Full deep dive: Claude Code Architecture Breakdown
Anthropic's agentic coding assistant, available as CLI, desktop app, web app, and IDE extensions. Uses a two-layer architecture (agentic harness + LLM backend) with no vector database or RAG — instead relying on direct file reads, keyword search, and large context windows (200K–1M tokens). Leads SWE-bench Verified at 80.8%.
Architecture Overview
Claude Code uses a two-layer design:
- Agentic Harness — manages the gather → act → verify loop, tools, permissions, context, and subagents
- LLM Backend — Claude Opus 4.6 / Sonnet 4.6 / Haiku 4.5 via Anthropic API (also supports Bedrock, Vertex AI, Microsoft Foundry)
All surfaces (CLI, desktop, web, IDE) share the same harness — no surface-specific logic.
Key Design Decisions
| Decision | Detail |
|---|---|
| No RAG / no vector DB | Direct file reads via Glob + Grep + Read; context windows large enough to search on demand |
| Context engineering | Auto-compaction at ~95% capacity; CLAUDE.md re-injected losslessly after compaction |
| Sandbox model | Application-level (permissions + hooks), not kernel-level |
| Config standard | CLAUDE.md (Claude-only, supports hierarchy + @import + hooks integration) |
| Extension model | MCP servers, hooks (17 lifecycle events), subagents, custom agents, skills |
| Token efficiency | ~4x higher usage than Codex — trades efficiency for thoroughness |
Models
| Model | Use Case | Context Window |
|---|---|---|
| Claude Opus 4.6 | Complex reasoning, architecture | 200K–1M |
| Claude Sonnet 4.6 | Day-to-day coding | 200K–1M |
| Claude Haiku 4.5 | Fast tasks, subagent work | 200K |
Why It's in Adopt
Claude Code's combination of highest benchmark scores (SWE-bench 80.8%), best-in-class reasoning quality, and a mature extension model (MCP, hooks, subagents) makes it the top choice for complex, multi-file coding tasks. The no-RAG approach eliminates setup complexity. The main tradeoff is higher token consumption and CLI-first design (though the web app and IDE extensions have closed the gap).
Key Characteristics
| Property | Value |
|---|---|
| Company | Anthropic |
| System | Claude Code (CLI + desktop + web + IDE) |
| Models | Claude Opus 4.6 / Sonnet 4.6 / Haiku 4.5 |
| Key innovations | No-RAG context engineering, auto-compaction, CLAUDE.md, hooks engine |
| Open source | Partial (Claude Code SDK, MCP protocol) |
| Sources | Overview, Context Engineering, Architecture Breakdown |