Claude Code: The AI-Powered CLI That Rewrites How You Build Software

What is Claude Code?

Claude Code is Anthropic's official agentic coding CLI. Unlike a code-completion autocomplete, Claude Code is a fully agentic tool: it reads your codebase, understands context, writes and edits files, runs shell commands, executes tests, and iterates until the task is done — all from your terminal.

┌─────────────────────────────────────────────────────────────┐
│                  Claude Code Architecture                    │
│                                                              │
│  Developer ──▶ Terminal (claude)                             │
│                    │                                         │
│                    ▼                                         │
│  ┌─────────────────────────────────────────┐                │
│  │         Claude Code Agent               │                │
│  │  ┌──────────┐  ┌──────────┐             │                │
│  │  │  File    │  │  Shell   │  + LLM      │                │
│  │  │  Read/   │  │  Exec    │  Reasoning  │                │
│  │  │  Write   │  │  Bash    │             │                │
│  │  └──────────┘  └──────────┘             │                │
│  └─────────────────────────────────────────┘                │
│                    │                                         │
│                    ▼                                         │
│         Your codebase (files, tests, CI)                     │
└─────────────────────────────────────────────────────────────┘

Installation and Setup

bash
1# Install via npm
2npm install -g @anthropic-ai/claude-code
3
4# Or use directly with npx
5npx @anthropic-ai/claude-code
6
7# Set your API key
8export ANTHROPIC_API_KEY="sk-ant-..."
9
10# Launch in your project
11cd my-project
12claude
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Core Capabilities

Agentic Code Tasks

bash
1# Claude reads all relevant files, implements the feature, writes tests, runs them
2> Add a rate-limiting middleware to the Express app. Use Redis with a sliding window algorithm. Add tests.
3
4# Output:
5# Reading src/app.ts... ✓
6# Reading src/middleware/... ✓  
7# Writing src/middleware/rateLimiter.ts
8# Writing tests/middleware/rateLimiter.test.ts
9# Running: npm test
10# Tests passed (3/3) ✓
11
12# Refactoring with full context
13> Refactor all API routes to use the new BaseController pattern from src/controllers/BaseController.ts
14
15# Multi-file bug fix
16> The user login flow is broken — trace through the auth flow from the login endpoint and fix it
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CLAUDE.md — Project Instructions

Create a CLAUDE.md at your project root to give Claude persistent context:

markdown
1# Project: E-Commerce API
2
3## Stack
4- Node.js + TypeScript + Express
5- PostgreSQL (via Prisma ORM)
6- Redis for caching and rate limiting
7- Jest for tests
8
9## Commands
10- `npm run dev` — start dev server
11- `npm test` — run all tests  
12- `npm run lint` — ESLint
13- `npm run db:migrate` — run Prisma migrations
14
15## Architecture
16- Feature-based structure under `src/features/`
17- All DB access through Prisma in `src/db/`
18- Use dependency injection via tsyringe
19- API responses must use the `ApiResponse<T>` type
20
21## Conventions
22- No `any` types — fix them properly
23- Every new endpoint needs an integration test
24- Use `logger.info/error` not `console.log`
25- Error messages must be user-friendly
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Slash Commands and Workflows

bash
1# Review the current branch changes
2/review
3
4# Run the app and visually verify a feature
5/run
6
7# Security review of pending changes  
8/security-review
9
10# Initialize CLAUDE.md for the project
11/init
12
13# Clear conversation context
14/clear
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Hooks — Automating Workflows

Hooks run shell commands triggered by Claude Code events. Configure in .claude/settings.json:

json
1{
2  "hooks": {
3    "PostToolUse": [
4      {
5        "matcher": "Edit|Write",
6        "hooks": [{
7          "type": "command",
8          "command": "npm run lint --fix 2>&1 | head -20"
9        }]
10      }
11    ],
12    "PreToolUse": [
13      {
14        "matcher": "Bash",
15        "hooks": [{
16          "type": "command",
17          "command": "echo 'Running: $CLAUDE_TOOL_INPUT_COMMAND'"
18        }]
19      }
20    ]
21  }
22}
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MCP Servers — Extending Claude Code

MCP (Model Context Protocol) lets you connect external data sources and tools to Claude Code:

json
1{
2  "mcpServers": {
3    "github": {
4      "command": "npx",
5      "args": ["-y", "@modelcontextprotocol/server-github"],
6      "env": { "GITHUB_TOKEN": "${GITHUB_TOKEN}" }
7    },
8    "postgres": {
9      "command": "npx",
10      "args": ["-y", "@modelcontextprotocol/server-postgres"],
11      "env": { "DATABASE_URL": "${DATABASE_URL}" }
12    },
13    "slack": {
14      "command": "npx",
15      "args": ["-y", "@modelcontextprotocol/server-slack"],
16      "env": { "SLACK_BOT_TOKEN": "${SLACK_BOT_TOKEN}" }
17    }
18  }
19}
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With the GitHub MCP, you can ask:

bash
1> Look at the open PRs on this repo and summarize what's in review
2> Create a GitHub issue for the bug we just found in the auth flow
3> What did the last 5 commits change?
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Permissions Model

Claude Code uses an explicit permissions system — it asks before running destructive operations:

bash
1# Claude will ASK before:
2# - git push
3# - npm install (new packages)
4# - Database migrations
5# - Deleting files
6
7# Auto-allowed (safe, read-only or easily reversible):
8# - Reading files
9# - Running tests
10# - Running the dev server
11# - npm run lint
12
13# Configure in .claude/settings.json
14{
15  "permissions": {
16    "allow": [
17      "Bash(npm test)",
18      "Bash(npm run lint*)",
19      "Bash(git diff*)",
20      "Bash(git log*)"
21    ],
22    "deny": [
23      "Bash(git push --force*)",
24      "Bash(rm -rf*)"
25    ]
26  }
27}
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Pro Tips for Power Users

1. Use /clear Aggressively

Context window fills up. When switching tasks, always /clear — fresh context = faster, cheaper, more focused responses.

2. Be Specific About Scope

bash
1# Vague (Claude will over-engineer)
2> Fix the login bug
3
4# Specific (Claude stays focused)
5> The /api/auth/login endpoint returns 500 when the email contains a + sign.
6> The bug is in src/features/auth/authService.ts in the validateEmail function.
7> Fix just that function and add a test for it.
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3. Let Claude Run Tests

Always ask Claude to run tests after changes. It catches its own mistakes:

bash
1> Implement the feature, then run the full test suite and fix any failures
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4. Sub-agents for Parallel Work

Claude Code can spin up sub-agents for independent tasks:

bash
1> In parallel: (1) add TypeScript strict mode to tsconfig and fix all type errors,
2  (2) update all dependencies to their latest minor versions and fix any breaking changes
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Ultrareview — AI Code Review

bash
1# Deep multi-agent review of your current branch vs main
2/ultrareview
3
4# Review a specific GitHub PR
5/ultrareview 142
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Ultrareview spawns multiple specialized agents to independently review security, performance, correctness, and architecture — then synthesizes findings into a prioritized report.