Strict Tool Use

Definition

Setting strict: true on a custom tool definition uses constrained decoding to guarantee that Claude's tool call inputs exactly match the tool's input_schema.

Key points

• strict: true is a field on the tool definition (sibling of name/description/input_schema), NOT on tool_choice.
• Combine tool_choice: {"type": "any"} + strict: true to guarantee both that a tool is called AND that its inputs are schema-valid.
• Constrained-decoding mechanism is the same family as Structured Outputs — the two can be combined.

Why it matters for the exam

• A classic distractor puts strict under tool_choice; recognising the correct placement is the tested skill.
• Pairing with any is the canonical "guaranteed valid tool call" answer.

Common gotchas

• Placing strict on tool_choice is wrong — it belongs on the tool.
• strict guarantees schema conformance, not that a tool is called; you still need any/tool to force a call — see Tool Choice.

See also

• Tool Definition Schema
• Tool Choice
• Structured Outputs

Sources

• docs-tools-agents
• netlight-study-guide