diff --git a/packages/client/news/129.bugfix b/packages/client/news/129.bugfix
new file mode 100644
index 000000000..d38f79e05
--- /dev/null
+++ b/packages/client/news/129.bugfix
@@ -0,0 +1 @@
+Stopped the content and user data schemas from silently stripping fields they do not declare (e.g. `changeNote`, custom dexterity fields, custom member properties). Unknown keys are now passed through to the backend, which is the authority on which fields exist. @jmevissen
diff --git a/packages/client/src/validation/content.ts b/packages/client/src/validation/content.ts
index 58e870902..1bbd06e92 100644
--- a/packages/client/src/validation/content.ts
+++ b/packages/client/src/validation/content.ts
@@ -60,57 +60,61 @@ export const RelatedItemPayloadSchema = z
     title: true,
   });
 
-export const createContentDataSchema = z.object({
-  '@id': z.string().optional(),
-  '@static_behaviors': z.unknown().optional(),
-  '@type': z.string(),
-  allow_discussion: z.boolean().optional(),
-  blocks: z.unknown().optional(),
-  blocks_layout: z.object({ items: z.array(z.string()) }).optional(),
-  contributors: z.array(z.string()).optional(),
-  creators: z.array(z.string()).optional(),
-  description: z.string().optional(),
-  effective: z.string().nullable().optional(),
-  exclude_from_nav: z.boolean().optional(),
-  expires: z.string().nullable().optional(),
-  file: z
-    .object({
-      'content-type': z.string(),
-      data: z.string(),
-      encoding: z.string(),
-      filename: z.string(),
-    })
-    .optional(),
-  id: z.string().optional(),
-  image: z
-    .object({
-      'content-type': z.string(),
-      data: z.string(),
-      encoding: z.string(),
-      filename: z.string(),
-    })
-    .optional(),
-  language: z.string().optional(),
-  preview_caption: z.string().optional(),
-  preview_image: z
-    .object({
-      'content-type': z.string(),
-      data: z.string(),
-      encoding: z.string(),
-      filename: z.string(),
-    })
-    .optional(),
-  relatedItems: z.array(RelatedItemPayloadSchema).optional(),
-  rights: z.string().nullable().optional(),
-  title: z.string(),
-  versioning_enabled: z.boolean().optional(),
-});
+export const createContentDataSchema = z
+  .object({
+    '@id': z.string().optional(),
+    '@static_behaviors': z.unknown().optional(),
+    '@type': z.string(),
+    allow_discussion: z.boolean().optional(),
+    blocks: z.unknown().optional(),
+    blocks_layout: z.object({ items: z.array(z.string()) }).optional(),
+    changeNote: z.string().optional(),
+    contributors: z.array(z.string()).optional(),
+    creators: z.array(z.string()).optional(),
+    description: z.string().optional(),
+    effective: z.string().nullable().optional(),
+    exclude_from_nav: z.boolean().optional(),
+    expires: z.string().nullable().optional(),
+    file: z
+      .object({
+        'content-type': z.string(),
+        data: z.string(),
+        encoding: z.string(),
+        filename: z.string(),
+      })
+      .optional(),
+    id: z.string().optional(),
+    image: z
+      .object({
+        'content-type': z.string(),
+        data: z.string(),
+        encoding: z.string(),
+        filename: z.string(),
+      })
+      .optional(),
+    language: z.string().optional(),
+    preview_caption: z.string().optional(),
+    preview_image: z
+      .object({
+        'content-type': z.string(),
+        data: z.string(),
+        encoding: z.string(),
+        filename: z.string(),
+      })
+      .optional(),
+    relatedItems: z.array(RelatedItemPayloadSchema).optional(),
+    rights: z.string().nullable().optional(),
+    title: z.string(),
+    versioning_enabled: z.boolean().optional(),
+  })
+  .passthrough();
 
 export const updateContentDataSchema = z
   .object({
     allow_discussion: z.boolean().optional(),
     blocks: z.unknown().optional(),
     blocks_layout: z.object({ items: z.array(z.string()) }).optional(),
+    changeNote: z.string().optional(),
     contributors: z.array(z.string()).optional(),
     creators: z.array(z.string()).optional(),
     description: z.string().optional(),
@@ -141,7 +145,8 @@ export const updateContentDataSchema = z
     title: z.string().optional(),
     versioning_enabled: z.boolean().optional(),
   })
-  .partial();
+  .partial()
+  .passthrough();
 
 export const copyMoveContentDataSchema = z.object({
   path: z.string(),
diff --git a/packages/client/src/validation/passthrough.test.ts b/packages/client/src/validation/passthrough.test.ts
new file mode 100644
index 000000000..e12b2bd6e
--- /dev/null
+++ b/packages/client/src/validation/passthrough.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, test } from 'vitest';
+import { createContentDataSchema, updateContentDataSchema } from './content';
+import { createUserDataSchema, updateUserDataSchema } from './users';
+
+describe('content data schemas', () => {
+  test('createContentDataSchema keeps unknown (custom dexterity) fields', () => {
+    const parsed = createContentDataSchema.parse({
+      '@type': 'Document',
+      title: 'My Page',
+      my_custom_field: 'custom value',
+    });
+
+    expect(parsed.my_custom_field).toBe('custom value');
+  });
+
+  test('updateContentDataSchema keeps unknown (custom dexterity) fields', () => {
+    const parsed = updateContentDataSchema.parse({
+      title: 'My Page',
+      my_custom_field: 'custom value',
+    });
+
+    expect(parsed.my_custom_field).toBe('custom value');
+  });
+
+  test('updateContentDataSchema still rejects mistyped declared fields', () => {
+    expect(() =>
+      updateContentDataSchema.parse({ exclude_from_nav: 'yes' }),
+    ).toThrow();
+  });
+});
+
+describe('user data schemas', () => {
+  test('createUserDataSchema keeps custom member properties', () => {
+    const parsed = createUserDataSchema.parse({
+      email: 'jane@example.com',
+      username: 'jane',
+      phone: '+1 555 0100',
+    });
+
+    expect(parsed.phone).toBe('+1 555 0100');
+  });
+
+  test('updateUserDataSchema keeps custom member properties', () => {
+    const parsed = updateUserDataSchema.parse({
+      fullname: 'Jane Doe',
+      department: 'Engineering',
+    });
+
+    expect(parsed.department).toBe('Engineering');
+  });
+
+  test('updateUserDataSchema still rejects mistyped declared fields', () => {
+    expect(() =>
+      updateUserDataSchema.parse({ email: 'not-an-email' }),
+    ).toThrow();
+  });
+});
diff --git a/packages/client/src/validation/users.ts b/packages/client/src/validation/users.ts
index 750818eb7..c726c5d42 100644
--- a/packages/client/src/validation/users.ts
+++ b/packages/client/src/validation/users.ts
@@ -1,16 +1,18 @@
 import { z } from 'zod';
 
-export const createUserDataSchema = z.object({
-  description: z.string().optional(),
-  email: z.string().email(),
-  fullname: z.string().optional(),
-  home_page: z.string().url().optional(),
-  location: z.string().optional(),
-  sendPasswordReset: z.boolean().optional(),
-  username: z.string(),
-  roles: z.array(z.string()).optional(),
-  password: z.string().optional(),
-});
+export const createUserDataSchema = z
+  .object({
+    description: z.string().optional(),
+    email: z.string().email(),
+    fullname: z.string().optional(),
+    home_page: z.string().url().optional(),
+    location: z.string().optional(),
+    sendPasswordReset: z.boolean().optional(),
+    username: z.string(),
+    roles: z.array(z.string()).optional(),
+    password: z.string().optional(),
+  })
+  .passthrough();
 
 const portraitSchema = z.object({
   'content-type': z.string(),
@@ -20,15 +22,17 @@ const portraitSchema = z.object({
   scale: z.boolean().optional(),
 });
 
-export const updateUserDataSchema = z.object({
-  description: z.string().optional(),
-  email: z.string().email().optional(),
-  fullname: z.string().optional(),
-  home_page: z.string().url().optional(),
-  location: z.string().optional(),
-  username: z.string().optional(),
-  portrait: portraitSchema.optional(),
-});
+export const updateUserDataSchema = z
+  .object({
+    description: z.string().optional(),
+    email: z.string().email().optional(),
+    fullname: z.string().optional(),
+    home_page: z.string().url().optional(),
+    location: z.string().optional(),
+    username: z.string().optional(),
+    portrait: portraitSchema.optional(),
+  })
+  .passthrough();
 
 export const resetPasswordWithTokenDataSchema = z.object({
   reset_token: z.string(),