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Everything else on the overview is a number. This one is the product. First-call success is the share of calls that were right the first time. Formally, a call counts when all three hold:
  1. outcome is ok
  2. is_empty is false
  3. no retry followed within 30 seconds
It answers the question an MCP author cannot otherwise ask: does the model understand my tools? A tool that works perfectly but is described badly has a high ok rate and a low first-call rate, and only the second one tells you there is a problem.

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How a retry is detected

Within one session, find the same tool_name called twice within 30 seconds. Then the args_hash decides: That distinction is the whole reason args_hash exists. Without it, a paginating client would look like a model in distress.
The hash is twelve characters of a SHA-256 over the arguments with keys sorted. It cannot be reversed, and the sorting is what stops the same arguments in a different order from looking like a different request. See What is sent.

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Sessions have to be real for this to work

Retries are detected within one session. If every request opens a new session, there is never a second call to compare against and the rate reads 100% forever. On streamable HTTP this is the mistake to avoid: the SDK shares one session and one buffer per process precisely so that calls stay grouped, even though a fresh McpServer is built per request.

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It lags by one day

Retries and first-call success are computed by the nightly pass at 02:00 UTC, which walks each session’s calls in order and applies the rule above. Today’s rows are always zero until it runs. So the metric is reported strictly through yesterday and labelled as of yesterday. Including today in the denominator would drag the number down every morning and recover it every night, and a figure that moves on its own is worse than one that is a day old. Two consequences:
  • The denominator is calls on days the pass has covered, not all calls in range. A range that is entirely today has no rate at all.
  • A range with calls but no covered days returns null, which renders as not yet computed rather than 0%. A zero there would claim every call failed.

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Reading the number

The retries figure sits directly under the card because it is the metric that moves this one. Per tool, the same rate appears in the tool health table and drives its status badge.

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What to do when it is low

See Improve first-call success. In short: read the tool’s description as though you had never seen the API, name the arguments after what they mean rather than what they are, and check whether the tool is returning empty results that the protocol is calling success.