A coding agent harness is a deterministic loop that wraps an LLM, providing it with tools, a system prompt, and filesystem access. The post traces a full task execution step by step and outlines the key features that distinguish top autonomous coding agents: customizable system prompts and rules, usage-based pricing, autonomous multi-phase workflows, background execution, MCP-based tool integrations, and support for multiple model providers. The harness acts as the final layer engineers control before the LLM takes over, analogous to rails guiding a train.
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