Tools overview

Tools are the capabilities Claude can invoke during a session — reading files, running commands, searching the codebase, fetching URLs, and more.

Tools are discrete, self-contained capabilities that Claude invokes during a session to accomplish tasks. Each tool has a defined input schema, a permission level, and an execution function. Claude selects the appropriate tool based on what the task requires, then waits for the result before continuing.

How the tool loop works

1. Claude selects a tool

   After analyzing the conversation, Claude emits a tool_use block specifying the tool name and its input arguments.

2. Permission check

   The permission system evaluates whether the tool call is allowed under the current permission mode (default, plan, bypassPermissions, etc.). Depending on the outcome, the tool call is automatically approved, denied, or presented to you for a yes/no decision.

3. Tool executes

   The tool runs — reading a file, executing a shell command, fetching a URL, spawning a sub-agent, and so on. Progress is streamed to the terminal UI in real time.

4. Result returned

   The tool returns a result, which is injected into the conversation context as a tool_result block. Claude then continues from where it left off, using the new information.

Permission levels

Each tool has a permission level that determines when you need to approve its use.

Level	Description	Examples
Read-only	Does not modify any files or system state. Automatically approved in most modes.	FileReadTool, GlobTool, GrepTool, WebFetchTool
Mutating	Creates or modifies files or state. Requires approval unless the path is in an allowlist or bypassPermissions mode is active.	FileWriteTool, FileEditTool, NotebookEditTool
Destructive / side-effect	Executes shell commands, spawns agents, or has network side-effects. Requires explicit approval unless pre-approved.	BashTool, AgentTool, TeamCreateTool, WebSearchTool

The permission model is documented in detail in Permission model. You can pre-approve specific paths or commands in your local settings to avoid repeated prompts.

Controlling available tools

You can restrict which tools Claude can use in a session using CLI flags.

Allow only specific tools:

claude --allowedTools FileReadTool,GlobTool,GrepTool

Block specific tools:

claude --disallowedTools BashTool,WebFetchTool

Both flags accept a comma-separated list of tool names. --allowedTools takes precedence — only the listed tools will be available. --disallowedTools removes specific tools from the default set.

Use --allowedTools for read-only sessions where you want Claude to explore the codebase without making changes.

Complete tool reference

The table below lists every built-in tool. Tools marked with a feature flag may not be available in all configurations.

Tool	Category	Description	Permission level
BashTool	Shell	Execute shell commands in a persistent session	Destructive
FileReadTool	File	Read files — text, images, PDFs, Jupyter notebooks	Read-only
FileWriteTool	File	Create or overwrite a file with new content	Mutating
FileEditTool	File	Partial file edit via exact string replacement	Mutating
GlobTool	Search	Find files by glob pattern	Read-only
GrepTool	Search	Search file contents with regex (ripgrep)	Read-only
WebFetchTool	Web	Fetch and extract content from a URL	Read-only
WebSearchTool	Web	Search the web for current information	Read-only
AgentTool	Agent	Spawn a sub-agent to handle a complex task	Destructive
SkillTool	Agent	Execute a named skill (reusable workflow)	Destructive
MCPTool	Agent	Invoke a tool from a connected MCP server	Varies
LSPTool	Agent	Language Server Protocol diagnostics and hover	Read-only
NotebookEditTool	File	Edit cells in a Jupyter notebook	Mutating
TaskCreateTool	Task	Create a task in the shared task list	Mutating
TaskUpdateTool	Task	Update task status, owner, or metadata	Mutating
TaskGetTool	Task	Retrieve a single task by ID	Read-only
TaskListTool	Task	List all tasks in the current task list	Read-only
TaskStopTool	Task	Stop a running task	Mutating
TaskOutputTool	Task	Read the output file of a completed agent task	Read-only
SendMessageTool	Agent	Send a message to another agent in a swarm	Mutating
TeamCreateTool	Agent	Create a named team for multi-agent coordination	Mutating
TeamDeleteTool	Agent	Delete a team and clean up its resources	Mutating
EnterPlanModeTool	Session	Switch to plan mode (propose without executing)	Mutating
ExitPlanModeTool	Session	Exit plan mode and resume normal execution	Mutating
EnterWorktreeTool	Session	Isolate work in a temporary git worktree	Mutating
ExitWorktreeTool	Session	Exit the current git worktree	Mutating
RemoteTriggerTool	Session	Register a remote trigger for proactive mode	Mutating
ScheduleCronTool	Session	Schedule a cron-style recurring trigger	Mutating
SleepTool	Session	Pause execution in proactive/KAIROS mode	Read-only
SyntheticOutputTool	Session	Emit structured output from the agent	Mutating
ToolSearchTool	Agent	Discover tools by name or description	Read-only
AskUserQuestionTool	Session	Ask you a clarifying question mid-task	Read-only
REPLTool	Shell	Interactive REPL for code evaluation (internal)	Destructive
PowerShellTool	Shell	Execute PowerShell commands on Windows	Destructive

Tool categories

File tools

Read, write, and edit files. Covers FileReadTool, FileWriteTool, FileEditTool, and NotebookEditTool.

Search tools

Find files and search content. Covers GlobTool, GrepTool, and search patterns with BashTool.

Agent tools

Multi-agent workflows, task management, and inter-agent messaging.

Web tools

Fetch URLs and search the web. Covers WebFetchTool and WebSearchTool.