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Claude Code: Architectural Overview

System Architecture​

Claude Code follows a five-layer architecture designed for extensibility, security, and seamless integration with development workflows.

┌─────────────────────────────────────────────────────────┐
│                   User Terminal                          │
│                  (claude command)                       │
└─────────────────────────────────────────────────────────┘
                             │
┌─────────────────────────────────────────────────────────┐
│                 CLI Interface Layer                     │
│  ┌─────────────┐ ┌─────────────┐ ┌─────────────────────┐│
│  │ Interactive │ │ Print Mode  │ │ Slash Commands      ││
│  │ Mode        │ │ (-p flag)   │ │ (/help, /mcp, etc.) ││
│  └─────────────┘ └─────────────┘ └─────────────────────┘│
└─────────────────────────────────────────────────────────┘
                             │
┌─────────────────────────────────────────────────────────┐
│                Extension Framework                      │
│  ┌─────────────────────┐    ┌─────────────────────────┐ │
│  │    Hook System      │    │     MCP Framework       │ │
│  │ • UserPromptSubmit  │    │ • stdio transport       │ │
│  │ • PreToolUse        │    │ • HTTP transport        │ │
│  │ • PreCompact        │    │ • SSE transport         │ │
│  │ • Stop/SubagentStop │    │ • OAuth auth            │ │
│  └─────────────────────┘    └─────────────────────────┘ │
└─────────────────────────────────────────────────────────┘
                             │
┌─────────────────────────────────────────────────────────┐
│                Tool Execution System                    │
│  ┌──────────────────────────────────────────────────────┐│
│  │                Built-in Tools                        ││
│  │ BashTool • SearchTool • WebTool • TaskTool          ││
│  │ ReadTool • EditTool • Git Integration               ││
│  └──────────────────────────────────────────────────────┘│
│  ┌──────────────────────────────────────────────────────┐│
│  │                 MCP Tools                            ││
│  │ mcp__github__* • mcp__jira__* • Custom MCP Servers  ││
│  └──────────────────────────────────────────────────────┘│
└─────────────────────────────────────────────────────────┘
                             │
┌─────────────────────────────────────────────────────────┐
│                AI Model Layer                           │
│  ┌─────────────┐ ┌─────────────┐ ┌─────────────────────┐│
│  │ Anthropic   │ │ AWS Bedrock │ │ Google Vertex AI    ││
│  │ API         │ │             │ │                     ││
│  └─────────────┘ └─────────────┘ └─────────────────────┘│
│  ┌─────────────────────────────────────────────────────┐ │
│  │ Models: Sonnet 3.5, Sonnet 4, Opus 4             │ │
│  └─────────────────────────────────────────────────────┘ │
└─────────────────────────────────────────────────────────┘
                             │
┌─────────────────────────────────────────────────────────┐
│              Configuration Management                   │
│  Environment Variables → settings.json → .claude/      │ 
│  • API Keys • Model Selection • Tool Permissions       │
└─────────────────────────────────────────────────────────┘

Core Components Deep Dive​

1. CLI Interface Layer​

The entry point for all user interactions, providing multiple modes of operation:

Interactive Mode: Persistent conversation sessions with context retention

claude
> Help me debug this authentication issue
> Now let's add unit tests for the fix

Print Mode: Single-shot command execution

claude -p "Refactor the user service to use dependency injection"

Slash Commands: System management and introspection

/mcp          # View MCP server status
/permissions  # Manage tool access
/doctor       # System diagnostics

2. Extension Framework​

Two parallel extension mechanisms providing powerful customization:

Hook System​

Event-driven extension points throughout the execution flow:

  • UserPromptSubmit: Intercept and modify user input
  • PreToolUse: Validate tool execution before running
  • PreCompact: Control conversation compression
  • Stop/SubagentStop: Post-execution cleanup and logging

MCP Framework​

Standardized protocol for external service integration:

  • Transport Layer: stdio, HTTP, SSE connections
  • Authentication: OAuth and token-based auth
  • Resource Management: Dynamic tool discovery and execution
  • Scoping: User, local, and project-level configurations

3. Tool Execution System​

Dual-tier tool architecture with built-in and extensible tools:

Built-in Tool Ecosystem​

BashTool
├── Command execution with progress tracking
├── Shell snapshot management for reliability  
└── Environment variable support

SearchTool (Grep)
├── Regex pattern matching across codebase
├── jq filter support for structured data
└── Context-aware result ranking

WebTool
├── URL content fetching with date context
├── HTML to markdown conversion
└── Caching for performance optimization

TaskTool
├── TODO list management and persistence
├── Task execution with status tracking
└── Integration with project management systems

ReadTool/EditTool
├── File I/O with line-specific operations
├── Intelligent indentation preservation
└── Change tracking and validation

Git Integration
├── Repository operations (status, diff, commit)
├── Branch management and merging
└── Workflow automation (PR creation, etc.)

MCP Tool Integration​

mcp__[server]__[tool] naming convention
├── mcp__github__get_issue
├── mcp__github__update_issue  
├── mcp__jira__create_ticket
└── mcp__custom__domain_specific_tool

4. AI Model Layer​

Multi-provider model access with intelligent selection:

Provider Architecture:

  • Anthropic API: Direct access to latest Claude models
  • AWS Bedrock: Enterprise deployment with additional security
  • Google Vertex AI: Integration with Google Cloud ecosystem

Model Selection Strategy:

Task Complexity Assessment
├── Simple tasks → Sonnet 3.5 (speed + cost optimization)
├── Complex reasoning → Opus 4 (maximum capability)
└── Latest features → Sonnet 4 (cutting-edge performance)

5. Configuration Management​

Hierarchical configuration system with clear precedence:

Priority Order (highest to lowest):
1. Environment Variables (ANTHROPIC_API_KEY, CLAUDE_MODEL, etc.)
2. User Settings (~/.claude/settings.json)
3. Project Settings (.claude/settings.json)
4. Default Values (built-in fallbacks)

Data Flow Architecture​

Request Processing Flow​

User Input → Hook Processing → Context Assembly → AI Processing → Tool Execution → Response Generation
     ↓              ↓                ↓               ↓               ↓                ↓
┌─────────┐ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐
│ Natural │ │UserPrompt   │ │ Codebase    │ │ Model       │ │ Tool        │ │ Formatted   │
│Language │ │Submit Hook  │ │ Context +   │ │ Inference   │ │ Execution   │ │ Output +    │
│Command  │ │Validation   │ │ Memory      │ │ + Tool      │ │ (Built-in   │ │ Context     │
│         │ │             │ │ Integration │ │ Selection   │ │ or MCP)     │ │ Updates     │
└─────────┘ └─────────────┘ └─────────────┘ └─────────────┘ └─────────────┘ └─────────────┘

Context Management​

Claude Code maintains rich context through multiple mechanisms:

Codebase Context:

  • File structure mapping
  • Dependency graph analysis
  • Recent change tracking
  • Pattern recognition across project

Conversation Context:

  • Multi-turn conversation memory
  • Task completion tracking
  • Error and solution history
  • User preference learning

System Context:

  • Environment state monitoring
  • Tool availability assessment
  • Permission and security constraints
  • Performance metrics tracking

Security Architecture​

Multi-Layer Security Model​

┌─────────────────────────────────────────────────────────┐
│                User-Controlled Permissions              │
│  /permissions command • Tool-specific access control   │
└─────────────────────────────────────────────────────────┘
                             │
┌─────────────────────────────────────────────────────────┐
│                 Hook-Based Validation                   │
│  PreToolUse hooks • Custom validation logic            │
└─────────────────────────────────────────────────────────┘
                             │  
┌─────────────────────────────────────────────────────────┐
│                Environment Isolation                    │
│  DevContainer support • Network restrictions           │
└─────────────────────────────────────────────────────────┘
                             │
┌─────────────────────────────────────────────────────────┐
│               Credential Management                     │
│  Keychain integration • Environment variable handling  │
└─────────────────────────────────────────────────────────┘

Data Privacy Safeguards​

  • Local Processing: Code analysis happens locally when possible
  • Minimal Data Retention: 30-day retention for feedback transcripts
  • No Training Data: User feedback not used for model training
  • Encryption in Transit: All API communications encrypted
  • Audit Logging: Comprehensive activity logging for compliance

Performance Architecture​

Optimization Strategies​

Context Optimization:

  • Intelligent context pruning
  • Incremental codebase analysis
  • Cached dependency resolution
  • Selective file inclusion

Token Efficiency:

  • Model-appropriate task routing
  • Context compression algorithms
  • Memory system for frequent patterns
  • Progressive disclosure of information

Network Optimization:

  • Connection pooling for MCP servers
  • Async tool execution where possible
  • Request batching for related operations
  • Intelligent retry mechanisms with backoff

Extensibility Patterns​

Plugin Development Model​

Custom Hook Development
├── Event-driven architecture
├── TypeScript/JavaScript support
├── Async operation handling
└── Error boundary management

MCP Server Development  
├── Standardized protocol implementation
├── Multiple transport options
├── OAuth integration support
└── Resource discovery mechanisms

Custom Command Development
├── Markdown-based command definition
├── Parameter handling and validation
├── Integration with existing tool ecosystem  
└── Documentation auto-generation

This architecture enables Claude Code to function as a comprehensive AI development platform while maintaining the flexibility and composability that developers expect from command-line tools.