[FEATURE] Let an app extension replace a chat message part renderer #2894

🤖 written by Claude (start)

📝 Feature Summary

Let an app extension replace the component that renders a chat message part, keyed by part kind.

❓ Problem Statement / Motivation

ChatMessageItem picks its component with a hardcoded ternary on part.dataKind (ChatMessageItem.tsx#L116). #2890 adds structured_output as a third branch. Every new part kind adds another.

The app extension system cannot reach it. All 13 points in EXTENSION_POINT_IDS are additive — ExtensionSlot mounts a component alongside the core one, never instead of it. Three of them sit in chat; none reaches part rendering.

So an extension that wants its own JSON viewer, its own tool-call card, or a domain renderer for an otherwise unknown payload has to fork ChatMessageItem.

💡 Proposed Solution

Mirror providerIcons (types.ts#L298): a map keyed by a string, merged over the core map, later extension wins, unknown key simply added.

/** Replacement renderers for chat message parts, keyed by ChatDataPart["dataKind"]. */
chatPartRenderers?: Readonly<
  Partial<Record<ChatDataPart["dataKind"], ComponentType<{ part: ChatDataPart }>>>
>;

ChatMessageItem looks the kind up and falls back to the core component. That removes the ternary rather than growing it, and covers tool_call, tool_result, structured_output and unknown at once.

Open questions:

• Key on ChatPart["kind"] as well, so text and approval records are replaceable?
• Does a renderer need turn context (taskId, sessionId), the way app_agents_agentChat_agentChatMessage_additionalActionsButton does?

🔄 Alternatives Considered

• A new additive slot per part kind. Wrong shape — the core card still renders, so the reader sees two.
• shell region replacement. Too coarse; it replaces navigation regions, not a leaf component.

🎯 Affected Service(s)

UI Service

📚 Additional Context

Prompted by #2890, which hardcodes StructuredOutputCard into that ternary. Best done after it merges, so the change has something to replace.

Extension guide: ui/docs/app-extensions.md

🤖 written by Claude (end)