@@ -61,6 +61,10 @@ let package = Package(
                 .headerSearchPath("headers"),
                 .define("DEBUG", .when(configuration: .debug)),
                 .define("NDEBUG", .when(configuration: .release)),
+                // Generated code: nobody can act on a C++ stable API "for
+                // frameworks" warning here. Libraries are held to the API in
+                // their own packages.
+                .define("RN_ALLOW_FRAMEWORKS"),
             ],
             linkerSettings: [
                 .linkedFramework("Foundation")
@@ -79,6 +83,8 @@ let package = Package(
                 // Match the prebuilt React.framework NDEBUG-gated C++ ABI (see above).
                 .define("DEBUG", .when(configuration: .debug)),
                 .define("NDEBUG", .when(configuration: .release)),
+                // Generated code (see ReactCodegen above).
+                .define("RN_ALLOW_FRAMEWORKS"),
             ],
             linkerSettings: [
                 .linkedFramework("Foundation")
 
@@ -59,6 +59,14 @@
  *   [advanced] --skip-codegen   Skip the react-native codegen step.
  *   [advanced] --config-command <json> Override the autolinking config command.
  *
+ * Environment ([add/update]; recorded in .spm-injected.json so Xcode builds
+ * reuse them, re-run `update` without one to turn it off):
+ *   RN_STRICT_API=1             Enforce React Native's C++ stable API: defines
+ *                               the RN_STRICT_API macro for the app target, its
+ *                               spm.modules packages and app codegen.
+ *   RN_ALLOW_FRAMEWORKS=1       Silence the stable API's "for frameworks"
+ *                               warning there: defines RN_ALLOW_FRAMEWORKS.
+ *
  * Steps performed (add/update):
  *   1. react-native codegen → build/generated/ios/ + install SPM codegen template
  *   2. generate-spm-autolinking-config.js → build/generated/autolinking/autolinking.json
@@ -99,6 +107,7 @@ const {
   findInjectedXcodeproj,
   injectSpmIntoExistingXcodeproj,
   readArtifactsVersionOverride,
+  readCxxApiDefines,
   readMarker,
   readPinnedConfigCommand,
   removeSpmInjection,
@@ -112,6 +121,7 @@ const {
   scaffoldAll,
 } = require('./spm/scaffold-package-swift');
 const {
+  CXX_API_DEFINES,
   RemoteVersionError,
   buildPerAppHeaderTree,
   defaultCacheDir,
@@ -420,17 +430,24 @@ function runCodegenStep(
   appRoot /*: string */,
   reactNativeRoot /*: string */,
   skipCodegen /*: boolean */,
+  cxxApiDefines /*: ReadonlyArray<string> */,
 ) /*: void */ {
   if (skipCodegen) {
     // Output dir may already exist from a previous run; still refresh the
     // SPM template so cache-slot changes propagate.
     log('Skipping codegen (--skip-codegen)');
-    installSpmCodegenTemplate(appRoot, reactNativeRoot, {log});
+    installSpmCodegenTemplate(appRoot, reactNativeRoot, {log}, {cxxApiDefines});
     return;
   }
   log('Running react-native codegen...');
   try {
-    runCodegenAndInstallTemplate(projectRoot, appRoot, reactNativeRoot, {log});
+    runCodegenAndInstallTemplate(
+      projectRoot,
+      appRoot,
+      reactNativeRoot,
+      {log},
+      {cxxApiDefines},
+    );
   } catch {
     logError('Codegen failed. Continuing anyway...');
   }
@@ -832,6 +849,7 @@ async function setupXcodeproj(
   appRoot /*: string */,
   reactNativeRoot /*: string */,
   action /*: string */,
+  cxxApiDefines /*: ReadonlyArray<string> */,
 ) /*: Promise<void> */ {
   const target = resolveInjectionTarget(args, appRoot);
   if (target.error != null) {
@@ -909,6 +927,7 @@ async function setupXcodeproj(
     // generate-spm-xcodeproj.js).
     artifactsVersionOverride: args.version ?? null,
     configCommand: resolveConfigCommandToPin(args),
+    cxxApiDefines,
   });
   if (result.status !== 'injected') {
     logError(`SPM injection failed: ${result.reason}`);
@@ -968,6 +987,22 @@ function logNextSteps(
   log('To remove SPM later: `npx react-native spm deinit`');
 }
 
+// The React Native C++ stable API macros this run defines for the app target,
+// its spm.modules packages and app codegen (CXX_API_DEFINES). `add`/`update`
+// read each from the env var of the same name (=1; unset turns it off, as with
+// `pod install`) and record the result in the injection marker; every other
+// action, including the build-time `sync`, reuses the recorded macros because
+// Xcode builds inherit neither the env vars nor the shell that set them.
+function resolveCxxApiDefines(
+  action /*: string */,
+  appRoot /*: string */,
+) /*: Array<string> */ {
+  if (action === 'add' || action === 'update') {
+    return CXX_API_DEFINES.filter(name => process.env[name] === '1');
+  }
+  return readCxxApiDefines(appRoot);
+}
+
 // The autolinking config command for this run: an explicit `--config-command`
 // first, then the value a previous `add`/`update` pinned into the injection
 // marker. undefined means "no explicit command", which is what makes
@@ -1160,6 +1195,18 @@ async function main(argv /*:: ?: Array<string> */) /*: Promise<void> */ {
   const version = determineVersion(args, reactNativeRoot, appRoot);
   log(`React Native version: ${version}`);
 
+  const cxxApiDefines = resolveCxxApiDefines(action, appRoot);
+  if (cxxApiDefines.length > 0) {
+    log(
+      `C++ stable API: defining ${cxxApiDefines.join(', ')} in the app ` +
+        'target, its spm.modules packages and app codegen',
+    );
+  }
+  const cxxApiDefineArgs = cxxApiDefines.flatMap(name => [
+    '--cxx-api-define',
+    name,
+  ]);
+
   // Resolve remote SPM mode ONCE up front. remotePackageConfig throws
   // RemoteVersionError when remote mode is active but no usable RN version can
   // be derived (e.g. the monorepo '1000.0.0' placeholder with no override).
@@ -1184,7 +1231,7 @@ async function main(argv /*:: ?: Array<string> */) /*: Promise<void> */ {
   // "Artifacts already present in ...".
 
   if (action === 'codegen') {
-    runCodegenStep(projectRoot, appRoot, reactNativeRoot, false);
+    runCodegenStep(projectRoot, appRoot, reactNativeRoot, false, cxxApiDefines);
     return;
   }
 
@@ -1198,6 +1245,7 @@ async function main(argv /*:: ?: Array<string> */) /*: Promise<void> */ {
         reactNativeRoot,
         '--ios-deployment-target',
         iosDeploymentTarget,
+        ...cxxApiDefineArgs,
       ]);
     } catch (e) {
       if (e instanceof MissingManifestError) {
@@ -1240,7 +1288,13 @@ async function main(argv /*:: ?: Array<string> */) /*: Promise<void> */ {
     );
   }
 
-  runCodegenStep(projectRoot, appRoot, reactNativeRoot, args.skipCodegen);
+  runCodegenStep(
+    projectRoot,
+    appRoot,
+    reactNativeRoot,
+    args.skipCodegen,
+    cxxApiDefines,
+  );
 
   // A manifest scaffolded before the app's deployment target changed (or by a
   // pre-v20 scaffolder) would pin a floor SwiftPM then refuses to link against.
@@ -1269,6 +1323,7 @@ async function main(argv /*:: ?: Array<string> */) /*: Promise<void> */ {
       reactNativeRoot,
       '--ios-deployment-target',
       iosDeploymentTarget,
+      ...cxxApiDefineArgs,
     ]);
   } catch (e) {
     if (e instanceof MissingManifestError) {
@@ -1310,7 +1365,7 @@ async function main(argv /*:: ?: Array<string> */) /*: Promise<void> */ {
   }
 
   // (Re)install the static codegen Package.swift template once build/generated/ios exists.
-  installSpmCodegenTemplate(appRoot, reactNativeRoot, {log});
+  installSpmCodegenTemplate(appRoot, reactNativeRoot, {log}, {cxxApiDefines});
 
   // Build the per-app generated-headers farm (vended as the ReactAppHeaders
   // SPM target inside the codegen package). React core headers need no trees
@@ -1327,7 +1382,7 @@ async function main(argv /*:: ?: Array<string> */) /*: Promise<void> */ {
   // Xcodeproj setup: in-place injection into the existing project (the only
   // strategy — no rename, no from-scratch; git is the safety net).
   try {
-    await setupXcodeproj(args, appRoot, reactNativeRoot, action);
+    await setupXcodeproj(args, appRoot, reactNativeRoot, action, cxxApiDefines);
   } catch (e) {
     logError(`xcodeproj setup failed: ${e.message}`);
     if (process.exitCode == null) {
@@ -1356,6 +1411,7 @@ module.exports = {
   resolveAppIosDeploymentTarget,
   resolveConfigCommandToPin,
   resolveExplicitConfigCommand,
+  resolveCxxApiDefines,
   shouldAutoDeintegrate,
   ensureBothArtifactFlavors,
 };
@@ -233,6 +233,28 @@ A floor set in an `.xcconfig` your configuration is based on is honored,
 manifests (they never create new ones) — if you persisted a scaffold with
 `patch-package`, re-run `npx patch-package <dep>` afterwards.
 
+### C++ stable API (`RN_STRICT_API`)
+
+`RN_STRICT_API=1` and `RN_ALLOW_FRAMEWORKS=1` on `add`/`update` turn on React
+Native's C++ stable API checks:
+
+```bash
+RN_STRICT_API=1 npx react-native spm update
+```
+
+Each one defines the macro of the same name for the app target
+(`GCC_PREPROCESSOR_DEFINITIONS`), the packages generated for your
+[local native modules](#local-native-modules), and the app codegen package
+(`build/generated/ios/Package.swift`), which also always defines
+`RN_ALLOW_FRAMEWORKS`. Libraries build from their own `Package.swift`, so they
+are only checked if they define the macros themselves.
+
+The choice is recorded in `.spm-injected.json` (as `cxxApiDefines`), because the
+**Sync SPM Autolinking** build phase regenerates those packages on every build
+without your shell's environment. Unlike the pins above, every `add`/`update`
+reads the variables again: run `update` without one to turn it off. `deinit`
+removes the app-target definitions with the rest of the injection.
+
 ## Files the tool touches
 
 Paths are relative to the Xcode project directory (`ios/`) unless noted.
@@ -251,7 +273,9 @@ Paths are relative to the Xcode project directory (`ios/`) unless noted.
 The two pinned settings are the `--version` pin (`artifactsVersionOverride`, see
 [Pinning the React Native version](#pinning-the-react-native-version)) and the
 [autolinking config command](#the-autolinking-config-command-is-remembered).
-Because `deinit` drops the marker, it drops both.
+Because `deinit` drops the marker, it drops both. The marker also records the
+[C++ stable API](#c-stable-api-rn_strict_api) opt-ins (`cxxApiDefines`), but
+those are not pins: every `add`/`update` sets them from the environment again.
 
 ### In your repo — generated, gitignored
 
 
@@ -283,6 +283,25 @@ describe('generateSynthPackageSwift', () => {
     );
   });
 
+  it('defines its C++ API macros once each, next to the config-gated ABI defines', () => {
+    expect(generateSynthPackageSwift(baseSpec())).not.toMatch(
+      /RN_(STRICT_API|ALLOW_FRAMEWORKS)/,
+    );
+    const result = generateSynthPackageSwift(
+      baseSpec({
+        cxxApiDefines: [
+          'RN_STRICT_API',
+          'RN_ALLOW_FRAMEWORKS',
+          'RN_ALLOW_FRAMEWORKS',
+        ],
+      }),
+    );
+    expect(result).toMatch(
+      /cxxSettings: \[[^\]]*\.define\("NDEBUG", \.when\(configuration: \.release\)\), \.define\("RN_STRICT_API"\), \.define\("RN_ALLOW_FRAMEWORKS"\)[,\]]/,
+    );
+    expect(result.match(/\.define\("RN_ALLOW_FRAMEWORKS"\)/g)).toHaveLength(1);
+  });
+
   it('depends on ReactNative via a fixed relative path (default synth depth)', () => {
     const result = generateSynthPackageSwift(baseSpec({hasReactDep: true}));
     expect(result).toContain(
@@ -538,6 +557,27 @@ describe('main() — --ios-deployment-target', () => {
     return {appRoot, rnRoot, autolinkDir};
   }
 
+  it("defines the app's C++ API macros (--cxx-api-define) in the synth manifest", () => {
+    const {appRoot, rnRoot, autolinkDir} = buildApp();
+    main([
+      '--app-root',
+      appRoot,
+      '--react-native-root',
+      rnRoot,
+      '--cxx-api-define',
+      'RN_STRICT_API',
+      '--cxx-api-define',
+      'RN_ALLOW_FRAMEWORKS',
+    ]);
+    const synth = fs.readFileSync(
+      path.join(autolinkDir, 'packages', 'MyNativeModule', 'Package.swift'),
+      'utf8',
+    );
+    expect(synth).toMatch(
+      /cxxSettings: \[[^\]]*\.define\("RN_STRICT_API"\), \.define\("RN_ALLOW_FRAMEWORKS"\)/,
+    );
+  });
+
   it('sanitizes the flag into both the aggregator and the synth manifest', () => {
     const {appRoot, rnRoot, autolinkDir} = buildApp();
     main([
 
@@ -14,6 +14,7 @@ const {
   SPM_INJECTED_MARKER,
   injectSpmIntoExistingXcodeproj,
   readArtifactsVersionOverride,
+  readCxxApiDefines,
   readPinnedConfigCommand,
   removeSpmInjection,
 } = require('../generate-spm-xcodeproj');
@@ -212,6 +213,52 @@ describe('injectSpmIntoExistingXcodeproj — HERMES_CLI_PATH', () => {
   });
 });
 
+describe('injectSpmIntoExistingXcodeproj — C++ stable API defines', () => {
+  const BOTH = ['RN_STRICT_API', 'RN_ALLOW_FRAMEWORKS'];
+  const inject = (ctx, cxxApiDefines) =>
+    injectSpmIntoExistingXcodeproj({
+      appRoot: ctx.appRoot,
+      reactNativeRoot: ctx.rnRoot,
+      xcodeprojPath: ctx.xcodeprojPath,
+      cxxApiDefines,
+    });
+
+  it('defines the opted-in macros in every app-target config and records them', () => {
+    const ctx = scaffoldApp();
+    expect(inject(ctx, BOTH).status).toBe('injected');
+    // Debug + Release, each seeded with $(inherited) so project-level
+    // definitions (DEBUG=1) still apply.
+    expect(
+      pbxprojOf(ctx.xcodeprojPath).match(
+        /GCC_PREPROCESSOR_DEFINITIONS = \(\s*"\$\(inherited\)",\s*RN_STRICT_API,\s*RN_ALLOW_FRAMEWORKS,\s*\);/g,
+      ),
+    ).toHaveLength(2);
+    expect(readCxxApiDefines(ctx.appRoot)).toEqual(BOTH);
+  });
+
+  it('removes them on an update without the opt-ins', () => {
+    const ctx = scaffoldApp();
+    inject(ctx, BOTH);
+    inject(ctx, []);
+    expect(pbxprojOf(ctx.xcodeprojPath)).not.toContain('RN_STRICT_API');
+    expect(pbxprojOf(ctx.xcodeprojPath)).not.toContain('RN_ALLOW_FRAMEWORKS');
+    expect(readCxxApiDefines(ctx.appRoot)).toEqual([]);
+  });
+
+  it('round-trips add (opted in) → deinit byte-for-byte', () => {
+    const ctx = scaffoldApp();
+    const before = pbxprojOf(ctx.xcodeprojPath);
+    inject(ctx, BOTH);
+    expect(
+      removeSpmInjection({
+        appRoot: ctx.appRoot,
+        xcodeprojPath: ctx.xcodeprojPath,
+      }).status,
+    ).toBe('removed');
+    expect(pbxprojOf(ctx.xcodeprojPath)).toBe(before);
+  });
+});
+
 describe('removeSpmInjection — the surgical inverse of add', () => {
   it('round-trips: add then deinit restores the pbxproj byte-for-byte', () => {
     const {appRoot, xcodeprojPath, rnRoot} = scaffoldApp();