1. Purpose, Problem, and Tech Stack

What this is

The tree at /workspaces/src/ is the source of Claude Code, Anthropic's official CLI coding agent. It is referenced by name in the canonical system prompt (constants/prompts.ts:452):

You are Claude Code, Anthropic's official CLI for Claude.

And externally:

• Product URL https://claude.com/claude-code — constants/product.ts:1
• Docs map https://code.claude.com/docs/en/claude_code_docs_map.md — constants/prompts.ts:103

It is distributed as a terminal application with three primary shapes:

1. Interactive TUI (React + Ink) — the default claude invocation. Entry entrypoints/cli.tsx → main.tsx → components/App.tsx.
2. Headless / print / SDK mode — programmable use via the Claude Agent SDK. Entry entrypoints/sdk/ + QueryEngine.ts. Emits SDKMessage events.
3. Remote / bridge / daemon modes — claude remote-control, claude daemon, claude ps|logs|attach|kill, claude environment-runner, claude self-hosted-runner. These turn the CLI into a long-lived session server that can execute work dispatched by Anthropic's CCR (Claude Code Remote) backend or a daemon supervisor (entrypoints/cli.tsx:100-246).

Problem solved

Claude Code turns an LLM into a general-purpose software engineering agent that can:

• Read and edit files on disk, run shell commands, run tests, make commits, open PRs.
• Spawn subagents for parallelizable work (AgentTool), isolate them in git worktrees, or run them on remote sandboxes.
• Consult and update long-lived memory (memdir/, CLAUDE.md) so context survives across sessions.
• Speak the Model Context Protocol (MCP) so external tools/resources/prompts plug in uniformly.
• Be driven from many surfaces: a terminal, the Claude.ai web UI over a WebSocket bridge, an IDE extension, a Chrome extension, an iPhone / iMessage / Telegram channel, a scheduled cron job.

Target users: software engineers and engineering teams. The product has explicit accommodations for Anthropic-internal users (USER_TYPE === 'ant' branches everywhere) plus the external 1P SKU.

Languages and runtime

• TypeScript with .ts / .tsx throughout — zod/v4 for runtime schemas (Tool.ts:343).
• Bun is the runtime and bundler. Files import from 'bun:bundle' and call feature(...) for compile-time dead code elimination (entrypoints/cli.tsx:1, QueryEngine.ts:1). Feature gates like feature('BRIDGE_MODE'), feature('DAEMON'), feature('HISTORY_SNIP'), feature('COORDINATOR_MODE') are folded at build time so external builds strip ant-only paths.
• React + Ink for the terminal UI (components/App.tsx, components/Messages.tsx). Virtualized message list + useSyncExternalStore for performance.
• Node APIs for process and filesystem (fs/promises, crypto, os). Polyfilled via utils/fsOperations behind a small abstraction to let the upstream-proxy relay and sandbox intercept.

Core dependencies (evident in the code)

Why these choices (as far as one can tell from the code)

• Bun lets the same file do top-level import { feature } from 'bun:bundle' with build-time evaluation, which is how the codebase maintains one source tree for internal-rich and external-minimal builds. The process.env.USER_TYPE === 'ant' conditionals everywhere are build-time constant-folded — see the explicit comment at constants/prompts.ts:617-619: "process.env.USER_TYPE === 'ant' is build-time --define. It MUST be inlined at each callsite (not hoisted to a const) so the bundler can constant-fold it."
• Ink was the natural choice in 2023 when this project began; it also means the same React hooks (hooks/useTextInput.ts, hooks/useCommandQueue.ts, etc.) compose naturally.
• TypeScript + Zod gives end-to-end type safety across the tool boundary: each tool declares an inputSchema: z.ZodType<…>, and both the permission layer and the API call see the same type (Tool.ts:362-400).
• MCP lets third parties add tools/resources/prompts without forking the binary; it is the default extension surface.

Size snapshot

Repo is a single flat src/ with ~329 files in utils/ alone. Notable sizes:

• main.tsx — 4,683 lines (giant dispatcher: boots the TUI, headless, resume, etc.).
• query.ts — 1,729 lines (the core agent loop).
• QueryEngine.ts — 1,295 lines (SDK/headless wrapper around query()).
• services/api/claude.ts — 3,419 lines (streaming, retry, caching, beta headers).
• services/compact/compact.ts — 1,705 lines (history compaction).
• Tool.ts — 792 lines (the tool contract).

You could implement a minimal clone in a fraction of this size. The weight is in production concerns: retries, cache optimization, permission modes, fleet rollouts, telemetry, multi-surface persistence.