Claude Agent Sdk

Build autonomous AI agents with Claude Agent SDK. Structured outputs guarantee JSON schema validation, with plugins system and hooks for event-driven workflows. Prevents 14 documented errors. Use when: building coding agents, SRE systems, security auditors, or troubleshooting CLI not found, structured output validation, session forking errors, MCP config issues, subagent cleanup.

# Claude Agent SDK - Structured Outputs & Error Prevention Guide

**Package**: @anthropic-ai/claude-agent-sdk@0.2.12
**Breaking Changes**: v0.1.45 - Structured outputs (Nov 2025), v0.1.0 - No default system prompt, settingSources required

---

## What's New in v0.1.45+ (Nov 2025)

**Major Features:**

### 1. Structured Outputs (v0.1.45, Nov 14, 2025)
- **JSON schema validation** - Guarantees responses match exact schemas
- **`outputFormat` parameter** - Define output structure with JSON schema or Zod
- **Access validated results** - Via `message.structured_output`
- **Beta header required**: `structured-outputs-2025-11-13`
- **Type safety** - Full TypeScript inference with Zod schemas

**Example:**
```typescript
import { query } from "@anthropic-ai/claude-agent-sdk";
import { z } from "zod";
import { zodToJsonSchema } from "zod-to-json-schema";

const schema = z.object({
  summary: z.string(),
  sentiment: z.enum(['positive', 'neutral', 'negative']),
  confidence: z.number().min(0).max(1)
});

const response = query({
  prompt: "Analyze this code review feedback",
  options: {
    model: "claude-sonnet-4-5",
    outputFormat: {
      type: "json_schema",
      json_schema: {
        name: "AnalysisResult",
        strict: true,
        schema: zodToJsonSchema(schema)
      }
    }
  }
});

for await (const message of response) {
  if (message.type === 'result' && message.structured_output) {
    // Guaranteed to match schema
    const validated = schema.parse(message.structured_output);
    console.log(`Sentiment: ${validated.sentiment}`);
  }
}
```

**Zod Compatibility (v0.1.71+):** SDK supports both Zod v3.24.1+ and Zod v4.0.0+ as peer dependencies. Import remains `import { z } from "zod"` for either version.

### 2. Plugins System (v0.1.27)
- **`plugins` array** - Load local plugin paths
- **Custom plugin support** - Extend agent capabilities

### 3. Hooks System (v0.1.0+)

**All 12 Hook Events:**

| Hook | When Fired | Use Case |
|------|------------|----------|
| `PreToolUse` | Before tool execution | Validate, modify, or block tool calls |
| `PostToolUse` | After tool execution | Log results, trigger side effects |
| `Notification` | Agent notifications | Display status updates |
| `UserPromptSubmit` | User prompt received | Pre-process or validate input |
| `SubagentStart` | Subagent spawned | Track delegation, log context |
| `SubagentStop` | Subagent completed | Aggregate results, cleanup |
| `PreCompact` | Before context compaction | Save state before truncation |
| `PermissionRequest` | Permission needed | Custom approval workflows |
| `Stop` | Agent stopping | Cleanup, final logging |
| `SessionStart` | Session begins | Initialize state |
| `SessionEnd` | Session ends | Persist state, cleanup |
| `Error` | Error occurred | Custom error handling |

**Hook Configuration:**
```typescript
const response = query({
  prompt: "...",
  options: {
    hooks: {
      PreToolUse: async (input) => {
        console.log(`Tool: ${input.toolName}`);
        return { allow: true };  // or { allow: false, message: "..." }
      },
      PostToolUse: async (input) => {
        await logToolUsage(input.toolName, input.result);
      }
    }
  }
});
```

### 4. Additional Options
- **`fallbackModel`** - Automatic model fallback on failures
- **`maxThinkingTokens`** - Control extended thinking budget
- **`strictMcpConfig`** - Strict MCP configuration validation
- **`continue`** - Resume with new prompt (differs from `resume`)
- **`permissionMode: 'plan'`** - New permission mode for planning workflows

📚 **Docs**: https://platform.claude.com/docs/en/agent-sdk/structured-outputs

---

## The Complete Claude Agent SDK Reference

## Table of Contents

1. [Core Query API](#core-query-api)
2. [Tool Integration](#tool-integration-built-in--custom)
3. [MCP Servers](#mcp-servers-model-context-protocol)
4. [Subagent Orchestration](#subagent-orchestration)
5. [Session Management](#session-management)
6. [Permission Control](#permission-control)
7. [Sandbox Settings](#sandbox-settings-security-critical)
8. [File Checkpointing](#file-checkpointing)
9. [Filesystem Settings](#filesystem-settings)
10. [Query Object Methods](#query-object-methods)
11. [Message Types & Streaming](#message-types--streaming)
12. [Error Handling](#error-handling)
13. [Known Issues](#known-issues-prevention)

---

## Core Query API

**Key signature:**
```typescript
query(prompt: string | AsyncIterable<SDKUserMessage>, options?: Options)
  -> AsyncGenerator<SDKMessage>
```

**Critical Options:**
- `outputFormat` - Structured JSON schema validation (v0.1.45+)
- `settingSources` - Filesystem settings loading ('user'|'project'|'local')
- `canUseTool` - Custom permission logic callback
- `agents` - Programmatic subagent definitions
- `mcpServers` - MCP server configuration
- `permissionMode` - 'default'|'acceptEdits'|'bypassPermissions'|'plan'
- `betas` - Enable beta features (e.g., 1M context window)
- `sandbox` - Sandbox settings for secure execution
- `enableFileCheckpointing` - Enable file state snapshots
- `systemPrompt` - System prompt (string or preset object)

### Extended Context (1M Tokens)

Enable 1 million token context window:

```typescript
const response = query({
  prompt: "Analyze this large codebase",
  options: {
    betas: ['context-1m-2025-08-07'],  // Enable 1M context
    model: "claude-sonnet-4-5"
  }
});
```

### System Prompt Configuration

Two forms of systemPrompt:

```typescript
// 1. Simple string
systemPrompt: "You are a helpful coding assistant."

// 2. Preset with optional append (preserves Claude Code defaults)
systemPrompt: {
  type: 'preset',
  preset: 'claude_code',
  append: "\n\nAdditional context: Focus on security."
}
```

**Use preset form** when you want Claude Code's default behaviors plus custom additions.

---

## Tool Integration (Built-in + Custom)

**Tool Control:**
- `allowedTools` - Whitelist (takes precedence)
- `disallowedTools` - Blacklist
- `canUseTool` - Custom permission callback (see Permission Control section)

**Built-in Tools:** Read, Write, Edit, Bash, Grep, Glob, WebSearch, WebFetch, Task, NotebookEdit, BashOutput, KillBash, ListMcpResources, ReadMcpResource, AskUserQuestion

### AskUserQuestion Tool (v0.1.71+)

Enable user interaction during agent execution:

```typescript
const response = query({
  prompt: "Review and refactor the codebase",
  options: {
    allowedTools: ["Read", "Write", "Edit", "AskUserQuestion"]
  }
});

// Agent can now ask clarifying questions
// Questions appear in message stream as tool_call with name "AskUserQuestion"
```

**Use cases:**
- Clarify ambiguous requirements mid-task
- Get user approval before destructive operations
- Present options and get selection

### Tools Configuration (v0.1.57+)

**Three forms of tool configuration:**

```typescript
// 1. Exact allowlist (string array)
tools: ["Read", "Write", "Grep"]

// 2. Disable all tools (empty array)
tools: []

// 3. Preset with defaults (object form)
tools: { type: 'preset', preset: 'claude_code' }
```

**Note:** `allowedTools` and `disallowedTools` still work but `tools` provides more flexibility.

---

## MCP Servers (Model Context Protocol)

**Server Types:**
- **In-process** - `createSdkMcpServer()` with `tool()` definitions
- **External** - stdio, HTTP, SSE transport

**Tool Definition:**
```typescript
tool(name: string, description: string, zodSchema, handler)
```

**Handler Return:**
```typescript
{ content: [{ type: "text", text: "..." }], isError?: boolean }
```

### External MCP Servers (stdio)

```typescript
const response = query({
  prompt: "List files and analyze Git history",
  options: {
    mcpServers: {
      // Filesystem server
      "filesystem": {
        command: "npx",
        args: ["@modelcontextprotocol/server-filesystem"],
        env: {
          ALLOWED_PATHS: "/Users/developer/projects:/tmp"
        }
      },
      // Git operations server
      "git": {
        command: "npx",
        args: ["@modelcontextprotocol/se