> ⚠️ **Postponed pending approval.** Part of the [Chat Fundamentals plan](https://git.drupalcode.org/project/ai/-/work_items/3586640); awaits maintainer approval ([approval issue](https://git.drupalcode.org/project/ai/-/work_items/3586639)). Targets the **1.6.x branch once it is created**. **Depends on:** [content blocks on ChatMessage (#3586643)](https://git.drupalcode.org/project/ai/-/work_items/3586643) and [streamed metadata fix (#3586641)](https://git.drupalcode.org/project/ai/-/work_items/3586641). ### Problem/Motivation The ChatProcessor plugin type (`Drupal\ai\Plugin\ChatProcessor\ChatProcessorInterface`, base `ChatProcessorBase`, manager `plugin.manager.ai.chat_processor`) is the contract between the ai module and chat frontends. It can hand a consumer exactly two things per turn: the response text, and `getPostResponseMarkup(): string` — trusted raw HTML appended after `Xss::filter()`, which the docblock itself warns must be hand-escaped by the plugin. There is no structured payload channel, so even once messages can carry MCP content blocks (#3586643), a frontend has no sanctioned way to receive them. History has the same gap: `getMessageHistory()` returns `[{role, message, timestamp}]` — a flat string record. Any structured payload attached to a live response would vanish when the thread re-renders on page reload. This shape is the persistence bottleneck for every rich-chat capability in this plan (cards, sources, suggested replies). Finally, individual messages have no stable identity. Threads do (`setThreadId()`), but a frontend cannot reference "this specific message" — needed for correlating late-arriving payloads and a prerequisite for anything like per-message feedback, which is deliberately left to frontends (see the meta issue). ### Proposed resolution All interface additions ship with defaults in `ChatProcessorBase` — the interface's own docblock sanctions this evolution pattern (implement by extending `ChatProcessorBase`, which provides backwards-compatible defaults). - **`getStructuredResponse(): array`** — shape `['content_blocks' => array<MCP block arrays>, '_meta' => array]`, default `['content_blocks' => [], '_meta' => []]`. The structured sibling of `getPostResponseMarkup()`, which stays as the legacy trusted-HTML channel (2.0.0 wishlist in the meta issue deprecates it). - **Non-streaming:** available after `execute()`. - **Streaming:** text streams as today; the structured response is defined to be available after `onStreamComplete()` has been called — blocks are an end-of-message manifest, since cards, citations and suggested replies are generally only known at completion. Mid-stream data reaches the processor via per-chunk `StreamedChatMessage` metadata, which #3586641 makes survive reconstruction. - **`getMessageHistory()` item shape extension** (doc-level, BC-safe): items may carry optional `id` (stable message identifier), `content_blocks` and `_meta` keys alongside `role`, `message`, `timestamp`. Consumers that replay history render blocks through the same path as live responses (renderer child issue). - **Reference implementation** in the AI Assistant processor (ai_chatbot's `AiAssistantApiProcessor` backed by ai_assistant_api): persist blocks/`_meta` alongside messages in the existing thread tempstore (`ai_assistant_threads`), return them from both `getStructuredResponse()` and `getMessageHistory()`. - Update `docs/developers/writing_a_chat_processor_plugin.md` with the new channel, the streaming timing contract, and a consumer-side sketch: a chatbot either serializes raw block JSON to a block-aware frontend or server-renders blocks via the `ai.chat_block_render` service — that choice belongs to the consumer, keeping the contract frontend-agnostic. **Tests:** kernel test with a fixture processor asserting the structured response round-trip in both streaming and non-streaming modes, and history items retaining blocks across two simulated requests on one thread. ### Backwards compatibility Interface additions are covered by `ChatProcessorBase` defaults; any processor implementing the interface directly (none known in contrib) must add the methods — flag in the change record. History shape gains optional keys only. ### AI Assistance Yes, AI was used to explore the codebase and draft this issue.