Glossary · Phase 10 · Agent harnesses

Append-only

Only ever adding to the end, never editing what's there. Agent transcripts and session logs work this way.

Treat the message list like a ship's log written in pen. Add each reply and each batch of results to the end, and don't edit or reorder what's already there. That's what append-only means.
From the lesson The core loop, built properly

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Watch delay double after each temporary error. The 200 ends the loop, so the third wait never happens.

Learn it properly

The core loop, built properly · Stop reasons, parallel tool calls, errors and turn budgets. About 35 minutes.

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Other terms from the same lesson

stop reason: The field in each Messages API reply that says why the model stopped, such as end_turn, tool_use or max_tokens. The loop branches on it.

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