// LAB
Build agents
from zero.
Hands-on series that ship real code. Currently running: Mini Claude Code — a working coding agent built in six episodes, no framework hiding the moving parts.
Mini Claude Code · Episode 01: A 40-Line REPL That Actually Remembers
Start of a hands-on series where we build a working coding agent from scratch in TypeScript. Episode 1 gets the smallest possible loop running — stdin, streaming, and a conversation that survives turns.
Mini Claude Code · Episode 03: Editing Files Without Wrecking the Repo
Episode 3 adds the apply_patch tool — the one where most agents quietly destroy your work. We build a strict unified-diff applier with dry-run preview, workspace guardrails, and a confirmation gate that survives a Claude in a bad mood.
Mini Claude Code · Episode 02: Teaching the Agent to Use Tools
Episode 2 gives our REPL its first three tools — read_file, list_dir, run_bash — and walks through the tool-use loop with the Anthropic SDK. We handle stop reasons, tool errors, and the surprisingly tricky part of what to put back in the conversation.
Mini Claude Code · Episode 06: The Eval Loop That Finally Tells You If You Made It Worse
Season finale. We build a 15-task SWE-lite harness that runs the agent end-to-end against verifiable code tasks, then wire in prompt caching so iteration is cheap. Pass rate, mean turns, and cost per task — pinned as a baseline you can regress against.
Mini Claude Code · Episode 05: Spawning Sub-Agents With Their Own Context
Episode 5 splits the agent in two. We add a spawn_subagent tool that dispatches a scoped task to a fresh Claude with its own tool set and its own context window, then returns a compact summary. The pattern behind Anthropic's 'token usage explains 80% of variance' finding, in ~90 lines.
Mini Claude Code · Episode 04: Making the Agent Forget on Purpose
Episode 4 attacks the token bill head-on. We implement observation masking (the JetBrains-style trick that beats LLM summarization on cost and quality) and add an auto-continue path so max_tokens truncation stops silently breaking tool sequences.