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Session Management

The Claude Agent SDK provides session management capabilities for handling conversation state and resumption. Sessions allow you to continue conversations across multiple interactions while maintaining full context.

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How Sessions Work

When you start a new query, the SDK automatically creates a session and returns a session ID in the initial system message. You can capture this ID to resume the session later.

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Getting the Session ID

import { query } from "@anthropic-ai/claude-agent-sdk"

let sessionId: string | undefined

const response = query({
  prompt: "Help me build a web application",
  options: {
    model: "claude-sonnet-4-5"
  }
})

for await (const message of response) {
  // The first message is a system init message with the session ID
  if (message.type === 'system' && message.subtype === 'init') {
    sessionId = message.session_id
    console.log(`Session started with ID: ${sessionId}`)
    // You can save this ID for later resumption
  }

  // Process other messages...
  console.log(message)
}

// Later, you can use the saved sessionId to resume
if (sessionId) {
  const resumedResponse = query({
    prompt: "Continue where we left off",
    options: {
      resume: sessionId
    }
  })
}

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Resuming Sessions

The SDK supports resuming sessions from previous conversation states, enabling continuous development workflows. Use the resume option with a session ID to continue a previous conversation. 
import { query } from "@anthropic-ai/claude-agent-sdk"

// Resume a previous session using its ID
const response = query({
  prompt: "Continue implementing the authentication system from where we left off",
  options: {
    resume: "session-xyz", // Session ID from previous conversation
    model: "claude-sonnet-4-5",
    allowedTools: ["Read", "Edit", "Write", "Glob", "Grep", "Bash"]
  }
})

// The conversation continues with full context from the previous session
for await (const message of response) {
  console.log(message)
}
The SDK automatically handles loading the conversation history and context when you resume a session, allowing Claude to continue exactly where it left off.

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Forking Sessions

When resuming a session, you can choose to either continue the original session or fork it into a new branch. By default, resuming continues the original session. Use the forkSession option (TypeScript) or fork_session option (Python) to create a new session ID that starts from the resumed state.

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When to Fork a Session

Forking is useful when you want to:
  • Explore different approaches from the same starting point
  • Create multiple conversation branches without modifying the original
  • Test changes without affecting the original session history
  • Maintain separate conversation paths for different experiments

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Forking vs Continuing

BehaviorforkSession: false (default)forkSession: true
Session IDSame as originalNew session ID generated
HistoryAppends to original sessionCreates new branch from resume point
Original SessionModifiedPreserved unchanged
Use CaseContinue linear conversationBranch to explore alternatives

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Example: Forking a Session

import { query } from "@anthropic-ai/claude-agent-sdk"

// First, capture the session ID
let sessionId: string | undefined

const response = query({
  prompt: "Help me design a REST API",
  options: { model: "claude-sonnet-4-5" }
})

for await (const message of response) {
  if (message.type === 'system' && message.subtype === 'init') {
    sessionId = message.session_id
    console.log(`Original session: ${sessionId}`)
  }
}

// Fork the session to try a different approach
const forkedResponse = query({
  prompt: "Now let's redesign this as a GraphQL API instead",
  options: {
    resume: sessionId,
    forkSession: true,  // Creates a new session ID
    model: "claude-sonnet-4-5"
  }
})

for await (const message of forkedResponse) {
  if (message.type === 'system' && message.subtype === 'init') {
    console.log(`Forked session: ${message.session_id}`)
    // This will be a different session ID
  }
}

// The original session remains unchanged and can still be resumed
const originalContinued = query({
  prompt: "Add authentication to the REST API",
  options: {
    resume: sessionId,
    forkSession: false,  // Continue original session (default)
    model: "claude-sonnet-4-5"
  }
})