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Structured Output for Tool Args

Tool arguments must be 100% valid JSON — schema enforcement + retry.

2 hours2 resources1 prereqs

Agent tool calls = JSON. Broken JSON or schema mismatch → agent crashes. Solutions:

  1. Native function calling: OpenAI/Anthropic/Gemini built-in. Validates schema, retries.
  2. Constraint decoding (open-source): Outlines, guidance — token-level enforcement.
  3. Pydantic + retry: on parse fail, re-prompt with the error. PydanticAI does this built-in.
  4. Few-shot examples: for complex schemas, add 1-2 input/output samples.

Best practice: in tool descriptions, explicitly mark required fields and state defaults for optional ones.

Prerequisites

Agent System Prompts
Agent system prompt = identity + tool-use rules + loop discipline + format. 5-section scaffold.
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Structured Output for Tool Args · AI Agent Engineer Roadmap | SYK