Tool Use & Function Calling

How do you handle tool call errors gracefully?

Last verified: July 2026 · 2 min read
QUICK ANSWER

To report a tool execution failure to Claude, return a user message block with the parameter 'is_error' set to true. This tells Claude the tool failed, prompting the model to fix its input arguments.

Error Response Block

{
  "type": "tool_result",
  "tool_use_id": "toolu_12345",
  "content": "Database error: connection timed out.",
  "is_error": true
}
Verified against: Anthropic API Tool Use v1

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