Agentic Harness Archaeology

Dating the layers Most of what we call an agentic harness is a fossil. It is the shape a workaround took when an older model could not do something. The workaround hardened into architecture, got a name, and kept being defended long after the model learned the trick on its own. You can date a harness the way you date rock layers. Show me a codebase with a vector store, a ReAct parser, a DAG runner, and a 4,000-line instruction file and I can tell you which quarter it was designed in. The uncomfortable part is that none of these were mistakes. Each one was the right engineering answer to a real weakness in the model. The mistake is keeping them after the weakness is gone. A workaround that no longer works around anything is just a wall between the model and the task. Small context: from RAG to agentic search When the window was 4k or 8k tokens, you physically could not put all your documents in the prompt. So we built retrieval: split the documents into chunks, embed the chunks, embed…