# Tessary tool error

tool_error is one of Tessary's built-in classifiers. It watches the rate at which each tool's calls
fail rather than judging individual failing calls. A call counts as failed on structural evidence
only: an error status on the span, a recorded exception, or a result that itself declares an error.
Output text that merely mentions the word "error" never counts.

It catches a tool going bad while the agent keeps running: an API that started refusing, a
dependency that broke, a rate limit that began to bite. Each tool's alert threshold comes from its
own baseline failure rate and an explicit false-alarm budget, and evidence accumulates across calls,
so a tool failing at its normal rate stays quiet and only a sustained shift fires. A detection is
one finding per tool: when the shift began, which failure patterns moved the rate, and which call
sites the traffic came through. If triage confirms a real deviation, the finding opens a case.

## Questions answered under this concept

- [Does tool_error count a tool's output as failed just because it mentions the word error?](https://tessary.ai/answers/tessary-tool-error/does-mentioning-error-in-output-trigger-tool-error)
- [Does tool_error flag a single failed tool call?](https://tessary.ai/answers/tessary-tool-error/does-tool-error-flag-a-single-failed-call)
- [How do I catch a tool that starts failing more often than it used to?](https://tessary.ai/answers/tessary-tool-error/how-do-i-catch-a-tool-failing-more-than-it-used-to)
- [Is tool_error turned on by default?](https://tessary.ai/answers/tessary-tool-error/is-tool-error-turned-on-by-default)
- [What happens when tool_error flags a tool?](https://tessary.ai/answers/tessary-tool-error/what-happens-when-tool-error-flags-a-tool)
- [What is Tessary's tool_error classifier?](https://tessary.ai/answers/tessary-tool-error/what-is-tessarys-tool-error-classifier)

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Source: https://tessary.ai/answers/tessary-tool-error
All concepts: https://tessary.ai/answers
From Tessary, agent reliability for AI agents in production: https://tessary.ai