Episode 3: Tool - Claude's Hands and Feet — Comprehensive Guide to Claude Code Extension Ecosystem

Tool is the smallest unit of operation Claude can perform. Built-in Tools and MCP Tools together constitute Claude's entire actual operational capability.

Built-in Tool List

Tool	Function	Security Level
`Read`	Read file content	Read-only
`Write`	Create/overwrite file	Write
`Edit`	Precise string replacement	Write
`Bash`	Execute Shell commands	Highest
`Glob`	File pattern search	Read-only
`Grep`	Content search	Read-only
`Agent`	Start sub-agent	Medium
`AskUserQuestion`	Ask user a question	Read-only
`WebSearch`	Web search	Read-only
`LSP`	Code intelligence (jump/refs)	Read-only

Tool Call Lifecycle

sequenceDiagram
    participant Claude as Claude AI
    participant Engine as Tool Engine
    participant Guard as Permission Check
    participant Hook as Hook System
    participant FS as File System/Shell

    Claude->>Engine: call Edit(file, old, new)
    Engine->>Guard: Check permissions
    alt Needs user confirmation
        Guard->>Guard: Pop-up confirmation
        Note over Guard: User denies → Return error
    end
    Guard->>Hook: PreToolUse Event
    Hook->>Hook: Run PreToolUse hooks
    Hook-->>Engine: Allow/Block
    Engine->>FS: Execute file modification
    FS-->>Engine: Return result
    Engine->>Hook: PostToolUse Event
    Hook->>Hook: Run PostToolUse hooks
    Engine-->>Claude: Return operation result

Key Points:

Tool is the only way Claude can "do" things. Without Tools, Claude can only output text.
Every call passes through: Permission Check → PreToolUse Hook → Execution → PostToolUse Hook.
Skill is not a Tool. A Skill is an instruction template that eventually executes via Tools.

MCP Tools — Extending Capabilities

MCP Servers expose additional Tools via protocol. To Claude, MCP Tools are used in the same way as built-in Tools.

flowchart LR
    subgraph Tools Available to Claude
        B[Read] 
        C[Write]
        D[Edit]
        E[Bash]
        F[...Built-in]
        G[mcp__search
claude-mem search]
        H[mcp__get_observations
claude-mem observations]
        I[mcp__analyze_image
Image analysis]
        J[mcp__webReader
Web page reading]
    end

    subgraph Sources
        CORE[Claude Code Built-in]
        MCP1[claude-mem MCP]
        MCP2[4_5v MCP]
        MCP3[web_reader MCP]
    end

    CORE --> B & C & D & E & F
    MCP1 --> G & H
    MCP2 --> I
    MCP3 --> J