The Harness Is the Machine: Task Contracts, Worktrees, Receipts
A long-form X post outlining a production architecture for coding agents. The thesis: when an agent fails a multi-file refactor, the problem is not the prompt or the model but the environment around it — the harness. The author lays out a seven-stage pipeline: Opus compiles a task_contract.json with path allowlists and a cost/turn/timeout budget; the harness spawns an isolated git worktree; Sonnet edits only inside the boundary; Haiku validates every tool call parameter; a separate model acts as adversarial skeptic against the diff; a pre-commit hook checks test exit codes and signs a receipt.json; failures crystallize into rules, skills, and regression evals. Includes contract schema, isolate_worktree.sh, pre-tool.sh, receipt format, and a 10-point pre-flight checklist. Caveat: the Opus/Sonnet/Haiku/Fable 5.5 model names and the 18%→95% success numbers are unsourced, and the post ends with a Telegram funnel link.