diff --git a/apps/website/content/docs/a2ui/guides/message-protocol.mdx b/apps/website/content/docs/a2ui/guides/message-protocol.mdx
index bea9f91b9..42b1e5062 100644
--- a/apps/website/content/docs/a2ui/guides/message-protocol.mdx
+++ b/apps/website/content/docs/a2ui/guides/message-protocol.mdx
@@ -120,7 +120,7 @@ Details worth pinning down:
 - **Context is resolved.** The inbound Button's `action.event.context` is a plain object whose values are dynamic values (often `{ path }` bindings). The outbound message's `action.context` is the same keys with each value already resolved against the current data model — here `{ "path": "/origin" }` became `["LAX"]`.
 - **`label` is a Threadplane extension.** It's derived from the source component's authored text — for a Button-with-Text-child, the child Text's bare literal string ("Search flights"). It's optional; the transcript renderer uses it to label the user bubble, and backends may ignore it.
 
-The client's current data model is only attached as `metadata.a2uiClientDataModel` when the surface's `createSurface` set `sendDataModel: true`. It's omitted otherwise. When present, it's an `A2uiClientDataModel` — `{ surfaces: Record<surfaceId, Record<string, unknown>> }`, the per-surface model keyed by `surfaceId`. See [the schema reference](/docs/a2ui/reference/schema#outbound-action-messages) for the full outbound shape.
+The client's current data model is only attached as `metadata.a2uiClientDataModel` when the surface's `createSurface` set `sendDataModel: true`. It's omitted otherwise. When present, it's an `A2uiClientDataModel` — `{ surfaces: Record<surfaceId, Record<string, unknown>> }`, the per-surface **live** model keyed by `surfaceId` — user edits included, renderer-internal keys stripped. See [the schema reference](/docs/a2ui/reference/schema#outbound-action-messages) for the full outbound shape.
 
 ## Relationship to Google's A2UI
 
diff --git a/apps/website/content/docs/chat/a2ui/overview.mdx b/apps/website/content/docs/chat/a2ui/overview.mdx
index db31c5c03..05d86672b 100644
--- a/apps/website/content/docs/chat/a2ui/overview.mdx
+++ b/apps/website/content/docs/chat/a2ui/overview.mdx
@@ -153,7 +153,7 @@ onInput(event: Event): void {
 }
 ```
 
-The write-back protocol is client-side state. If the surface's `createSurface` set `sendDataModel: true`, outgoing action messages also include the current surface data model snapshot under `metadata.a2uiClientDataModel`.
+The write-back protocol is client-side state. If the surface's `createSurface` set `sendDataModel: true`, outgoing action messages also include the current **live** surface data model (user edits included) under `metadata.a2uiClientDataModel`.
 
 ## Actions
 
@@ -186,7 +186,7 @@ The surface-to-spec conversion turns this into a render `click` binding that cal
 }
 ```
 
-`label` is a Threadplane extension, derived from the Button's child Text; transcripts use it to label the user bubble. If the surface has `sendDataModel: true`, the emitted message also includes `metadata.a2uiClientDataModel` with the current surface data model snapshot.
+`label` is a Threadplane extension, derived from the Button's child Text; transcripts use it to label the user bubble. If the surface has `sendDataModel: true`, the emitted message also includes `metadata.a2uiClientDataModel` with the live surface data model (user edits included).
 
 The other action form, `{ "functionCall": { "call": ..., "args": ... } }`, executes a client-side function locally instead of round-tripping to the agent — wired to the surface component's `a2ui:localAction` handler, with `openUrl` (new tab, `noopener`) built in.
 
diff --git a/apps/website/content/docs/chat/api/api-docs.json b/apps/website/content/docs/chat/api/api-docs.json
index b208c2883..db1ae3566 100644
--- a/apps/website/content/docs/chat/api/api-docs.json
+++ b/apps/website/content/docs/chat/api/api-docs.json
@@ -8639,6 +8639,20 @@
       "```ts\nproviders: [provideViews({ ...a2uiBasicCatalog(), MyWidget: MyWidgetComponent })]\n```"
     ]
   },
+  {
+    "name": "a2uiClientCapabilities",
+    "kind": "function",
+    "description": "The A2UI client capabilities this renderer supports — the typed\n`a2uiClientCapabilities` metadata a host attaches to agent requests so\nthe agent knows which component catalogs it may target\n(catalog negotiation, A2UI v0.9 transport metadata).",
+    "signature": "a2uiClientCapabilities(): A2uiClientCapabilities",
+    "params": [],
+    "returns": {
+      "type": "A2uiClientCapabilities",
+      "description": ""
+    },
+    "examples": [
+      "```ts\nconst metadata = { a2uiClientCapabilities: a2uiClientCapabilities() };\n```"
+    ]
+  },
   {
     "name": "action",
     "kind": "function",
diff --git a/docs/superpowers/plans/2026-08-17-a2ui-v09-phase4-senddatamodel.md b/docs/superpowers/plans/2026-08-17-a2ui-v09-phase4-senddatamodel.md
new file mode 100644
index 000000000..94758d4f9
--- /dev/null
+++ b/docs/superpowers/plans/2026-08-17-a2ui-v09-phase4-senddatamodel.md
@@ -0,0 +1,32 @@
+# A2UI v0.9 Phase 4 — sendDataModel Round-Trip Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Complete the `createSurface.sendDataModel` round-trip: outbound action messages attach the **live** per-surface data model (user edits included, internal keys stripped), and hosts get a typed `a2uiClientCapabilities()` helper for catalog negotiation metadata.
+
+**Architecture:** Phase 1 already attaches `metadata.a2uiClientDataModel` when `surface.sendDataModel` — but from the agent-seeded snapshot. Phase 3 gave `<a2ui-surface>` the live store; this phase passes the merged live model (minus the reserved `_a2uiChecks` key) into `buildA2uiActionMessage`, and adds a tiny capabilities helper. Base: main after #819.
+
+---
+
+### Task 1: live model in the action metadata
+
+**Files:** Modify `libs/chat/src/lib/a2ui/surface.component.ts` (+spec).
+
+- [ ] Failing spec: a surface created with `sendDataModel: true` whose bound TextField was edited via the live store → emitted action's `metadata.a2uiClientDataModel.surfaces[surfaceId]` contains the live-typed value and does NOT contain `_a2uiChecks`.
+- [ ] Implement: in the `a2ui:event` success path, call `buildA2uiActionMessage({...params, context}, { ...surf, dataModel: publicModel })` where `publicModel` = merged live model with `_a2uiChecks` removed. Green + commit.
+
+### Task 2: capabilities helper
+
+**Files:** Create `libs/chat/src/lib/a2ui/capabilities.ts` (+spec); modify `libs/chat/src/public-api.ts`.
+
+- [ ] `a2uiClientCapabilities(): A2uiClientCapabilities` returning `{ supportedCatalogIds: [A2UI_BASIC_CATALOG_ID] }` — the typed metadata hosts attach to requests for catalog negotiation. Export + api-docs.
+
+### Task 3: prompts + docs
+
+**Files:** `examples/*/python/src/schemas/a2ui_v09.py` twins (sendDataModel note), `apps/website/content/docs/a2ui/guides/message-protocol.mdx` + `chat/a2ui/overview.mdx` (live-model metadata wording), `npm run generate-api-docs`.
+
+### Task 4: verification + PR
+
+- [ ] `nx run-many -t lint test build -p a2ui chat`; pytest twins; affected sweep.
+- [ ] Live Chrome smoke: prompt for a `sendDataModel: true` form; submit and verify (via thread state) that the outbound action carries `metadata.a2uiClientDataModel` with the live-typed values.
+- [ ] PR `feat(a2ui): live sendDataModel round-trip + capabilities helper (Phase 4)`; merge on green.
diff --git a/examples/ag-ui/python/src/schemas/a2ui_v09.py b/examples/ag-ui/python/src/schemas/a2ui_v09.py
index 52b787aed..b090d8750 100644
--- a/examples/ag-ui/python/src/schemas/a2ui_v09.py
+++ b/examples/ag-ui/python/src/schemas/a2ui_v09.py
@@ -75,7 +75,11 @@
 event action fires: failing checks block the action, display "message" under
 the offending input, and send the agent an error message
 {"version":"v0.9","error":{"code":"VALIDATION_FAILED",...}} instead. Bind the
-checked value to the same {"path": ...} as the input's value. Example:
+checked value to the same {"path": ...} as the input's value.
+
+Set "sendDataModel": true on createSurface when the agent should receive the
+surface's full LIVE data model (including user edits) in the metadata of
+every action message ({"metadata":{"a2uiClientDataModel":{"surfaces":{...}}}}). Example:
 
   {"id": "email", "component": "TextField", "label": "Email",
    "value": {"path": "/email"},
diff --git a/examples/chat/python/src/schemas/a2ui_v09.py b/examples/chat/python/src/schemas/a2ui_v09.py
index 52b787aed..b090d8750 100644
--- a/examples/chat/python/src/schemas/a2ui_v09.py
+++ b/examples/chat/python/src/schemas/a2ui_v09.py
@@ -75,7 +75,11 @@
 event action fires: failing checks block the action, display "message" under
 the offending input, and send the agent an error message
 {"version":"v0.9","error":{"code":"VALIDATION_FAILED",...}} instead. Bind the
-checked value to the same {"path": ...} as the input's value. Example:
+checked value to the same {"path": ...} as the input's value.
+
+Set "sendDataModel": true on createSurface when the agent should receive the
+surface's full LIVE data model (including user edits) in the metadata of
+every action message ({"metadata":{"a2uiClientDataModel":{"surfaces":{...}}}}). Example:
 
   {"id": "email", "component": "TextField", "label": "Email",
    "value": {"path": "/email"},
diff --git a/libs/chat/src/lib/a2ui/capabilities.spec.ts b/libs/chat/src/lib/a2ui/capabilities.spec.ts
new file mode 100644
index 000000000..fac7f0d5f
--- /dev/null
+++ b/libs/chat/src/lib/a2ui/capabilities.spec.ts
@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: MIT
+import { describe, expect, it } from 'vitest';
+import { A2UI_BASIC_CATALOG_ID } from '@threadplane/a2ui';
+import { a2uiClientCapabilities } from './capabilities';
+
+describe('a2uiClientCapabilities', () => {
+  it('advertises the basic catalog', () => {
+    expect(a2uiClientCapabilities()).toEqual({
+      supportedCatalogIds: [A2UI_BASIC_CATALOG_ID],
+    });
+  });
+});
diff --git a/libs/chat/src/lib/a2ui/capabilities.ts b/libs/chat/src/lib/a2ui/capabilities.ts
new file mode 100644
index 000000000..12ee0369d
--- /dev/null
+++ b/libs/chat/src/lib/a2ui/capabilities.ts
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+import { A2UI_BASIC_CATALOG_ID, type A2uiClientCapabilities } from '@threadplane/a2ui';
+
+/**
+ * The A2UI client capabilities this renderer supports — the typed
+ * `a2uiClientCapabilities` metadata a host attaches to agent requests so
+ * the agent knows which component catalogs it may target
+ * (catalog negotiation, A2UI v0.9 transport metadata).
+ *
+ * @example
+ * ```ts
+ * const metadata = { a2uiClientCapabilities: a2uiClientCapabilities() };
+ * ```
+ */
+export function a2uiClientCapabilities(): A2uiClientCapabilities {
+  return { supportedCatalogIds: [A2UI_BASIC_CATALOG_ID] };
+}
diff --git a/libs/chat/src/lib/a2ui/surface.component.spec.ts b/libs/chat/src/lib/a2ui/surface.component.spec.ts
index bcf15000d..bd2271aed 100644
--- a/libs/chat/src/lib/a2ui/surface.component.spec.ts
+++ b/libs/chat/src/lib/a2ui/surface.component.spec.ts
@@ -195,3 +195,43 @@ describe('A2uiSurfaceComponent — validation gate + live context (Phase 3)', ()
     expect(fx.componentInstance.liveStore.get('/email')).toBe('user@typed.io');
   });
 });
+
+describe('A2uiSurfaceComponent — sendDataModel live round-trip (Phase 4)', () => {
+  beforeEach(() => TestBed.configureTestingModule({ imports: [A2uiSurfaceComponent] }));
+
+  it('attaches the live data model (internal keys stripped) when sendDataModel is true', () => {
+    const store = createA2uiSurfaceStore();
+    store.apply({ version: 'v0.9', createSurface: {
+      surfaceId: 's3', catalogId: 'basic', sendDataModel: true,
+    } } as never);
+    store.apply({ version: 'v0.9', updateComponents: {
+      surfaceId: 's3',
+      components: [
+        { id: 'root', component: 'Column', children: ['name', 'go'] },
+        { id: 'name', component: 'TextField', label: 'Name', value: { path: '/name' },
+          checks: [{ condition: { call: 'required', args: { value: { path: '/name' } } }, message: 'Required' }] },
+        { id: 'go', component: 'Button', child: 'go-lbl', action: { event: { name: 'save' } } },
+        { id: 'go-lbl', component: 'Text', text: 'Save' },
+      ],
+    } } as never);
+    store.apply({ version: 'v0.9', updateDataModel: { surfaceId: 's3', path: '/name', value: '' } } as never);
+
+    const fx = TestBed.createComponent(A2uiSurfaceComponent);
+    fx.componentRef.setInput('state', store.surfaceState('s3')()!);
+    fx.componentRef.setInput('catalog', a2uiBasicCatalog());
+    const actions: { metadata?: { a2uiClientDataModel?: { surfaces: Record<string, Record<string, unknown>> } } }[] = [];
+    fx.componentInstance.action.subscribe((a) => actions.push(a));
+    fx.detectChanges();
+
+    // User edits the bound field via the live store.
+    fx.componentInstance.liveStore.set('/name', 'Ada Lovelace');
+
+    const handler = fx.componentInstance.internalHandlers()['a2ui:event'];
+    handler({ surfaceId: 's3', sourceComponentId: 'go', name: 'save', context: {} });
+
+    expect(actions).toHaveLength(1);
+    const model = actions[0].metadata!.a2uiClientDataModel!.surfaces['s3'];
+    expect(model['name']).toBe('Ada Lovelace');
+    expect('_a2uiChecks' in model).toBe(false);
+  });
+});
diff --git a/libs/chat/src/lib/a2ui/surface.component.ts b/libs/chat/src/lib/a2ui/surface.component.ts
index 17d38712f..218e461f3 100644
--- a/libs/chat/src/lib/a2ui/surface.component.ts
+++ b/libs/chat/src/lib/a2ui/surface.component.ts
@@ -200,7 +200,14 @@ export class A2uiSurfaceComponent {
           }
         }
 
-        const message = buildA2uiActionMessage({ ...params, context }, surf);
+        // sendDataModel metadata must carry the LIVE model (user edits
+        // included), minus renderer-internal keys.
+        const { _a2uiChecks, ...publicModel } = liveModel;
+        void _a2uiChecks;
+        const message = buildA2uiActionMessage(
+          { ...params, context },
+          { ...surf, dataModel: publicModel },
+        );
         this.action.emit(message);
         return message;
       },
diff --git a/libs/chat/src/public-api.ts b/libs/chat/src/public-api.ts
index 1aaf7eb91..e9d68ff53 100644
--- a/libs/chat/src/public-api.ts
+++ b/libs/chat/src/public-api.ts
@@ -209,6 +209,7 @@ export { A2uiSurfaceComponent } from './lib/a2ui/surface.component';
 export { buildA2uiActionMessage } from './lib/a2ui/build-action-message';
 export { a2uiBasicCatalog } from './lib/a2ui/catalog/index';
 export { emitBinding } from './lib/a2ui/catalog/emit-binding';
+export { a2uiClientCapabilities } from './lib/a2ui/capabilities';
 
 // A2UI catalog components (for custom catalog composition via withViews)
 export { A2uiTextFieldComponent } from './lib/a2ui/catalog/text-field.component';