The heuristic scorer has no way to say "I don't know". Its seven dimensions are a
roughly 100-word software-vocabulary whitelist, so when a prompt matches none of them
every dimension contributes 0, the weighted sum is 0.0, and 0.0 sits below
simple_medium (0.15). Absence of evidence was being scored as evidence of simplicity,
and unclassifiable traffic went to the cheapest tier. On a graded 809-question
benchmark that no-signal mass is 50.2% of prompts; on a 257-session agent transcript
corpus it is 36.6% of turns
_score_and_classify now returns config.default_tier before the band mapping when
nothing was recognised, under its own cause=no_signal_default with
signals=['no-signal'], so a spend log row says the score did not choose the tier, the
way reasoning_override already does. default_tier is new on ComplexityRouterConfig and
defaults to MEDIUM; setting it to SIMPLE restores the previous behavior exactly. The
LLM classifier falls back to this scorer on timeout or error, so the same setting
decides where unmatched traffic lands during a classifier outage
The branch tests the signals rather than the score, and rather than the individual
dimension scores. A weighted score of zero does not mean nothing was recognised, since
contributions cancel: "hi, quick python question" comes to zero with three dimensions
firing and is real evidence of a simple request. A dimension scoring zero does not mean
it stayed silent either, because _score_keyword_match takes the no-match score as a
parameter, so a future dimension with a nonzero baseline would silently kill the
branch. A signal is the one thing a dimension emits only when it recognised something,
and a test pins that invariant across every scorer
An explicitly configured default_tier with no model behind it is rejected at load
rather than surfacing as a routing error on the first unmatched request. The check is
skipped when the field is left implicit, so a partial tiers map keeps loading as it
does today instead of turning an upgrade into a startup failure
The router e2e config pins default_tier: SIMPLE. That test tells a live LLM classifier
apart from a silent fallback by which backend served the request, and its prompt has no
scoring signal, so leaving the new default in place would have made both arms land on
the same backend and turned the test into a false green
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