Documentation Index

Fetch the complete documentation index at: https://code.claude.com/docs/llms.txt Use this file to discover all available pages before exploring further.

How the agent loop works

Understand the message lifecycle, tool execution, context window, and architecture that power your SDK agents.

The Agent SDK lets you embed Claude Code's autonomous agent loop in your own applications. The SDK is a standalone package that gives you programmatic control over tools, permissions, cost limits, and output. You don't need the Claude Code CLI installed to use it.

When you start an agent, the SDK runs the same execution loop that powers Claude Code: Claude evaluates your prompt, calls tools to take action, receives the results, and repeats until the task is complete. This page explains what happens inside that loop so you can build, debug, and optimize your agents effectively.

The loop at a glance

Every agent session follows the same cycle:

<img src="https://mintcdn.com/claude-code/gvy2DIUELtNA8qD3/images/agent-loop-diagram.svg?fit=max&auto=format&n=gvy2DIUELtNA8qD3&q=85&s=192e1bd6c8a2950a16e5ee0b94e27e26" alt="Agent loop: prompt enters, Claude evaluates, branches to tool calls or final answer" width="680" height="150" data-path="images/agent-loop-diagram.svg" />

1. Receive prompt. Claude receives your prompt, along with the system prompt, tool definitions, and conversation history. The SDK yields a SystemMessage with subtype "init" containing session metadata.
2. Evaluate and respond. Claude evaluates the current state and determines how to proceed. It may respond with text, request one or more tool calls, or both. The SDK yields an AssistantMessage containing the text and any tool call requests.
3. Execute tools. The SDK runs each requested tool and collects the results. Each set of tool results feeds back to Claude for the next decision. You can use hooks to intercept, modify, or block tool calls before they run.
4. Repeat. Steps 2 and 3 repeat as a cycle. Each full cycle is one turn. Claude continues calling tools and processing results until it produces a response with no tool calls.
5. Return result. The SDK yields a final AssistantMessage with the text response (no tool calls), followed by a ResultMessage with the final text, token usage, cost, and session ID.

A quick question ("what files are here?") might take one or two turns of calling Glob and responding with the results. A complex task ("refactor the auth module and update the tests") can chain dozens of tool calls across many turns, reading files, editing code, and running tests, with Claude adjusting its approach based on each result.

Turns and messages

A turn is one round trip inside the loop: Claude produces output that includes tool calls, the SDK executes those tools, and the results feed back to Claude automatically. This happens without yielding control back to your code. Turns continue until Claude produces output with no tool calls, at which point the loop ends and the final result is delivered.

Consider what a full session might look like for the prompt "Fix the failing tests in auth.ts".

First, the SDK sends your prompt to Claude and yields a SystemMessage with the session metadata. Then the loop begins:

1. Turn 1: Claude calls Bash to run npm test. The SDK yields an AssistantMessage with the tool call, executes the command, then yields a UserMessage with the output (three failures).
2. Turn 2: Claude calls Read on auth.ts and auth.test.ts. The SDK returns the file contents and yields an AssistantMessage.
3. Turn 3: Claude calls Edit to fix auth.ts, then calls Bash to re-run npm test. All three tests pass. The SDK yields an AssistantMessage.
4. Final turn: Claude produces a text-only response with no tool calls: "Fixed the auth bug, all three tests pass now." The SDK yields a final AssistantMessage with this text, then a ResultMessage with the same text plus cost and usage.

That was four turns: three with tool calls, one final text-only response.

You can cap the loop with max_turns / maxTurns, which counts tool-use turns only. For example, max_turns=2 in the loop above would have stopped before the edit step. You can also use max_budget_usd / maxBudgetUsd to cap turns based on a spend threshold.

Without limits, the loop runs until Claude finishes on its own, which is fine for well-scoped tasks but can run long on open-ended prompts ("improve this codebase"). Setting a budget is a good default for production agents. See Turns and budget below for the option reference.

Message types

As the loop runs, the SDK yields a stream of messages. Each message carries a type that tells you what stage of the loop it came from. The five core types are:

• SystemMessage: session lifecycle events. The subtype field distinguishes them: "init" is the first message (session metadata), and "compact_boundary" fires after compaction. In TypeScript, the compact boundary is its own SDKCompactBoundaryMessage type rather than a subtype of SDKSystemMessage.
• AssistantMessage: emitted after each Claude response, including the final text-only one. Contains text content blocks and tool call blocks from that turn.
• UserMessage: emitted after each tool execution with the tool result content sent back to Claude. Also emitted for any user inputs you stream mid-loop.
• StreamEvent: only emitted when partial messages are enabled. Contains raw API streaming events (text deltas, tool input chunks). See Stream responses.
• ResultMessage: marks the end of the agent loop. Contains the final text result, token usage, cost, and session ID. Check the subtype field to determine whether the task succeeded or hit a limit. A small number of trailing system events, such as prompt_suggestion, can arrive after it, so iterate the stream to completion rather than breaking on the result. See Handle the result.

These five types cover the full agent loop lifecycle in both SDKs. The TypeScript SDK also yields additional observability events (hook events, tool progress, rate limits, task notifications) that provide extra detail but are not required to drive the loop. See the Python message types reference and TypeScript message types reference for the complete lists.

Handle messages

Which messages you handle depends on what you're building:

• Final results only: handle ResultMessage to get the output, cost, and whether the task succeeded or hit a limit.
• Progress updates: handle AssistantMessage to see what Claude is doing each turn, including which tools it called.
• Live streaming: enable partial messages (include_partial_messages in Python, includePartialMessages in TypeScript) to get StreamEvent messages in real time. See Stream responses in real-time.

How you check message types depends on the SDK:

• Python: check message types with isinstance() against classes imported from claude_agent_sdk (for example, isinstance(message, ResultMessage)).

• TypeScript: check the type string field (for example, message.type === "result"). AssistantMessage and UserMessage wrap the raw API message in a .message field, so content blocks are at message.message.content, not message.content.

  from claude_agent_sdk import query, AssistantMessage, ResultMessage
  
  async for message in query(prompt="Summarize this project"):
      if isinstance(message, AssistantMessage):
          print(f"Turn completed: {len(message.content)} content blocks")
      if isinstance(message, ResultMessage):
          if message.subtype == "success":
              print(message.result)
          else:
              print(f"Stopped: {message.subtype}")
  

  
  for await (const message of query({ prompt: "Summarize this project" })) {
    if (message.type === "assistant") {
      console.log(`Turn completed: ${message.message.content.length} content blocks`);
    }
    if (message.type === "result") {
      if (message.subtype === "success") {
        console.log(message.result);
      } else {
        console.log(`Stopped: ${message.subtype}`);
      }
    }
  }
  

Tool execution

Tools give your agent the ability to take action. Without tools, Claude can only respond with text. With tools, Claude can read files, run commands, search code, and interact with external services.

Built-in tools

The SDK includes the same tools that power Claude Code:

Beyond built-in tools, you can:

• Connect external services with MCP servers (databases, browsers, APIs)
• Define custom tools with custom tool handlers
• Load project skills via setting sources for reusable workflows

Tool permissions

Claude determines which tools to call based on the task, but you control whether those calls are allowed to execute. You can auto-approve specific tools, block others entirely, or require approval for everything. Three options work together to determine what runs:

• allowed_tools / allowedTools auto-approves listed tools. A read-only agent with ["Read", "Glob", "Grep"] in its allowed tools list runs those tools without prompting. Tools not listed are still available but require permission.
• disallowed_tools / disallowedTools blocks listed tools, regardless of other settings. See Permissions for the order that rules are checked before a tool runs.
• permission_mode / permissionMode controls what happens to tools that aren't covered by allow or deny rules. See Permission mode for available modes.

You can also scope individual tools with rules like "Bash(npm *)" to allow only specific commands. See Permissions for the full rule syntax.

When a tool is denied, Claude receives a rejection message as the tool result and typically attempts a different approach or reports that it couldn't proceed.

Parallel tool execution

When Claude requests multiple tool calls in a single turn, both SDKs can run them concurrently or sequentially depending on the tool. Read-only tools (like Read, Glob, Grep, and MCP tools marked as read-only) can run concurrently. Tools that modify state (like Edit, Write, and Bash) run sequentially to avoid conflicts.

Custom tools default to sequential execution. To enable parallel execution for a custom tool, mark it as read-only in its annotations: readOnly in TypeScript or readOnlyHint in Python.

Control how the loop runs

You can limit how many turns the loop takes, how much it costs, how deeply Claude reasons, and whether tools require approval before running. All of these are fields on ClaudeAgentOptions (Python) / Options (TypeScript).

Turns and budget

When either limit is hit, the SDK returns a ResultMessage with a corresponding error subtype (error_max_turns or error_max_budget_usd). See Handle the result for how to check these subtypes and ClaudeAgentOptions / Options for syntax.

Effort level

The effort option controls how much reasoning Claude applies. Lower effort levels use fewer tokens per turn and reduce cost. Not all models support the effort parameter. See Effort for which models support it.

If you don't set effort, the Python SDK leaves the parameter unset and defers to the model's default behavior. The TypeScript SDK defaults to "high".

effort trades late