A pre-tool hook blocks a tool call by returning an isError result. Wha — Claude Certified Architect – Foundations Practice — Claude Cert Academy

A pre-tool hook blocks a tool call by returning an isError result. What does the model receive, and how does it respond?

• The model receives nothing — blocked calls are silently dropped from the conversation
• The model receives the isError result as a tool_result block and decides how to recover, just as it would for any other tool failure
• The model raises an exception that must be caught by the orchestrator
• The model retries the tool call automatically until the hook approves it

Answer: The model receives the isError result as a tool_result block and decides how to recover, just as it would for any other tool failure

A pre-tool hook that returns isError inserts the error as a tool_result block in the conversation. From the model's perspective, the tool was called and failed — it has no way to distinguish a hook-blocked call from a tool that ran and returned an error. The model applies its normal recovery logic: retry, use a different tool, or ask for clarification. This is intentional — it means hook blocks are handled gracefully without special-case orchestrator code.

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