[BUG]: context hook messages drop metadata, so per-message plugin context never reaches the model #63252

Description

session.hook("context") receives messages that only carry { id, role, content }. Since #61018 ("refactor(ai): remove unused metadata fields", first released in 2.2.6) Message.metadata no longer exists on those objects, so a plugin cannot attach context to a specific stored user message any more.

That is the documented v2 pattern for per-message context injection: query on the prompt hook, store the result on event.metadata, then inject it before that same message on every model step. The host still persists metadata with the stored message and ctx.session.context() still returns it, but the context hook payload no longer has it, so the injection silently no-ops.

The asymmetry is the whole bug: the field is writable on prompt, readable through session.context(), and absent on the one payload where plugins need it.

Plugins

@openviking/opencode-plugin 2026.11.10 (session-start profile + per-prompt recall injected this way). Any v2 plugin using prompt -> context metadata is affected the same way.

OpenCode version

2.2.7. First release with the change: 2.2.6.

Steps to reproduce

Minimal plugin:

export default {
  id: "canary",
  async setup(ctx) {
    await ctx.session.hook("prompt", async (event) => {
      event.metadata = { ...event.metadata, canary: { context: ["CANARY-TOKEN"] } }
    })
    await ctx.session.hook("context", async (event) => {
      for (const message of event.messages) {
        const blocks = message.metadata?.canary?.context
        console.log("hook metadata?", Boolean(blocks))
        if (message.role === "user" && blocks) {
          message.content.unshift({ type: "text", text: blocks.join() })
        }
      }
    })
  },
  async server() { return {} },
}

1. Load the plugin and ask the model to repeat CANARY-TOKEN if it can see it.
2. hook metadata? false on every step, and the model never sees the token.
3. ctx.session.context({ sessionID }) for the same session returns that message with metadata.canary intact, so the data exists and is reachable, just not from the hook.

Expected

Keep metadata on the messages handed to context (a read-only pass-through is enough), or expose another supported way for a plugin to address stored message metadata from the hook. Writing on prompt and reading back through a full session.context() fetch is a costly substitute, and plugging that hole is what every affected plugin now has to reimplement.

Actual

Silently dropped. No error, no warning, no deprecation notice.

Extra context

• refactor(ai): remove unused metadata fields #61018 documents this as a breaking change for plugins, but only in the PR body; the 2.2.6 and 2.2.7 release notes do not mention it, so downstream plugins had no signal to update.
• I am not asking to restore @opencode/ai's Message.metadata; the request is about what the hook payload carries.
• Local workaround used by the plugin: cache the blocks by message id in process memory, and fall back to ctx.session.context() once per session when a message id is unknown (restart/resume/fork). It works, but the storage round trip is exactly what the metadata field used to avoid.

System

• OpenCode 2.2.7, macOS (Apple Silicon), Ghostty/iTerm2, plugin API v2 only (no v1 hooks in play).