structured-output-as-message-part.md

@tanstack/ai	minor
@tanstack/ai-client	minor
@tanstack/ai-react	minor
@tanstack/ai-vue	minor
@tanstack/ai-solid	minor
@tanstack/ai-svelte	minor

feat: structured-output as a typed MessagePart on each assistant UIMessage

useChat({ outputSchema }) (React, Vue, Solid) and createChat({ outputSchema }) (Svelte) previously kept a single hook-level partial/final slot, so multi-turn structured chats lost every prior turn's response as soon as a new one streamed in. Each assistant turn now carries its own typed structured-output MessagePart on the UIMessage it belongs to. History walks messages and finds the typed part on each turn; the hook-level partial and final are derived from the latest assistant message's part and continue to work as before. Applies to all four framework hook packages.

The structured-output part type is generic over the schema's inferred data type:

• StructuredOutputPart<TData = unknown> in @tanstack/ai carries data: TData, partial: DeepPartial<TData>, raw: string, plus status: 'streaming' | 'complete' | 'error' and an optional errorMessage.
• MessagePart<TTools, TData> and UIMessage<TTools, TData> in @tanstack/ai-client thread the generic through the message types.
• Each framework hook's return (UseChatReturn<TTools, TSchema> for React / Vue / Solid, CreateChatReturn<TTools, TSchema> for Svelte) substitutes TData = InferSchemaType<TSchema> when a schema is supplied, so messages[i].parts.find(p => p.type === 'structured-output').data is typed by the schema with no cast required.

Default TData = unknown keeps every existing consumer that doesn't pass a schema source-compatible.

Server-side chat({ outputSchema, stream: true }) emits a new structured-output.start CUSTOM event before the JSON deltas so the client processor can route them into the StructuredOutputPart instead of building a TextPart. The wire converter serializes the part's raw JSON back as assistant content, so multi-turn structured chats stay coherent (the LLM sees its own prior structured responses on follow-up turns). For adapters without native JSON-schema streaming (Anthropic, Gemini, Ollama), the existing fallback path emits one terminal structured-output.complete event and the same per-turn typed part lands on the message — consumer code is identical.

A new example route demonstrating the multi-turn pattern is at /generations/structured-chat in the ts-react-chat example.

Breaking-shape note (minor, not major): When outputSchema is set, TEXT_MESSAGE_CONTENT deltas no longer create a TextPart on the assistant message — they accumulate into the StructuredOutputPart. Consumers that iterated message.parts and explicitly filtered out TextParts to hide raw JSON (the workaround documented prior to this change) can remove that filter; doing nothing is also safe because no TextPart is produced in the first place.