diff --git a/compiler/forget/packages/playground/lib/patchUseMemoCache.ts b/compiler/forget/packages/playground/lib/patchUseMemoCache.ts
index 9e790133a8..481c6d0636 100644
--- a/compiler/forget/packages/playground/lib/patchUseMemoCache.ts
+++ b/compiler/forget/packages/playground/lib/patchUseMemoCache.ts
@@ -28,6 +28,108 @@ export default function patchUseMemoCache(reactSource: string) {
       return value;
     };
 
+    // Utility functions
+    function isWriteable(desc) {
+      return (desc.writable || desc.set) && desc.configurable;
+    }
+    function getOrInsertDefault(m, k) {
+      const entry = m.get(k);
+      if (entry) {
+        return { existed: true, entry };
+      }
+      else {
+        const newEntry = new Map();
+        m.set(k, newEntry);
+        return { existed: false, entry: newEntry };
+      }
+    }
+    function buildMakeReadOnly(logger, skippedClasses, isInROMode) {
+      // All saved proxys
+      const savedROObjects = new WeakMap();
+      // Overwrites an object property with its proxy and saves its original value
+      function addProperty(obj, key, prop, savedEntries) {
+        const proxy = {
+          get() {
+            // read from backing cache entry
+            return makeReadOnly(savedEntries.get(key).savedVal);
+          },
+          set(newVal) {
+            if (isInROMode()) {
+              logger("FORGET_MUTATE_IMMUT", key, newVal);
+            }
+            // update backing cache entry
+            savedEntries.get(key).savedVal = newVal;
+          },
+        };
+        if (prop.configurable != null) {
+          proxy.configurable = prop.configurable;
+        }
+        if (prop.enumerable != null) {
+          proxy.enumerable = prop.enumerable;
+        }
+        savedEntries.set(key, { savedVal: obj[key], getter: proxy.get });
+        Object.defineProperty(obj, key, proxy);
+      }
+      // Changes an object to be read-only, returns its input
+      function makeReadOnly(o) {
+        if (typeof o !== "object" || o == null) {
+          return o;
+        }
+        else if (skippedClasses.includes(o.constructor.name)) {
+          return o;
+        }
+        const { existed, entry: cache } = getOrInsertDefault(savedROObjects, o);
+        for (const [k, entry] of cache.entries()) {
+          const currentProp = Object.getOwnPropertyDescriptor(o, k);
+          if (currentProp && !isWriteable(currentProp)) {
+            continue;
+          }
+          const currentPropGetter = currentProp?.get;
+          const cachedGetter = entry.getter;
+          if (currentPropGetter !== cachedGetter) {
+            // cache is currently holding an old property
+            //  - it may have been deleted
+            //  - it may have been deleted + re-set
+            //  (meaning that new value is not proxied,
+            //   and the current proxied value is stale)
+            cache.delete(k);
+            if (!currentProp && isInROMode()) {
+              logger("FORGET_DELETE_PROP_IMMUT", k);
+            }
+            else if (currentProp) {
+              if (isInROMode()) {
+                logger("FORGET_CHANGE_PROP_IMMUT", k);
+              }
+              addProperty(o, k, currentProp, cache);
+            }
+          }
+        }
+        for (const [k, prop] of Object.entries(Object.getOwnPropertyDescriptors(o))) {
+          if (!cache.has(k) && isWriteable(prop)) {
+            if (isInROMode() && existed) {
+              logger("FORGET_ADD_PROP_IMMUT", k);
+            }
+            addProperty(o, k, prop, cache);
+          }
+        }
+        return o;
+      }
+      return makeReadOnly;
+    }
+    
+    useMemoCache.makeReadOnly = buildMakeReadOnly(
+      console.log,
+      [
+        // some immutVals have a transitive reference to global (e.g. LexicalEditor)
+        'Window',
+        'Storage', // useBrowserStorage
+        // some immutVals (e.g. ref.current) have a ref to DOM nodes (e.g. BaseDOMContainer)
+        //  - since FiberNodes -should- be interior mutable, this is safe
+        'FiberNode',
+      ],
+      () => true, // always read-only
+    );
+
     React.useMemoCache = useMemoCache;
 
     export default React;