Claude Agent SDK - Structured Outputs & Error Prevention Guide

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}`);
  }
}

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);
      }
    }
  }
});

query(prompt: string | AsyncIterable<SDKUserMessage>, options?: Options)
  -> AsyncGenerator<SDKMessage>

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

// 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."
}

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"

// 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' }

tool(name: string, description: string, zodSchema, handler)

{ content: [{ type: "text", text: "..." }], isError?: boolean }

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/server-git"],
        env: {
          GIT_REPO_PATH: "/Users/developer/projects/my-repo"
        }
      }
    },
    allowedTools: [
      "mcp__filesystem__list_files",
      "mcp__filesystem__read_file",
      "mcp__git__log",
      "mcp__git__diff"
    ]
  }
});

const response = query({
  prompt: "Analyze data from remote service",
  options: {
    mcpServers: {
      "remote-service": {
        url: "https://api.example.com/mcp",
        headers: {
          "Authorization": "Bearer your-token-here",
          "Content-Type": "application/json"
        }
      }
    },
    allowedTools: ["mcp__remote-service__analyze"]
  }
});

type AgentDefinition = {
  description: string;        // When to use this agent
  prompt: string;             // System prompt for agent
  tools?: string[];           // Allowed tools (optional)
  model?: 'sonnet' | 'opus' | 'haiku' | 'inherit';  // Model (optional)
  skills?: string[];          // Skills to load (v0.2.10+)
  maxTurns?: number;          // Maximum turns before stopping (v0.2.10+)
}

agents: {
  "security-checker": {
    description: "Security audits and vulnerability scanning",
    prompt: "You check security. Scan for secrets, verify OWASP compliance.",
    tools: ["Read", "Grep", "Bash"],
    model: "sonnet",
    skills: ["security-best-practices"],  // Load specific skills
    maxTurns: 10  // Limit to 10 turns
  }
}

const response = query({
  prompt: "Deploy to production",
  options: {
    agents: {
      "deployer": {
        description: "Handle deployments",
        prompt: "Deploy the application",
        tools: ["Bash"]
      }
    },
    hooks: {
      Stop: async (input) => {
        // Manual cleanup of spawned processes
        console.log("Parent stopped - cleaning up subagents");
        // Implement process tracking and termination
      }
    }
  }
});

// Explore alternative without modifying original
const forked = query({
  prompt: "Try GraphQL instead of REST",
  options: {
    resume: sessionId,
    forkSession: true  // Creates new branch, original session unchanged
  }
});

for await (const message of response) {
  if (message.type === 'system' && message.subtype === 'init') {
    sessionId = message.session_id;  // Save for later resume/fork
  }
}

import {
  unstable_v2_createSession,
  unstable_v2_resumeSession,
  unstable_v2_prompt
} from "@anthropic-ai/claude-agent-sdk";

// Create a new session
const session = await unstable_v2_createSession({
  model: "claude-sonnet-4-5",
  workingDirectory: process.cwd(),
  allowedTools: ["Read", "Grep", "Glob"]
});

// Send prompts and stream responses
const stream = unstable_v2_prompt(session, "Analyze the codebase structure");
for await (const message of stream) {
  console.log(message);
}

// Continue conversation in same session
const stream2 = unstable_v2_prompt(session, "Now suggest improvements");
for await (const message of stream2) {
  console.log(message);
}

// Resume a previous session
const resumedSession = await unstable_v2_resumeSession(session.sessionId);

type PermissionMode = "default" | "acceptEdits" | "bypassPermissions" | "plan";

const response = query({
  prompt: "Deploy application to production",
  options: {
    permissionMode: "default",
    canUseTool: async (toolName, input) => {
      // Allow read-only operations
      if (['Read', 'Grep', 'Glob'].includes(toolName)) {
        return { behavior: "allow" };
      }

      // Deny destructive bash commands
      if (toolName === 'Bash') {
        const dangerous = ['rm -rf', 'dd if=', 'mkfs', '> /dev/'];
        if (dangerous.some(pattern => input.command.includes(pattern))) {
          return {
            behavior: "deny",
            message: "Destructive command blocked for safety"
          };
        }
      }

      // Require confirmation for deployments
      if (input.command?.includes('deploy') || input.command?.includes('kubectl apply')) {
        return {
          behavior: "ask",
          message: "Confirm deployment to production?"
        };
      }

      // Allow by default
      return { behavior: "allow" };
    }
  }
});

type CanUseToolCallback = (
  toolName: string,
  input: any
) => Promise<PermissionDecision>;

type PermissionDecision =
  | { behavior: "allow" }
  | { behavior: "deny"; message?: string }
  | { behavior: "ask"; message?: string };

// Block all file writes
canUseTool: async (toolName, input) => {
  if (toolName === 'Write' || toolName === 'Edit') {
    return { behavior: "deny", message: "No file modifications allowed" };
  }
  return { behavior: "allow" };
}

// Require confirmation for specific files
canUseTool: async (toolName, input) => {
  const sensitivePaths = ['/etc/', '/root/', '.env', 'credentials.json'];
  if ((toolName === 'Write' || toolName === 'Edit') &&
      sensitivePaths.some(path => input.file_path?.includes(path))) {
    return {
      behavior: "ask",
      message: `Modify sensitive file ${input.file_path}?`
    };
  }
  return { behavior: "allow" };
}

// Log all tool usage
canUseTool: async (toolName, input) => {
  console.log(`Tool requested: ${toolName}`, input);
  await logToDatabase(toolName, input);
  return { behavior: "allow" };
}

const response = query({
  prompt: "Run system diagnostics",
  options: {
    sandbox: {
      enabled: true,
      autoAllowBashIfSandboxed: true,  // Auto-approve bash in sandbox
      excludedCommands: ["rm", "dd", "mkfs"],  // Never auto-approve these
      allowUnsandboxedCommands: false  // Deny unsandboxable commands
    }
  }
});

type SandboxSettings = {
  enabled: boolean;
  autoAllowBashIfSandboxed?: boolean;  // Default: false
  excludedCommands?: string[];
  allowUnsandboxedCommands?: boolean;  // Default: false
  network?: NetworkSandboxSettings;
  ignoreViolations?: SandboxIgnoreViolations;
};

type NetworkSandboxSettings = {
  enabled: boolean;
  proxyUrl?: string;  // HTTP proxy for network requests
};

const response = query({
  prompt: "Refactor the authentication module",
  options: {
    enableFileCheckpointing: true  // Enable file snapshots
  }
});

// Later: rewind file changes to a specific point
for await (const message of response) {
  if (message.type === 'user' && message.uuid) {
    // Can rewind to this point later
    const userMessageUuid = message.uuid;

    // To rewind (call on Query object)
    await response.rewindFiles(userMessageUuid);
  }
}

type SettingSource = 'user' | 'project' | 'local';

// .claude/settings.json
{
  "allowedTools": ["Read", "Write", "Edit"]
}

// .claude/settings.local.json
{
  "allowedTools": ["Read"]  // Overrides project settings
}

// Programmatic
const response = query({
  options: {
    settingSources: ["project", "local"],
    allowedTools: ["Read", "Grep"]  // ← This wins
  }
});

// Actual allowedTools: ["Read", "Grep"]

const q = query({ prompt: "..." });

// Async iteration (primary usage)
for await (const message of q) { ... }

// Runtime model control
await q.setModel("claude-opus-4-5");           // Change model mid-session
await q.setMaxThinkingTokens(4096);            // Set thinking budget

// Introspection
const models = await q.supportedModels();     // List available models
const commands = await q.supportedCommands(); // List available commands
const account = await q.accountInfo();        // Get account details

// MCP status
const status = await q.mcpServerStatus();     // Check MCP server status
// Returns: { [serverName]: { status: 'connected' | 'failed', error?: string } }

// File operations (requires enableFileCheckpointing)
await q.rewindFiles(userMessageUuid);         // Rewind to checkpoint

for await (const message of response) {
  if (message.type === 'system' && message.subtype === 'init') {
    sessionId = message.session_id;  // Capture for resume/fork
  }
  if (message.type === 'result' && message.structured_output) {
    // Structured output available (v0.1.45+)
    const validated = schema.parse(message.structured_output);
  }
}

// 1. Proactive session forking (create checkpoints before hitting limit)
const checkpoint = query({
  prompt: "Checkpoint current state",
  options: {
    resume: sessionId,
    forkSession: true  // Create branch before hitting limit
  }
});

// 2. Monitor time and rotate sessions proactively
const MAX_SESSION_TIME = 80 * 60 * 1000;  // 80 minutes (before 90-min crash)
let sessionStartTime = Date.now();

function shouldRotateSession() {
  return Date.now() - sessionStartTime > MAX_SESSION_TIME;
}

// 3. Start new sessions before hitting context limits
if (shouldRotateSession()) {
  const summary = await getSummary(currentSession);
  const newSession = query({
    prompt: `Continue with context: ${summary}`
  });
  sessionStartTime = Date.now();
}

// ❌ Wrong - missing type field (causes cryptic exit code 1)
mcpServers: {
  "my-server": {
    url: "https://api.example.com/mcp"
  }
}

// ✅ Correct - type field required for URL-based servers
mcpServers: {
  "my-server": {
    url: "https://api.example.com/mcp",
    type: "http"  // or "sse" for Server-Sent Events
  }
}

// MCP tool handler - sanitize external data
tool("fetch_content", "Fetch text content", {}, async (args) => {
  const content = await fetchExternalData();

  // ✅ Sanitize Unicode line/paragraph separators
  const sanitized = content
    .replace(/\u2028/g, '\\u2028')
    .replace(/\u2029/g, '\\u2029');

  return {
    content: [{ type: "text", text: sanitized }]
  };
});