Compaction User Prefix

An opencode plugin that works around a compaction bug where a freshly compacted conversation can start with an assistant tool-call message, causing Anthropic to reject the request with:

messages.N: `tool_use` ids were found without `tool_result` blocks immediately after.

The plugin prepends a synthetic user text message at the experimental.chat.messages.transform boundary so the body sent to the model always begins with a user turn.

The bug

When manual or auto compaction runs after a previous compaction whose tail_start_id landed mid-turn on an assistant tool-call, compaction.process builds selected.head as a chain of consecutive assistant tool-call messages with no user message at the start.

MessageV2.toModelMessagesEffect then faithfully converts that to a model message array whose first entry is assistant tool_use, which Anthropic rejects.

Upstream reference: opencode session message msg_e5550bcfb001EjDfz6hFE3lpHB (compaction failure on session ses_1ae999d1dffeOLwEseaIFPDg0V).

How it works

The plugin hooks experimental.chat.messages.transform, which receives the same WithParts[] opencode is about to pass into toModelMessages. If the first message is an assistant, it prepends a synthetic user text part. opencode recognises text parts in user messages, so the prepended turn survives the conversion and the resulting request body starts with a user message.

This is safe on the normal chat path because real chat already begins with a user message, in which case the !== "assistant" check short-circuits and nothing is prepended.

Use

Register the plugin in your opencode config (opencode.json or ~/.config/opencode/opencode.json):

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": ["opencode-compaction-user-prefix"]
}

License

MIT © Quentin Nivelais