Building a coding agent in Swift from scratch

Summary

Exploring the architecture of coding agents by rebuilding a Claude Code-style CLI from scratch in Swift. A complete 9-part learning series is available on ivanmagda.dev. Start the series → Claude Code feels unusually effective compared to other coding agents, and I suspect most of it comes from architectural restraint rather than architectural complexity. I studied the tool surface, traced the interaction loop, and tried to isolate which design choices actually matter. My working theory: coding agents benefit more from a small set of excellent tools and tight loop design than from large orchestration layers. Claude Code doesn't have many tools. The tools it does have are simple: a search tool, a file editing tool. But those tools are really good. And the system leans on the model far more than most agent implementations — less scaffolding, more trust in the LLM to do the heavy lifting. This project tests that idea by rebuilding the core mechanics from scratch in Swift, one stage at a time, to see how little architecture you actually need. This project tests a few specific ideas about coding agents: A small number of high-quality tools beats a large tool catalog The model should do most of the heavy lifting — thin orchestration, not thick Explicit task state improves reliability more than prompt-only planning Controlled context injection matters more than persistent memory Context compaction is a product feature, not just a token optimization Each stage is designed to isolate one mechanism and see what it enables. The whole thing boils down to one loop: func run(query: String) async throws -> String { messages.append(.user(query)) while true { let request = APIRequest( model: model, system: systemPrompt, messages: messages, tools: Self.toolDefinitions ) let response = try await apiClient.createMessage(request) messages.append(Message(role: .assistant, content: response.content)) guard response.stopReason == .toolUse else { return response.content.textContent } var re...