Claude Code

Claude Code is Anthropic's official agentic coding tool — now available as a CLI, desktop app (Mac/Windows), web app (claude.ai/code), and IDE extensions for VS Code and JetBrains. It autonomously reads, writes, and runs code across your entire project.

Architecture Deep Dive → Claude Code Architecture Breakdown — two-layer harness design, context engineering strategies, permission model, MCP extension points, and how it compares to Codex's kernel-level sandboxing.

Why It's in Adopt

Claude Code has evolved significantly since its initial CLI-only release. It remains one of the most capable agentic coding tools available, now with multiple interfaces:

• Terminal CLI — the original and still the most powerful interface for shell-heavy workflows
• Desktop app — native Mac and Windows app for a more visual experience
• Web app — claude.ai/code for browser-based access
• IDE extensions — VS Code and JetBrains integrations
• Background agents — run tasks asynchronously while you work on other things
• Scheduled tasks — recurring agents on a cron schedule for automated maintenance
• Hooks — 17 programmable hook events that execute shell commands in response to Claude Code actions (tool calls, file edits, command execution, etc.)
• Agent Teams (research preview) — spawn multiple sub-agents, each with a dedicated context window and git worktree, with shared task lists and inter-agent messaging
• MCP server support — connect to external tools, databases, and APIs

Claude Code now authors ~4% of all public GitHub commits (~135K/day) and leads SWE-bench Verified at 80.8% — the highest score of any coding agent on complex, real-world bug fixes.

It's particularly well-suited to tasks that span many files or require running code to verify correctness — things that in-editor tools struggle with.

How It Works

# Install
npm install -g @anthropic-ai/claude-code

# Run in your project directory
claude

# Then describe what you want in plain English
> Add rate limiting to the /api/login endpoint

Claude Code reads your files, plans a solution, makes the changes, and can run your tests to verify. You review and approve changes before they're committed.

Safety Architecture

Claude Code enforces safety at the application layer — a fundamentally different approach from OpenAI Codex's OS kernel-level sandboxing (Landlock, seccomp, Seatbelt). The trade-off:

This means Claude Code is more expressive for teams that want to encode policy (e.g. "run ESLint before every file write", "block commits to main"), while Codex is stronger against adversarial escape. Both approaches are valid — they optimize for different concerns.

Configuration: CLAUDE.md vs AGENTS.md

CLAUDE.md is Claude Code's project configuration file — more powerful within Claude (persistent context, coding conventions, build commands, project structure) but only works with Claude Code. AGENTS.md is the cross-tool open standard (Linux Foundation / AAIF) that works across Codex, Cursor, Copilot, Windsurf, Gemini CLI, and others. Teams using multiple tools benefit from AGENTS.md portability; teams committed to Claude Code get more depth from CLAUDE.md. Both can coexist in the same repo.

Key Concepts for Newcomers

• Agentic: Unlike a chat interface, Claude Code takes actions — it doesn't just tell you what to do, it does it.
• Permission model: Claude Code asks before running commands or making changes you haven't approved.
• Context: It automatically reads relevant files rather than requiring you to paste them in.
• CLAUDE.md: A project file that gives Claude Code persistent context about your codebase — coding conventions, build commands, project structure.

Key Characteristics

Cost of Ownership

Claude Code's pricing model is fundamentally different from its competitors — and the difference matters more than the headline price.

The real cost question is cost per completed task, not monthly subscription. Claude Code on API billing uses ~4x more tokens than Codex per task (trades efficiency for thoroughness), but its higher SWE-bench score means fewer failed attempts and less human rework. A task that takes Claude Code $2 in tokens but succeeds on the first try is cheaper than a $0.50 attempt that requires a human to finish.

For teams: GitHub Copilot at $19–39/seat is the cheapest all-you-can-eat option and the right choice if your primary use case is completions and light agent work. Claude Code on API billing makes sense for heavy agentic usage where per-task success rate matters most. Cursor's credit model is the most complex to budget — the "unlimited auto-mode" is unlimited only when Cursor picks the model.

Claude Code Security (Research Preview)

Anthropic launched Claude Code Security on February 20, 2026 — an AI-driven vulnerability scanning capability built on top of Claude Code (web). Unlike traditional SAST tools, it reasons about code rather than pattern-matching: it traces data flows across files, understands context, and identifies multi-component vulnerability patterns that rule-based scanners miss.

Key characteristics:

• Uses Claude Opus 4.6 as the scanning engine
• Multi-stage verification: findings are re-analyzed to filter false positives before surfacing
• Human-in-the-loop: each finding includes a confidence rating; patches are suggested but require human approval before application
• Identified 500+ high-risk vulnerabilities across production OSS codebases in its initial runs — some issues had existed for decades, surviving many rounds of manual review
• Available to Enterprise and Team customers, with expedited access for OSS maintainers

(Anthropic announcement, Help Net Security)

Further Reading

• Claude Code Architecture Breakdown — full deep dive into the agentic harness, context engineering, and safety model
• Claude Code Security — official product page
• Effective Context Engineering for AI Agents (Anthropic)
• Building Effective Agents (Anthropic)