facebook · josephsavona · Oct 2, 2025 · Oct 2, 2025
diff --git a/compiler/packages/babel-plugin-react-compiler/src/HIR/Environment.ts b/compiler/packages/babel-plugin-react-compiler/src/HIR/Environment.ts
@@ -210,7 +210,7 @@ export const EnvironmentConfigSchema = z.object({
    * that if a useEffect or useCallback references a function value, that function value will be
    * considered frozen, and in turn all of its referenced variables will be considered frozen as well.
    */
-  enablePreserveExistingMemoizationGuarantees: z.boolean().default(false),
+  enablePreserveExistingMemoizationGuarantees: z.boolean().default(true),
 
   /**
    * Validates that all useMemo/useCallback values are also memoized by Forget. This mode can be

diff --git a/...-compiler/src/__tests__/fixtures/compiler/allocating-primitive-as-dep.expect.md b/...-compiler/src/__tests__/fixtures/compiler/allocating-primitive-as-dep.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 // bar(props.b) is an allocating expression that produces a primitive, which means
 // that Forget should memoize it.
 // Correctness:
@@ -16,7 +17,8 @@ function AllocatingPrimitiveAsDep(props) {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime"; // bar(props.b) is an allocating expression that produces a primitive, which means
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
+// bar(props.b) is an allocating expression that produces a primitive, which means
 // that Forget should memoize it.
 // Correctness:
 //   - y depends on either bar(props.b) or bar(props.b) + 1

diff --git a/...abel-plugin-react-compiler/src/__tests__/fixtures/compiler/allocating-primitive-as-dep.js b/...abel-plugin-react-compiler/src/__tests__/fixtures/compiler/allocating-primitive-as-dep.js
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 // bar(props.b) is an allocating expression that produces a primitive, which means
 // that Forget should memoize it.
 // Correctness:

diff --git a/...r/src/__tests__/fixtures/compiler/allow-modify-global-in-callback-jsx.expect.md b/...r/src/__tests__/fixtures/compiler/allow-modify-global-in-callback-jsx.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 
 const someGlobal = {value: 0};
@@ -32,7 +33,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import { useMemo } from "react";
 
 const someGlobal = { value: 0 };

diff --git a/...gin-react-compiler/src/__tests__/fixtures/compiler/allow-modify-global-in-callback-jsx.js b/...gin-react-compiler/src/__tests__/fixtures/compiler/allow-modify-global-in-callback-jsx.js
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 
 const someGlobal = {value: 0};

diff --git a/...src/__tests__/fixtures/compiler/computed-load-primitive-as-dependency.expect.md b/...src/__tests__/fixtures/compiler/computed-load-primitive-as-dependency.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   let a = foo();
   // freeze `a` so we know the next line cannot mutate it
@@ -17,7 +18,7 @@ function Component(props) {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const $ = _c(2);
   const a = foo();

diff --git a/...n-react-compiler/src/__tests__/fixtures/compiler/computed-load-primitive-as-dependency.js b/...n-react-compiler/src/__tests__/fixtures/compiler/computed-load-primitive-as-dependency.js
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   let a = foo();
   // freeze `a` so we know the next line cannot mutate it

diff --git a/...src/__tests__/fixtures/compiler/constant-propagation-template-literal.expect.md b/...src/__tests__/fixtures/compiler/constant-propagation-template-literal.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import {Stringify, identity} from 'shared-runtime';
 
 function foo() {
@@ -64,7 +65,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import { Stringify, identity } from "shared-runtime";
 
 function foo() {

diff --git a/...n-react-compiler/src/__tests__/fixtures/compiler/constant-propagation-template-literal.js b/...n-react-compiler/src/__tests__/fixtures/compiler/constant-propagation-template-literal.js
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import {Stringify, identity} from 'shared-runtime';
 
 function foo() {

diff --git a/...r/src/__tests__/fixtures/compiler/context-variable-as-jsx-element-tag.expect.md b/...r/src/__tests__/fixtures/compiler/context-variable-as-jsx-element-tag.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {Stringify} from 'shared-runtime';
 
@@ -25,7 +26,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import { useMemo } from "react";
 import { Stringify } from "shared-runtime";
 

diff --git a/...gin-react-compiler/src/__tests__/fixtures/compiler/context-variable-as-jsx-element-tag.js b/...gin-react-compiler/src/__tests__/fixtures/compiler/context-variable-as-jsx-element-tag.js
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {Stringify} from 'shared-runtime';
 

diff --git a/...piler/src/__tests__/fixtures/compiler/destructure-direct-reassignment.expect.md b/...piler/src/__tests__/fixtures/compiler/destructure-direct-reassignment.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function foo(props) {
   let x, y;
   ({x, y} = {x: props.a, y: props.b});
@@ -21,6 +22,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function foo(props) {
   let x;
   let y;

diff --git a/...-plugin-react-compiler/src/__tests__/fixtures/compiler/destructure-direct-reassignment.js b/...-plugin-react-compiler/src/__tests__/fixtures/compiler/destructure-direct-reassignment.js
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 function foo(props) {
   let x, y;
   ({x, y} = {x: props.a, y: props.b});

diff --git a/...alled-function-capturing-mutable-values-memoizes-with-captures-values.expect.md b/...alled-function-capturing-mutable-values-memoizes-with-captures-values.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @flow @enableNewMutationAliasingModel
+// @flow @enableNewMutationAliasingModel @enablePreserveExistingMemoizationGuarantees:false
 /**
  * This hook returns a function that when called with an input object,
  * will return the result of mapping that input with the supplied map

diff --git a/...error.invalid-uncalled-function-capturing-mutable-values-memoizes-with-captures-values.js b/...error.invalid-uncalled-function-capturing-mutable-values-memoizes-with-captures-values.js
@@ -1,4 +1,4 @@
-// @flow @enableNewMutationAliasingModel
+// @flow @enableNewMutationAliasingModel @enablePreserveExistingMemoizationGuarantees:false
 /**
  * This hook returns a function that when called with an input object,
  * will return the result of mapping that input with the supplied map

diff --git a/...zation-inner-destructured-value-mistaken-as-dependency-later-mutation.expect.md b/...zation-inner-destructured-value-mistaken-as-dependency-later-mutation.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 
 /**
  * Repro from https://github.com/facebook/react/issues/34262

diff --git a/...ro-preserve-memoization-inner-destructured-value-mistaken-as-dependency-later-mutation.js b/...ro-preserve-memoization-inner-destructured-value-mistaken-as-dependency-later-mutation.js
@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 
 /**
  * Repro from https://github.com/facebook/react/issues/34262

diff --git a/...moization-inner-destructured-value-mistaken-as-dependency-mutated-dep.expect.md b/...moization-inner-destructured-value-mistaken-as-dependency-mutated-dep.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 
 import {identity, Stringify, useHook} from 'shared-runtime';
 

diff --git a/...repro-preserve-memoization-inner-destructured-value-mistaken-as-dependency-mutated-dep.js b/...repro-preserve-memoization-inner-destructured-value-mistaken-as-dependency-mutated-dep.js
@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 
 import {identity, Stringify, useHook} from 'shared-runtime';
 

diff --git a/...sed-memoization-from-capture-in-invoked-function-inferred-as-mutation.expect.md b/...sed-memoization-from-capture-in-invoked-function-inferred-as-mutation.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @flow @validatePreserveExistingMemoizationGuarantees
+// @flow @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {logValue, useFragment, useHook, typedLog} from 'shared-runtime';
 

diff --git a/...or.todo-repro-missed-memoization-from-capture-in-invoked-function-inferred-as-mutation.js b/...or.todo-repro-missed-memoization-from-capture-in-invoked-function-inferred-as-mutation.js
@@ -1,4 +1,4 @@
-// @flow @validatePreserveExistingMemoizationGuarantees
+// @flow @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {logValue, useFragment, useHook, typedLog} from 'shared-runtime';
 

diff --git a/.../error.todo-repro-missed-memoization-from-inferred-mutation-in-logger.expect.md b/.../error.todo-repro-missed-memoization-from-inferred-mutation-in-logger.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @flow @validatePreserveExistingMemoizationGuarantees
+// @flow @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useFragment} from 'react-relay';
 import LogEvent from 'LogEvent';
 import {useCallback, useMemo} from 'react';

diff --git a/...fixtures/compiler/error.todo-repro-missed-memoization-from-inferred-mutation-in-logger.js b/...fixtures/compiler/error.todo-repro-missed-memoization-from-inferred-mutation-in-logger.js
@@ -1,4 +1,4 @@
-// @flow @validatePreserveExistingMemoizationGuarantees
+// @flow @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useFragment} from 'react-relay';
 import LogEvent from 'LogEvent';
 import {useCallback, useMemo} from 'react';

diff --git a/...ler/error.todo-repro-unmemoized-callback-captured-in-context-variable.expect.md b/...ler/error.todo-repro-unmemoized-callback-captured-in-context-variable.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {ValidateMemoization, useHook} from 'shared-runtime';
 

diff --git a/..._/fixtures/compiler/error.todo-repro-unmemoized-callback-captured-in-context-variable.tsx b/..._/fixtures/compiler/error.todo-repro-unmemoized-callback-captured-in-context-variable.tsx
@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo} from 'react';
 import {ValidateMemoization, useHook} from 'shared-runtime';
 

diff --git a/...rc/__tests__/fixtures/compiler/error.validate-object-entries-mutation.expect.md b/...rc/__tests__/fixtures/compiler/error.validate-object-entries-mutation.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {makeObject_Primitives, Stringify} from 'shared-runtime';
 
 function Component(props) {

diff --git a/...-react-compiler/src/__tests__/fixtures/compiler/error.validate-object-entries-mutation.js b/...-react-compiler/src/__tests__/fixtures/compiler/error.validate-object-entries-mutation.js
@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {makeObject_Primitives, Stringify} from 'shared-runtime';
 
 function Component(props) {

diff --git a/...src/__tests__/fixtures/compiler/error.validate-object-values-mutation.expect.md b/...src/__tests__/fixtures/compiler/error.validate-object-values-mutation.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {makeObject_Primitives, Stringify} from 'shared-runtime';
 
 function Component(props) {

diff --git a/...n-react-compiler/src/__tests__/fixtures/compiler/error.validate-object-values-mutation.js b/...n-react-compiler/src/__tests__/fixtures/compiler/error.validate-object-values-mutation.js
@@ -1,4 +1,4 @@
-// @validatePreserveExistingMemoizationGuarantees
+// @validatePreserveExistingMemoizationGuarantees @enablePreserveExistingMemoizationGuarantees:false
 import {makeObject_Primitives, Stringify} from 'shared-runtime';
 
 function Component(props) {

diff --git a/...mpiler/src/__tests__/fixtures/compiler/existing-variables-with-c-name.expect.md b/...mpiler/src/__tests__/fixtures/compiler/existing-variables-with-c-name.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo, useState} from 'react';
 import {ValidateMemoization} from 'shared-runtime';
 
@@ -28,7 +29,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c2 } from "react/compiler-runtime";
+import { c as _c2 } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import { useMemo, useState } from "react";
 import { ValidateMemoization } from "shared-runtime";
 

diff --git a/...l-plugin-react-compiler/src/__tests__/fixtures/compiler/existing-variables-with-c-name.js b/...l-plugin-react-compiler/src/__tests__/fixtures/compiler/existing-variables-with-c-name.js
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import {useMemo, useState} from 'react';
 import {ValidateMemoization} from 'shared-runtime';
 

diff --git a/..._tests__/fixtures/compiler/fbt/bug-fbt-plural-multiple-function-calls.expect.md b/..._tests__/fixtures/compiler/fbt/bug-fbt-plural-multiple-function-calls.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 import fbt from 'fbt';
 
 /**
@@ -35,7 +36,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime";
+import { c as _c } from "react/compiler-runtime"; // @enablePreserveExistingMemoizationGuarantees:false
 import fbt from "fbt";
 
 /**

diff --git a/...ct-compiler/src/__tests__/fixtures/compiler/fbt/bug-fbt-plural-multiple-function-calls.ts b/...ct-compiler/src/__tests__/fixtures/compiler/fbt/bug-fbt-plural-multiple-function-calls.ts
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 import fbt from 'fbt';
 
 /**

diff --git a/...piler/src/__tests__/fixtures/compiler/for-in-statement-type-inference.expect.md b/...piler/src/__tests__/fixtures/compiler/for-in-statement-type-inference.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 const {identity, mutate} = require('shared-runtime');
 
 function Component(props) {
@@ -24,6 +25,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 const { identity, mutate } = require("shared-runtime");
 
 function Component(props) {

diff --git a/...-plugin-react-compiler/src/__tests__/fixtures/compiler/for-in-statement-type-inference.js b/...-plugin-react-compiler/src/__tests__/fixtures/compiler/for-in-statement-type-inference.js
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 const {identity, mutate} = require('shared-runtime');
 
 function Component(props) {

diff --git a/...ugin-react-compiler/src/__tests__/fixtures/compiler/hooks-with-prefix.expect.md b/...ugin-react-compiler/src/__tests__/fixtures/compiler/hooks-with-prefix.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @hookPattern:".*\b(use[^$]+)$"
+// @hookPattern:".*\b(use[^$]+)$" @enablePreserveExistingMemoizationGuarantees:false
 
 import * as React from 'react';
 import {makeArray, useHook} from 'shared-runtime';
@@ -36,7 +36,7 @@ export const FIXTURE_ENTRYPOINT = {
 ## Code
 
 ```javascript
-import { c as _c } from "react/compiler-runtime"; // @hookPattern:".*\b(use[^$]+)$"
+import { c as _c } from "react/compiler-runtime"; // @hookPattern:".*\b(use[^$]+)$" @enablePreserveExistingMemoizationGuarantees:false
 
 import * as React from "react";
 import { makeArray, useHook } from "shared-runtime";

diff --git a/...packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/hooks-with-prefix.js b/...packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/hooks-with-prefix.js
@@ -1,4 +1,4 @@
-// @hookPattern:".*\b(use[^$]+)$"
+// @hookPattern:".*\b(use[^$]+)$" @enablePreserveExistingMemoizationGuarantees:false
 
 import * as React from 'react';
 import {makeArray, useHook} from 'shared-runtime';

diff --git a/...-react-compiler/src/__tests__/fixtures/compiler/infer-computed-delete.expect.md b/...-react-compiler/src/__tests__/fixtures/compiler/infer-computed-delete.expect.md
@@ -2,7 +2,7 @@
 ## Input
 
 ```javascript
-// @debug
+// @debug @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x[props.value];
@@ -14,7 +14,7 @@ function Component(props) {
 ## Code
 
 ```javascript
-// @debug
+// @debug @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x[props.value];

diff --git a/...ages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/infer-computed-delete.js b/...ages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/infer-computed-delete.js
@@ -1,4 +1,4 @@
-// @debug
+// @debug @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x[props.value];

diff --git a/...-react-compiler/src/__tests__/fixtures/compiler/infer-property-delete.expect.md b/...-react-compiler/src/__tests__/fixtures/compiler/infer-property-delete.expect.md
@@ -2,6 +2,7 @@
 ## Input
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x.value;
@@ -13,6 +14,7 @@ function Component(props) {
 ## Code
 
 ```javascript
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x.value;

diff --git a/...ages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/infer-property-delete.js b/...ages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/infer-property-delete.js
@@ -1,3 +1,4 @@
+// @enablePreserveExistingMemoizationGuarantees:false
 function Component(props) {
   const x = makeObject();
   const y = delete x.value;