Feature: Include user message text in experimental.chat.system.transform input #17637

Problem

The experimental.chat.system.transform hook currently receives { sessionID, model } in its input, but does not include the user's message text. This makes it impossible for plugins to do targeted context injection based on the user's current prompt.

Use Case

I've built a memory recall plugin that automatically injects relevant context from a persistent knowledge base into the system prompt. The ideal flow:

1. User types a message about topic X
2. system.transform hook reads the message
3. Plugin queries the knowledge base for topic X
4. Plugin pushes relevant context into output.system
5. The LLM sees the context on the same turn the user asked about it

Currently, the user's message text is only available in chat.message which fires after system.transform, so the targeted context can only be injected on the next turn.

Current Workaround

Two-phase approach:

• Phase 1 (session.created): Broad recall using project name
• Phase 2 (chat.message): Captures user text, does targeted recall, rewrites an instructions file that OpenCode re-reads on the next turn

This means turn 1 gets generic context and turn 2+ gets targeted context. Works but adds a one-turn delay.

Proposed Change

Include the user's message content in system.transform input:

await Plugin.trigger("experimental.chat.system.transform", {
  sessionID: input.sessionID,
  model: input.model,
  userMessage: currentUserMessage,  // add this
}, { system });

Or include the full messages array:

await Plugin.trigger("experimental.chat.system.transform", {
  sessionID: input.sessionID,
  model: input.model,
  messages: msgs,  // the messages being sent to the LLM
}, { system });

Verification

Confirmed via strings analysis of the OpenCode 1.2.26 binary that:

• experimental.chat.system.transform exists and fires during prompt construction
• chat.message exists and fires after system prompt is built
• The user message is available in memory at system.transform time but isn't passed to the hook

Environment

• OpenCode 1.2.26
• Plugin: TypeScript, uses event + chat.message + experimental.chat.system.transform hooks
• Use case: Persistent memory integration (cognitive-stack with vector store + knowledge graph)