Complete "Giving errors back to model" pattern documentation

Adds JSDoc example `App_updateModelContext_reportError` showing how to
report runtime errors (e.g., microphone permission denied) back to the
model using `updateModelContext`.

Updates patterns.md with clearer framing:
- Server-side (tool handler): return `{ isError: true, ... }`
- Client-side (app): use `updateModelContext` for runtime errors

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/patterns.md

@@ -20,12 +20,13 @@ Set {@link types!McpUiToolMeta.visibility Tool.\_meta.ui.visibility} to `["app"]
 
 {@includeCode ./patterns.tsx#authenticatedCalls}
 
-## [TODO] Giving errors back to model
+## Giving errors back to model
 
-- Before app runs: validate inputs in tool call
+**Server-side**: Tool handler validates inputs and returns `{ isError: true, content: [...] }`. The model receives this error through the normal tool call response.
-- After it runs: use `updateModelContext`
 
-{@includeCode ./patterns.tsx#errorsToModel}
+**Client-side**: If a runtime error occurs (e.g., API failure, permission denied, resource unavailable), use {@link app!App.updateModelContext updateModelContext} to inform the model:
+
+{@includeCode ../src/app.examples.ts#App_updateModelContext_reportError}
 
 ## Matching host styling (CSS variables, theme, and fonts)
 

docs/patterns.tsx

@@ -27,17 +27,6 @@ function authenticatedCalls(app: App) {
   //#endregion authenticatedCalls
 }
 
-/**
- * Example: Giving errors back to model
- */
-function errorsToModel(app: App) {
-  //#region errorsToModel
-  // Before app runs: validate inputs in tool call
-  // After it runs: updateModelContext
-  // TODO: Complete implementation
-  //#endregion errorsToModel
-}
-
 /**
  * Example: Unified host styling (theme, CSS variables, fonts)
  */
@@ -132,7 +121,6 @@ function migrateFromOpenai() {
 
 // Suppress unused variable warnings
 void authenticatedCalls;
-void errorsToModel;
 void hostStylingVanillaJs;
 void hostStylingReact;
 void persistData;

src/app.examples.ts

@@ -364,6 +364,28 @@ async function App_updateModelContext_structuredContent(app: App) {
   //#endregion App_updateModelContext_structuredContent
 }
 
+/**
+ * Example: Report runtime error to model.
+ */
+async function App_updateModelContext_reportError(app: App) {
+  //#region App_updateModelContext_reportError
+  try {
+    const _stream = await navigator.mediaDevices.getUserMedia({ audio: true });
+    // ... use _stream for transcription
+  } catch (err) {
+    // Inform the model that the app is in a degraded state
+    await app.updateModelContext({
+      content: [
+        {
+          type: "text",
+          text: "Error: transcription unavailable",
+        },
+      ],
+    });
+  }
+  //#endregion App_updateModelContext_reportError
+}
+
 /**
  * Example: Open documentation link.
  */

src/app.ts

@@ -674,6 +674,9 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
    * @example Update with structured content
    * {@includeCode ./app.examples.ts#App_updateModelContext_structuredContent}
    *
+   * @example Report runtime error to model
+   * {@includeCode ./app.examples.ts#App_updateModelContext_reportError}
+   *
    * @returns Promise that resolves when the context update is acknowledged
    */
   updateModelContext(