INTERMEDIATE · 14 lessons · ~6 hours

Claude Code Guide

A 14-lesson course on Claude Code internals: harness, context window, prompt cache, skills, hooks, MCP, subagents, models, anti-patterns. Built around an end-to-end Travel Agent example.

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  1. 01 01. What is Claude Code: harness, agent loop, and your place in it Claude Code is a harness around an LLM, not the model itself. The model decides which tool to call; the harness executes it, returns the result, and runs the agent loop until the final answer.
  2. 02 02. Context window and prompt cache Context window is both money and quality. Opus/Sonnet 4.6 standard — 200k tokens; 1M available via alias opus[1m]/sonnet[1m]. Prompt cache cuts the bill 10x, but lives only 5 minutes by default.
  3. 03 03. CLAUDE.md: levels, imports, auto-memory CLAUDE.md is a "paste this every time" in a nice wrapper: an automatic prefix for each session. It has five levels (managed, user, project, local, subdirectory), they concatenate rather than overwrite each other.
  4. 04 04. Skills: SKILL.md, scripts, references Skill is a directory with SKILL.md and optional scripts/references/templates, not a single markdown file. A brief description of the skill goes into each request; the full SKILL.md "unfolds" only when the task matches.
  5. 05 05. Hooks: deterministic control over agent loop Hooks are git hooks for Claude Code: points in the lifecycle where the harness is guaranteed to execute your script. In Claude Code v2.1.89 there are 28+ events, exit code 2 blocks model action, JSON response {"action":"block"} does the same.
  6. 06 06. MCP Servers MCP (Model Context Protocol) — "USB-C for AI integrations": write a server once, and it's consumed by Claude Code, Claude Desktop, Cursor, Continue, custom SDK. Three transports (stdio, SSE, HTTP); exports tools, resources, prompts, elicitation.
  7. 07 07. Plugins: packaging skills + hooks + agents + MCP Plugin — npm package for Claude Code: packaging skills, hooks, subagents, slash-commands, and MCP-config into a single versioned entity. Manifest .claude-plugin/plugin.json, installation via /plugin install, marketplaces.
  8. 08 08. Tool calls and agent loop under the hood Tool call — a fundamental mechanism that transforms an LLM from a chatbot into an agent. The model returns not text, but a tool_use block; the harness executes the call and returns tool_result. Understanding this cycle, you understand 80% of how any AI-agent works.
  9. 09 09. Subagents: isolated agent cycles Subagent is a mini-session of Claude Code with its own context and system prompt. Only the final summary is returned to the main context. It saves from context overflow on browse-heavy tasks and causes unexpected bills if launched without restraint.
  10. 10 10. Agent Teams (experimental) If subagents are "sent an assistant on a business trip, waited for the result", then Agent Teams is "assembled a team of several developers, gave them a task list and they work in parallel"
  11. 11 11. Models and pricing Model selection is a speed/cost/quality trade-off, not "always Opus". Cache reads cost 0.1× input (10× cheaper), output ~5× input. Opus 4.7 has a new tokenizer (+35% tokens on the same texts), on Bedrock/Vertex default aliases are shifted back one version.
  12. 12 12. Travel Agent from Scratch: blueprint Bringing it all together. One real project. One monorepo. Specific files, configs, structure. From here on, you copy-paste and adapt.
  13. 13 13. Best practices: daily routine and anti-patterns All previous chapters are about mechanics. This one is about discipline. Without it, even a perfect configuration eventually turns into a mess: cache misses, skills get stale, hooks fail silently,
  14. 14 14. Verifying Claims from the Original Thread This chapter is a line-by-line breakdown of the ~20 points that started the guide preparation. Each claim has been verified against official documentation (`docs.claude.com`, `code.claude.com/docs`),