A mixed document inbox contains invoices, receipts, and service contracts. You defined three extraction tools (extract_invoice, extract_receipt, extract_contract) and call the API with tool_choice set to "auto". About 4% of responses contain only prose, such as "This appears to be a service contract with…", and no tool call, so they break ingestion. What is the most effective fix?

Answer: A — Set tool_choice to "any" so Claude must call one of the extraction tools while still choosing which tool fits the document type.

With "auto", the model is allowed to answer in text. "any" guarantees a tool call and still lets the model choose which tool fits, which is the case section 6 describes for multiple schemas and an unknown document type. (b) guarantees a tool call but forces the invoice schema onto receipts and contracts. (c) parses natural language to drive control flow and adds a second request. (d) is a prompt-only fix for something the API can enforce.