# Learn Claude Code Architecture

> A deep-dive educational guide into the internal architecture of Anthropic's Claude Code CLI — the agentic coding assistant that leaked via a `.map` file in March 2026.

**Internal Codename:** Tengu  
**Runtime:** Bun  
**UI Framework:** React + Ink (React for CLI)  
**Language:** TypeScript (strict)  
**Scale:** ~1,900 files, 512,000+ lines of code

---

## Why This Guide Exists

Claude Code is one of the most sophisticated agentic AI systems ever built. Its architecture contains lessons in:

- **Agentic loop design** — How to build a multi-turn, tool-calling AI agent
- **Terminal UI at scale** — 113 React components running in a terminal
- **Permission systems** — Layered security for autonomous code execution
- **Context management** — Keeping conversations within token limits
- **Extension architectures** — Plugins, skills, hooks, and sub-agents

Whether you're building your own AI agent, contributing to open-source AI tools, or just curious about how the sausage is made — this guide is for you.

---

## Guide Structure

Start from the top and work down, or jump to whatever interests you:

| # | Guide | What You'll Learn |
|---|-------|-------------------|
| 1 | [System Overview](./01-system-overview.md) | Bird's-eye view of every layer — entries, UI, core engine, tools, state |
| 2 | [The Agentic Loop](./02-agentic-loop.md) | How `query.ts` drives the model → tool → model cycle |
| 3 | [Tool System](./03-tool-system.md) | How 42 built-in tools are defined, validated, and executed |
| 4 | [Permission System](./04-permission-system.md) | Deny rules, allow rules, hooks, classifier, user prompts |
| 5 | [Context Management](./05-context-management.md) | Snip, micro, auto, reactive compact — the full pipeline |
| 6 | [State Management](./06-state-management.md) | AppState store, immutability, and reactive side effects |
| 7 | [Extension Model](./07-extension-model.md) | Skills, plugins, hooks, sub-agents, and swarms |
| 8 | [API Client](./08-api-client.md) | `claude.ts` — streaming, retries, caching, and model fallback |
| 9 | [UI Architecture](./09-ui-architecture.md) | React Ink, 113 components, and the 896KB REPL |

---

## Key Files Quick Reference

| File | Size | Role |
|------|------|------|
| `src/main.tsx` | 804KB | CLI entrypoint — Commander.js parser + React/Ink bootstrap |
| `src/screens/REPL.tsx` | 896KB | Interactive terminal shell — the heart of the UI |
| `src/query.ts` | 1,730 lines | The agentic loop — model ↔ tool cycling |
| `src/QueryEngine.ts` | 1,296 lines | Session lifecycle owner — wraps `query()` |
| `src/Tool.ts` | 793 lines | Tool interface definition — every tool implements this |
| `src/commands.ts` | ~25K | Slash command registry |
| `src/state/AppStateStore.ts` | 570 lines | Immutable store with 50+ fields |
| `src/services/compact/` | 11 files | The entire compaction pipeline |

---

## Architecture At A Glance

```mermaid
%%{init: {'theme': 'dark', 'themeVariables': { 'primaryColor': '#1a1a2e', 'primaryTextColor': '#e0e0e0', 'primaryBorderColor': '#4a9eff', 'lineColor': '#4a9eff', 'secondaryColor': '#16213e', 'tertiaryColor': '#0f3460', 'edgeLabelBackground': '#1a1a2e'}}}%%
graph TB
    CLI["CLI Entry<br/>main.tsx — 804KB"]:::entry
    SDK["SDK Entry<br/>Programmatic API"]:::entry
    MCP_S["MCP Server<br/>Expose as MCP"]:::entry

    REPL["REPL.tsx — 896KB<br/>Interactive Terminal Shell"]:::ui
    Comps["113 Components<br/>Messages, Diffs, Dialogs"]:::ui
    Hooks["83 React Hooks<br/>Permissions, Input, IDE"]:::ui

    QE["QueryEngine.ts<br/>Session Lifecycle Owner"]:::core
    QL["query.ts — 1730 lines<br/>Agentic Loop"]:::core
    CL["claude.ts — 3420 lines<br/>Anthropic API Client"]:::core

    TD["Tool Interface<br/>Tool.ts"]:::tool
    BT["42 Built-in Tools"]:::tool
    MT["MCP Tools — dynamic"]:::tool
    TO["Tool Orchestration<br/>Parallel Execution"]:::tool

    Compact["Compaction Pipeline<br/>snip / micro / auto /<br/>reactive / collapse"]:::ctx

    Rules["Allow + Deny Rules"]:::perm
    HK["PreToolUse Hooks"]:::perm
    Classifier["Auto-mode Classifier"]:::perm

    AS["AppState Store<br/>Immutable — 50+ fields"]:::state
    SS["Session Storage<br/>Transcripts + Resume"]:::state
    Cfg["Config Layer<br/>Global / Project / CLAUDE.md"]:::state

    Skills["Skills"]:::ext
    Plugins["Plugins"]:::ext
    Agents["Sub-agents + Swarms"]:::ext

    API["Anthropic Messages API"]:::external
    MCP_Ext["External MCP Servers"]:::external
    GrowthBook["GrowthBook + Statsig"]:::external

    CLI --> REPL
    SDK --> QE
    MCP_S --> QE

    REPL --> Comps
    REPL --> Hooks
    REPL --> QE
    REPL --> AS

    QE --> QL
    QL --> CL
    QL --> Compact
    QL --> TO

    CL --> API
    CL --> GrowthBook

    TO --> TD
    TD --> BT
    TD --> MT
    BT --> Rules
    BT --> HK
    MT --> Rules

    Rules --> Classifier

    QE --> SS
    REPL --> Cfg

    Skills --> TD
    Plugins --> TD
    Plugins --> MCP_Ext
    Agents --> QL
    MCP_Ext --> MT

    classDef entry fill:#0d4f4f,stroke:#17a2b8,color:#e0e0e0,stroke-width:2px
    classDef ui fill:#1a1a4e,stroke:#6f42c1,color:#e0e0e0,stroke-width:2px
    classDef core fill:#2d1b4e,stroke:#e83e8c,color:#e0e0e0,stroke-width:2px
    classDef tool fill:#1b3a1b,stroke:#28a745,color:#e0e0e0,stroke-width:2px
    classDef ctx fill:#3d2b00,stroke:#fd7e14,color:#e0e0e0,stroke-width:2px
    classDef perm fill:#4a1a1a,stroke:#dc3545,color:#e0e0e0,stroke-width:2px
    classDef state fill:#1a2d4a,stroke:#4a9eff,color:#e0e0e0,stroke-width:2px
    classDef ext fill:#2d2d0d,stroke:#ffc107,color:#e0e0e0,stroke-width:2px
    classDef external fill:#333,stroke:#888,color:#aaa,stroke-width:1px,stroke-dasharray: 5 5
```

---

## Prerequisites

To get the most out of these guides, you should be comfortable with:

- **TypeScript** — The entire codebase is strict TypeScript
- **React** — The UI uses React (via Ink for the terminal)
- **Async generators** — The agentic loop and streaming are built on `async function*`
- **LLM APIs** — Familiarity with the Anthropic Messages API helps

---