Parallel Executions & Exceptions — Advanced Tool Use

High-Throughput Action Loops

Claude Series 5 models (Fable 5.1, Opus 5.5, Sonnet 5.5, and Haiku 5.5) natively support Parallel Tool Use, allowing the model to trigger multiple tools (e.g., searching 3 distinct databases) in a single turn. If your backend cannot handle concurrency, set disable_parallel_tool_use: true inside the tool_choice object (e.g., tool_choice: { type: "auto", disable_parallel_tool_use: true }) to force sequential execution.

Handling Terminal Failures

When a tool call fails in your code (e.g., a database timeout), you must feed that error back to Claude using the is_error: true property in the tool_result object. This prevents Claude from hallucinating fake data and instead triggers a 'recovery' reasoning path where it might try a different tool or notify the user.

Tool Definition Structure

{
  "tools": [
    {
      "name": "lookup_customer",
      "description": "Look up customer account details by ID",
      "input_schema": {
        "type": "object",
        "properties": {
          "customer_id": { "type": "string", "description": "Unique customer identifier" }
        },
        "required": ["customer_id"]
      }
    }
  ]
}