReactFlightClientHostConfig$$' -> 'forks/ReactFlightClientHostConfig.${serverRenderer}'
+module.name_mapper='react-devtools-feature-flags' -> 'react-devtools-shared/src/config/DevToolsFeatureFlags.default'
     `.trim(),
     )
     .replace('%REACT_RENDERER_FLOW_IGNORES%', ignoredPaths.join('\n'));
diff --git a/scripts/flow/react-devtools.js b/scripts/flow/react-devtools.js
index bab0373511..c8a8789de4 100644
--- a/scripts/flow/react-devtools.js
+++ b/scripts/flow/react-devtools.js
@@ -7,4 +7,5 @@
  * @flow
  */
 
-// No types
+declare var __EXTENSION__: boolean;
+declare var __TEST__: boolean;
diff --git a/scripts/jest/config.build-devtools.js b/scripts/jest/config.build-devtools.js
index 494cc4a8e7..7f1cd88559 100644
--- a/scripts/jest/config.build-devtools.js
+++ b/scripts/jest/config.build-devtools.js
@@ -26,6 +26,9 @@ const packages = readdirSync(packagesRoot).filter(dir => {
 // Create a module map to point React packages to the build output
 const moduleNameMapper = {};
 
+moduleNameMapper['react-devtools-feature-flags'] =
+  '<rootDir>/packages/react-devtools-shared/src/config/DevToolsFeatureFlags.default';
+
 // Map packages to bundles
 packages.forEach(name => {
   // Root entry point
diff --git a/scripts/jest/setupEnvironment.js b/scripts/jest/setupEnvironment.js
index 456645cdda..2ba88b1561 100644
--- a/scripts/jest/setupEnvironment.js
+++ b/scripts/jest/setupEnvironment.js
@@ -5,6 +5,8 @@ if (NODE_ENV !== 'development' && NODE_ENV !== 'production') {
   throw new Error('NODE_ENV must either be set to development or production.');
 }
 global.__DEV__ = NODE_ENV === 'development';
+global.__EXTENSION__ = false;
+global.__TEST__ = NODE_ENV === 'test';
 global.__PROFILE__ = NODE_ENV === 'development';
 global.__UMD__ = false;
 
diff --git a/scripts/jest/typescript/jest.d.ts b/scripts/jest/typescript/jest.d.ts
index ef5ec332bb..ca68247fb9 100644
--- a/scripts/jest/typescript/jest.d.ts
+++ b/scripts/jest/typescript/jest.d.ts
@@ -1,5 +1,7 @@
 declare var jasmine: any;
 declare var __DEV__: boolean;
+declare var __TEST__: boolean;
+declare var __EXTENSION__: boolean;
 
 declare function afterEach(fn: any): any;
 declare function beforeEach(fn: any): any;