@@ -19,13 +19,15 @@ const {
 const fs = require('node:fs');
 
 /*::
+type TestInventoryHeader = {
+  naturalPath: string,
+  bucket: string,
+  lang: string,
+  identities: Array<{source: string}>,
+  includes?: {internal: Array<{naturalPath: string, cxxGuarded: boolean}>},
+};
 type TestInventoryManifest = {
-  headers: Array<{
-    naturalPath: string,
-    bucket: string,
-    lang: string,
-    identities: Array<{source: string}>,
-  }>,
+  headers: Array<TestInventoryHeader>,
 };
 */
 
@@ -45,7 +47,7 @@ const entry = (
   naturalPath /*: string */,
   bucket /*: string */,
   source /*:: ?: string */,
-) => ({
+) /*: TestInventoryHeader */ => ({
   naturalPath,
   bucket,
   lang: 'objc',
@@ -209,6 +211,108 @@ describe('R5 invalid-identifier exemption assert (H5)', () => {
   });
 });
 
+describe('R13 stable API guards are textual module members', () => {
+  const includes = (
+    ...deps /*: Array<[string, boolean]> */
+  ) /*: {internal: Array<{naturalPath: string, cxxGuarded: boolean}>} */ => ({
+    internal: deps.map(([naturalPath, cxxGuarded]) => ({
+      naturalPath,
+      cxxGuarded,
+    })),
+  });
+  const guardedManifest = () => {
+    const m = validManifest();
+    m.headers.push(
+      entry('react/cxxstableapi/UmbrellaGuard.h', 'objc-modular-candidate'),
+      {
+        ...entry('react/debug/flags.h', 'objc-modular-candidate'),
+        includes: includes(['react/cxxstableapi/UmbrellaGuard.h', false]),
+      },
+      {
+        ...entry('react/debug/UsesFlags.h', 'objc-modular-candidate'),
+        includes: includes(['react/debug/flags.h', false]),
+      },
+      {
+        ...entry('react/runtime/CApi.h', 'objc-modular-candidate'),
+        includes: includes(['react/cxxstableapi/UmbrellaGuard.h', true]),
+      },
+      entry('react/runtime/Plain.h', 'objc-modular-candidate'),
+    );
+    return m;
+  };
+
+  test('guards and members that reach one stay in the module, marked textual', () => {
+    const plan = planFromInventoryForTest(guardedManifest());
+    expect(plan.namespaceModules.react).toEqual([
+      'react/cxxstableapi/UmbrellaGuard.h',
+      'react/debug/UsesFlags.h',
+      'react/debug/flags.h',
+      'react/runtime/CApi.h',
+      'react/runtime/Plain.h',
+    ]);
+    expect(plan.textualNamespaceHeaders).toEqual([
+      'react/cxxstableapi/UmbrellaGuard.h',
+      'react/debug/UsesFlags.h',
+      'react/debug/flags.h',
+    ]);
+  });
+
+  test('a guard reached only under #ifdef __cplusplus does not count', () => {
+    const plan = planFromInventoryForTest(guardedManifest());
+    expect(plan.textualNamespaceHeaders).not.toContain('react/runtime/CApi.h');
+  });
+
+  test('module map declares them `textual header`', () => {
+    const plan = planFromInventoryForTest(guardedManifest());
+    const mm = renderNamespaceModuleMap(
+      plan.namespaceModules,
+      plan.textualNamespaceHeaders,
+    );
+    expect(mm).toContain('module ReactNativeHeaders_react {');
+    expect(mm).toContain(
+      '  textual header "react/cxxstableapi/UmbrellaGuard.h"',
+    );
+    expect(mm).toContain('  textual header "react/debug/flags.h"');
+    expect(mm).not.toMatch(/^\s*header "react\/debug\/flags\.h"/m);
+    expect(mm).toContain('  header "react/runtime/Plain.h"');
+  });
+
+  test('without stable API guards every member stays modular', () => {
+    const plan = planFromInventoryForTest(validManifest());
+    expect(plan.textualNamespaceHeaders).toEqual([]);
+    expect(
+      renderNamespaceModuleMap(
+        plan.namespaceModules,
+        plan.textualNamespaceHeaders,
+      ),
+    ).not.toContain('textual header');
+  });
+
+  test('ObjC verify fixture imports a modular member, never a textual one', () => {
+    const {renderObjcFixture} = require('../headers-verify');
+    const m = guardedManifest();
+    m.headers.push(
+      entry('reactperflogger/Guarded.h', 'objc-modular-candidate'),
+    );
+    const guardedOnly = m.headers.find(
+      x => x.naturalPath === 'reactperflogger/Guarded.h',
+    );
+    if (guardedOnly == null) {
+      throw new Error('fixture missing reactperflogger/Guarded.h');
+    }
+    guardedOnly.includes = includes([
+      'react/cxxstableapi/UmbrellaGuard.h',
+      false,
+    ]);
+    const tu = renderObjcFixture(planFromInventoryForTest(m));
+    // First modular member in sort order (the textual ones sort before it).
+    expect(tu).toContain('#import <react/runtime/CApi.h>');
+    expect(tu).not.toContain('#import <react/cxxstableapi/');
+    // A module whose members are all textual has nothing to precompile.
+    expect(tu).not.toContain('#import <reactperflogger/');
+  });
+});
+
 describe('R11 redirect shims for dual-identity headers', () => {
   test('RNH spelling of a source that also ships as React/ becomes a shim', () => {
     const m = validManifest();
 
@@ -292,7 +292,10 @@ function buildReactNativeHeadersXcframework(
   }
   fs.writeFileSync(
     path.join(stage, 'module.modulemap'),
-    renderNamespaceModuleMap(plan.namespaceModules),
+    renderNamespaceModuleMap(
+      plan.namespaceModules,
+      plan.textualNamespaceHeaders,
+    ),
   );
 
   // ---- compose (stub archives + create-xcframework) ----
 
@@ -47,10 +47,9 @@
  *     non-modular framework headers; yoga + RCTDeprecation found
  *     empirically). Namespaces whose name is not a valid module identifier
  *     (e.g. jsinspector-modern) are exempt — they have no candidates today;
- *     planFromInventory FAILS CLOSED if that changes. `react/` is also exempt: its few
- *     objc-modular-candidates stay textual (as they already were inside
- *     React.framework) so no `react` module aliases the `React` framework
- *     module.
+ *     planFromInventory FAILS CLOSED if that changes. `react/` gets one
+ *     too, named ReactNativeHeaders_react so it cannot alias the `React`
+ *     framework module on a case-insensitive filesystem.
  * R6. Bare root aliases are servable only as `<React/X>` — bare angle forms
  *     (`#import <RCTAppDelegate.h>`) have no framework spelling. This is the
  *     accepted, measured consumer migration (~4 lines ecosystem-wide).
@@ -93,6 +92,16 @@
  *     within one artifact, so no module edge is created. These umbrellas are
  *     objc-blocked by construction (they re-export their module's C++
  *     surface), so they were never R4 umbrella or R5 module members anyway.
+ * R13. The C++ stable API guards (react/cxxstableapi/*Guard.h) are
+ *     `textual header` members of their R5 module, as is every R5 member that
+ *     reaches one through unguarded includes. A guard decides at each
+ *     #include, from the includer's RN_BUILDING / RN_UMBRELLA_CONTEXT, whether
+ *     that include is allowed. A module member is instead compiled once, on
+ *     its own, with the consumer's -D macros and no includer at all, so under
+ *     RN_STRICT_API the guard fires against the module itself and every
+ *     `import React` fails to build. A textual member still belongs to the
+ *     module, so a framework module may include it (R5), but it is expanded
+ *     at each include site, where the guard sees its real includer.
  */
 
 const fs = require('node:fs');
@@ -124,6 +133,8 @@ export type HeadersSpecPlan = {
   umbrella: Array<string>,
   // R5: plain modules for ReactNativeHeaders' module.modulemap
   namespaceModules: {[ns: string]: Array<string>},
+  // R13: the namespaceModules members declared `textual header`.
+  textualNamespaceHeaders: Array<string>,
   // R10: per-namespace umbrella headers emitted into ReactNativeHeaders.
   namespaceUmbrellas: Array<{relPath: string, content: string}>,
   // R9: private headers added to the React module map (allowlist).
@@ -268,6 +279,39 @@ function isUmbrellaSafe(h /*: any */, rnRoot /*: string */) /*: boolean */ {
   }
 }
 
+// R13: natural-path prefix of the C++ stable API guard headers.
+const STABLE_API_GUARD_PREFIX = 'react/cxxstableapi/';
+
+// R13: the inventory headers that are a stable API guard or reach one through
+// unguarded includes — what compiling the header on its own in Objective-C
+// (a namespace module build) evaluates.
+function stableApiGuardedHeaders(manifest /*: any */) /*: Set<string> */ {
+  const includes /*: Map<string, Array<string>> */ = new Map();
+  for (const h of manifest.headers) {
+    includes.set(
+      h.naturalPath,
+      (h.includes?.internal ?? [])
+        .filter(i => !i.cxxGuarded)
+        .map(i => i.naturalPath),
+    );
+  }
+  const isGuard = (np /*: string */) => np.startsWith(STABLE_API_GUARD_PREFIX);
+  const guarded /*: Set<string> */ = new Set(
+    Array.from(includes.keys()).filter(isGuard),
+  );
+  let changed = true;
+  while (changed) {
+    changed = false;
+    for (const [np, deps] of includes) {
+      if (!guarded.has(np) && deps.some(d => isGuard(d) || guarded.has(d))) {
+        guarded.add(np);
+        changed = true;
+      }
+    }
+  }
+  return guarded;
+}
+
 // R10: per-namespace umbrella headers. Some consumers (e.g. Expo's
 // RCTAppDelegateUmbrella.h) probe
 // `<React_RCTAppDelegate/React_RCTAppDelegate-umbrella.h>` via __has_include.
@@ -319,6 +363,8 @@ function planFromInventory(
   const reactNativeHeaders /*: Array<SpecEntry> */ = [];
   const umbrella /*: Array<string> */ = [];
   const namespaceModules /*: {[string]: Array<string>} */ = {};
+  const textualNamespaceHeaders /*: Array<string> */ = [];
+  const guardedHeaders = stableApiGuardedHeaders(manifest);
   const collisions /*: Array<string> */ = [];
   const seen /*: Map<string, string> */ = new Map();
 
@@ -409,6 +455,9 @@ function planFromInventory(
           namespaceModules[ns] = [];
         }
         namespaceModules[ns].push(np);
+        if (guardedHeaders.has(np)) {
+          textualNamespaceHeaders.push(np);
+        }
       }
     }
   }
@@ -417,6 +466,7 @@ function planFromInventory(
   for (const ns of Object.keys(namespaceModules)) {
     namespaceModules[ns].sort();
   }
+  textualNamespaceHeaders.sort();
 
   // R11: assign redirect shims for duplicate spellings of one source.
   // Owner precedence: the React/ form (module React / canonical textual home)
@@ -486,6 +536,7 @@ function planFromInventory(
     depsNamespaces: DEPS_NAMESPACES,
     umbrella,
     namespaceModules,
+    textualNamespaceHeaders,
     namespaceUmbrellas,
     privateReactHeaders: PRIVATE_REACT_HEADERS,
     collisions,
@@ -525,10 +576,13 @@ function renderUmbrellaHeader(umbrella /*: Array<string> */) /*: string */ {
  * modules, one per namespace with modular candidates — discovered implicitly
  * by clang via the auto-added header search path. Headers are referenced by
  * their path relative to the Headers root (= the modulemap's directory).
+ * Members in `textualHeaders` (R13) are declared `textual header`.
  */
 function renderNamespaceModuleMap(
   namespaceModules /*: {[string]: Array<string>} */,
+  textualHeaders /*: ReadonlyArray<string> */ = [],
 ) /*: string */ {
+  const textual = new Set(textualHeaders);
   // The module NAME is internal to clang's module graph (consumers never
   // `@import` these; they `#import <ns/...>` and clang maps the header to its
   // module). It only has to be unique and must not alias the `React` framework
@@ -539,7 +593,9 @@ function renderNamespaceModuleMap(
     ns === 'react' ? 'ReactNativeHeaders_react' : ns;
   const blocks = [];
   for (const ns of Object.keys(namespaceModules).sort()) {
-    const headerLines = namespaceModules[ns].map(hh => `  header "${hh}"`);
+    const headerLines = namespaceModules[ns].map(hh =>
+      textual.has(hh) ? `  textual header "${hh}"` : `  header "${hh}"`,
+    );
     // R10: the per-namespace umbrella is itself a module member, so importing
     // it stays modular (otherwise it re-trips -Wnon-modular-include inside the
     // consumer's framework module).
 
@@ -30,7 +30,9 @@
  *     b. A privileged-consumer ObjC++ TU (the Expo shape): textually imports
  *        every R9 textual Fabric header against ReactNativeHeaders.
  *     c. A Swift TU: `import React` + `RCTBridge.moduleRegistry` — the Expo
- *        Swift case, proving the R9 modular header is module-visible.
+ *        Swift case, proving the R9 modular header is module-visible. It is
+ *        compiled twice, the second time with RN_STRICT_API defined, so the
+ *        modules also build for apps that opt into the C++ stable API (R13).
  *     d. Swift C++-interop TUs that import React plus every importable module
  *        declared by the composed ReactNativeHeaders module map, then the
  *        shipped inspector umbrella as an isolated probe module, forcing
@@ -233,7 +235,10 @@ function verifyStructural(
   const rnhHeaders = path.join(rnhSlice, 'Headers');
   assertFileEquals(
     path.join(rnhHeaders, 'module.modulemap'),
-    renderNamespaceModuleMap(plan.namespaceModules),
+    renderNamespaceModuleMap(
+      plan.namespaceModules,
+      plan.textualNamespaceHeaders,
+    ),
     'ReactNativeHeaders module map',
   );
   for (const u of plan.namespaceUmbrellas) {
@@ -290,9 +295,16 @@ function renderObjcFixture(plan /*: HeadersSpecPlan */) /*: string */ {
   for (const name of plan.privateReactHeaders.modular) {
     lines.push(`#import <React/${name}>`);
   }
-  // One header per R5 namespace module precompiles the whole module.
+  // One header per R5 namespace module precompiles the whole module. A textual
+  // member (R13) is expanded in place rather than importing its module, so
+  // pick a modular one; a module with only textual members has nothing to
+  // precompile.
+  const textual = new Set(plan.textualNamespaceHeaders);
   for (const ns of Object.keys(plan.namespaceModules).sort()) {
-    lines.push(`#import <${plan.namespaceModules[ns][0]}>`);
+    const member = plan.namespaceModules[ns].find(h => !textual.has(h));
+    if (member != null) {
+      lines.push(`#import <${member}>`);
+    }
   }
   for (const u of plan.namespaceUmbrellas) {
     lines.push(`#import <${u.relPath}>`);
@@ -438,27 +450,47 @@ function runCompileGates(
 
     const swift = path.join(tmp, 'gate-swift.swift');
     fs.writeFileSync(swift, renderSwiftFixture());
-    xcrun(
-      [
-        'swiftc',
-        '-typecheck',
-        '-sdk',
-        sdk,
-        '-target',
-        SIM_TARGET,
-        '-module-cache-path',
-        path.join(tmp, 'mc-swift'),
-        '-F',
-        reactSlice,
-        '-I',
-        rnhHeaders,
-        '-I',
-        depsHeaders,
-        swift,
-      ],
+    const swiftGate = (
+      moduleCache /*: string */,
+      extraArgs /*: Array<string> */,
+      what /*: string */,
+    ) =>
+      xcrun(
+        [
+          'swiftc',
+          '-typecheck',
+          '-sdk',
+          sdk,
+          '-target',
+          SIM_TARGET,
+          '-module-cache-path',
+          path.join(tmp, moduleCache),
+          '-F',
+          reactSlice,
+          '-I',
+          rnhHeaders,
+          '-I',
+          depsHeaders,
+          ...extraArgs,
+          swift,
+        ],
+        what,
+      );
+    swiftGate(
+      'mc-swift',
+      [],
       'Swift gate (import React + RCTBridge.moduleRegistry)',
     );
     log('compile: Swift moduleRegistry fixture OK.');
+    // R13: an app that opts into the C++ stable API defines RN_STRICT_API for
+    // its own code, and the modules it imports are built with that define. A
+    // guard compiled as a module member would fail this build.
+    swiftGate(
+      'mc-swift-strict',
+      ['-Xcc', '-DRN_STRICT_API'],
+      'Swift strict C++ stable API gate (import React with RN_STRICT_API)',
+    );
+    log('compile: Swift fixture with RN_STRICT_API OK.');
 
     const moduleMap = path.join(rnhHeaders, 'module.modulemap');
     const moduleNames = parseModuleNames(moduleMap);