Installation

Install Claude Code via the pre-built CLI or build from source using esbuild.

There are three ways to get Claude Code running. Choose based on what you need.

Option A: Pre-built CLI (recommended)

The npm package already contains a compiled cli.js. This is the fastest path and requires no build tooling beyond Node.js.

Prerequisites

node --version   # >= 18

Run directly

cd /path/to/claude-code-source-code
node cli.js --version
# → 2.1.88 (Claude Code)

Install globally

npm install -g .
claude --version
# → 2.1.88 (Claude Code)

For automation and scripting, prefer node cli.js directly. Global install is convenient for interactive use.

Option B: Build from source (esbuild)

A best-effort source build using esbuild gets approximately 95% complete. Some internal modules are unavailable (see warning below).

Prerequisites

node --version   # >= 18
npm --version    # >= 9

Steps

cd claude-code-source-code/

# 1. Install the build dependency
npm install --save-dev esbuild

# 2. Run the build script
node scripts/build.mjs

# 3. Run the output
node dist/cli.js --version

The source build is incomplete by design. 108 modules referenced by feature()-gated branches are not included in the published npm package — they exist only in Anthropic’s internal monorepo and were dead-code-eliminated at compile time. They cannot be recovered from cli.js, sdk-tools.d.ts, or any published artifact. The esbuild build creates stubs for these modules automatically, but some functionality will be absent at runtime.

What the build script does

Phase	Action
1. Copy	src/ → build-src/ (original source is untouched)
2. Transform	feature('X') → false to enable dead code elimination
2b. Transform	MACRO.VERSION → '2.1.88' (compile-time version injection)
2c. Transform	import from 'bun:bundle' → stub import
3. Entry	Create a wrapper that injects MACRO globals
4. Bundle	esbuild with iterative stub creation for missing modules

Known issues

The source uses Bun compile-time intrinsics that esbuild cannot fully replicate:

1. feature('FLAG') from bun:bundle — Bun resolves this at compile time to true/false and eliminates dead branches. The build script replaces these with false, but esbuild still attempts to resolve require() calls inside those branches.

2. MACRO.X — Bun’s --define replaces these at compile time. The build script uses string replacement, which works for most cases but can miss edge cases in complex expressions.

3. 108 missing modules — Feature-gated internal modules (daemon, bridge assistant, context collapse, etc.) that don’t exist in the published source. Normally dead-code-eliminated by Bun, but esbuild can’t eliminate them because the require() calls remain syntactically present.

4. bun:ffi — Used for native proxy support. Stubbed out in the build.

5. import type from generated files — Some generated type files are not in the published source.

Diagnosing remaining issues

# Check what's still missing after a build attempt
npx esbuild build-src/entry.ts --bundle --platform=node \
  --packages=external --external:'bun:*' \
  --log-level=error --log-limit=0 --outfile=/dev/null 2>&1 | \
  grep "Could not resolve" | sort -u

For each unresolved module, create a stub file in build-src/src/ that exports empty functions, then re-run node scripts/build.mjs.

Option C: Build with Bun (requires internal access)

A full rebuild requires Bun for its compile-time intrinsics (feature(), MACRO, bun:bundle). The internal build configuration is not included in the published package — you would need access to Anthropic’s internal monorepo.

This option is documented here for completeness and for Anthropic engineers.

Install Bun

curl -fsSL https://bun.sh/install | bash

The real build command (reference only)

# bun build src/entrypoints/cli.tsx \
#   --define:feature='(flag) => flag === "SOME_FLAG"' \
#   --define:MACRO.VERSION='"2.1.88"' \
#   --target=bun \
#   --outfile=dist/cli.js

The internal build configuration, feature flag definitions, and the 108 internal modules are not included in this repository. Running the command above without Anthropic’s internal monorepo will fail.

Why Bun is required

Bun’s feature() is a compile-time intrinsic:

• Returns true in Anthropic’s internal build → code is kept in the bundle
• Returns false in the published build → code is dead-code-eliminated
• The 108 missing modules simply do not exist anywhere in the published artifact

Anthropic Internal Monorepo              Published npm Package
──────────────────────────               ─────────────────────
feature('DAEMON') → true    ──build──→   feature('DAEMON') → false
↓                                         ↓
daemon/main.js  ← INCLUDED    ──bundle─→  daemon/main.js  ← DELETED (DCE)
tools/REPLTool  ← INCLUDED    ──bundle─→  tools/REPLTool  ← DELETED (DCE)

Project structure

claude-code-source-code/
├── src/                  # Original TypeScript source (1,884 files, 512K LOC)
├── stubs/                # Build stubs for Bun compile-time intrinsics
│   ├── bun-bundle.ts     #   feature() stub → always returns false
│   ├── macros.ts         #   MACRO version constants
│   └── global.d.ts       #   Global type declarations
├── scripts/
│   └── build.mjs         # Build script (esbuild-based)
├── node_modules/         # 192 npm dependencies
├── vendor/               # Native module source stubs
├── build-src/            # Created by build script (transformed copy)
├── dist/                 # Build output (created by build script)
└── cli.js                # Pre-built CLI bundle (~12 MB)