MessagesTools

Handle tool calls

Parse tool_use blocks, format tool_result responses, and handle errors with is_error.

This page covers the tool-call lifecycle: reading tool_use blocks from Claude's response, formatting tool_result blocks in your reply, and signaling errors. For the SDK abstraction that handles this automatically, see Tool Runner.

Simpler with Tool Runner: The manual tool handling described on this page is automatically managed by Tool Runner. Use this page when you need custom control over tool execution.

Claude's response differs based on whether it uses a client or server tool.

Handling results from client tools

The response will have a stop_reason of tool_use and one or more tool_use content blocks that include:

id: A unique identifier for this particular tool use block. This will be used to match up the tool results later.
name: The name of the tool being used.
input: An object containing the input being passed to the tool, conforming to the tool's input_schema.

When you receive a tool use response for a client tool, you should:

Extract the name, id, and input from the tool_use block.
Run the actual tool in your codebase corresponding to that tool name, passing in the tool input.
Continue the conversation by sending a new message with the role of user, and a content block containing the tool_result type and the following information:
- tool_use_id: The id of the tool use request this is a result for.
- content (optional): The result of the tool, as a string (for example, "content": "15 degrees"), a list of nested content blocks (for example, "content": [{"type": "text", "text": "15 degrees"}]), or a list of document blocks (for example, "content": [{"type": "document", "source": {"type": "text", "media_type": "text/plain", "data": "15 degrees"}}]). These content blocks can use the text, image, document, or search_result types.
- is_error (optional): Set to true if the tool execution resulted in an error.

Important formatting requirements:

Tool result blocks must immediately follow their corresponding tool use blocks in the message history. You cannot include any messages between the assistant's tool use message and the user's tool result message.
In the user message containing tool results, the tool_result blocks must come FIRST in the content array. Any text must come AFTER all tool results.
If the assistant turn also called a server tool that has no result block yet, the user message must contain only tool_result blocks. Text after the results ends the turn early; for a server tool Claude called directly, the request then fails with a 400 error that names the unresolved server tool. See Stop reasons and fallback.

For example, this will cause a 400 error:

{
  "role": "user",
  "content": [
    { "type": "text", "text": "Here are the results:" }, // ❌ Text before tool_result
    { "type": "tool_result", "tool_use_id": "toolu_01" /* ... */ }
  ]
}

This is correct when the assistant turn calls only client tools:

{
  "role": "user",
  "content": [
    { "type": "tool_result", "tool_use_id": "toolu_01" /* ... */ },
    { "type": "text", "text": "What should I do next?" } // ✅ Text after tool_result
  ]
}

If you receive an error like "tool_use ids were found without tool_result blocks immediately after", check that your tool results are formatted correctly.

Tool results often carry content from sources outside your control: web pages, inbound email, user uploads, third-party APIs. Treat that content as untrusted: an attacker who can influence it may embed instructions that try to redirect Claude (indirect prompt injection). Keep untrusted content inside tool_result blocks rather than system prompts or plain user text blocks, and see Mitigate jailbreaks and prompt injections for further hardening.

After receiving the tool result, Claude will use that information to continue generating a response to the original user prompt.

Handling results from server tools

Claude executes the tool internally and incorporates the results directly into its response without requiring additional user interaction.

A response can contain both a client tool_use block and a server_tool_use block that has no result block. That server tool call is not finished yet, and its result block arrives in a later response. Reply with a user message that contains only the tool_result blocks for the client tools and keep the same tools array; for a server tool Claude called directly, the API runs it on that request and the next response starts with its result block. See Stop reasons and fallback.

Differences from other APIs

Unlike APIs that separate tool use or use special roles like tool or function, the Claude API integrates tools directly into the user and assistant message structure.

Messages contain arrays of text, image, tool_use, and tool_result blocks. user messages include client content and tool_result, while assistant messages contain AI-generated content and tool_use.

Handling errors with is_error

There are a few different types of errors that can occur when using tools with Claude:

Next steps

Parallel tool use

Handle responses where Claude calls several tools in a single turn.

Tool Runner (SDK)

Let the SDK manage the tool_use loop, result formatting, and retries for you.

Define tools

Write schemas and descriptions that steer Claude toward the right tool.

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Handling results from client tools

The response will have a stop_reason of tool_use and one or more tool_use content blocks that include:

id: A unique identifier for this particular tool use block. This will be used to match up the tool results later.

name: The name of the tool being used.

input: An object containing the input being passed to the tool, conforming to the tool's input_schema.

When you receive a tool use response for a client tool, you should:

Extract the name, id, and input from the tool_use block.

Run the actual tool in your codebase corresponding to that tool name, passing in the tool input.

Continue the conversation by sending a new message with the role of user, and a content block containing the tool_result type and the following information:

tool_use_id: The id of the tool use request this is a result for.
content (optional): The result of the tool, as a string (for example, "content": "15 degrees"), a list of nested content blocks (for example, "content": [{"type": "text", "text": "15 degrees"}]), or a list of document blocks (for example, "content": [{"type": "document", "source": {"type": "text", "media_type": "text/plain", "data": "15 degrees"}}]). These content blocks can use the text, image, document, or search_result types.
is_error (optional): Set to true if the tool execution resulted in an error.

Important formatting requirements:

Tool result blocks must immediately follow their corresponding tool use blocks in the message history. You cannot include any messages between the assistant's tool use message and the user's tool result message.
In the user message containing tool results, the tool_result blocks must come FIRST in the content array. Any text must come AFTER all tool results.
If the assistant turn also called a server tool that has no result block yet, the user message must contain only tool_result blocks. Text after the results ends the turn early; for a server tool Claude called directly, the request then fails with a 400 error that names the unresolved server tool. See Stop reasons and fallback.

For example, this will cause a 400 error:

{
  "role": "user",
  "content": [
    { "type": "text", "text": "Here are the results:" }, // ❌ Text before tool_result
    { "type": "tool_result", "tool_use_id": "toolu_01" /* ... */ }
  ]
}

This is correct when the assistant turn calls only client tools:

{
  "role": "user",
  "content": [
    { "type": "tool_result", "tool_use_id": "toolu_01" /* ... */ },
    { "type": "text", "text": "What should I do next?" } // ✅ Text after tool_result
  ]
}

If you receive an error like "tool_use ids were found without tool_result blocks immediately after", check that your tool results are formatted correctly.

After receiving the tool result, Claude will use that information to continue generating a response to the original user prompt.

Handling results from server tools

Claude executes the tool internally and incorporates the results directly into its response without requiring additional user interaction.

Differences from other APIs

Unlike APIs that separate tool use or use special roles like tool or function, the Claude API integrates tools directly into the user and assistant message structure.

Handling results from client tools

Example API response with a `tool_use` content block

Example of successful tool result

Example of tool result with images

Example of empty tool result

Example of tool result with documents

Handling results from server tools

Handling errors with is_error

Tool execution error

Invalid tool name

Server tool errors

Next steps

Handling results from client tools

Example API response with a `tool_use` content block

Example of successful tool result

Example of tool result with images

Example of empty tool result

Example of tool result with documents

Handling results from server tools

Handling errors with is_error

Tool execution error

Invalid tool name

Server tool errors

Next steps

Handling results from client tools

Handling results from server tools

Handling errors with is_error

Next steps

Handling results from client tools

Handling results from server tools

Handling errors with is_error

Next steps