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214 calls to find_customer returned empty with no error — 24% of 892 calls. The agent was told these succeeded.

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When it fires

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What it means

The tool ran, the protocol reported success, and the model got an empty array, an empty object or an empty string. Nothing anywhere reported a problem. See Empty results for exactly what counts. This is the quietest failure a server has. It never appears in an error rate, it never shows up in logs, and the only symptom the author sees is that people stop using the server.

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The three causes

An id that does not exist is not “no results” — it is a mistake the model can correct. Returning [] tells it nothing. Returning isError: true with “no customer with id 4821” tells it exactly what happened, and it will search instead of retrying blindly.This is the most common cause and the highest-value fix.
A filter that silently ANDs everything when the model expected OR. A date argument expecting YYYY-MM-DD receiving a natural-language date and matching nothing. A case-sensitive match on a name.Look at the outcome breakdown: if bad_args is near zero and empty is high, the schema is accepting things it should be rejecting.
“Search customers” that only searches by exact email will return nothing most of the time, and the model has no way to know why. Narrow the description or widen the tool.

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Why 5%

Below 5%, an empty result is ordinary. Searches genuinely miss. Above it, the pattern is usually one of the three above rather than the shape of the data — and the rule’s 20-call floor keeps a couple of unlucky calls from firing it.

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It drags first-call success with it

An empty call can never be a first-call success, by definition. A tool showing both insights has one problem, not two, and fixing the empties fixes both.