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.