diff --git a/apps/website/content/docs/a2ui/api/api-docs.json b/apps/website/content/docs/a2ui/api/api-docs.json
index ed322a41f..15fb63358 100644
--- a/apps/website/content/docs/a2ui/api/api-docs.json
+++ b/apps/website/content/docs/a2ui/api/api-docs.json
@@ -181,22 +181,16 @@
     "description": "",
     "properties": [
       {
-        "name": "args",
-        "type": "Record<string, unknown>",
-        "description": "",
-        "optional": true
-      },
-      {
-        "name": "call",
-        "type": "string",
-        "description": "",
+        "name": "condition",
+        "type": "unknown",
+        "description": "A DynamicBoolean — typically a validator function call (`required`,\n`regex`, `length`, `numeric`, `email`) or a logic combinator. The rule\npasses when the condition resolves to `true`.",
         "optional": false
       },
       {
         "name": "message",
         "type": "string",
-        "description": "",
-        "optional": true
+        "description": "Error message displayed when the check fails.",
+        "optional": false
       }
     ],
     "examples": []
diff --git a/apps/website/content/docs/a2ui/reference/schema.mdx b/apps/website/content/docs/a2ui/reference/schema.mdx
index a463bd7f8..aa666d5c2 100644
--- a/apps/website/content/docs/a2ui/reference/schema.mdx
+++ b/apps/website/content/docs/a2ui/reference/schema.mdx
@@ -83,7 +83,7 @@ interface A2uiComponentBase {
 }
 ```
 
-Input components additionally mix in `A2uiCheckable` (`checks?: A2uiCheck[]`) for client-side validation rules (typed now, enforced in a later phase).
+Input components additionally mix in `A2uiCheckable` (`checks?: A2uiCheck[]`). Each rule is the spec `CheckRule` shape — `{ condition: DynamicBoolean, message: string }` — where `condition` is typically a validator call (`required`, `regex`, `length`, `numeric`, `email`) or a logic combination. Renderers evaluate rules against the live data model and block event actions while any rule fails.
 
 The basic-catalog component shapes are:
 
@@ -130,7 +130,7 @@ Several fields are constrained to a fixed enum. Emit one of the listed values 
 
 The union of the basic-catalog shapes is exported as `A2uiCatalogComponent`. The broader `A2uiComponent` also admits non-basic-catalog components (`A2uiComponentBase & Record<string, unknown>`) — renderers treat unknown `component` strings as unrenderable and fall back gracefully.
 
-The schema exposes `validationRegexp` on `TextField` and `checks` on input components, but validation execution is not implemented in this package. Treat schema fields as protocol data until a renderer wires behavior.
+The schema exposes `validationRegexp` on `TextField` and `checks` on input components; this package supplies the validator functions in `createA2uiFunctionRegistry()`, and `@threadplane/chat`'s surface renderer enforces the rules (inline messages, blocked event actions, `VALIDATION_FAILED` error messages).
 
 ## Message envelopes
 
diff --git a/apps/website/content/docs/chat/a2ui/catalog.mdx b/apps/website/content/docs/chat/a2ui/catalog.mdx
index 36e6b75d1..808eebb43 100644
--- a/apps/website/content/docs/chat/a2ui/catalog.mdx
+++ b/apps/website/content/docs/chat/a2ui/catalog.mdx
@@ -431,7 +431,7 @@ Renders an HTML5 `<audio>` element with native controls.
 
 ## Client-Side Functions
 
-The v0.9 basic catalog defines typed client-side functions that can appear in `{ "call": ... }` dynamic values and `checks` rules: validation (`required`, `regex`, `length`, `numeric`, `email`), formatting (`formatString`, `formatNumber`, `formatCurrency`, `formatDate`, `pluralize`), logic (`and`, `or`, `not`), and `openUrl`. The formatting and logic functions (`formatString` with `${...}` interpolation, `formatNumber`, `formatCurrency`, `formatDate`, `pluralize`, `and`, `or`, `not`) **execute client-side** via `createA2uiFunctionRegistry()`, which `surfaceToSpec` applies to every dynamic value. `openUrl` runs as a local action through the surface component's built-in `a2ui:localAction` fallback (new tab, `noopener`). The validation functions (`required`, `regex`, `length`, `numeric`, `email`) are typed on the wire but not yet enforced — validation `checks` ship in an upcoming release.
+The v0.9 basic catalog defines typed client-side functions that can appear in `{ "call": ... }` dynamic values and `checks` rules: validation (`required`, `regex`, `length`, `numeric`, `email`), formatting (`formatString`, `formatNumber`, `formatCurrency`, `formatDate`, `pluralize`), logic (`and`, `or`, `not`), and `openUrl`. The formatting and logic functions (`formatString` with `${...}` interpolation, `formatNumber`, `formatCurrency`, `formatDate`, `pluralize`, `and`, `or`, `not`) **execute client-side** via `createA2uiFunctionRegistry()`, which `surfaceToSpec` applies to every dynamic value. `openUrl` runs as a local action through the surface component's built-in `a2ui:localAction` fallback (new tab, `noopener`). The validation functions (`required`, `regex`, `length`, `numeric`, `email`) power `checks` rules (`{ condition, message }`) on input components: the surface evaluates every rule against the **live** data model (user edits included) when an event action fires — failing checks block the action, render `message` under the offending input, and emit a `VALIDATION_FAILED` error message through the surface component's `validationError` output. A `TextField.validationRegexp` with a bound value contributes an implicit regex rule.
 
 ## Component Summary
 
diff --git a/apps/website/content/docs/chat/api/api-docs.json b/apps/website/content/docs/chat/api/api-docs.json
index 07b667e8e..b208c2883 100644
--- a/apps/website/content/docs/chat/api/api-docs.json
+++ b/apps/website/content/docs/chat/api/api-docs.json
@@ -181,6 +181,12 @@
         "description": "",
         "optional": false
       },
+      {
+        "name": "errorText",
+        "type": "InputSignal<string>",
+        "description": "Live validation message written by the surface's check gate\n(bound to /_a2uiChecks/<id>); empty when valid.",
+        "optional": false
+      },
       {
         "name": "label",
         "type": "InputSignal<string>",
@@ -259,6 +265,12 @@
         "description": "v0.9 prop: render as 'checkbox' rows (default) or 'chips'.",
         "optional": false
       },
+      {
+        "name": "errorText",
+        "type": "InputSignal<string>",
+        "description": "Live validation message written by the surface's check gate\n(bound to /_a2uiChecks/<id>); empty when valid.",
+        "optional": false
+      },
       {
         "name": "filterable",
         "type": "InputSignal<boolean>",
@@ -512,6 +524,12 @@
         "description": "v0.9 prop: enableTime — include time portion.",
         "optional": false
       },
+      {
+        "name": "errorText",
+        "type": "InputSignal<string>",
+        "description": "Live validation message written by the surface's check gate\n(bound to /_a2uiChecks/<id>); empty when valid.",
+        "optional": false
+      },
       {
         "name": "htmlInputType",
         "type": "Signal<string>",
@@ -998,6 +1016,12 @@
         "description": "",
         "optional": false
       },
+      {
+        "name": "errorText",
+        "type": "InputSignal<string>",
+        "description": "Live validation message written by the surface's check gate\n(bound to /_a2uiChecks/<id>); empty when valid.",
+        "optional": false
+      },
       {
         "name": "label",
         "type": "InputSignal<string>",
@@ -1088,6 +1112,12 @@
         "description": "Merge built-in A2UI handlers with consumer-provided handlers.",
         "optional": false
       },
+      {
+        "name": "liveStore",
+        "type": "StateStore",
+        "description": "Surface-owned live state store: `$bindState` props read it and input\ncomponents write user edits into it, so event-time logic (checks,\naction context) sees CURRENT values instead of the agent-seeded\nsnapshot. Seeded from spec.state with user edits preserved. Public so\nhosts (and tests) can read the live values of a rendered surface.",
+        "optional": false
+      },
       {
         "name": "primaryColor",
         "type": "Signal<string | null>",
@@ -1129,6 +1159,12 @@
         "type": "InputSignal<Type<unknown> | undefined>",
         "description": "Optional top-level placeholder when the surface has no components\nyet. Defaults to A2uiDefaultFallbackComponent.",
         "optional": false
+      },
+      {
+        "name": "validationError",
+        "type": "OutputEmitterRef<A2uiErrorMessage>",
+        "description": "Emitted when a submit is blocked by failing validation checks —\nthe spec client → agent error message (code VALIDATION_FAILED).",
+        "optional": false
       }
     ],
     "methods": [
@@ -1309,6 +1345,12 @@
         "description": "",
         "optional": false
       },
+      {
+        "name": "errorText",
+        "type": "InputSignal<string>",
+        "description": "Live validation message written by the surface's check gate\n(bound to /_a2uiChecks/<id>); empty when valid.",
+        "optional": false
+      },
       {
         "name": "htmlInputType",
         "type": "Signal<string>",
@@ -1342,7 +1384,7 @@
       {
         "name": "validationRegexp",
         "type": "InputSignal<string>",
-        "description": "Stored but not yet enforced beyond the native pattern attribute.",
+        "description": "Enforced by the surface's check gate as an implicit regex rule (plus the native pattern attribute).",
         "optional": false
       },
       {
diff --git a/apps/website/next-env.d.ts b/apps/website/next-env.d.ts
index c4b7818fb..fdbfe5258 100644
--- a/apps/website/next-env.d.ts
+++ b/apps/website/next-env.d.ts
@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/dev/types/routes.d.ts";
+import "./../../dist/apps/website/.next/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.
diff --git a/docs/superpowers/plans/2026-08-17-a2ui-v09-phase3-checks.md b/docs/superpowers/plans/2026-08-17-a2ui-v09-phase3-checks.md
new file mode 100644
index 000000000..85ae4d8ce
--- /dev/null
+++ b/docs/superpowers/plans/2026-08-17-a2ui-v09-phase3-checks.md
@@ -0,0 +1,45 @@
+# A2UI v0.9 Phase 3 — Validation Checks 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:** Implement A2UI v0.9 client-side validation: `checks` rules on input components evaluate against **live user input**, failing checks display inline errors and block event actions, and the client emits the spec `error` message (`VALIDATION_FAILED`).
+
+**Architecture:** Correct `A2uiCheck` to the official `CheckRule` shape (`{condition: DynamicBoolean, message}`); add the validator functions (`required`/`regex`/`length`/`numeric`/`email`) to the Phase-2 registry; `<a2ui-surface>` takes ownership of the render `StateStore` (seed-preserve pattern from `chat-generative-ui`) so event-time logic sees live values; at `a2ui:event` time the surface evaluates all checks against `dataModel ∪ store.getSnapshot()`, blocks + emits `A2uiErrorMessage` on failure, writes per-component messages to reserved store paths (`/_a2uiChecks/<id>`) that catalog inputs bind for reactive display, and resolves `{path}` action-context values live (fixing the pre-existing stale-context gap). Base: main after #818.
+
+**Key discovered facts:** official `CheckRule` = `{condition, message}` (NOT `{call,args,message}` as typed in Phase 1 — unused, safe to correct); `render-spec` accepts a `[store]` input and `signalStateStore` exposes `getSnapshot()`; `invokeHandlers` passes action params raw (no dispatch-time binding resolution) — hence the surface must resolve live values itself.
+
+---
+
+### Task 1: types + validators (`libs/a2ui`)
+
+**Files:** Modify `libs/a2ui/src/lib/types.ts` (+types.spec), `functions.ts` (+functions.spec), `index.ts`.
+
+- [ ] `A2uiCheck` → `{ condition: DynamicValue; message: string }` (spec CheckRule); update the `A2uiCheckable` JSDoc.
+- [ ] Failing specs for validators (exact arg schemas from `scratchpad/basic-catalog.json`): `required {value}` (false for null/undefined/''/[] — true otherwise), `regex {value, pattern}` (RegExp.test, string value required, invalid pattern → false), `length {value, min?, max?}`, `numeric {value, min?, max?}` (accepts numeric strings), `email {value}` (linear-safe pattern).
+- [ ] Implement in the standard registry. Green + commit.
+
+### Task 2: surface store ownership + live models
+
+**Files:** Modify `libs/chat/src/lib/a2ui/surface.component.ts` (+spec).
+
+- [ ] `<a2ui-surface>` creates one internal `signalStateStore({})`, seeds it from `spec().state` with the seeded-map preserve-user-edits semantics (copy of `chat-generative-ui`), passes `[store]` to `<render-spec>`.
+- [ ] Spec: user write survives a spec re-emission; agent update to an untouched path lands.
+
+### Task 3: live checks + event gating + error output
+
+**Files:** Modify `libs/chat/src/lib/a2ui/surface.component.ts` (+spec), `surface-to-spec.ts` (+spec), checkable catalog components (`text-field`, `check-box`, `choice-picker`, `slider`, `date-time-input`) + specs.
+
+- [ ] surface-to-spec: `{path}` **action-context** values stay as `{ $bindState: path }` markers (no build-time resolution); checkable components with `checks` (or TextField `validationRegexp` + path-bound value → synthesized regex rule) get `errorText: { $bindState: '/_a2uiChecks/<id>' }` prop.
+- [ ] surface.component `a2ui:event` handler: build live model = `{...surface.dataModel, ...store.getSnapshot()}` (deep merge by pointer for written paths); resolve `$bindState` markers in `params.context` from the live model; evaluate every component's check rules (`resolveDynamic(rule.condition, liveModel, undefined, registry) === true` passes); on failure: write each failing component's first message to `/_a2uiChecks/<id>`, emit `error` output (`{version:'v0.9', error:{code:'VALIDATION_FAILED', surfaceId, path?, message}}`), do NOT emit the action; on success: clear `/_a2uiChecks/*` and emit as today.
+- [ ] Catalog checkable components render `errorText` (small `--ds-*` error line + invalid styling) when non-empty.
+- [ ] Specs cover: failing required check blocks + displays + emits error; fixing the value then re-clicking emits the action with the live context value.
+
+### Task 4: prompts + docs + api-docs
+
+**Files:** `examples/*/python/src/schemas/a2ui_v09.py` (byte-identical twins; add checks section with the CheckRule shape + validators), a2ui/chat docs pages that say checks are "typed but not enforced", `libs/a2ui/README.md`, `npm run generate-api-docs`.
+
+### Task 5: verification + PR
+
+- [ ] `nx run-many -t lint test build -p a2ui chat`; pytest twins; `nx affected -t lint test build`.
+- [ ] Live Chrome smoke: prompt for a form with a required + email check; submit empty → inline error, no agent turn; fill valid → action round-trips with the typed values in context (verify via thread state).
+- [ ] PR `feat(a2ui): validation checks + client error message (Phase 3)`; 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 53dddf917..52b787aed 100644
--- a/examples/ag-ui/python/src/schemas/a2ui_v09.py
+++ b/examples/ag-ui/python/src/schemas/a2ui_v09.py
@@ -64,6 +64,27 @@
 Action function (only inside {"functionCall": ...} actions): openUrl with
 args {url} — opens the URL in a new tab.
 
+### Validation checks
+
+Input components (TextField, CheckBox, ChoicePicker, Slider, DateTimeInput)
+accept a "checks" array of rules {"condition": DynamicBoolean, "message": str}.
+The condition is typically a validator call — required {value}, regex {value,
+pattern}, length {value, min?, max?} (min and/or max required), numeric
+{value, min?, max?}, email {value} — or an and/or/not combination. The client
+evaluates checks against the LIVE data model (including user edits) when an
+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:
+
+  {"id": "email", "component": "TextField", "label": "Email",
+   "value": {"path": "/email"},
+   "checks": [
+     {"condition": {"call": "required", "args": {"value": {"path": "/email"}}},
+      "message": "Email is required"},
+     {"condition": {"call": "email", "args": {"value": {"path": "/email"}}},
+      "message": "Enter a valid email address"}]}
+
 ### Children
 
 Container components ("children" property) accept either:
diff --git a/examples/chat/python/src/schemas/a2ui_v09.py b/examples/chat/python/src/schemas/a2ui_v09.py
index 53dddf917..52b787aed 100644
--- a/examples/chat/python/src/schemas/a2ui_v09.py
+++ b/examples/chat/python/src/schemas/a2ui_v09.py
@@ -64,6 +64,27 @@
 Action function (only inside {"functionCall": ...} actions): openUrl with
 args {url} — opens the URL in a new tab.
 
+### Validation checks
+
+Input components (TextField, CheckBox, ChoicePicker, Slider, DateTimeInput)
+accept a "checks" array of rules {"condition": DynamicBoolean, "message": str}.
+The condition is typically a validator call — required {value}, regex {value,
+pattern}, length {value, min?, max?} (min and/or max required), numeric
+{value, min?, max?}, email {value} — or an and/or/not combination. The client
+evaluates checks against the LIVE data model (including user edits) when an
+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:
+
+  {"id": "email", "component": "TextField", "label": "Email",
+   "value": {"path": "/email"},
+   "checks": [
+     {"condition": {"call": "required", "args": {"value": {"path": "/email"}}},
+      "message": "Email is required"},
+     {"condition": {"call": "email", "args": {"value": {"path": "/email"}}},
+      "message": "Enter a valid email address"}]}
+
 ### Children
 
 Container components ("children" property) accept either:
diff --git a/libs/a2ui/src/lib/functions.spec.ts b/libs/a2ui/src/lib/functions.spec.ts
index e4915c74e..0e10c2471 100644
--- a/libs/a2ui/src/lib/functions.spec.ts
+++ b/libs/a2ui/src/lib/functions.spec.ts
@@ -178,3 +178,56 @@ describe('registry behavior', () => {
     expect(run('not', { value: { call: 'and', args: { values: [true, true] } } })).toBe(false);
   });
 });
+
+describe('validators', () => {
+  test('required', () => {
+    expect(run('required', { value: 'x' })).toBe(true);
+    expect(run('required', { value: { path: '/name' } })).toBe(true);
+    expect(run('required', { value: '' })).toBe(false);
+    expect(run('required', { value: null })).toBe(false);
+    expect(run('required', { value: { path: '/missing' } })).toBe(false);
+    expect(run('required', { value: [] })).toBe(false);
+    expect(run('required', { value: ['a'] })).toBe(true);
+    expect(run('required', { value: 0 })).toBe(true);
+    expect(run('required', { value: false })).toBe(true);
+  });
+
+  test('regex', () => {
+    expect(run('regex', { value: 'abc-12', pattern: '^[a-z]+-\\d+$' })).toBe(true);
+    expect(run('regex', { value: 'nope', pattern: '^[a-z]+-\\d+$' })).toBe(false);
+    expect(run('regex', { value: 42, pattern: '\\d+' })).toBe(false);
+    expect(run('regex', { value: 'x', pattern: '(' })).toBe(false); // invalid pattern
+  });
+
+  test('length', () => {
+    expect(run('length', { value: 'hello', min: 2 })).toBe(true);
+    expect(run('length', { value: 'h', min: 2 })).toBe(false);
+    expect(run('length', { value: 'hello', max: 4 })).toBe(false);
+    expect(run('length', { value: 'hi', min: 1, max: 4 })).toBe(true);
+    expect(run('length', { value: 7, min: 1 })).toBe(false);
+  });
+
+  test('numeric', () => {
+    expect(run('numeric', { value: 5, min: 1, max: 10 })).toBe(true);
+    expect(run('numeric', { value: '5', min: 1 })).toBe(true);
+    expect(run('numeric', { value: 0, min: 1 })).toBe(false);
+    expect(run('numeric', { value: 11, max: 10 })).toBe(false);
+    expect(run('numeric', { value: 'abc', min: 0 })).toBe(false);
+  });
+
+  test('email', () => {
+    expect(run('email', { value: 'ada@example.com' })).toBe(true);
+    expect(run('email', { value: 'ada@sub.example.co' })).toBe(true);
+    expect(run('email', { value: 'not-an-email' })).toBe(false);
+    expect(run('email', { value: 'a@b' })).toBe(false);
+    expect(run('email', { value: '' })).toBe(false);
+    expect(run('email', { value: 7 })).toBe(false);
+  });
+
+  test('validators compose with logic functions in check conditions', () => {
+    expect(run('and', { values: [
+      { call: 'required', args: { value: { path: '/name' } } },
+      { call: 'length', args: { value: { path: '/name' }, min: 2 } },
+    ] })).toBe(true);
+  });
+});
diff --git a/libs/a2ui/src/lib/functions.ts b/libs/a2ui/src/lib/functions.ts
index 9fb7278b6..b27442a06 100644
--- a/libs/a2ui/src/lib/functions.ts
+++ b/libs/a2ui/src/lib/functions.ts
@@ -321,6 +321,53 @@ const STANDARD_FUNCTIONS: Record<string, A2uiFunctionImpl> = {
   not(args, ctx) {
     return ctx.resolveArg(args['value']) !== true;
   },
+  // --- Validators (check-rule conditions) ---
+  required(args, ctx) {
+    const value = ctx.resolveArg(args['value']);
+    if (value == null) return false;
+    if (typeof value === 'string') return value.length > 0;
+    if (Array.isArray(value)) return value.length > 0;
+    return true;
+  },
+  regex(args, ctx) {
+    const value = ctx.resolveArg(args['value']);
+    const pattern = ctx.resolveArg(args['pattern']);
+    if (typeof value !== 'string' || typeof pattern !== 'string') return false;
+    try {
+      return new RegExp(pattern).test(value);
+    } catch {
+      return false;
+    }
+  },
+  length(args, ctx) {
+    const value = ctx.resolveArg(args['value']);
+    if (typeof value !== 'string') return false;
+    const min = toNumber(ctx.resolveArg(args['min']));
+    const max = toNumber(ctx.resolveArg(args['max']));
+    if (min !== undefined && value.length < min) return false;
+    if (max !== undefined && value.length > max) return false;
+    return true;
+  },
+  numeric(args, ctx) {
+    const value = toNumber(ctx.resolveArg(args['value']));
+    if (value === undefined) return false;
+    const min = toNumber(ctx.resolveArg(args['min']));
+    const max = toNumber(ctx.resolveArg(args['max']));
+    if (min !== undefined && value < min) return false;
+    if (max !== undefined && value > max) return false;
+    return true;
+  },
+  email(args, ctx) {
+    const value = ctx.resolveArg(args['value']);
+    if (typeof value !== 'string') return false;
+    // Linear-time shape check: local@domain.tld with a dotted domain.
+    const at = value.indexOf('@');
+    if (at <= 0 || at !== value.lastIndexOf('@') || at === value.length - 1) return false;
+    const domain = value.slice(at + 1);
+    const dot = domain.lastIndexOf('.');
+    if (dot <= 0 || dot === domain.length - 1) return false;
+    return !/\s/.test(value);
+  },
 };
 
 // formatString needs the registry that owns it to evaluate nested calls;
diff --git a/libs/a2ui/src/lib/types.ts b/libs/a2ui/src/lib/types.ts
index 5b734abcf..72d51664f 100644
--- a/libs/a2ui/src/lib/types.ts
+++ b/libs/a2ui/src/lib/types.ts
@@ -55,12 +55,15 @@ export interface A2uiFunctionAction {
 
 export type A2uiAction = A2uiEventAction | A2uiFunctionAction;
 
-// --- Validation checks (typed in Phase 1, enforced in Phase 3) ---
+// --- Validation checks (spec `CheckRule`) ---
 
 export interface A2uiCheck {
-  call: string;
-  args?: Record<string, DynamicValue>;
-  message?: string;
+  /** A DynamicBoolean — typically a validator function call (`required`,
+   * `regex`, `length`, `numeric`, `email`) or a logic combinator. The rule
+   * passes when the condition resolves to `true`. */
+  condition: DynamicValue;
+  /** Error message displayed when the check fails. */
+  message: string;
 }
 
 // --- Components (flat, discriminated by the `component` string) ---
diff --git a/libs/chat/src/lib/a2ui/catalog/check-box.component.ts b/libs/chat/src/lib/a2ui/catalog/check-box.component.ts
index b47189bb5..3df1b489c 100644
--- a/libs/chat/src/lib/a2ui/catalog/check-box.component.ts
+++ b/libs/chat/src/lib/a2ui/catalog/check-box.component.ts
@@ -13,6 +13,9 @@ import { emitBinding } from './emit-binding';
       <input type="checkbox" [checked]="value()" (change)="onChange($event)" class="a2ui-cb__input" />
       {{ label() }}
     </label>
+    @if (errorText()) {
+      <div class="a2ui-check-error" role="alert">{{ errorText() }}</div>
+    }
   `,
   styles: [`
     .a2ui-cb {
@@ -29,7 +32,11 @@ import { emitBinding } from './emit-binding';
       cursor: pointer;
       accent-color: var(--a2ui-primary);
     }
-  `],
+      .a2ui-check-error {
+      font-size: var(--a2ui-typography-label-size);
+      color: var(--a2ui-error, #d33d55);
+    }
+`],
 })
 export class A2uiCheckBoxComponent {
   private readonly host = injectRenderHost();
@@ -37,6 +44,9 @@ export class A2uiCheckBoxComponent {
   readonly label = input<string>('');
   /** v0.9 prop: boolean checked state. */
   readonly value = input<boolean>(false);
+  /** Live validation message written by the surface's check gate
+   * (bound to /_a2uiChecks/<id>); empty when valid. */
+  readonly errorText = input<string>('');
   readonly _bindings = input<Record<string, string>>({});
   // Framework inputs required by the render harness.
   readonly bindings = input<Record<string, string>>({});
diff --git a/libs/chat/src/lib/a2ui/catalog/choice-picker.component.ts b/libs/chat/src/lib/a2ui/catalog/choice-picker.component.ts
index 648468f5c..c0c906899 100644
--- a/libs/chat/src/lib/a2ui/catalog/choice-picker.component.ts
+++ b/libs/chat/src/lib/a2ui/catalog/choice-picker.component.ts
@@ -59,6 +59,9 @@ interface ResolvedOption {
           }
         </div>
       }
+      @if (errorText()) {
+      <div class="a2ui-check-error" role="alert">{{ errorText() }}</div>
+    }
     </div>
   `,
   styles: [`
@@ -119,7 +122,11 @@ interface ResolvedOption {
       color: var(--a2ui-on-primary);
       border-color: var(--a2ui-primary);
     }
-  `],
+      .a2ui-check-error {
+      font-size: var(--a2ui-typography-label-size);
+      color: var(--a2ui-error, #d33d55);
+    }
+`],
 })
 export class A2uiChoicePickerComponent {
   private static _idCounter = 0;
@@ -141,6 +148,9 @@ export class A2uiChoicePickerComponent {
   readonly displayStyle = input<'checkbox' | 'chips'>('checkbox');
   /** v0.9 prop: when true, show a client-side option filter input. */
   readonly filterable = input<boolean>(false);
+  /** Live validation message written by the surface's check gate
+   * (bound to /_a2uiChecks/<id>); empty when valid. */
+  readonly errorText = input<string>('');
   readonly _bindings = input<Record<string, string>>({});
   // Framework inputs required by the render harness.
   readonly bindings = input<Record<string, string>>({});
diff --git a/libs/chat/src/lib/a2ui/catalog/date-time-input.component.ts b/libs/chat/src/lib/a2ui/catalog/date-time-input.component.ts
index cd17e613b..0c47ec4f4 100644
--- a/libs/chat/src/lib/a2ui/catalog/date-time-input.component.ts
+++ b/libs/chat/src/lib/a2ui/catalog/date-time-input.component.ts
@@ -22,6 +22,9 @@ import { emitBinding } from './emit-binding';
         class="a2ui-dti__input"
         (change)="onChange($event)"
       />
+      @if (errorText()) {
+      <div class="a2ui-check-error" role="alert">{{ errorText() }}</div>
+    }
     </div>
   `,
   styles: [`
@@ -46,7 +49,11 @@ import { emitBinding } from './emit-binding';
       outline-offset: 2px;
       border-color: var(--a2ui-primary);
     }
-  `],
+      .a2ui-check-error {
+      font-size: var(--a2ui-typography-label-size);
+      color: var(--a2ui-error, #d33d55);
+    }
+`],
 })
 export class A2uiDateTimeInputComponent {
   private static _idCounter = 0;
@@ -65,6 +72,9 @@ export class A2uiDateTimeInputComponent {
   readonly min = input<string | undefined>(undefined);
   /** v0.9 prop: ISO upper bound mapped to the native input's max. */
   readonly max = input<string | undefined>(undefined);
+  /** Live validation message written by the surface's check gate
+   * (bound to /_a2uiChecks/<id>); empty when valid. */
+  readonly errorText = input<string>('');
   readonly _bindings = input<Record<string, string>>({});
   // Framework inputs required by the render harness.
   readonly bindings = input<Record<string, string>>({});
diff --git a/libs/chat/src/lib/a2ui/catalog/slider.component.ts b/libs/chat/src/lib/a2ui/catalog/slider.component.ts
index 60f550dc3..050591801 100644
--- a/libs/chat/src/lib/a2ui/catalog/slider.component.ts
+++ b/libs/chat/src/lib/a2ui/catalog/slider.component.ts
@@ -22,6 +22,9 @@ import { emitBinding } from './emit-binding';
         [value]="value()"
         (input)="onInput($event)"
       />
+      @if (errorText()) {
+      <div class="a2ui-check-error" role="alert">{{ errorText() }}</div>
+    }
     </div>
   `,
   styles: [`
@@ -36,7 +39,11 @@ import { emitBinding } from './emit-binding';
       cursor: pointer;
       accent-color: var(--a2ui-primary);
     }
-  `],
+      .a2ui-check-error {
+      font-size: var(--a2ui-typography-label-size);
+      color: var(--a2ui-error, #d33d55);
+    }
+`],
 })
 export class A2uiSliderComponent {
   private static _idCounter = 0;
@@ -51,6 +58,9 @@ export class A2uiSliderComponent {
   readonly min = input<number>(0);
   /** v0.9 prop: upper bound. */
   readonly max = input<number>(100);
+  /** Live validation message written by the surface's check gate
+   * (bound to /_a2uiChecks/<id>); empty when valid. */
+  readonly errorText = input<string>('');
   readonly _bindings = input<Record<string, string>>({});
   // Framework inputs required by the render harness.
   readonly bindings = input<Record<string, string>>({});
diff --git a/libs/chat/src/lib/a2ui/catalog/text-field.component.ts b/libs/chat/src/lib/a2ui/catalog/text-field.component.ts
index 58f93f249..6518c512b 100644
--- a/libs/chat/src/lib/a2ui/catalog/text-field.component.ts
+++ b/libs/chat/src/lib/a2ui/catalog/text-field.component.ts
@@ -44,6 +44,9 @@ const TYPE_MAP: Record<TextFieldVariant, string> = {
           (input)="onInput($event)"
         />
       }
+      @if (errorText()) {
+        <div class="a2ui-check-error" role="alert">{{ errorText() }}</div>
+      }
     </div>
   `,
   styles: [`
@@ -69,7 +72,11 @@ const TYPE_MAP: Record<TextFieldVariant, string> = {
       outline-offset: 2px;
       border-color: var(--a2ui-primary);
     }
-  `],
+      .a2ui-check-error {
+      font-size: var(--a2ui-typography-label-size);
+      color: var(--a2ui-error, #d33d55);
+    }
+`],
 })
 export class A2uiTextFieldComponent {
   private static _idCounter = 0;
@@ -83,8 +90,11 @@ export class A2uiTextFieldComponent {
   readonly placeholder = input<string>('');
   /** v0.9 prop: input variant (default 'shortText'). */
   readonly variant = input<TextFieldVariant>('shortText');
-  /** Stored but not yet enforced beyond the native pattern attribute. */
+  /** Enforced by the surface's check gate as an implicit regex rule (plus the native pattern attribute). */
   readonly validationRegexp = input<string>('');
+  /** Live validation message written by the surface's check gate
+   * (bound to /_a2uiChecks/<id>); empty when valid. */
+  readonly errorText = input<string>('');
   readonly _bindings = input<Record<string, string>>({});
   // Framework inputs required by the render harness.
   readonly bindings = input<Record<string, string>>({});
diff --git a/libs/chat/src/lib/a2ui/surface-to-spec.spec.ts b/libs/chat/src/lib/a2ui/surface-to-spec.spec.ts
index bcf07fd53..ab976d767 100644
--- a/libs/chat/src/lib/a2ui/surface-to-spec.spec.ts
+++ b/libs/chat/src/lib/a2ui/surface-to-spec.spec.ts
@@ -166,13 +166,16 @@ describe('surfaceToSpec (v0.9)', () => {
     });
   });
 
-  it('resolves action context path bindings against the data model', () => {
+  it('keeps action context path bindings as live $bindState markers', () => {
+    // The surface component substitutes the CURRENT store value at dispatch
+    // time so user edits reach the agent (build-time resolution would freeze
+    // the agent-seeded snapshot).
     const surface = makeSurface(
       [
         c({ id: 'root', component: 'Column', children: ['btn'] }),
         c({
           id: 'btn', component: 'Button', child: 'lbl',
-          action: { event: { name: 'submit', context: { email: { path: '/email' } } } },
+          action: { event: { name: 'submit', context: { email: { path: '/email' }, formId: 'signup' } } },
         }),
         c({ id: 'lbl', component: 'Text', text: 'Go' }),
       ],
@@ -180,7 +183,20 @@ describe('surfaceToSpec (v0.9)', () => {
     );
     const spec = surfaceToSpec(surface)!;
     const params = spec.elements['btn'].on!['click'].params;
-    expect(params['context']).toEqual({ email: 'alice@example.com' });
+    expect(params['context']).toEqual({ email: { $bindState: '/email' }, formId: 'signup' });
+  });
+
+  it('adds errorText bindings + state seeds for checkable components', () => {
+    const surface = makeSurface(
+      [c({
+        id: 'root', component: 'TextField', label: 'Email', value: { path: '/email' },
+        checks: [{ condition: { call: 'email', args: { value: { path: '/email' } } }, message: 'Invalid email' }],
+      })],
+      { email: '' },
+    );
+    const spec = surfaceToSpec(surface)!;
+    expect(spec.elements['root'].props['errorText']).toEqual({ $bindState: '/_a2uiChecks/root' });
+    expect((spec.state as Record<string, Record<string, unknown>>)['_a2uiChecks']).toEqual({ root: '' });
   });
 
   it('functionCall actions wire to the local-action handler', () => {
diff --git a/libs/chat/src/lib/a2ui/surface-to-spec.ts b/libs/chat/src/lib/a2ui/surface-to-spec.ts
index 737b048c7..5f59a3342 100644
--- a/libs/chat/src/lib/a2ui/surface-to-spec.ts
+++ b/libs/chat/src/lib/a2ui/surface-to-spec.ts
@@ -48,7 +48,14 @@ function resolveAction(
   const resolvedContext: Record<string, unknown> = {};
   if (event.context && typeof event.context === 'object') {
     for (const [key, value] of Object.entries(event.context)) {
-      resolvedContext[key] = resolveDynamic(value, surface.dataModel, undefined, A2UI_FUNCTIONS);
+      if (isPathRef(value)) {
+        // Live marker: the surface component substitutes the CURRENT value
+        // (user edits included) from its state store at dispatch time —
+        // build-time resolution would freeze the agent-seeded snapshot.
+        resolvedContext[key] = { $bindState: value.path };
+      } else {
+        resolvedContext[key] = resolveDynamic(value, surface.dataModel, undefined, A2UI_FUNCTIONS);
+      }
     }
   }
   return {
@@ -118,6 +125,12 @@ export function surfaceToSpec(surface: A2uiSurface): Spec | null {
       resolvedProps['_bindings'] = bindings;
     }
 
+    // Checkable components surface their live validation message through a
+    // reserved store path the surface component writes on failed submits.
+    if (componentHasChecks(rawProps)) {
+      resolvedProps['errorText'] = { $bindState: `/_a2uiChecks/${id}` };
+    }
+
     const action = (rawProps as { action?: A2uiAction }).action;
     const on = resolveAction(action, surface, id);
 
@@ -185,5 +198,24 @@ export function surfaceToSpec(surface: A2uiSurface): Spec | null {
     ? 'root'
     : (surface.components.keys().next().value as string);
 
-  return { root, elements, state: surface.dataModel } as Spec;
+  // Seed empty check messages so errorText $bindState bindings resolve
+  // (render-element defers mounting while any bound prop is undefined).
+  const checkSeeds: Record<string, string> = {};
+  for (const [id, comp] of surface.components) {
+    if (componentHasChecks(comp as unknown as Record<string, unknown>)) checkSeeds[id] = '';
+  }
+  const state = Object.keys(checkSeeds).length > 0
+    ? { ...surface.dataModel, _a2uiChecks: { ...checkSeeds, ...(surface.dataModel['_a2uiChecks'] as Record<string, unknown> ?? {}) } }
+    : surface.dataModel;
+
+  return { root, elements, state } as Spec;
+}
+
+/** True when the component carries validation rules the renderer enforces:
+ * explicit `checks`, or a TextField `validationRegexp` with a bound value. */
+export function componentHasChecks(raw: Record<string, unknown>): boolean {
+  if (Array.isArray(raw['checks']) && raw['checks'].length > 0) return true;
+  return typeof raw['validationRegexp'] === 'string'
+    && raw['validationRegexp'].length > 0
+    && isPathRef(raw['value']);
 }
diff --git a/libs/chat/src/lib/a2ui/surface.component.spec.ts b/libs/chat/src/lib/a2ui/surface.component.spec.ts
index 5744971bb..bcf15000d 100644
--- a/libs/chat/src/lib/a2ui/surface.component.spec.ts
+++ b/libs/chat/src/lib/a2ui/surface.component.spec.ts
@@ -84,3 +84,114 @@ describe('A2uiSurfaceComponent — nested children with real catalog (regression
     expect(fx.nativeElement.textContent).toContain('Hello');
   });
 });
+
+describe('A2uiSurfaceComponent — validation gate + live context (Phase 3)', () => {
+  beforeEach(() => TestBed.configureTestingModule({ imports: [A2uiSurfaceComponent] }));
+
+  function makeCheckedSurfaceState() {
+    const store = createA2uiSurfaceStore();
+    store.apply({ version: 'v0.9', createSurface: { surfaceId: 's1', catalogId: 'basic' } } as never);
+    store.apply({ version: 'v0.9', updateComponents: {
+      surfaceId: 's1',
+      components: [
+        { id: 'root', component: 'Column', children: ['email', 'go'] },
+        { id: 'email', component: 'TextField', label: 'Email', value: { path: '/email' },
+          checks: [
+            { condition: { call: 'required', args: { value: { path: '/email' } } }, message: 'Email is required' },
+            { condition: { call: 'email', args: { value: { path: '/email' } } }, message: 'Invalid email' },
+          ] },
+        { id: 'go', component: 'Button', child: 'go-lbl',
+          action: { event: { name: 'submit', context: { email: { path: '/email' } } } } },
+        { id: 'go-lbl', component: 'Text', text: 'Send' },
+      ],
+    } } as never);
+    store.apply({ version: 'v0.9', updateDataModel: { surfaceId: 's1', path: '/email', value: '' } } as never);
+    return store.surfaceState('s1')()!;
+  }
+
+  it('blocks the action, writes the check message, and emits VALIDATION_FAILED when checks fail', () => {
+    const fx = TestBed.createComponent(A2uiSurfaceComponent);
+    fx.componentRef.setInput('state', makeCheckedSurfaceState());
+    fx.componentRef.setInput('catalog', a2uiBasicCatalog());
+    const actions: unknown[] = [];
+    const errors: { error: { code: string; message?: string } }[] = [];
+    fx.componentInstance.action.subscribe((a) => actions.push(a));
+    fx.componentInstance.validationError.subscribe((e) => errors.push(e));
+    fx.detectChanges();
+
+    const handler = fx.componentInstance.internalHandlers()['a2ui:event'];
+    const result = handler({
+      surfaceId: 's1', sourceComponentId: 'go', name: 'submit',
+      context: { email: { $bindState: '/email' } },
+    });
+
+    expect(result).toBeUndefined();
+    expect(actions).toHaveLength(0);
+    expect(errors).toHaveLength(1);
+    expect(errors[0].error.code).toBe('VALIDATION_FAILED');
+    expect(errors[0].error.message).toBe('Email is required');
+    expect(fx.componentInstance.liveStore.get('/_a2uiChecks/email')).toBe('Email is required');
+  });
+
+  it('emits the action with live-typed context values once checks pass', () => {
+    const fx = TestBed.createComponent(A2uiSurfaceComponent);
+    fx.componentRef.setInput('state', makeCheckedSurfaceState());
+    fx.componentRef.setInput('catalog', a2uiBasicCatalog());
+    const actions: { action: { context?: Record<string, unknown> } }[] = [];
+    fx.componentInstance.action.subscribe((a) => actions.push(a));
+    fx.detectChanges();
+
+    // Simulate the user typing into the bound TextField (writes the store).
+    fx.componentInstance.liveStore.set('/email', 'ada@example.com');
+
+    const handler = fx.componentInstance.internalHandlers()['a2ui:event'];
+    handler({
+      surfaceId: 's1', sourceComponentId: 'go', name: 'submit',
+      context: { email: { $bindState: '/email' } },
+    });
+
+    expect(actions).toHaveLength(1);
+    expect(actions[0].action.context).toEqual({ email: 'ada@example.com' });
+    expect(fx.componentInstance.liveStore.get('/_a2uiChecks/email')).toBe('');
+  });
+
+  it('enforces TextField validationRegexp as an implicit check', () => {
+    const store = createA2uiSurfaceStore();
+    store.apply({ version: 'v0.9', createSurface: { surfaceId: 's2', catalogId: 'basic' } } as never);
+    store.apply({ version: 'v0.9', updateComponents: {
+      surfaceId: 's2',
+      components: [
+        { id: 'root', component: 'Column', children: ['code'] },
+        { id: 'code', component: 'TextField', label: 'Code', value: { path: '/code' },
+          validationRegexp: '^[A-Z]{3}$' },
+      ],
+    } } as never);
+    store.apply({ version: 'v0.9', updateDataModel: { surfaceId: 's2', path: '/code', value: 'nope' } } as never);
+
+    const fx = TestBed.createComponent(A2uiSurfaceComponent);
+    fx.componentRef.setInput('state', store.surfaceState('s2')()!);
+    fx.componentRef.setInput('catalog', a2uiBasicCatalog());
+    const errors: { error: { code: string } }[] = [];
+    fx.componentInstance.validationError.subscribe((e) => errors.push(e));
+    fx.detectChanges();
+
+    const handler = fx.componentInstance.internalHandlers()['a2ui:event'];
+    const result = handler({ surfaceId: 's2', sourceComponentId: 'root', name: 'submit', context: {} });
+    expect(result).toBeUndefined();
+    expect(errors).toHaveLength(1);
+    expect(fx.componentInstance.liveStore.get('/_a2uiChecks/code')).toBe('Invalid format');
+  });
+
+  it('preserves user edits in the live store across spec re-emissions', () => {
+    const state = makeCheckedSurfaceState();
+    const fx = TestBed.createComponent(A2uiSurfaceComponent);
+    fx.componentRef.setInput('state', state);
+    fx.componentRef.setInput('catalog', a2uiBasicCatalog());
+    fx.detectChanges();
+    fx.componentInstance.liveStore.set('/email', 'user@typed.io');
+    // Re-emit the same state (streaming re-materializes surfaces).
+    fx.componentRef.setInput('state', { ...state });
+    fx.detectChanges();
+    expect(fx.componentInstance.liveStore.get('/email')).toBe('user@typed.io');
+  });
+});
diff --git a/libs/chat/src/lib/a2ui/surface.component.ts b/libs/chat/src/lib/a2ui/surface.component.ts
index 6ed485f6f..17d38712f 100644
--- a/libs/chat/src/lib/a2ui/surface.component.ts
+++ b/libs/chat/src/lib/a2ui/surface.component.ts
@@ -1,12 +1,15 @@
 // SPDX-License-Identifier: MIT
 import {
-  Component, computed, input, output, ChangeDetectionStrategy, Type,
+  Component, computed, effect, input, output, untracked, ChangeDetectionStrategy, Type,
 } from '@angular/core';
 import { NgComponentOutlet } from '@angular/common';
-import type { A2uiSurface, A2uiActionMessage } from '@threadplane/a2ui';
-import { RenderSpecComponent, toRenderRegistry } from '@threadplane/render';
+import type { A2uiSurface, A2uiActionMessage, A2uiErrorMessage, A2uiCheck } from '@threadplane/a2ui';
+import {
+  A2UI_WIRE_VERSION, createA2uiFunctionRegistry, getByPointer, isPathRef, resolveDynamic,
+} from '@threadplane/a2ui';
+import { RenderSpecComponent, toRenderRegistry, signalStateStore } from '@threadplane/render';
 import type { ViewRegistry, RenderEvent } from '@threadplane/render';
-import { surfaceToSpec } from './surface-to-spec';
+import { surfaceToSpec, componentHasChecks } from './surface-to-spec';
 import { buildA2uiActionMessage } from './build-action-message';
 import { A2uiDefaultFallbackComponent } from './a2ui-default-fallback.component';
 import type { A2uiSurfaceState } from './surface-store';
@@ -32,6 +35,7 @@ import type { A2uiViews } from './views';
       <render-spec
         [spec]="s"
         [registry]="registry()"
+        [store]="liveStore"
         [handlers]="internalHandlers()"
         (events)="onRenderEvent($event)"
       />
@@ -68,6 +72,41 @@ export class A2uiSurfaceComponent {
   readonly surfaceFallback = input<Type<unknown> | undefined>(undefined);
   readonly events = output<RenderEvent>();
   readonly action = output<A2uiActionMessage>();
+  /** Emitted when a submit is blocked by failing validation checks —
+   * the spec client → agent error message (code VALIDATION_FAILED). */
+  readonly validationError = output<A2uiErrorMessage>();
+
+  /** Surface-owned live state store: `$bindState` props read it and input
+   * components write user edits into it, so event-time logic (checks,
+   * action context) sees CURRENT values instead of the agent-seeded
+   * snapshot. Seeded from spec.state with user edits preserved. Public so
+   * hosts (and tests) can read the live values of a rendered surface. */
+  readonly liveStore = signalStateStore({});
+
+  /** Last value this component seeded per state path (see chat-generative-ui:
+   * distinguishes "still our seed — safe to overwrite" from "user edited"). */
+  private readonly seeded = new Map<string, unknown>();
+
+  constructor() {
+    effect(() => {
+      const s = this.spec();
+      const state = s?.state as Record<string, unknown> | undefined;
+      if (!state) return;
+      untracked(() => {
+        for (const [key, value] of Object.entries(state)) {
+          const path = key.startsWith('/') ? key : `/${key}`;
+          const current = this.liveStore.get(path);
+          const untouched =
+            current === undefined ||
+            (this.seeded.has(path) && current === this.seeded.get(path));
+          if (untouched) {
+            if (current !== value) this.liveStore.set(path, value);
+            this.seeded.set(path, value);
+          }
+        }
+      });
+    });
+  }
 
   /** Agent-set primary color from `createSurface.theme.primaryColor`.
    * Returns null when unset so the host binding doesn't override the
@@ -119,7 +158,49 @@ export class A2uiSurfaceComponent {
         // a mismatched id is also bound.
         const surf = this.state()?.surface ?? this.surface();
         if (!surf) return undefined;
-        const message = buildA2uiActionMessage(params, surf);
+
+        // Live model: user edits in the store overlay the agent-seeded model.
+        const liveModel = this.mergedLiveModel(surf);
+
+        // Validation gate: every check rule on the surface must pass before
+        // an event action dispatches (spec CheckRule semantics).
+        const failures = evaluateSurfaceChecks(surf, liveModel);
+        if (failures.length > 0) {
+          for (const f of failures) {
+            this.liveStore.set(`/_a2uiChecks/${f.componentId}`, f.message);
+          }
+          const first = failures[0];
+          this.validationError.emit({
+            version: A2UI_WIRE_VERSION,
+            error: {
+              code: 'VALIDATION_FAILED',
+              surfaceId: surf.surfaceId,
+              ...(first.path ? { path: first.path } : {}),
+              message: first.message,
+            },
+          });
+          return undefined;
+        }
+        // Clear any stale messages from a previous failed submit.
+        for (const [id, comp] of surf.components) {
+          if (componentHasChecks(comp as unknown as Record<string, unknown>)) {
+            this.liveStore.set(`/_a2uiChecks/${id}`, '');
+          }
+        }
+
+        // Substitute live-context markers with current values.
+        const rawContext = (params['context'] as Record<string, unknown>) ?? {};
+        const context: Record<string, unknown> = {};
+        for (const [k, v] of Object.entries(rawContext)) {
+          if (v != null && typeof v === 'object' && '$bindState' in (v as Record<string, unknown>)) {
+            const path = String((v as Record<string, unknown>)['$bindState']);
+            context[k] = getByPointer(liveModel, path);
+          } else {
+            context[k] = v;
+          }
+        }
+
+        const message = buildA2uiActionMessage({ ...params, context }, surf);
         this.action.emit(message);
         return message;
       },
@@ -144,4 +225,74 @@ export class A2uiSurfaceComponent {
   onRenderEvent(event: RenderEvent): void {
     this.events.emit(event);
   }
+
+  /** Agent-seeded data model overlaid with the store's current state
+   * (user edits + check messages). Shallow per-key merge is sufficient:
+   * store snapshots hold whole top-level values written via pointers. */
+  private mergedLiveModel(surf: A2uiSurface): Record<string, unknown> {
+    const snapshot = this.liveStore.getSnapshot() as Record<string, unknown>;
+    return deepOverlay(surf.dataModel, snapshot);
+  }
+}
+
+/** Recursively overlay `top` onto `base` (plain objects merge; anything
+ * else in `top` wins). */
+function deepOverlay(
+  base: Record<string, unknown>,
+  top: Record<string, unknown>,
+): Record<string, unknown> {
+  const out: Record<string, unknown> = { ...base };
+  for (const [k, v] of Object.entries(top)) {
+    const prev = out[k];
+    if (
+      v != null && typeof v === 'object' && !Array.isArray(v)
+      && prev != null && typeof prev === 'object' && !Array.isArray(prev)
+    ) {
+      out[k] = deepOverlay(prev as Record<string, unknown>, v as Record<string, unknown>);
+    } else {
+      out[k] = v;
+    }
+  }
+  return out;
+}
+
+const CHECK_FUNCTIONS = createA2uiFunctionRegistry();
+
+interface CheckFailure {
+  componentId: string;
+  message: string;
+  /** Data-model pointer of the checked value, when determinable. */
+  path?: string;
+}
+
+/** Evaluate every check rule on the surface against the live model.
+ * A rule passes when its condition resolves to exactly `true`. TextField
+ * `validationRegexp` (with a bound value) contributes an implicit rule. */
+function evaluateSurfaceChecks(
+  surf: A2uiSurface,
+  liveModel: Record<string, unknown>,
+): CheckFailure[] {
+  const failures: CheckFailure[] = [];
+  for (const [id, comp] of surf.components) {
+    const raw = comp as unknown as Record<string, unknown>;
+    const boundPath = isPathRef(raw['value']) ? (raw['value'] as { path: string }).path : undefined;
+    const rules: A2uiCheck[] = Array.isArray(raw['checks']) ? [...(raw['checks'] as A2uiCheck[])] : [];
+    if (
+      typeof raw['validationRegexp'] === 'string' && raw['validationRegexp'].length > 0 && boundPath
+    ) {
+      rules.push({
+        condition: { call: 'regex', args: { value: { path: boundPath }, pattern: raw['validationRegexp'] } },
+        message: 'Invalid format',
+      });
+    }
+    for (const rule of rules) {
+      if (!rule || typeof rule.message !== 'string') continue;
+      const passed = resolveDynamic(rule.condition, liveModel, undefined, CHECK_FUNCTIONS) === true;
+      if (!passed) {
+        failures.push({ componentId: id, message: rule.message, ...(boundPath ? { path: boundPath } : {}) });
+        break; // first failing rule per component
+      }
+    }
+  }
+  return failures;
 }