diff --git a/packages/react-native/ReactCommon/jsi/jsi/decorator.h b/packages/react-native/ReactCommon/jsi/jsi/decorator.h
index f101c1832e61..b78aa14214dd 100644
--- a/packages/react-native/ReactCommon/jsi/jsi/decorator.h
+++ b/packages/react-native/ReactCommon/jsi/jsi/decorator.h
@@ -243,6 +243,10 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
     return plain_.utf16(sym);
   }
 
+  size_t length(const String& str) override {
+    return plain_.length(str);
+  }
+
   void getStringData(
       const jsi::String& str,
       void* ctx,
@@ -822,6 +826,11 @@ class WithRuntimeDecorator : public RuntimeDecorator<Plain, Base> {
     return RD::utf16(sym);
   }
 
+  size_t length(const String& str) override {
+    Around around{with_};
+    return RD::length(str);
+  }
+
   void getStringData(
       const jsi::String& str,
       void* ctx,
diff --git a/packages/react-native/ReactCommon/jsi/jsi/test/testlib.cpp b/packages/react-native/ReactCommon/jsi/jsi/test/testlib.cpp
index 9c056ef03790..38e2fc328b1b 100644
--- a/packages/react-native/ReactCommon/jsi/jsi/test/testlib.cpp
+++ b/packages/react-native/ReactCommon/jsi/jsi/test/testlib.cpp
@@ -138,6 +138,61 @@ TEST_P(JSITest, StringLengthTest) {
   EXPECT_EQ(invalid.length(rt), 2);
 }
 
+TEST_P(JSITest, DecoratorForwardsStringLength) {
+  // Decorators should forward length(const String&) to the decorated runtime
+  // instead of falling back to the default Runtime::length, which copies the
+  // string via utf16() and bypasses the runtime's own implementation.
+  class Inner : public RuntimeDecorator<Runtime, Runtime> {
+   public:
+    explicit Inner(Runtime& rt) : RuntimeDecorator(rt) {}
+
+    size_t length(const String&) override {
+      ++lengthCalls;
+      return 42;
+    }
+
+    std::u16string utf16(const String& str) override {
+      ++utf16Calls;
+      return RuntimeDecorator::utf16(str);
+    }
+
+    int lengthCalls = 0;
+    int utf16Calls = 0;
+  };
+
+  class Outer : public RuntimeDecorator<Runtime, Runtime> {
+   public:
+    explicit Outer(Runtime& rt) : RuntimeDecorator(rt) {}
+  };
+
+  struct Count {
+    void before() {
+      ++beforeCalls;
+    }
+    void after() {
+      ++afterCalls;
+    }
+    int beforeCalls = 0;
+    int afterCalls = 0;
+  };
+
+  Inner inner(rt);
+  String str = String::createFromAscii(inner, "hello");
+
+  Outer outer(inner);
+  EXPECT_EQ(str.length(outer), 42);
+  EXPECT_EQ(inner.lengthCalls, 1);
+  EXPECT_EQ(inner.utf16Calls, 0);
+
+  Count count;
+  WithRuntimeDecorator<Count> with(inner, count);
+  EXPECT_EQ(str.length(with), 42);
+  EXPECT_EQ(inner.lengthCalls, 2);
+  EXPECT_EQ(inner.utf16Calls, 0);
+  EXPECT_EQ(count.beforeCalls, 1);
+  EXPECT_EQ(count.afterCalls, 1);
+}
+
 TEST_P(JSITest, ObjectTest) {
   eval("x = {1:2, '3':4, 5:'six', 'seven':['eight', 'nine']}");
   Object x = rt.global().getPropertyAsObject(rt, "x");
diff --git a/scripts/cxx-api/api-snapshots/ReactAndroidDebugCxx.api b/scripts/cxx-api/api-snapshots/ReactAndroidDebugCxx.api
index 5ee27dc3ab0a..0d655df28e51 100644
--- a/scripts/cxx-api/api-snapshots/ReactAndroidDebugCxx.api
+++ b/scripts/cxx-api/api-snapshots/ReactAndroidDebugCxx.api
@@ -12775,6 +12775,7 @@ class facebook::jsi::RuntimeDecorator : public facebook::jsi::Runtime {
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
@@ -12893,6 +12894,7 @@ class facebook::jsi::WithRuntimeDecorator : public facebook::jsi::RuntimeDecorat
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
diff --git a/scripts/cxx-api/api-snapshots/ReactAndroidNewarchCxx.api b/scripts/cxx-api/api-snapshots/ReactAndroidNewarchCxx.api
index 8126c975bda3..5f1932553e10 100644
--- a/scripts/cxx-api/api-snapshots/ReactAndroidNewarchCxx.api
+++ b/scripts/cxx-api/api-snapshots/ReactAndroidNewarchCxx.api
@@ -12391,6 +12391,7 @@ class facebook::jsi::RuntimeDecorator : public facebook::jsi::Runtime {
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
@@ -12509,6 +12510,7 @@ class facebook::jsi::WithRuntimeDecorator : public facebook::jsi::RuntimeDecorat
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
diff --git a/scripts/cxx-api/api-snapshots/ReactAndroidReleaseCxx.api b/scripts/cxx-api/api-snapshots/ReactAndroidReleaseCxx.api
index 2130ccfe23fa..d209696f6f7e 100644
--- a/scripts/cxx-api/api-snapshots/ReactAndroidReleaseCxx.api
+++ b/scripts/cxx-api/api-snapshots/ReactAndroidReleaseCxx.api
@@ -12622,6 +12622,7 @@ class facebook::jsi::RuntimeDecorator : public facebook::jsi::Runtime {
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
@@ -12740,6 +12741,7 @@ class facebook::jsi::WithRuntimeDecorator : public facebook::jsi::RuntimeDecorat
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
diff --git a/scripts/cxx-api/api-snapshots/ReactAppleDebugCxx.api b/scripts/cxx-api/api-snapshots/ReactAppleDebugCxx.api
index 0168eac13d1b..59bb2ff0982a 100644
--- a/scripts/cxx-api/api-snapshots/ReactAppleDebugCxx.api
+++ b/scripts/cxx-api/api-snapshots/ReactAppleDebugCxx.api
@@ -14544,6 +14544,7 @@ class facebook::jsi::RuntimeDecorator : public facebook::jsi::Runtime {
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
@@ -14662,6 +14663,7 @@ class facebook::jsi::WithRuntimeDecorator : public facebook::jsi::RuntimeDecorat
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
diff --git a/scripts/cxx-api/api-snapshots/ReactAppleNewarchCxx.api b/scripts/cxx-api/api-snapshots/ReactAppleNewarchCxx.api
index bdc9f5eaffe0..5b75d59f8cd2 100644
--- a/scripts/cxx-api/api-snapshots/ReactAppleNewarchCxx.api
+++ b/scripts/cxx-api/api-snapshots/ReactAppleNewarchCxx.api
@@ -14226,6 +14226,7 @@ class facebook::jsi::RuntimeDecorator : public facebook::jsi::Runtime {
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
@@ -14344,6 +14345,7 @@ class facebook::jsi::WithRuntimeDecorator : public facebook::jsi::RuntimeDecorat
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
diff --git a/scripts/cxx-api/api-snapshots/ReactAppleReleaseCxx.api b/scripts/cxx-api/api-snapshots/ReactAppleReleaseCxx.api
index 70e0dc1d72ac..f8c5c2af6d36 100644
--- a/scripts/cxx-api/api-snapshots/ReactAppleReleaseCxx.api
+++ b/scripts/cxx-api/api-snapshots/ReactAppleReleaseCxx.api
@@ -14401,6 +14401,7 @@ class facebook::jsi::RuntimeDecorator : public facebook::jsi::Runtime {
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
@@ -14519,6 +14520,7 @@ class facebook::jsi::WithRuntimeDecorator : public facebook::jsi::RuntimeDecorat
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
diff --git a/scripts/cxx-api/api-snapshots/ReactCommonDebugCxx.api b/scripts/cxx-api/api-snapshots/ReactCommonDebugCxx.api
index a5f5489a6076..a15e1c68fe48 100644
--- a/scripts/cxx-api/api-snapshots/ReactCommonDebugCxx.api
+++ b/scripts/cxx-api/api-snapshots/ReactCommonDebugCxx.api
@@ -9730,6 +9730,7 @@ class facebook::jsi::RuntimeDecorator : public facebook::jsi::Runtime {
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
@@ -9848,6 +9849,7 @@ class facebook::jsi::WithRuntimeDecorator : public facebook::jsi::RuntimeDecorat
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
diff --git a/scripts/cxx-api/api-snapshots/ReactCommonNewarchCxx.api b/scripts/cxx-api/api-snapshots/ReactCommonNewarchCxx.api
index 3637899b15e7..0867f18fcd01 100644
--- a/scripts/cxx-api/api-snapshots/ReactCommonNewarchCxx.api
+++ b/scripts/cxx-api/api-snapshots/ReactCommonNewarchCxx.api
@@ -9554,6 +9554,7 @@ class facebook::jsi::RuntimeDecorator : public facebook::jsi::Runtime {
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
@@ -9672,6 +9673,7 @@ class facebook::jsi::WithRuntimeDecorator : public facebook::jsi::RuntimeDecorat
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
diff --git a/scripts/cxx-api/api-snapshots/ReactCommonReleaseCxx.api b/scripts/cxx-api/api-snapshots/ReactCommonReleaseCxx.api
index caa417e05392..025efa0c8459 100644
--- a/scripts/cxx-api/api-snapshots/ReactCommonReleaseCxx.api
+++ b/scripts/cxx-api/api-snapshots/ReactCommonReleaseCxx.api
@@ -9721,6 +9721,7 @@ class facebook::jsi::RuntimeDecorator : public facebook::jsi::Runtime {
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;
@@ -9839,6 +9840,7 @@ class facebook::jsi::WithRuntimeDecorator : public facebook::jsi::RuntimeDecorat
   protected virtual facebook::jsi::WeakObject createWeakObject(const facebook::jsi::Object& o) override;
   protected virtual size_t byteLength(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t byteOffset(const facebook::jsi::TypedArray& typedArray) override;
+  protected virtual size_t length(const facebook::jsi::String& str) override;
   protected virtual size_t length(const facebook::jsi::TypedArray& typedArray) override;
   protected virtual size_t push(const facebook::jsi::Array& a, const facebook::jsi::Value* elements, size_t count) override;
   protected virtual size_t size(const facebook::jsi::Array& a) override;