`cosine`'s docstring claimed its guard was load-bearing because "a NaN reaching the
ranking sort key would corrupt ordering silently rather than failing loudly" — but the
guard tested `norm == 0.0` only, which a NaN or inf norm passes straight through. The
claim was prose, not behaviour.
Measured, not assumed: `cosine(unit, nan_vector)` AND `cosine(unit, inf_vector)` both
returned `nan`, and a NaN sort key made ranking INPUT-ORDER-DEPENDENT — six permutations
of the same three candidates produced four distinct orderings. That defeats the total
order `HybridRanker` documents ("`id` makes the result independent of input order").
Refuse rather than coerce, and deliberately NOT symmetric with the zero-norm branch: a
zero vector is a legitimate handled state (`FakeEmbedder` returns `np.zeros` by design),
whereas a non-finite component only ever means the INJECTED embedder is broken. Scoring
it `0.0` would launder that into "no semantic similarity" while ranking proceeded on a
forged signal — validation, never repair, mirroring `read_spend`.
Reachable via the documented `Embedder` extension point, not the shipped fake; scoped to
the norms (90% principle — a finite-normed dot-product overflow is not chased).
Also corrects `docs/extending.md`, which stated `SEMANTIC_WEIGHT_DEFAULT = 0.5` while the
code has said `0.25` since the weight was lowered.
625 -> 630 tests. Load-bearing MEASURED against the WHOLE suite, five mutations all red:
detach the guard entirely · coerce to 0.0 instead of raising · check only the first norm ·
drop "non-finite" from the message · (control) detach the zero-norm branch, which fails
ONLY the zero-norm test — the new guard does not mask the existing one.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018V9vNBmxAmgJ2JMoHByiHS
