You are building a structured data extraction system using Claude.

A tool definition converts the desired extraction structure into an explicit machine-readable contract. Claude returns the event information inside a tool_use block, with the tool arguments corresponding to the properties defined by the tool’s input_schema . Anthropic specifies that custom tool parameters are described using JSON Schema, allowing the application to extract the structured arguments directly rather than attempting to recover JSON from ordinary prose. For current implementations, adding strict: true to the tool definition provides guaranteed conformance of tool-call inputs to the declared schema. ( https://docs.anthropic.com/en/docs/agents-and-tools/tool-use/implement-tool-use )

Options A, B, and D remain prompt-based formatting techniques. They may improve the probability of valid JSON, but none creates the same schema-enforced interface. Prefilling an opening brace constrains the beginning of the response without guaranteeing valid field names, required properties, or data types. Retry logic detects failures only after generation and adds latency. Detailed formatting instructions can still produce malformed or structurally incorrect output.

Anthropic now also provides Structured Outputs for direct, schema-validated JSON responses. Within the options presented, however, a schema-defined tool is the only approach that establishes an explicit structured-output boundary rather than relying primarily on text-generation compliance. ( https://docs.anthropic.com/en/docs/test-and-evaluate/strengthen-guardrails/increase-consistency )

Official references/topics: Tool Definitions, JSON Schema Input Contracts, Strict Tool Use, Structured Outputs.