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diff --git a/‎.changeset/10156-edit-form-writes-only-changed-fields.md‎
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diff --git a/‎content/docs/plugins/plugin-form.mdx‎
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diff --git a/‎packages/plugin-form/README.md‎
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diff --git a/‎packages/plugin-form/src/DrawerForm.tsx‎
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diff --git a/‎packages/plugin-form/src/ModalForm.tsx‎
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@@ -13,7 +13,7 @@ The verdict that answers 「may this caller edit this field」 already existed a
 
 **What changed, in observable terms.**
 
-- The field-level verdict now arrives at the ONE outbound filter as a predicate (`sanitizeFormData`'s `canEdit`), instead of as a strip loop written out after each container's call. ⚠️ `DrawerForm` previously sent every displayed field regardless of the caller's field permissions; it no longer does. `ObjectForm` and `ModalForm` produce the same payloads they did before — their loops were correct, they were just copies.
+- The field-level verdict now arrives at the ONE outbound filter as a predicate (`sanitizeFormData`'s `canEdit`), instead of as a strip loop written out after each container's call. ⚠️ `DrawerForm` previously sent every displayed field regardless of the caller's field permissions; it no longer does. `ObjectForm` and `ModalForm` already withheld the refused field, and still do — their loops were correct, they were just copies.
 - The render pass is likewise one function for all three containers. ⚠️ `DrawerForm` previously drew a field the caller may read but not edit as a live input; it now draws it read-only and disabled, exactly as the other two already did. A field the caller may not READ is dropped, also as before.
 - Both halves stay fail-open with no `PermissionProvider` / `MePermissionsProvider` mounted, unchanged: a standalone form, a designer preview and a guest surface have no resolvable principal, and the server still enforces.
 - A lookup's selected chip no longer offers its remove ✕ when the field is disabled. ⚠️ This is how BOTH refusals reach the widget — a field the object declares `readonly` is folded into `disabled` by the form's section builder, and a field the permission set refuses is marked disabled by the pass above — so a reporter could previously clear a master-detail parent the server would then refuse to unset. The trigger and the browse button were already disabled; the chip's ✕ was the one control the gate had missed. The chips themselves stay: the value is readable, only the affordance goes.
 
@@ -0,0 +1,23 @@
+---
+'@object-ui/plugin-form': patch
+'@object-ui/types': patch
+---
+
+An edit form now writes only the fields that changed (objectui#10156).
+
+**Clause-②: no.** No exported symbol, type or prop changes. `@object-ui/plugin-form` publishes `.` only, from `index.tsx`, and `index.tsx` re-exports neither `sanitize` nor `masterDetailTx`. After a build, `dist/index.d.ts` names none of the new helpers. The change is in what the client sends, and ⚠️ in what a host `submitHandler` receives in edit mode (see below).
+
+**Before.** A master-detail child row already sent only the cells that differed from its loaded snapshot (objectui#10108). The other two edit payloads did not. The plain record edit `PATCH` and the parent operation of a master-detail batch sent every sanitized field on every save, including fields the user never touched. With the concurrency guard, a `409` followed by **Overwrite** therefore rewrote every field, not only the ones this user changed.
+
+**What changed, in observable terms.**
+
+- In edit mode, `ObjectForm`, `ModalForm` and `DrawerForm` compare their save with the record they read through `findOne`, and write only the fields that differ. The parent operation of a master-detail batch follows, because its header is a simple `ObjectForm`.
+- There is one comparison, shared with the master-detail child rows. It sends anything it cannot prove unchanged. `null` and `undefined` count as the same value. `null` and `''` are different. So are `5` and `'5'`, a lookup id and its expanded object, a `Date` and a date string, and two objects whose keys come in a different order. A field the form changed by itself after the read, such as a cascade clear, is sent.
+- A save with nothing changed still sends the full sanitized payload. It stays a real request, with the same concurrency guard and a real server record for `onSuccess`.
+- A form that did not read the record itself still sends every field. That covers a create, a record supplied as `initialData`, and inline `customFields`.
+- After a successful save, the form counts the fields it just wrote as saved. A form that stays open compares its next save with the record as it is now. Changing a field back to its first-read value is therefore still sent.
+- The concurrency guard is unchanged. The update still carries `ifMatch` = the `updated_at` the form read, and a `409` still offers **Keep editing** or **Overwrite**. **Overwrite** now resends only the changed fields.
+- ⚠️ A host `submitHandler` on an edit form receives the payload the form would have written. That is the changed fields, or the full sanitized payload when nothing changed. A host that needs the whole record must read it itself. In this repository, only `MasterDetailForm` passes a `submitHandler` to an edit form. Its header form receives the changed fields. Its row editor has no `recordId`, so it still receives every value.
+- The JSDoc of `ObjectFormSchema.submitHandler` in `@object-ui/types`, and its copies on `ModalFormSchema` and `DrawerFormSchema`, now say what an edit-mode handler receives.
+
+**Not covered.** The `tabbed`, `wizard` and `split` variants have save paths of their own and still send every value they hold. That includes a master-detail header laid out `tabbed`, and a simple form whose mobile `stepper` option shows it one step at a time through the wizard.
@@ -413,6 +413,20 @@ The components are on the package's export surface — `ObjectForm`,
 (`TabbedFormSchema`, `WizardFormSchema`, `ModalFormSchema`, …). There is no
 aggregate map among them.
 
+## What an edit save writes
+
+An `object-form` in `mode: 'edit'` reads its record with `findOne`, and its save
+writes only the fields that differ from that read. That covers the simple form,
+the `modal` and `drawer` variants, and the parent operation of a master-detail
+form. The `tabbed`, `wizard` and `split` variants still send every value the form
+holds, and so does a simple form whose mobile `stepper` option routes it through
+the wizard. A field whose sameness cannot be proven is sent: `5` and `'5'`, `null` and
+`''`, and a lookup id and its expanded object all count as different. A save with
+nothing changed still sends the full payload, as it always has. The `ifMatch`
+concurrency guard is unchanged, and its **Overwrite** choice now resends only the
+changed fields. A host `submitHandler` receives the same payload in edit mode.
+The package README has the full rule, under "What an edit save writes".
+
 ## Examples
 
 ### Form with Validation
 
@@ -869,6 +869,57 @@ returning, which reported a save that never happened and, through
 (objectui#6300). A declared `submitHandler` is consulted first, so a host that
 said it owns the write is never bypassed for want of an adapter it never needed.
 
+## What an edit save writes
+
+An `object-form` in `mode: 'edit'` with a `recordId` reads the record with
+`dataSource.findOne`, and its save writes **only the fields that differ from
+that read** (objectui#10156). The simple form, the `modal` and the `drawer`
+variants do this, and so does the parent operation of a master-detail form,
+whose header is a simple form. A master-detail child row already worked this
+way (objectui#10108), and all of them use the same comparison. The `tabbed`,
+`wizard` and `split` variants are not covered. That includes a master-detail
+header laid out `tabbed`, and a simple form whose mobile `stepper` option shows it
+one step at a time through the wizard. They still send every value the form
+holds.
+
+The comparison sends every field it cannot prove unchanged, because a field
+wrongly judged unchanged would lose the user's edit while the server still
+answers 200:
+
+- `null` and `undefined` count as the same value. `''` is not a blank here, so
+  `null` and `''` are different.
+- A number and a numeric string are different (`5` and `'5'`).
+- A lookup id and the expanded lookup object are different.
+- A `Date` and a date string are different. So are two date strings written in
+  different formats.
+- Objects and arrays are equal only when they serialize identically.
+  Reordering their keys or elements makes them different.
+
+A field the form changed by itself after the read is a change, so it is sent.
+That covers a cascade clear or a value cleared when its field was hidden.
+
+Some saves still send the full payload:
+
+- **A save with nothing changed.** It sends every field, as it always has.
+  That keeps it a real request, with the same concurrency guard and a real
+  server record for `onSuccess`.
+- **A form with no record read of its own.** This includes a create, a record
+  given as `initialData` or through inline `customFields`, and a save made
+  while a new record is still loading. Each of these sends every field.
+
+After a successful save, the form treats the fields it just wrote as saved. A
+form that stays open therefore compares its next save with the record as it
+stands now, not as it was first read.
+
+The concurrency guard is unchanged. The update still carries
+`ifMatch` = the `updated_at` the form read, and a `409` still offers
+**Keep editing** or **Overwrite**. **Overwrite** now resends only the changed
+fields, so it no longer rewrites fields this user never touched.
+
+⚠️ A host `submitHandler` gets the same payload in edit mode. Normally that is
+the changed fields; after a save with nothing changed, it is the full payload.
+A host that needs the whole record must read it itself.
+
 ## Integration with Data Sources
 
 **The adapter is not a schema key.** A schema is a serialisable document; a live
 
@@ -50,7 +50,13 @@ import {
   CONTAINER_GRID_COLS,
 } from './autoLayout';
 import { deriveFieldGroupSections, projectSectionDivider, resolveSectionCollapse } from './fieldGroups';
-import { sanitizeFormData } from './sanitize';
+import {
+  sanitizeFormData,
+  dirtyEditPayload,
+  snapshotLoadedRecord,
+  advanceLoadedRecord,
+  type LoadedRecordSnapshot,
+} from './sanitize';
 import { applyFieldPermissions, fieldWriteGate } from './fieldWriteGate';
 import { seedCreateValues, omitServerResolvedDefaults } from './schemaDefaults';
 import { resolveInitialRecord } from './initialRecord';
@@ -158,6 +164,10 @@ export interface DrawerFormSchema {
    * When supplied, the form validates and hands the collected values
    * to this handler INSTEAD of calling `dataSource.create` /
    * `dataSource.update`; the returned record is passed on to `onSuccess`.
+   * In `edit` mode, for a record this form read itself, it hands over what it
+   * would have written: the fields that differ from that read, or the full
+   * sanitized payload when nothing changed (objectui#10156; the whole rule is
+   * on `ObjectFormSchema['submitHandler']`).
    *
    * `MasterDetailForm` supplies it to route the parent AND its child
    * collections through one atomic `batchTransaction` (#2679 / ADR-0034
@@ -273,6 +283,12 @@ export const DrawerForm: React.FC<DrawerFormProps> = ({
   // `initialData`/`initialValues` are objects callers commonly rebuild every
   // render, and flashing the loading state for those would thrash.
   const loadedRecordIdRef = useRef<string | number | undefined>(undefined);
+  // The record itself as read — the baseline an edit save diffs against, so
+  // only the fields that changed are written (objectui#10156). Kept apart from
+  // `formData`, which seeds the form and supplies the OCC token: advancing it
+  // after a save would reseed the one and move the other. Set by the `findOne`
+  // below and nowhere else, so a caller-supplied record is never a baseline.
+  const loadedRecordRef = useRef<LoadedRecordSnapshot | null>(null);
 
   // Fetch initial data
   useEffect(() => {
@@ -288,6 +304,8 @@ export const DrawerForm: React.FC<DrawerFormProps> = ({
     let cancelled = false;
     const fetchData = async () => {
       if (schema.mode === 'create' || !schema.recordId) {
+        // Seeded from something other than a read: no baseline to diff against.
+        loadedRecordRef.current = null;
         // Declared static defaults are this form's opening values (#4047) —
         // see `schemaDefaults` for the create-only boundary and for why
         // runtime defaults are left to the server.
@@ -297,6 +315,7 @@ export const DrawerForm: React.FC<DrawerFormProps> = ({
       }
 
       if (!dataSource) {
+        loadedRecordRef.current = null;
         setFormData(resolveInitialRecord(schema));
         setLoading(false);
         return;
@@ -316,6 +335,7 @@ export const DrawerForm: React.FC<DrawerFormProps> = ({
         const data = await dataSource.findOne(schema.objectName, schema.recordId);
         if (cancelled) return;
         loadedRecordIdRef.current = schema.recordId;
+        loadedRecordRef.current = snapshotLoadedRecord(schema, data);
         setFormData(data || {});
       } catch (err) {
         if (cancelled) return;
@@ -471,11 +491,14 @@ export const DrawerForm: React.FC<DrawerFormProps> = ({
       // Omit the fields the producer owns (#4069) — see
       // `omitServerResolvedDefaults` for why an empty key is not the same as
       // no key at insert time. Create only: on an edit form a cleared column is
-      // a real removal. Computed ONCE so every persistence route below — the
-      // host-owned seam included — writes the identical payload.
+      // a real removal. An EDIT writes only the fields that differ from the
+      // record this form read (objectui#10156; `dirtyEditPayload` holds the
+      // rule, and sends whatever it cannot settle). Computed ONCE so every
+      // persistence route below — the host-owned seam included — writes the
+      // identical payload.
       const writePayload = schema.mode === 'create'
         ? omitServerResolvedDefaults(payload, objectSchema)
-        : payload;
+        : dirtyEditPayload(payload, loadedRecordRef.current, schema);
 
       if (schema.submitHandler) {
         // The host owns persistence (e.g. MasterDetailForm batching the parent
@@ -498,12 +521,15 @@ export const DrawerForm: React.FC<DrawerFormProps> = ({
           dataSource,
           objectName: schema.objectName,
           recordId: schema.recordId,
-          payload,
+          payload: writePayload,
           baseRecord: formData,
         });
         if (outcome.status === 'cancelled') return;
         result = outcome.result;
       }
+      // The write landed: a save from this still-open drawer diffs against the
+      // record as it now stands, not as first read.
+      loadedRecordRef.current = advanceLoadedRecord(loadedRecordRef.current, schema, writePayload);
       if (schema.onSuccess) {
         await schema.onSuccess(result);
       }
 
@@ -50,7 +50,13 @@ import {
   CONTAINER_GRID_COLS,
 } from './autoLayout';
 import { deriveFieldGroupSections, projectSectionDivider, resolveSectionCollapse } from './fieldGroups';
-import { sanitizeFormData } from './sanitize';
+import {
+  sanitizeFormData,
+  dirtyEditPayload,
+  snapshotLoadedRecord,
+  advanceLoadedRecord,
+  type LoadedRecordSnapshot,
+} from './sanitize';
 import { applyFieldPermissions, fieldWriteGate } from './fieldWriteGate';
 import { seedCreateValues, omitServerResolvedDefaults } from './schemaDefaults';
 import { resolveInitialRecord } from './initialRecord';
@@ -176,6 +182,10 @@ export interface ModalFormSchema {
    * When supplied, the form validates and hands the collected values
    * to this handler INSTEAD of calling `dataSource.create` /
    * `dataSource.update`; the returned record is passed on to `onSuccess`.
+   * In `edit` mode, for a record this form read itself, it hands over what it
+   * would have written: the fields that differ from that read, or the full
+   * sanitized payload when nothing changed (objectui#10156; the whole rule is
+   * on `ObjectFormSchema['submitHandler']`).
    *
    * `MasterDetailForm` supplies it to route the parent AND its child
    * collections through one atomic `batchTransaction` (#2679 / ADR-0034
@@ -349,6 +359,12 @@ export const ModalForm: React.FC<ModalFormProps> = ({
   // `initialData`/`initialValues` are objects callers commonly rebuild every
   // render, and flashing the loading state for those would thrash.
   const loadedRecordIdRef = useRef<string | number | undefined>(undefined);
+  // The record itself as read — the baseline an edit save diffs against, so
+  // only the fields that changed are written (objectui#10156). Kept apart from
+  // `formData`, which seeds the form and supplies the OCC token: advancing it
+  // after a save would reseed the one and move the other. Set by the `findOne`
+  // below and nowhere else, so a caller-supplied record is never a baseline.
+  const loadedRecordRef = useRef<LoadedRecordSnapshot | null>(null);
 
   // Fetch initial data
   useEffect(() => {
@@ -364,6 +380,8 @@ export const ModalForm: React.FC<ModalFormProps> = ({
     let cancelled = false;
     const fetchData = async () => {
       if (schema.mode === 'create' || !schema.recordId) {
+        // Seeded from something other than a read: no baseline to diff against.
+        loadedRecordRef.current = null;
         // No persisted record to show, so the object's declared static
         // `defaultValue`s are the form's opening values (#4047) — caller-
         // supplied initial values still win. See `schemaDefaults` for why
@@ -375,6 +393,7 @@ export const ModalForm: React.FC<ModalFormProps> = ({
       }
 
       if (!dataSource) {
+        loadedRecordRef.current = null;
         setFormData(resolveInitialRecord(schema));
         setLoading(false);
         return;
@@ -394,6 +413,7 @@ export const ModalForm: React.FC<ModalFormProps> = ({
         const data = await dataSource.findOne(schema.objectName, schema.recordId);
         if (cancelled) return;
         loadedRecordIdRef.current = schema.recordId;
+        loadedRecordRef.current = snapshotLoadedRecord(schema, data);
         setFormData(data || {});
       } catch (err) {
         if (cancelled) return;
@@ -501,12 +521,14 @@ export const ModalForm: React.FC<ModalFormProps> = ({
       // Omit the fields the producer owns (#4069) — see
       // `omitServerResolvedDefaults` for why an empty key is not the same as
       // no key at insert time. Create only: on an edit form a cleared column is
-      // a real removal. Computed ONCE (after the FLS strip above) so every
-      // persistence route below — the host-owned seam included — writes the
-      // identical payload.
+      // a real removal. An EDIT writes only the fields that differ from the
+      // record this form read (objectui#10156; `dirtyEditPayload` holds the
+      // rule, and sends whatever it cannot settle). Computed ONCE (after the
+      // FLS strip above) so every persistence route below — the host-owned
+      // seam included — writes the identical payload.
       const writePayload = schema.mode === 'create'
         ? omitServerResolvedDefaults(payload, objectSchema)
-        : payload;
+        : dirtyEditPayload(payload, loadedRecordRef.current, schema);
 
       if (schema.submitHandler) {
         // The host owns persistence (e.g. MasterDetailForm batching the parent
@@ -529,12 +551,15 @@ export const ModalForm: React.FC<ModalFormProps> = ({
           dataSource,
           objectName: schema.objectName,
           recordId: schema.recordId,
-          payload,
+          payload: writePayload,
           baseRecord: formData,
         });
         if (outcome.status === 'cancelled') return;
         result = outcome.result;
       }
+      // The write landed: a save from this still-open modal diffs against the
+      // record as it now stands, not as first read.
+      loadedRecordRef.current = advanceLoadedRecord(loadedRecordRef.current, schema, writePayload);
       if (schema.onSuccess) {
         await schema.onSuccess(result);
       }