AI Guides › Skills & Agents
The Difference Between Testing A Skill Once And Trusting It Forever
By Nigel Guy · 3 min read
A skill that passed its initial tests earns real trust — that part's not
in question. What's less examined is how far that trust is meant to
stretch afterwards. In practice it tends to stretch indefinitely: every
clean run after the original test quietly adds to a sense that the skill is
reliable, until "it passed testing once" and "I've trusted it for a year
without incident" start to feel like the same claim. They aren't. The
second one is describing a streak, and streaks describe luck at least as
much as they describe reliability.
The rule: a skill has earned exactly the trust its testing actually
covered — not open-ended trust that keeps compounding every time it runs
cleanly afterwards — so treat any real use outside what was tested as
untested, however long the streak since has run.
The mechanism
- Record what the original test actually covered — which inputs,
roughly what scale, over what timeframe. This is worth writing down at
the time, because six months later you'll remember that you "tested it
thoroughly" without remembering the specifics of what that meant.
- Treat any use outside that recorded coverage as untested, not as
probably fine. If you tested against modest volumes and it's now
running at ten times that, or tested for a narrow case that's since
broadened, the streak of success at the old scale says nothing reliable
about the new one.
- Set a default decay assumption rather than assuming trust holds
indefinitely. Confidence in an untouched skill should reduce by
default after a fixed time or after a fixed number of changes to
anything it depends on — not because the skill necessarily got worse,
but because your evidence about it is older and covers less of its
current reality.
- Name the actual re-test triggers, rather than leaving "re-test
sometime" as a vague intention: a dependency change, a meaningful jump
in scale, or a jump in stakes — the same skill now being used for
something that matters more than what it was originally tested against.
- Don't let an unbroken streak substitute for a re-test when one of
those triggers fires. The streak is real evidence, but it's evidence
about the past under the old conditions, not evidence that the new
conditions are covered.
What to skip
Skip re-testing on every single run — that's diminishing returns for
anything that's genuinely stable, and the whole point of testing is to
avoid needing to re-verify every instance individually. And skip treating
an old test as covering a new use case just because the skill's own file
hasn't changed; the skill can be identical while everything around it —
scale, stakes, dependencies — has moved.
Guardrails
- This complements, rather than replaces, the discipline of testing a
skill thoroughly before first use — that earlier guide covers earning
trust initially; this one covers what happens to that trust afterwards,
and both matter.
- A long streak with no incidents is genuinely useful information; the
point here isn't to discount it, only to stop treating it as
interchangeable with a deliberate re-test against current conditions.
- How aggressively to decay trust by default is a judgement call that
should scale with stakes — a skill whose mistakes are cheap to spot and
fix can run on a longer leash than one whose mistakes are expensive or
hard to notice.
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