Operator, 2026-09-10: started radio from a song and "literally all of the
songs in the playlist after that were from a single artist which was not
expected."
There was no per-artist cap anywhere in the radio path. radio.go built the
pool and handed it straight to Shuffle, which scores, sorts and takes the
top N — nothing between those steps bounded any artist's share, so a pool
dominated by one artist produced an output dominated by it. The asymmetry
was the tell: discover.go, you_might_like.go and home.go all cap; radio
never got one.
With the fixture that reproduces it — 20 liked tracks by one artist plus 10
by ten others — the old path returns 10 tracks from 1 artist. It now returns
10 from 8.
TWO PASSES, and that is the whole design. A hard cap was the easy mistake:
radio asks for 50 tracks by default and 200 at most, so capping at three per
artist over a concentrated pool would hand back a six-track "radio". Pass
one takes candidates that fit under the caps; pass two fills any remaining
slots from those it skipped, still in score order. The result always holds
min(limit, len(candidates)) — the caps change WHICH tracks are picked, never
HOW MANY. Rule 131's principle past the system mixes it was written for.
The caps SCALE with the requested length rather than being a constant.
Three-per-artist is a sensible 12% of a 25-track mix and an absurd 1.5% of a
200-track radio, where every selection would sit in the relaxation path and
the cap would be decorative. RadioDiversityCaps holds the system mixes'
proportion at any length: 3/2 at 25, 6/4 at 50, 24/16 at 200, with floors so
a very short radio is not capped down to one track per artist.
A BOUND, NOT AN EXCLUSION — the operator asked for the opposite of removal:
"again it should be able to add songs from the same artist." The dominant
artist still appears, just not exclusively. Guarded, because the tempting
wrong fix is the filter songs-like used to carry.
Shuffle grew the parameter rather than gaining a capped twin: radio is its
only production caller, so a second function would have left the original
dead (rule 22).
Falsified against each named regression: uncapped gives 10/10 to one artist;
a hard cap returns 3 of 10 on a single-artist pool; a cap-as-exclusion drops
the artist entirely; a fixed cap stays 3 where the scaled one reaches 24.
Caught while writing the guards: the artist-key constant was hand-written
hex and wrong — the fixture's artist UUID carries 0001 in its fourth group,
so the lookup missed and the assertion measured nothing. Derived from the
same construction the fixture uses now.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH
Composes Score over a candidate slice, sorts descending, truncates.
Pure — no IO. Liked-rank-higher and high-skip-ranks-last behaviors
are stable across RNG seeds (jitter band is smaller than LikeBoost
and SkipPenalty).
bvandeusenandClaude Opus 5