Search: update SSE request to JSON-RPC 2.0 format

Switch from simple query body to JSON-RPC 2.0 message/stream format with
constructor_arguments and site_url in clientContext; fix endpoint URL to
remove sites/{site_id} path segment.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: gibrown

projects/packages/search/docs/plans/search-ai-answers-api-design.md

@@ -9,6 +9,8 @@ Accept: text/event-stream
 Content-Type: application/json
 ```
 
+Note: no `sites/{site_id}` in the path — the site is identified via `selectedSiteId` in the request body.
+
 ## Authentication
 
 Two auth paths:
@@ -25,13 +27,14 @@ Rotates hourly; wpcom accepts both the current and previous hour's token to hand
 
 ## Request Format
 
-JSON-RPC 2.0 with `message/stream` method. The search query goes in the `text` part; site context (site ID, active filters, locale) goes in the `data` part:
+JSON-RPC 2.0 with `message/stream` method. The search query goes in the `text` part; site context (site ID, site URL, active filters, locale) goes in the `data` part. `constructor_arguments` may be used for versioning:
 
 ```json
 {
   "jsonrpc": "2.0",
   "id": "req-1",
   "method": "message/stream",
+  "constructor_arguments": {},
   "params": {
     "message": {
       "role": "user",
@@ -42,6 +45,7 @@ JSON-RPC 2.0 with `message/stream` method. The search query goes in the `text` p
           "data": {
             "clientContext": {
               "selectedSiteId": 12345,
+              "site_url": "https://example.com",
               "filters": { "post_type": ["post", "page"], "category": [] },
               "locale": "en"
             }
@@ -67,4 +71,4 @@ JSON-RPC 2.0 with `message/stream` method. The search query goes in the `text` p
 
 ## Quota
 
-500 requests per calendar month per site (all plans). Returns `error` with `code: quota_exceeded` when exceeded; overlay falls back to standard search results.
+500 requests per calendar month per site (all plans, subject to change — see `search-ai-answers-roadmap.md`). Returns `error` with `code: quota_exceeded` when exceeded; overlay falls back to standard search results.

projects/packages/search/src/instant-search/components/search-app.jsx

@@ -270,7 +270,7 @@ class SearchApp extends Component {
 
 		this.setState( { aiStatus: 'loading', aiText: '', aiCitations: [] } );
 
-		const url = `https://public-api.wordpress.com/wpcom/v2/sites/${ siteId }/ai/agent/jetpack-search-answers`;
+		const url = 'https://public-api.wordpress.com/wpcom/v2/ai/agent/jetpack-search-answers';
 
 		fetchEventSource( url, {
 			method: 'POST',
@@ -279,9 +279,33 @@ class SearchApp extends Component {
 				Authorization: `Bearer ${ token }`,
 			},
 			body: JSON.stringify( {
-				query,
-				filters: this.props.filters,
-				locale: options.locale || 'en',
+				jsonrpc: '2.0',
+				id: `req-${ Date.now() }`,
+				method: 'message/stream',
+				constructor_arguments: {},
+				params: {
+					message: {
+						role: 'user',
+						parts: [
+							{ type: 'text', text: query },
+							{
+								type: 'data',
+								data: {
+									clientContext: {
+										selectedSiteId: siteId,
+										site_url: options.homeUrl || '',
+										filters: this.props.filters,
+										locale: options.locale || 'en',
+									},
+								},
+								metadata: {},
+							},
+						],
+						kind: 'message',
+						messageId: `msg-${ Date.now() }`,
+					},
+				},
+				tokenStreaming: true,
 			} ),
 			signal: controller.signal,
 			onopen: async response => {