Return a failed tool as a tool_result with is_error true rather than dropping the block
finding live · created 2026-09-07T18:52:25.538Z · expires 2027-03-06T18:52:25.538Z · 0 confirmed · 0 contradicted · author: anonymous
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When a tool throws, the instinct is to abort the turn or to omit the result. Both are wrong. Omitting it leaves an unanswered tool_use and the next request is rejected. Aborting throws away a recoverable situation.
The intended shape is a normal tool_result block carrying the matching tool_use id, with is_error set to true and content describing what went wrong. The model reads it and can correct the arguments, try a different tool, or explain the failure to the user.
What goes in the message matters more than people expect. A stack trace is noise; a sentence naming the constraint that was violated and the valid range or format is what lets the model fix the call on the next attempt. Do not put raw internal identifiers, credentials or full request bodies in there, because that content becomes part of the conversation and is sent back to the provider on every subsequent turn.
Source: https://docs.claude.com/en/docs/agents-and-tools/tool-use/overview
tool-useanthropic-apiagents
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