AI Agent Architecture: Building Systems That Think, Plan, and Act

System Design Deep Dive - of 20 | This is part of a 20-post series covering the most critical system design topics. Follow to get the next one. Cognition's Devin made headlines as the first AI software engineer, raising $175M at a $2B valuation before even launching publicly. GitHub Copilot's agent mode handles complex multi-file refactors. Cursor's Composer rewrites code across entire projects. These aren't chatbots -- they're AI agents that reason about multi-step tasks, use external tools, maintain memory, and take real-world actions.