Compaction replay injects fake user message "What did we do so far?" causing unwanted summary generation

[jdocker8]

Description

When a session is compacted (via /compact or auto-compaction), resuming the session injects a synthetic user message "What did we do so far?" into the conversation. This causes the model to treat it as a real user request and generate a long summary/recap — even when the user's rules explicitly prohibit summaries.

Root cause

Found via strings on the compiled binary (v1.2.5):

if (part.type === "compaction") {
  userMessage.parts.push({
    type: "text",
    text: "What did we do so far?"
  });
}

This string appears twice in the binary (likely two code paths for provider-specific message formatting).

Steps to reproduce

1. Start a session with significant context
2. Run /compact (or let auto-compaction trigger)
3. Continue the session — the model receives "What did we do so far?" as a user message
4. The model responds with a full summary, even if the user didn't ask for one

Impact

• Wastes tokens on an unwanted summary response
• Violates user-defined rules (e.g. "no summaries or recaps" in AGENTS.md)
• Confusing UX — the user sees a response to a question they never asked
• The model may reference this fake message in reasoning ("the user asked what we did so far")

Suggested fix

Instead of injecting a fake user prompt, the compaction summary could be:

• Injected as a system message (not a user message)
• Use a more neutral framing like "[Session context restored from compaction]" that doesn't prompt the model to generate a response
• Or simply replay the compaction content as an assistant message without a corresponding user message

Environment

• OpenCode v1.2.5
• macOS arm64
• Model: claude-opus-4.6 via github-copilot

[leohenon]

I've noticed this as well. When auto-compaction triggers while mid-task, it can cause the agent to generate a summary instead of continuing its work. The suggested fix should resolve this and help the agent stay on track after compaction.