diff --git a/.changeset/slow-phones-fix.md b/.changeset/slow-phones-fix.md
new file mode 100644
index 000000000..83cf3de15
--- /dev/null
+++ b/.changeset/slow-phones-fix.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/db': patch
+---
+
+Fix update() replacing untouched custom class instances with plain objects.
diff --git a/packages/db/src/proxy.ts b/packages/db/src/proxy.ts
index 57723e3cc..2cb5ee385 100644
--- a/packages/db/src/proxy.ts
+++ b/packages/db/src/proxy.ts
@@ -54,18 +54,27 @@ const MAP_SET_ITERATOR_METHODS = new Set([
   `forEach`,
 ])
 
+function isPlainObject(value: object): boolean {
+  const prototype = Object.getPrototypeOf(value)
+  return prototype === Object.prototype || prototype === null
+}
+
 /**
- * Check if a value is a proxiable object (not Date, RegExp, or Temporal)
+ * Check if a value can be safely proxied without changing its semantics.
  */
 function isProxiableObject(
   value: unknown,
 ): value is Record<string | symbol, unknown> {
+  if (value === null || typeof value !== `object`) {
+    return false
+  }
+
   return (
-    value !== null &&
-    typeof value === `object` &&
-    !((value as any) instanceof Date) &&
-    !((value as any) instanceof RegExp) &&
-    !isTemporal(value)
+    isPlainObject(value) ||
+    Array.isArray(value) ||
+    value instanceof Map ||
+    value instanceof Set ||
+    (ArrayBuffer.isView(value) && !(value instanceof DataView))
   )
 }
 
@@ -589,7 +598,16 @@ function deepClone<T extends unknown>(
     return obj
   }
 
-  const clone = {} as Record<string | symbol, unknown>
+  // Preserve non-plain objects by reference. Proxying or cloning an arbitrary
+  // class instance as a plain object strips its prototype and internal state.
+  if (!isPlainObject(obj)) {
+    return obj
+  }
+
+  const clone = Object.create(Object.getPrototypeOf(obj)) as Record<
+    string | symbol,
+    unknown
+  >
   visited.set(obj as object, clone)
 
   for (const key in obj) {
@@ -897,7 +915,7 @@ export function createChangeProxy<
           return value.bind(ptarget)
         }
 
-        // If the value is an object (but not Date, RegExp, or Temporal), create a proxy for it
+        // Proxy only values whose semantics are preserved by our draft handling.
         if (isProxiableObject(value)) {
           // Create a parent reference for the nested object
           const nestedParent = {
diff --git a/packages/db/tests/collection.test.ts b/packages/db/tests/collection.test.ts
index e96994db6..a12a0d565 100644
--- a/packages/db/tests/collection.test.ts
+++ b/packages/db/tests/collection.test.ts
@@ -253,6 +253,48 @@ describe(`Collection`, () => {
     }).toThrow(KeyUpdateNotAllowedError)
   })
 
+  it(`should preserve untouched custom class instances during updates`, async () => {
+    class Money {
+      constructor(public cents: number) {}
+    }
+
+    type Product = {
+      id: string
+      details: { name: string; price: Money }
+    }
+
+    const price = new Money(500)
+    const collection = createCollection<Product>({
+      id: `custom-class-update-test`,
+      getKey: (item) => item.id,
+      sync: {
+        sync: ({ begin, write, commit, markReady }) => {
+          begin()
+          write({
+            type: `insert`,
+            value: {
+              id: `product-1`,
+              details: { name: `Widget`, price },
+            },
+          })
+          commit()
+          markReady()
+        },
+      },
+      onUpdate: async () => {},
+    })
+
+    await collection.stateWhenReady()
+
+    collection.update(`product-1`, (draft) => {
+      draft.details.name = `Gadget`
+    })
+
+    const updated = collection.get(`product-1`)
+    expect(updated?.details.price).toBe(price)
+    expect(updated?.details.price).toBeInstanceOf(Money)
+  })
+
   it(`It shouldn't expose any state until the initial sync is finished`, () => {
     // Create a collection with a mock sync plugin
     createCollection<{ name: string }>({
diff --git a/packages/db/tests/proxy.test.ts b/packages/db/tests/proxy.test.ts
index bbac0151a..8ff3ed11d 100644
--- a/packages/db/tests/proxy.test.ts
+++ b/packages/db/tests/proxy.test.ts
@@ -1370,6 +1370,23 @@ describe(`Proxy Library`, () => {
         age: 30,
       })
     })
+
+    it(`should preserve untouched custom class instances in changed objects`, () => {
+      class Money {
+        constructor(public cents: number) {}
+      }
+
+      const price = new Money(500)
+      const obj = { details: { name: `Widget`, price } }
+
+      const changes = withChangeTracking(obj, (proxy) => {
+        proxy.details.name = `Gadget`
+      })
+
+      const changedDetails = changes.details as typeof obj.details
+      expect(changedDetails.price).toBe(price)
+      expect(changedDetails.price).toBeInstanceOf(Money)
+    })
   })
 
   describe(`withArrayChangeTracking`, () => {