fix: address CodeRabbit findings + e2e structured-output renderer

- e2e ChatUI: render `structured-output` parts so the assistant message
  has visible text content. The Phase 1 redesign routes structured-output
  JSON deltas into a dedicated part instead of a text part, so the e2e
  selector `getLastAssistantMessage` was returning an empty string and the
  structured-output-stream specs failed.

- processor: reconcile cumulative `chunk.content` against the existing
  StructuredOutputPart.raw before appending, mirroring the dedup the text
  path already does. Adapters that emit cumulative content (rather than
  incremental delta) on a structured run would otherwise duplicate the
  JSON buffer and corrupt the partial parse. Adds a focused unit test.

- example: fix the recipe metadata block in /generations/structured-chat
  so a valid `estimatedCostUsd === 0` renders `$0.00` instead of being
  hidden by a truthy check. Minor CodeRabbit catch.

The remaining CodeRabbit findings were either stale (raw fallback already
addressed in the prior fix commit) or pre-existing (initialMessages
mount-only hydration was on main before this branch — out of scope).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tombeckenham

examples/ts-react-chat/src/routes/generations.structured-chat.tsx

@@ -186,11 +186,13 @@ function RecipeCard({ part }: { part: StructuredOutputPart }) {
           <span className="inline-block w-2 h-2 mt-2 rounded-full bg-orange-400 animate-pulse" />
         )}
       </div>
-      {(recipe.cuisine || recipe.servings || recipe.estimatedCostUsd) && (
+      {(recipe.cuisine ||
+        recipe.servings ||
+        recipe.estimatedCostUsd !== undefined) && (
         <p className="text-xs text-gray-400 mt-1">
           {recipe.cuisine}
           {recipe.servings ? ` · serves ${recipe.servings}` : ''}
-          {recipe.estimatedCostUsd
+          {recipe.estimatedCostUsd !== undefined
             ? ` · ~$${recipe.estimatedCostUsd.toFixed(2)}`
             : ''}
         </p>

packages/typescript/ai/src/activities/chat/stream/processor.ts

@@ -855,11 +855,28 @@ export class StreamProcessor {
     this.completeAllToolCallsForMessage(messageId)
 
     if (this.structuredMessageIds.has(messageId)) {
-      const delta =
-        chunk.delta ||
-        (chunk.content !== undefined && chunk.content !== ''
-          ? chunk.content
-          : '')
+      // `chunk.delta` is incremental; `chunk.content` is sometimes cumulative
+      // (mirrors what the plain-text branch handles below). Reconcile against
+      // the existing raw buffer so adapters that emit cumulative content
+      // don't duplicate the JSON.
+      let delta = chunk.delta || ''
+      if (delta === '' && chunk.content !== undefined && chunk.content !== '') {
+        const existingRaw = (
+          this.messages
+            .find((m) => m.id === messageId)
+            ?.parts.find(
+              (p): p is Extract<MessagePart, { type: 'structured-output' }> =>
+                p.type === 'structured-output',
+            ) ?? { raw: '' }
+        ).raw
+        if (chunk.content.startsWith(existingRaw)) {
+          delta = chunk.content.slice(existingRaw.length)
+        } else if (existingRaw.startsWith(chunk.content)) {
+          delta = ''
+        } else {
+          delta = chunk.content
+        }
+      }
       if (delta !== '') {
         this.messages = appendStructuredOutputDelta(
           this.messages,

packages/typescript/ai/tests/stream-processor.test.ts

@@ -3551,6 +3551,40 @@ describe('StreamProcessor', () => {
       expect(sop?.data).toEqual({ name: 'A' })
     })
 
+    it('reconciles cumulative TEXT_MESSAGE_CONTENT against existing raw without duplication', () => {
+      // Some adapters emit `content` (cumulative buffer) instead of `delta`.
+      // The structured branch must dedup it against the part's existing raw,
+      // otherwise we'd append the whole buffer each time and corrupt the JSON.
+      const processor = new StreamProcessor()
+
+      processor.processChunk(ev.runStarted())
+      processor.processChunk(ev.textStart('msg-1'))
+      processor.processChunk(
+        chunk(EventType.CUSTOM, {
+          name: 'structured-output.start',
+          value: { messageId: 'msg-1' },
+        }),
+      )
+      processor.processChunk(
+        chunk(EventType.TEXT_MESSAGE_CONTENT, {
+          messageId: 'msg-1',
+          content: '{"name":',
+        }),
+      )
+      processor.processChunk(
+        chunk(EventType.TEXT_MESSAGE_CONTENT, {
+          messageId: 'msg-1',
+          content: '{"name":"Alice"}',
+        }),
+      )
+
+      const sop = processor
+        .getMessages()
+        .find((m) => m.role === 'assistant')!
+        .parts.find((p) => p.type === 'structured-output') as any
+      expect(sop.raw).toBe('{"name":"Alice"}')
+    })
+
     it('clears structuredMessageIds for messages dropped by removeMessagesAfter (reload)', () => {
       // Reload removes the last assistant message and re-streams from the
       // last user message. If the dropped assistant's routing entry lingers

testing/e2e/src/components/ChatUI.tsx

@@ -158,6 +158,26 @@ export function ChatUI({
                   </div>
                 )
               }
+              if (part.type === 'structured-output') {
+                // Render the streamed JSON so the assistant message has
+                // visible content for selectors (e.g. `getLastAssistantMessage`).
+                // Previously this content arrived as a `text` part — the new
+                // routing puts it on a `structured-output` part instead.
+                const sop = part as any
+                const text =
+                  sop.raw ||
+                  (sop.data !== undefined ? JSON.stringify(sop.data) : '')
+                if (text === '') return null
+                return (
+                  <div
+                    key={i}
+                    data-testid="structured-output-part"
+                    className="prose prose-invert prose-sm max-w-none whitespace-pre-wrap break-words"
+                  >
+                    {text}
+                  </div>
+                )
+              }
               return null
             })}
           </div>