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Codex vs Claude Code vs Copilot: How the Agent Harness Became the Product (2025-2026)
In 2025, AI coding tools competed on model choice and basic autonomy. Through 2026 the differences that mattered moved outward into the harness: instruction files, delegation, hooks, approvals, sandboxing, and where the agent actually runs. A dated, sourced comparison of OpenAI Codex, Anthropic Claude Code and GitHub Copilot.