The Complete Source of Claude Code Exposed

01 Streaming Tool Execution

Traditional AI tools wait for the model to finish generating, then execute tools, then continue. This creates noticeable latency. Claude Code's StreamingToolExecutor starts executing tools while the model is still generating.

// Start tools during streaming
for (const block of toolBlocks) {
  executor.addTool(block);  // Execute immediately
}

// Get results as they complete
for (const r of executor.getCompletedResults()) {
  yield r.message;
}

02 5-Layer Context Management

Context limits are the Achilles' heel of AI assistants. Claude Code implements five cascading strategies that work together. Users never see "context limit exceeded."

• Layer 1: History Snip — Remove old conversation turns
• Layer 2: Microcompact — Compress verbose tool results
• Layer 3: Context Collapse — Summarize sections while preserving detail
• Layer 4: Auto-Compact — Proactive summarization before hitting limits
• Layer 5: Reactive Compact — Emergency compaction on 413 errors

03 Build-Time Feature Elimination

Using Bun's feature() function, Anthropic can completely remove code from builds based on feature flags. Not disabled — eliminated.

import { feature } from 'bun:bundle';

const buddyModule = feature('BUDDY') 
  ? require('./commands/buddy/index.js') 
  : null;

// If BUDDY=false, the entire module is stripped from the bundle

The codebase has 44+ feature flags including KAIROS (autonomous mode), BUDDY (digital pet!), VOICE_MODE, and PROACTIVE.

04 Multi-Layer Permission System

AI that can execute code is inherently risky. Claude Code's permission system includes:

• Permission Modes: Default (ask everything), Plan (read-only), Auto (approve safe actions)
• Per-Tool Rules: Always allow, always deny, always ask — per tool/command
• Denial Tracking: After N denials, stop asking and assume "no"
• Security Classification: Every tool feeds into an automated classifier

05 Async Generator State Machine

The agentic loop isn't a simple while loop. It's a sophisticated state machine using async generators:

async function* queryLoop(params): AsyncGenerator<Event, Terminal> {
  while (true) {
    // Stream from model
    for await (const msg of callModel(state)) {
      yield msg;
    }
    
    // Execute tools
    for await (const result of executeTools()) {
      yield result;
    }
    
    if (!needsFollowUp) return { reason: 'completed' };
    state = computeNextState(state);
  }
}

Generators provide natural streaming, backpressure handling, clean cancellation via .return(), and composability.

06 Skills System

Claude Code is extensible through prompts, not just code. Skills are markdown files with YAML frontmatter:

---
name: debug
description: Help debug issues systematically
---

# Debug Skill

When debugging, follow these steps:
1. Reproduce the issue
2. Isolate the cause
3. Form a hypothesis
4. Test the hypothesis
5. Fix and verify

Skills can be bundled (ships with Claude Code), global (~/.claude/skills/), or project-specific (.claude/skills/).

07-10 More Innovations

• Terminal UI with React: Built using Ink, rendering React components to the terminal
• MCP Protocol: External tool discovery and integration via Model Context Protocol
• Startup Optimization: Aggressive parallel initialization, lazy imports, memory prefetch
• Error Recovery: Max output tokens recovery, prompt-too-long recovery, model fallback