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Tools

Tools are what make Claude Code agentic. Without tools, Claude can only respond with text. With tools, Claude can act: read code, edit files, run commands, search the web, and interact with external services.

Each tool use returns information that feeds back into the agentic loop, informing Claude's next decision.

The agentic loop

When you give Claude a task, it works through three phases: gather context, take action, and verify results. Claude uses tools throughout all phases, chaining dozens of actions together and course-correcting along the way. For example, when asked to "fix the failing tests":

  1. Bash — Run the test suite to see failures
  2. Read — Read the error output and relevant source files
  3. Edit — Fix the code
  4. Bash — Run tests again to verify

You can interrupt at any point to steer Claude in a different direction.

Built-in tools

File operations

Tool Description Permission
Read Read file contents No
Edit Targeted edits to specific files Yes
Write Create or overwrite files Yes
NotebookEdit Edit Jupyter notebook cells Yes
Tool Description Permission
Glob Find files by pattern matching (e.g., **/*.yaml) No
Grep Search file contents with regex No

Execution

Tool Description Permission
Bash Execute shell commands in your environment Yes

Web

Tool Description Permission
WebSearch Search the web for information No
WebFetch Fetch and process content from a URL No

Orchestration

Tool Description Permission
Task Spawn subagents for delegated work No
TaskOutput Retrieve output from background tasks No
AskUserQuestion Ask the user for clarification No
ExitPlanMode Exit plan mode and start implementing Yes

Extending tools

The built-in tools are the foundation. Claude Code can be extended with additional tools through:

  • MCP servers: Connect external services (databases, APIs, custom tools) via the Model Context Protocol. See MCP servers
  • Skills: Domain-specific workflows that orchestrate tool usage. See Skills
  • Subagents: Delegated workers with their own tool access and context. See Agents
  • Hooks: Shell commands that run automatically on tool events. See Hooks

Permissions

Tools that modify state (Edit, Write, Bash) require permission. Claude asks before using them unless you pre-approve in settings.json.

Permission modes

Cycle through modes with Shift+Tab:

  • Default: Claude asks before file edits and shell commands
  • Auto-accept edits: Claude edits files without asking, still asks for commands
  • Plan mode: Read-only tools only, creates a plan for approval

Permission rules in settings

{
  "$schema": "https://json.schemastore.org/claude-code-settings.json",
  "permissions": {
    "allow": ["Bash(npm run lint)", "Read(~/.zshrc)"],
    "deny": ["Bash(curl *)", "Read(./.env)"],
    "ask": ["Bash(git push *)"],
    "additionalDirectories": ["../docs/"],
    "defaultMode": "acceptEdits"
  },
  "env": { "MY_VAR": "value" },
  "hooks": {},
  "cleanupPeriodDays": 30,
  "language": "english"
}

Evaluation order: deny → ask → allow (first match wins). Rules support glob patterns for command arguments.

Claude Code automatically creates timestamped backups of config files, retaining the 5 most recent.