Title generation runs experimental.chat.system.transform, so plugin-added context is re-sent on every new session

Description

Session title generation streams the built-in title agent through the same LLMRequestPrep.prepare as chat turns (packages/opencode/src/session/prompt.ts: llm.stream({ agent: title, system: [], small: true, sessionID: <parent session id> })), so experimental.chat.system.transform fires for it. The hook input is { sessionID, model } with the parent's id, so a plugin cannot tell the title request from the first real turn.

Whatever a plugin appends in that hook is therefore sent again in a request whose only output is a short title. With a plugin that appends ~45K tokens of instructions for the main agent, every new session's title call was a 46K-token prompt, ~80s time-to-first-token on a local model, running in parallel with the real first turn. doobidoo/mcp-memory-service injects memories in the same hook keyed only on input.sessionID, so it pays the same.

Proposed fix: do not run experimental.chat.system.transform for the title agent. experimental.chat.messages.transform already never runs for it, and on v2 title generation has its own session.title hook with no agent or tools (#47663, #48212). Users who want a different title prompt have agent: { title: { prompt, model } }, and agent: { title: { disable: true } } still turns the call off.

Why skip the hook rather than make the title request share the main request's cached prefix: what plugins add in this hook differs per user and per plugin, so the title request cannot rely on a matching cached prefix, and the title call's latency and cost still land on every new session. Not running the hook for the title is the smallest change and sends the fewest tokens. I have a one-line PR ready and will open it against this issue.

Related: #23114 (plugin context reaching the title model through messages), #46115 (a redaction plugin wants title requests to run messages.transform), #46371 (title calls billed but absent from session cost).

Plugins

A local plugin that appends project context in experimental.chat.system.transform.

OpenCode version

v1.18.32 (dev @ fe3f3a41f7)

Steps to reproduce

1. In an empty directory, save repro.js:

   import { appendFileSync } from "node:fs"
   export default async () => ({
     "experimental.chat.system.transform": async (input, output) => {
       appendFileSync("hook.log", JSON.stringify({ inputKeys: Object.keys(input), sessionID: input.sessionID, system: output.system[0].slice(0, 26) }) + "\n")
     },
   })
   

2. opencode.json in the same directory: { "plugin": ["file:///absolute/path/to/repro.js"] }
3. opencode run "hello" </dev/null
4. hook.log has two lines with the same sessionID and inputKeys: ["sessionID","model"]; one system is the chat prompt, the other starts with You are a title generator. Nothing in the input lets the plugin skip the second one.

Screenshot and/or share link

n/a

Operating System

macOS 26.3

Terminal

iTerm2