bvandeusenandClaude Opus 5
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Fixes the integration failure from f367eeaa:
"same-day rebuild produced different track lists".
Cutting RandomFill 30→10 for Songs-like broke
TestBuildSystemPlaylists_DailyNonceDeterminism, and the reason is worth
stating because the number is not the bug.
`likes_overlap` and `random_fill` end in a bare `ORDER BY random()` with no
daily seed (recommendation.sql:118, :161). Such an arm returns a STABLE set
only while its LIMIT exceeds the rows eligible for it — then it returns all
of them, and the random order stops mattering because scoreAndSortCandidates
sorts by track id before drawing jitter. Below that threshold the arm
returns a random SUBSET, and two builds on the same day draw different ones.
So the test was green by accident. It seeds ~20 tracks against a default
RandomFill of 30; the limit exceeded the library, so the arm returned
everything. Determinism held for a reason unrelated to the code being right.
Which means it does NOT hold in production. Any real library is larger than
30, so same-day rebuilds have been drawing different mixes since that arm
was written — invisible, because a mix changing after a refresh looks like a
feature. Filed as #3889; the fix is a seeded ordering per (user, day), which
needs a .sql change and sqlc regeneration and so cannot land from here.
The correction: grow an arm freely, never shrink one whose ordering is
unseeded random. LikesOverlap and RandomFill go back to the defaults;
TasteOverlap stays halved because it sorts by `tpa.weight DESC, t.id` and is
genuinely deterministic. Guarded by a test that names the reasoning, so the
next person to trim these has to read why first — and it should be DELETED
once #3889 lands rather than worked around.
The cost is honest: the seed-independent share of the Songs-like pool falls
from 29% to 20% instead of the intended cut. That matters less than it
sounds. The pool only biases the draw; the songs_like WEIGHTS are what
actually demote sim_score-0 candidates, and they are untouched here — a
perfect match still scores 5.00 against an unrelated favourite's 2.00.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH