diff --git a/packages/react-native-renderer/src/ReactNativeAttributePayloadFabric.js b/packages/react-native-renderer/src/ReactNativeAttributePayloadFabric.js
index 0143ab28431c9..eed17b799e7cc 100644
--- a/packages/react-native-renderer/src/ReactNativeAttributePayloadFabric.js
+++ b/packages/react-native-renderer/src/ReactNativeAttributePayloadFabric.js
@@ -449,17 +449,24 @@ function fastAddProperties(
   props: Object,
   validAttributes: AttributeConfiguration,
 ): null | Object {
-  let attributeConfig;
-  let prop;
+  // Flatten nested style props.
+  if (isArray(props)) {
+    for (let i = 0; i < props.length; i++) {
+      payload = fastAddProperties(payload, props[i], validAttributes);
+    }
+    return payload;
+  }
 
   for (const propKey in props) {
-    prop = props[propKey];
+    const prop = props[propKey];
 
     if (prop === undefined) {
       continue;
     }
 
-    attributeConfig = ((validAttributes[propKey]: any): AttributeConfiguration);
+    const attributeConfig = ((validAttributes[
+      propKey
+    ]: any): AttributeConfiguration);
 
     if (attributeConfig == null) {
       continue;
@@ -477,7 +484,7 @@ function fastAddProperties(
       // An atomic prop with custom processing.
       newValue = attributeConfig.process(prop);
     } else if (typeof attributeConfig.diff === 'function') {
-      // An atomic prop with custom diffing. We don't do diffing here.
+      // An atomic prop with custom diffing. We don't need to do diffing when adding props.
       newValue = prop;
     }
 
@@ -489,17 +496,6 @@ function fastAddProperties(
       continue;
     }
 
-    // Not-atomic prop that needs to be flattened. Likely it's the 'style' prop.
-
-    // It can be an array.
-    if (isArray(prop)) {
-      for (let i = 0; i < prop.length; i++) {
-        payload = fastAddProperties(payload, prop[i], attributeConfig);
-      }
-      continue;
-    }
-
-    // Or it can be an object.
     payload = fastAddProperties(payload, prop, attributeConfig);
   }
 
@@ -514,11 +510,7 @@ function addProperties(
   props: Object,
   validAttributes: AttributeConfiguration,
 ): null | Object {
-  if (enableAddPropertiesFastPath) {
-    return fastAddProperties(updatePayload, props, validAttributes);
-  } else {
-    return diffProperties(updatePayload, emptyObject, props, validAttributes);
-  }
+  return diffProperties(updatePayload, emptyObject, props, validAttributes);
 }
 
 /**
@@ -538,11 +530,11 @@ export function create(
   props: Object,
   validAttributes: AttributeConfiguration,
 ): null | Object {
-  return addProperties(
-    null, // updatePayload
-    props,
-    validAttributes,
-  );
+  if (enableAddPropertiesFastPath) {
+    return fastAddProperties(null, props, validAttributes);
+  } else {
+    return addProperties(null, props, validAttributes);
+  }
 }
 
 export function diff(
diff --git a/packages/react-native-renderer/src/__tests__/ReactNativeAttributePayloadFabric-test.internal.js b/packages/react-native-renderer/src/__tests__/ReactNativeAttributePayloadFabric-test.internal.js
new file mode 100644
index 0000000000000..4df4507a93d38
--- /dev/null
+++ b/packages/react-native-renderer/src/__tests__/ReactNativeAttributePayloadFabric-test.internal.js
@@ -0,0 +1,446 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @jest-environment node
+ */
+'use strict';
+
+const {diff, create} = require('../ReactNativeAttributePayloadFabric');
+
+describe('ReactNativeAttributePayload.create', () => {
+  it('should work with simple example', () => {
+    expect(create({b: 2, c: 3}, {a: true, b: true})).toEqual({
+      b: 2,
+    });
+  });
+
+  it('should work with complex example', () => {
+    const validAttributes = {
+      style: {
+        position: true,
+        zIndex: true,
+        flexGrow: true,
+        flexShrink: true,
+        flexDirection: true,
+        overflow: true,
+        backgroundColor: true,
+      },
+    };
+
+    expect(
+      create(
+        {
+          style: [
+            {
+              flexGrow: 1,
+              flexShrink: 1,
+              flexDirection: 'row',
+              overflow: 'scroll',
+            },
+            [
+              {position: 'relative', zIndex: 2},
+              {flexGrow: 0},
+              {backgroundColor: 'red'},
+            ],
+          ],
+        },
+        validAttributes,
+      ),
+    ).toEqual({
+      flexGrow: 0,
+      flexShrink: 1,
+      flexDirection: 'row',
+      overflow: 'scroll',
+      position: 'relative',
+      zIndex: 2,
+      backgroundColor: 'red',
+    });
+  });
+
+  it('should ignore fields that are set to undefined', () => {
+    expect(create({}, {a: true})).toEqual(null);
+    expect(create({a: undefined}, {a: true})).toEqual(null);
+    expect(create({a: undefined, b: undefined}, {a: true, b: true})).toEqual(
+      null,
+    );
+    expect(
+      create({a: undefined, b: undefined, c: 1}, {a: true, b: true}),
+    ).toEqual(null);
+    expect(
+      create({a: undefined, b: undefined, c: 1}, {a: true, b: true, c: true}),
+    ).toEqual({c: 1});
+    expect(
+      create({a: 1, b: undefined, c: 2}, {a: true, b: true, c: true}),
+    ).toEqual({a: 1, c: 2});
+  });
+
+  it('should ignore invalid fields', () => {
+    expect(create({b: 2}, {})).toEqual(null);
+  });
+
+  it('should not use the diff attribute', () => {
+    const diffA = jest.fn();
+    expect(create({a: [2]}, {a: {diff: diffA}})).toEqual({a: [2]});
+    expect(diffA).not.toBeCalled();
+  });
+
+  it('should use the process attribute', () => {
+    const processA = jest.fn(a => a + 1);
+    expect(create({a: 2}, {a: {process: processA}})).toEqual({a: 3});
+    expect(processA).toBeCalledWith(2);
+  });
+
+  it('should work with undefined styles', () => {
+    expect(create({style: undefined}, {style: {b: true}})).toEqual(null);
+    expect(create({style: {a: '#ffffff', b: 1}}, {style: {b: true}})).toEqual({
+      b: 1,
+    });
+  });
+
+  it('should flatten nested styles and predefined styles', () => {
+    const validStyleAttribute = {someStyle: {foo: true, bar: true}};
+    expect(
+      create({someStyle: [{foo: 1}, {bar: 2}]}, validStyleAttribute),
+    ).toEqual({foo: 1, bar: 2});
+    expect(create({}, validStyleAttribute)).toEqual(null);
+    const barStyle = {
+      bar: 3,
+    };
+    expect(
+      create(
+        {someStyle: [[{foo: 1}, {foo: 2}], barStyle]},
+        validStyleAttribute,
+      ),
+    ).toEqual({foo: 2, bar: 3});
+  });
+
+  it('should not flatten nested props if attribute config is a primitive or only has diff/process', () => {
+    expect(create({a: {foo: 1, bar: 2}}, {a: true})).toEqual({
+      a: {foo: 1, bar: 2},
+    });
+    expect(create({a: [{foo: 1}, {bar: 2}]}, {a: true})).toEqual({
+      a: [{foo: 1}, {bar: 2}],
+    });
+    expect(create({a: {foo: 1, bar: 2}}, {a: {diff: a => a}})).toEqual({
+      a: {foo: 1, bar: 2},
+    });
+    expect(
+      create({a: [{foo: 1}, {bar: 2}]}, {a: {diff: a => a, process: a => a}}),
+    ).toEqual({a: [{foo: 1}, {bar: 2}]});
+  });
+
+  it('handles attributes defined multiple times', () => {
+    const validAttributes = {foo: true, style: {foo: true}};
+    expect(create({foo: 4, style: {foo: 2}}, validAttributes)).toEqual({
+      foo: 2,
+    });
+    expect(create({style: {foo: 2}}, validAttributes)).toEqual({
+      foo: 2,
+    });
+    expect(create({style: {foo: 2}, foo: 4}, validAttributes)).toEqual({
+      foo: 4,
+    });
+    expect(create({foo: 4, style: {foo: null}}, validAttributes)).toEqual({
+      foo: null, // this should ideally be null.
+    });
+    expect(
+      create({foo: 4, style: [{foo: null}, {foo: 5}]}, validAttributes),
+    ).toEqual({
+      foo: 5,
+    });
+  });
+
+  // Function properties are just markers to native that events should be sent.
+  it('should convert functions to booleans', () => {
+    expect(
+      create(
+        {
+          a: function () {
+            return 9;
+          },
+          b: function () {
+            return 3;
+          },
+        },
+        {a: true, b: true},
+      ),
+    ).toEqual({a: true, b: true});
+  });
+});
+
+describe('ReactNativeAttributePayload.diff', () => {
+  it('should work with simple example', () => {
+    expect(diff({a: 1, c: 3}, {b: 2, c: 3}, {a: true, b: true})).toEqual({
+      a: null,
+      b: 2,
+    });
+  });
+
+  it('should skip fields that are equal', () => {
+    expect(
+      diff(
+        {a: 1, b: 'two', c: true, d: false, e: undefined, f: 0},
+        {a: 1, b: 'two', c: true, d: false, e: undefined, f: 0},
+        {a: true, b: true, c: true, d: true, e: true, f: true},
+      ),
+    ).toEqual(null);
+  });
+
+  it('should remove fields', () => {
+    expect(diff({a: 1}, {}, {a: true})).toEqual({a: null});
+  });
+
+  it('should remove fields that are set to undefined', () => {
+    expect(diff({a: 1}, {a: undefined}, {a: true})).toEqual({a: null});
+  });
+
+  it('should ignore invalid fields', () => {
+    expect(diff({a: 1}, {b: 2}, {})).toEqual(null);
+  });
+
+  it('should use the diff attribute', () => {
+    const diffA = jest.fn((a, b) => true);
+    const diffB = jest.fn((a, b) => false);
+    expect(
+      diff(
+        {a: [1], b: [3]},
+        {a: [2], b: [4]},
+        {a: {diff: diffA}, b: {diff: diffB}},
+      ),
+    ).toEqual({a: [2]});
+    expect(diffA).toBeCalledWith([1], [2]);
+    expect(diffB).toBeCalledWith([3], [4]);
+  });
+
+  it('should not use the diff attribute on addition/removal', () => {
+    const diffA = jest.fn();
+    const diffB = jest.fn();
+    expect(
+      diff({a: [1]}, {b: [2]}, {a: {diff: diffA}, b: {diff: diffB}}),
+    ).toEqual({a: null, b: [2]});
+    expect(diffA).not.toBeCalled();
+    expect(diffB).not.toBeCalled();
+  });
+
+  it('should do deep diffs of Objects by default', () => {
+    expect(
+      diff(
+        {a: [1], b: {k: [3, 4]}, c: {k: [4, 4]}},
+        {a: [2], b: {k: [3, 4]}, c: {k: [4, 5]}},
+        {a: true, b: true, c: true},
+      ),
+    ).toEqual({a: [2], c: {k: [4, 5]}});
+  });
+
+  it('should work with undefined styles', () => {
+    expect(
+      diff(
+        {style: {a: '#ffffff', b: 1}},
+        {style: undefined},
+        {style: {b: true}},
+      ),
+    ).toEqual({b: null});
+    expect(
+      diff(
+        {style: undefined},
+        {style: {a: '#ffffff', b: 1}},
+        {style: {b: true}},
+      ),
+    ).toEqual({b: 1});
+    expect(
+      diff({style: undefined}, {style: undefined}, {style: {b: true}}),
+    ).toEqual(null);
+  });
+
+  it('should work with empty styles', () => {
+    expect(diff({a: 1, c: 3}, {}, {a: true, b: true})).toEqual({a: null});
+    expect(diff({}, {a: 1, c: 3}, {a: true, b: true})).toEqual({a: 1});
+    expect(diff({}, {}, {a: true, b: true})).toEqual(null);
+  });
+
+  it('should flatten nested styles and predefined styles', () => {
+    const validStyleAttribute = {someStyle: {foo: true, bar: true}};
+
+    expect(
+      diff({}, {someStyle: [{foo: 1}, {bar: 2}]}, validStyleAttribute),
+    ).toEqual({foo: 1, bar: 2});
+
+    expect(
+      diff({someStyle: [{foo: 1}, {bar: 2}]}, {}, validStyleAttribute),
+    ).toEqual({foo: null, bar: null});
+
+    const barStyle = {
+      bar: 3,
+    };
+
+    expect(
+      diff(
+        {},
+        {someStyle: [[{foo: 1}, {foo: 2}], barStyle]},
+        validStyleAttribute,
+      ),
+    ).toEqual({foo: 2, bar: 3});
+  });
+
+  it('should reset a value to a previous if it is removed', () => {
+    const validStyleAttribute = {someStyle: {foo: true, bar: true}};
+
+    expect(
+      diff(
+        {someStyle: [{foo: 1}, {foo: 3}]},
+        {someStyle: [{foo: 1}, {bar: 2}]},
+        validStyleAttribute,
+      ),
+    ).toEqual({foo: 1, bar: 2});
+  });
+
+  it('should not clear removed props if they are still in another slot', () => {
+    const validStyleAttribute = {someStyle: {foo: true, bar: true}};
+
+    expect(
+      diff(
+        {someStyle: [{}, {foo: 3, bar: 2}]},
+        {someStyle: [{foo: 3}, {bar: 2}]},
+        validStyleAttribute,
+      ),
+    ).toEqual({foo: 3}); // this should ideally be null. heuristic tradeoff.
+
+    expect(
+      diff(
+        {someStyle: [{}, {foo: 3, bar: 2}]},
+        {someStyle: [{foo: 1, bar: 1}, {bar: 2}]},
+        validStyleAttribute,
+      ),
+    ).toEqual({bar: 2, foo: 1});
+  });
+
+  it('should clear a prop if a later style is explicit null/undefined', () => {
+    const validStyleAttribute = {someStyle: {foo: true, bar: true}};
+    expect(
+      diff(
+        {someStyle: [{}, {foo: 3, bar: 2}]},
+        {someStyle: [{foo: 1}, {bar: 2, foo: null}]},
+        validStyleAttribute,
+      ),
+    ).toEqual({foo: null});
+
+    expect(
+      diff(
+        {someStyle: [{foo: 3}, {foo: null, bar: 2}]},
+        {someStyle: [{foo: null}, {bar: 2}]},
+        validStyleAttribute,
+      ),
+    ).toEqual({foo: null});
+
+    expect(
+      diff(
+        {someStyle: [{foo: 1}, {foo: null}]},
+        {someStyle: [{foo: 2}, {foo: null}]},
+        validStyleAttribute,
+      ),
+    ).toEqual({foo: null}); // this should ideally be null. heuristic.
+
+    // Test the same case with object equality because an early bailout doesn't
+    // work in this case.
+    const fooObj = {foo: 3};
+    expect(
+      diff(
+        {someStyle: [{foo: 1}, fooObj]},
+        {someStyle: [{foo: 2}, fooObj]},
+        validStyleAttribute,
+      ),
+    ).toEqual({foo: 3}); // this should ideally be null. heuristic.
+
+    expect(
+      diff(
+        {someStyle: [{foo: 1}, {foo: 3}]},
+        {someStyle: [{foo: 2}, {foo: undefined}]},
+        validStyleAttribute,
+      ),
+    ).toEqual({foo: null}); // this should ideally be null. heuristic.
+  });
+
+  it('handles attributes defined multiple times', () => {
+    const validAttributes = {foo: true, style: {foo: true}};
+    expect(diff({}, {foo: 4, style: {foo: 2}}, validAttributes)).toEqual({
+      foo: 2,
+    });
+    expect(diff({foo: 4}, {style: {foo: 2}}, validAttributes)).toEqual({
+      foo: 2,
+    });
+    expect(diff({style: {foo: 2}}, {foo: 4}, validAttributes)).toEqual({
+      foo: 4,
+    });
+  });
+
+  // Function properties are just markers to native that events should be sent.
+  it('should convert functions to booleans', () => {
+    // Note that if the property changes from one function to another, we don't
+    // need to send an update.
+    expect(
+      diff(
+        {
+          a: function () {
+            return 1;
+          },
+          b: function () {
+            return 2;
+          },
+          c: 3,
+        },
+        {
+          b: function () {
+            return 9;
+          },
+          c: function () {
+            return 3;
+          },
+        },
+        {a: true, b: true, c: true},
+      ),
+    ).toEqual({a: null, c: true});
+  });
+
+  it('should skip changed functions', () => {
+    expect(
+      diff(
+        {
+          a: function () {
+            return 1;
+          },
+        },
+        {
+          a: function () {
+            return 9;
+          },
+        },
+        {a: true},
+      ),
+    ).toEqual(null);
+  });
+
+  it('should skip deeply-nested changed functions', () => {
+    expect(
+      diff(
+        {
+          wrapper: {
+            a: function () {
+              return 1;
+            },
+          },
+        },
+        {
+          wrapper: {
+            a: function () {
+              return 9;
+            },
+          },
+        },
+        {wrapper: true},
+      ),
+    ).toEqual(null);
+  });
+});