feat(taste): household co-play similarity — #1533

Milestone #160 Opt 5. A collaborative candidate arm: tracks by artists
co-played across the instance with the seed's artist.

Minstrel is a single shared-library, multi-user server (no per-user
library ACL — verified: no owner/share/group model), so the "household"
is the whole instance's user set; the rule #47 scoping is satisfied by
the shared-library boundary. Single-user servers produce no edges.

- No migration: source='user_cooccurrence' was pre-whitelisted in the
  0009 similarity CHECK from day one.
- internal/db/queries/coplay.sql: Delete + Insert artist co-play edges.
  Score = Jaccard of the two artists' distinct-player sets (controls for
  globally-popular artists); >= 2 co-players AND Jaccard >= floor kept
  (the floor also self-limits hub artists). Completed plays, 365d window.
- internal/coplay: periodic worker (6h) that atomic-replaces the
  user_cooccurrence edge set from play_events — pure local SQL, no
  external calls. Wired in main.go alongside the similarity worker.
- LoadRadioCandidatesV2: new coplay_artists arm (source='user_cooccurrence',
  seed-artist based, 0.5 damp like similar_artists) + $11 limit;
  CandidateSourceLimits.UserCoplay (default 20, For-You 40).
- Integration tests: perfect-overlap Jaccard=1.0 edge + single-user
  empty-set gate.

Device axis and AcousticBrainz (Opt 4) are separately tracked; this
closes the milestone-#160 sequential options.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/coplay/worker.go

@@ -0,0 +1,86 @@
+// Package coplay builds household co-play similarity edges (#1533, milestone
+// #160 Opt 5). Minstrel is a single shared-library, multi-user server (no
+// per-user library ACL), so the "household" is the whole instance's user set.
+// A periodic worker recomputes artist–artist co-occurrence from play_events
+// entirely in SQL (no external API) and stores the edges in artist_similarity
+// under source='user_cooccurrence' — the pre-provisioned co-occurrence source
+// from the 0009 schema — which the radio/mix candidate pool consumes as a
+// collaborative arm. Single-user servers produce no edges (the >= 2 co-player
+// gate), so the feature is a graceful no-op there.
+package coplay
+
+import (
+	"context"
+	"log/slog"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// minJaccard is the co-play edge floor: prune weak pairs and self-limit hub
+// artists, whose large combined-audience denominators pull their Jaccard down.
+// 0.1 keeps only pairs whose co-player overlap is a real fraction of their
+// combined audience.
+const minJaccard = 0.1
+
+// Worker periodically rebuilds the household co-play edge set. Pure local SQL,
+// so there are no rate limits — but the pair self-join is roughly
+// O(users × artists_per_user²), so it ticks slowly: co-listening shifts over
+// days, not minutes, and a frequent rebuild would burn CPU for no fresher
+// signal.
+type Worker struct {
+	pool   *pgxpool.Pool
+	logger *slog.Logger
+	tick   time.Duration
+}
+
+// NewWorker constructs a worker with the production default 6h tick.
+func NewWorker(pool *pgxpool.Pool, logger *slog.Logger) *Worker {
+	return &Worker{pool: pool, logger: logger, tick: 6 * time.Hour}
+}
+
+// Run rebuilds once at startup (so edges exist without waiting a full tick),
+// then every w.tick until ctx is cancelled. Runs in its own goroutine, so a
+// slow initial rebuild never blocks boot.
+func (w *Worker) Run(ctx context.Context) {
+	if err := w.rebuild(ctx); err != nil {
+		w.logger.Error("coplay: initial rebuild failed", "err", err)
+	}
+	t := time.NewTicker(w.tick)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			if err := w.rebuild(ctx); err != nil {
+				w.logger.Error("coplay: rebuild failed", "err", err)
+			}
+		}
+	}
+}
+
+// rebuild atomic-replaces the user_cooccurrence edge set inside one
+// transaction — delete the old edges, recompute from current play history — so
+// the candidate pool never observes a half-built set.
+func (w *Worker) rebuild(ctx context.Context) error {
+	tx, err := w.pool.Begin(ctx)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = tx.Rollback(ctx) }()
+	q := dbq.New(tx)
+	if err := q.DeleteArtistCoplayEdges(ctx); err != nil {
+		return err
+	}
+	if err := q.InsertArtistCoplayEdges(ctx, minJaccard); err != nil {
+		return err
+	}
+	if err := tx.Commit(ctx); err != nil {
+		return err
+	}
+	w.logger.Debug("coplay: edges rebuilt")
+	return nil
+}

internal/coplay/worker_integration_test.go

@@ -0,0 +1,162 @@
+package coplay
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/dbtest"
+)
+
+func testPool(t *testing.T) *pgxpool.Pool {
+	t.Helper()
+	if testing.Short() {
+		t.Skip("skipping in -short mode")
+	}
+	dsn := os.Getenv("MINSTREL_TEST_DATABASE_URL")
+	if dsn == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	if err := db.Migrate(dsn, slog.New(slog.NewTextHandler(io.Discard, nil))); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	pool, err := pgxpool.New(context.Background(), dsn)
+	if err != nil {
+		t.Fatalf("pool: %v", err)
+	}
+	t.Cleanup(pool.Close)
+	dbtest.ResetDB(t, pool)
+	return pool
+}
+
+func discardLogger() *slog.Logger { return slog.New(slog.NewTextHandler(io.Discard, nil)) }
+
+func seedUser(t *testing.T, q *dbq.Queries, name string) pgtype.UUID {
+	t.Helper()
+	u, err := q.CreateUser(context.Background(), dbq.CreateUserParams{
+		Username: dbtest.TestUserPrefix + name, PasswordHash: "x", ApiToken: name + "-tok", IsAdmin: false,
+	})
+	if err != nil {
+		t.Fatalf("seed user %s: %v", name, err)
+	}
+	return u.ID
+}
+
+// seedArtistTrack creates an artist + album + one track, returning both ids.
+func seedArtistTrack(t *testing.T, q *dbq.Queries, name string) (pgtype.UUID, pgtype.UUID) {
+	t.Helper()
+	ctx := context.Background()
+	a, err := q.UpsertArtist(ctx, dbq.UpsertArtistParams{Name: name, SortName: name})
+	if err != nil {
+		t.Fatalf("seed artist: %v", err)
+	}
+	al, err := q.UpsertAlbum(ctx, dbq.UpsertAlbumParams{Title: name, SortTitle: name, ArtistID: a.ID})
+	if err != nil {
+		t.Fatalf("seed album: %v", err)
+	}
+	tr, err := q.UpsertTrack(ctx, dbq.UpsertTrackParams{
+		Title: name, AlbumID: al.ID, ArtistID: a.ID,
+		DurationMs: 1000, FilePath: "/tmp/coplay-" + name + ".mp3",
+		FileSize: 1, FileFormat: "mp3",
+	})
+	if err != nil {
+		t.Fatalf("seed track: %v", err)
+	}
+	return a.ID, tr.ID
+}
+
+func play(t *testing.T, pool *pgxpool.Pool, userID, trackID pgtype.UUID) {
+	t.Helper()
+	ctx := context.Background()
+	at := time.Now()
+	var sessionID pgtype.UUID
+	if err := pool.QueryRow(ctx,
+		`INSERT INTO play_sessions (user_id, started_at, last_event_at, client_id)
+		 VALUES ($1, $2, $2, 'coplay-test') RETURNING id`,
+		userID, at).Scan(&sessionID); err != nil {
+		t.Fatalf("insert play_session: %v", err)
+	}
+	if _, err := pool.Exec(ctx,
+		`INSERT INTO play_events (user_id, track_id, session_id, started_at, was_skipped)
+		 VALUES ($1, $2, $3, $4, false)`,
+		userID, trackID, sessionID, at); err != nil {
+		t.Fatalf("insert play_event: %v", err)
+	}
+}
+
+func coplayEdgeCount(t *testing.T, pool *pgxpool.Pool) int {
+	t.Helper()
+	var n int
+	if err := pool.QueryRow(context.Background(),
+		`SELECT count(*) FROM artist_similarity WHERE source='user_cooccurrence'`).Scan(&n); err != nil {
+		t.Fatalf("count edges: %v", err)
+	}
+	return n
+}
+
+// TestCoplayWorker_BuildsEdges: two users who both play two artists produce a
+// perfect-overlap co-play edge (Jaccard 1.0) in each direction.
+func TestCoplayWorker_BuildsEdges(t *testing.T) {
+	pool := testPool(t)
+	ctx := context.Background()
+	q := dbq.New(pool)
+
+	u1 := seedUser(t, q, "cop1")
+	u2 := seedUser(t, q, "cop2")
+	aA, tA := seedArtistTrack(t, q, "ArtA")
+	aB, tB := seedArtistTrack(t, q, "ArtB")
+
+	for _, u := range []pgtype.UUID{u1, u2} {
+		play(t, pool, u, tA)
+		play(t, pool, u, tB)
+	}
+
+	if err := NewWorker(pool, discardLogger()).rebuild(ctx); err != nil {
+		t.Fatalf("rebuild: %v", err)
+	}
+
+	var score float64
+	if err := pool.QueryRow(ctx,
+		`SELECT score FROM artist_similarity
+		 WHERE artist_a_id=$1 AND artist_b_id=$2 AND source='user_cooccurrence'`,
+		aA, aB).Scan(&score); err != nil {
+		t.Fatalf("expected co-play edge ArtA→ArtB: %v", err)
+	}
+	// Jaccard of perfectly-overlapping player sets (both users play both):
+	// coplayers=2, players each=2 → 2/(2+2-2) = 1.0.
+	if score != 1.0 {
+		t.Errorf("edge score = %v, want 1.0 (perfect overlap)", score)
+	}
+	if got := coplayEdgeCount(t, pool); got != 2 {
+		t.Errorf("edge count = %d, want 2 (A→B and B→A)", got)
+	}
+}
+
+// TestCoplayWorker_SingleUserNoEdges: one user's own co-listening yields no
+// edges — the >= 2 distinct-co-player gate keeps single-user servers empty.
+func TestCoplayWorker_SingleUserNoEdges(t *testing.T) {
+	pool := testPool(t)
+	ctx := context.Background()
+	q := dbq.New(pool)
+
+	u := seedUser(t, q, "solo")
+	_, tA := seedArtistTrack(t, q, "SoloA")
+	_, tB := seedArtistTrack(t, q, "SoloB")
+	play(t, pool, u, tA)
+	play(t, pool, u, tB)
+
+	if err := NewWorker(pool, discardLogger()).rebuild(ctx); err != nil {
+		t.Fatalf("rebuild: %v", err)
+	}
+	if got := coplayEdgeCount(t, pool); got != 0 {
+		t.Errorf("single-user edge count = %d, want 0 (>= 2 co-player gate)", got)
+	}
+}

internal/db/dbq/coplay.sql.go

@@ -0,0 +1,78 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+// source: coplay.sql
+
+package dbq
+
+import (
+	"context"
+)
+
+const deleteArtistCoplayEdges = `-- name: DeleteArtistCoplayEdges :exec
+
+DELETE FROM artist_similarity WHERE source = 'user_cooccurrence'
+`
+
+// Household co-play similarity (#1533, milestone #160 Opt 5). Minstrel is a
+// single shared-library, multi-user server (no per-user library ACL), so the
+// "household" is the whole instance's user set. These queries recompute
+// artist–artist co-occurrence edges from play_events and store them in
+// artist_similarity under the pre-provisioned source = 'user_cooccurrence'
+// (whitelisted in the 0009 CHECK from day one). A periodic worker DELETEs then
+// re-INSERTs so edges that fall below threshold disappear. Single-user servers
+// produce no edges (the >= 2 co-player gate), so the arm is empty there.
+func (q *Queries) DeleteArtistCoplayEdges(ctx context.Context) error {
+	_, err := q.db.Exec(ctx, deleteArtistCoplayEdges)
+	return err
+}
+
+const insertArtistCoplayEdges = `-- name: InsertArtistCoplayEdges :exec
+WITH user_artists AS (
+    SELECT DISTINCT pe.user_id, t.artist_id
+    FROM play_events pe
+    JOIN tracks t ON t.id = pe.track_id
+    WHERE pe.was_skipped = false
+      AND pe.started_at > now() - interval '365 days'
+),
+artist_players AS (
+    SELECT artist_id, count(*)::float8 AS players
+    FROM user_artists
+    GROUP BY artist_id
+),
+pairs AS (
+    SELECT ua.artist_id AS a_id,
+           ub.artist_id AS b_id,
+           count(*)::float8 AS coplayers
+    FROM user_artists ua
+    JOIN user_artists ub
+      ON ua.user_id = ub.user_id AND ua.artist_id <> ub.artist_id
+    GROUP BY ua.artist_id, ub.artist_id
+    HAVING count(*) >= 2
+),
+scored AS (
+    SELECT p.a_id, p.b_id,
+           p.coplayers / (pa.players + pb.players - p.coplayers) AS score
+    FROM pairs p
+    JOIN artist_players pa ON pa.artist_id = p.a_id
+    JOIN artist_players pb ON pb.artist_id = p.b_id
+)
+INSERT INTO artist_similarity (artist_a_id, artist_b_id, score, source, fetched_at)
+SELECT a_id, b_id, score, 'user_cooccurrence', now()
+FROM scored
+WHERE score >= $1
+ON CONFLICT (artist_a_id, artist_b_id, source)
+DO UPDATE SET score = EXCLUDED.score, fetched_at = EXCLUDED.fetched_at
+`
+
+// Two artists are "co-played" when the same users play both. score is the
+// Jaccard of their distinct-player sets — coplayers / (playersA + playersB −
+// coplayers) in (0,1] — which controls for globally-popular artists (an artist
+// everyone plays would otherwise co-occur with everything). Only pairs with
+// >= 2 distinct co-players AND Jaccard >= $1 (a floor that both prunes weak
+// edges and self-limits hub artists, whose large denominators drag their
+// Jaccard down) are kept. Completed plays only, 365-day window.
+func (q *Queries) InsertArtistCoplayEdges(ctx context.Context, score float64) error {
+	_, err := q.db.Exec(ctx, insertArtistCoplayEdges, score)
+	return err
+}

internal/db/dbq/recommendation.sql.go

@@ -835,6 +835,22 @@ taste_overlap AS (
     ORDER BY tpa.weight DESC, t.id
     LIMIT $10
 ),
+coplay_artists AS (
+    -- Household co-play (#1533): tracks by artists co-played across the
+    -- instance with the seed's artist (source='user_cooccurrence', built by
+    -- the coplay worker). Mirrors similar_artists but from local co-occurrence
+    -- instead of ListenBrainz; empty on single-user servers. Same 0.5 damp as
+    -- similar_artists since it's artist-level.
+    SELECT t.id AS track_id, asim.score * 0.5 AS sim_score
+    FROM artist_similarity asim
+    JOIN tracks t ON t.artist_id = asim.artist_b_id
+    JOIN seed_artist sa ON asim.artist_a_id = sa.artist_id
+    WHERE asim.source = 'user_cooccurrence'
+      AND t.id NOT IN (SELECT id FROM excluded_ids)
+      AND t.id <> $2
+    ORDER BY asim.score DESC, random()
+    LIMIT $11
+),
 random_fill AS (
     SELECT t.id AS track_id, 0.0::float8 AS sim_score
     FROM tracks t
@@ -846,6 +862,7 @@ random_fill AS (
           UNION SELECT track_id FROM tag_overlap
           UNION SELECT track_id FROM likes_overlap
           UNION SELECT track_id FROM taste_overlap
+          UNION SELECT track_id FROM coplay_artists
       )
     ORDER BY random()
     LIMIT $9
@@ -864,6 +881,7 @@ FROM (
     UNION ALL SELECT track_id, sim_score FROM tag_overlap
     UNION ALL SELECT track_id, sim_score FROM likes_overlap
     UNION ALL SELECT track_id, sim_score FROM taste_overlap
+    UNION ALL SELECT track_id, sim_score FROM coplay_artists
     UNION ALL SELECT track_id, sim_score FROM random_fill
 ) u
 JOIN tracks t ON t.id = u.track_id
@@ -894,6 +912,7 @@ type LoadRadioCandidatesV2Params struct {
 	Limit_4 int32
 	Limit_5 int32
 	Limit_6 int32
+	Limit_7 int32
 }
 
 type LoadRadioCandidatesV2Row struct {
@@ -914,7 +933,9 @@ type LoadRadioCandidatesV2Row struct {
 //	$10 taste_overlap K (#796 phase 2b — tracks by the user's top
 //	positively-weighted taste-profile artists, so taste-relevant tracks
 //	enter the pool even when the similarity/random arms miss them; scored
-//	in Go via TasteMatch, so sim_score here is 0 pool-inclusion).
+//	in Go via TasteMatch, so sim_score here is 0 pool-inclusion),
+//	$11 coplay_artists K (#1533 — tracks by artists co-played across the
+//	instance with the seed's artist; source='user_cooccurrence').
 //
 // Returns same shape as LoadRadioCandidates plus similarity_score column.
 func (q *Queries) LoadRadioCandidatesV2(ctx context.Context, arg LoadRadioCandidatesV2Params) ([]LoadRadioCandidatesV2Row, error) {
@@ -929,6 +950,7 @@ func (q *Queries) LoadRadioCandidatesV2(ctx context.Context, arg LoadRadioCandid
 		arg.Limit_4,
 		arg.Limit_5,
 		arg.Limit_6,
+		arg.Limit_7,
 	)
 	if err != nil {
 		return nil, err

internal/db/queries/coplay.sql

@@ -0,0 +1,55 @@
+-- Household co-play similarity (#1533, milestone #160 Opt 5). Minstrel is a
+-- single shared-library, multi-user server (no per-user library ACL), so the
+-- "household" is the whole instance's user set. These queries recompute
+-- artist–artist co-occurrence edges from play_events and store them in
+-- artist_similarity under the pre-provisioned source = 'user_cooccurrence'
+-- (whitelisted in the 0009 CHECK from day one). A periodic worker DELETEs then
+-- re-INSERTs so edges that fall below threshold disappear. Single-user servers
+-- produce no edges (the >= 2 co-player gate), so the arm is empty there.
+
+-- name: DeleteArtistCoplayEdges :exec
+DELETE FROM artist_similarity WHERE source = 'user_cooccurrence';
+
+-- name: InsertArtistCoplayEdges :exec
+-- Two artists are "co-played" when the same users play both. score is the
+-- Jaccard of their distinct-player sets — coplayers / (playersA + playersB −
+-- coplayers) in (0,1] — which controls for globally-popular artists (an artist
+-- everyone plays would otherwise co-occur with everything). Only pairs with
+-- >= 2 distinct co-players AND Jaccard >= $1 (a floor that both prunes weak
+-- edges and self-limits hub artists, whose large denominators drag their
+-- Jaccard down) are kept. Completed plays only, 365-day window.
+WITH user_artists AS (
+    SELECT DISTINCT pe.user_id, t.artist_id
+    FROM play_events pe
+    JOIN tracks t ON t.id = pe.track_id
+    WHERE pe.was_skipped = false
+      AND pe.started_at > now() - interval '365 days'
+),
+artist_players AS (
+    SELECT artist_id, count(*)::float8 AS players
+    FROM user_artists
+    GROUP BY artist_id
+),
+pairs AS (
+    SELECT ua.artist_id AS a_id,
+           ub.artist_id AS b_id,
+           count(*)::float8 AS coplayers
+    FROM user_artists ua
+    JOIN user_artists ub
+      ON ua.user_id = ub.user_id AND ua.artist_id <> ub.artist_id
+    GROUP BY ua.artist_id, ub.artist_id
+    HAVING count(*) >= 2
+),
+scored AS (
+    SELECT p.a_id, p.b_id,
+           p.coplayers / (pa.players + pb.players - p.coplayers) AS score
+    FROM pairs p
+    JOIN artist_players pa ON pa.artist_id = p.a_id
+    JOIN artist_players pb ON pb.artist_id = p.b_id
+)
+INSERT INTO artist_similarity (artist_a_id, artist_b_id, score, source, fetched_at)
+SELECT a_id, b_id, score, 'user_cooccurrence', now()
+FROM scored
+WHERE score >= $1
+ON CONFLICT (artist_a_id, artist_b_id, source)
+DO UPDATE SET score = EXCLUDED.score, fetched_at = EXCLUDED.fetched_at;

internal/db/queries/recommendation.sql

@@ -42,7 +42,9 @@ WHERE t.id <> $2
 --   $10 taste_overlap K (#796 phase 2b — tracks by the user's top
 --   positively-weighted taste-profile artists, so taste-relevant tracks
 --   enter the pool even when the similarity/random arms miss them; scored
---   in Go via TasteMatch, so sim_score here is 0 pool-inclusion).
+--   in Go via TasteMatch, so sim_score here is 0 pool-inclusion),
+--   $11 coplay_artists K (#1533 — tracks by artists co-played across the
+--   instance with the seed's artist; source='user_cooccurrence').
 -- Returns same shape as LoadRadioCandidates plus similarity_score column.
 
 WITH
@@ -126,6 +128,22 @@ taste_overlap AS (
     ORDER BY tpa.weight DESC, t.id
     LIMIT $10
 ),
+coplay_artists AS (
+    -- Household co-play (#1533): tracks by artists co-played across the
+    -- instance with the seed's artist (source='user_cooccurrence', built by
+    -- the coplay worker). Mirrors similar_artists but from local co-occurrence
+    -- instead of ListenBrainz; empty on single-user servers. Same 0.5 damp as
+    -- similar_artists since it's artist-level.
+    SELECT t.id AS track_id, asim.score * 0.5 AS sim_score
+    FROM artist_similarity asim
+    JOIN tracks t ON t.artist_id = asim.artist_b_id
+    JOIN seed_artist sa ON asim.artist_a_id = sa.artist_id
+    WHERE asim.source = 'user_cooccurrence'
+      AND t.id NOT IN (SELECT id FROM excluded_ids)
+      AND t.id <> $2
+    ORDER BY asim.score DESC, random()
+    LIMIT $11
+),
 random_fill AS (
     SELECT t.id AS track_id, 0.0::float8 AS sim_score
     FROM tracks t
@@ -137,6 +155,7 @@ random_fill AS (
           UNION SELECT track_id FROM tag_overlap
           UNION SELECT track_id FROM likes_overlap
           UNION SELECT track_id FROM taste_overlap
+          UNION SELECT track_id FROM coplay_artists
       )
     ORDER BY random()
     LIMIT $9
@@ -155,6 +174,7 @@ FROM (
     UNION ALL SELECT track_id, sim_score FROM tag_overlap
     UNION ALL SELECT track_id, sim_score FROM likes_overlap
     UNION ALL SELECT track_id, sim_score FROM taste_overlap
+    UNION ALL SELECT track_id, sim_score FROM coplay_artists
     UNION ALL SELECT track_id, sim_score FROM random_fill
 ) u
 JOIN tracks t ON t.id = u.track_id

internal/playlists/system.go

@@ -546,6 +546,9 @@ func systemForYouSourceLimits() recommendation.CandidateSourceLimits {
 		// deep slice of the user's top taste-profile artists into the pool
 		// (#796 phase 2b). Empty for cold-start users (no profile yet).
 		TasteOverlap: 80,
+		// Household co-play (#1533): a deeper collaborative slice for the
+		// taste surfaces. Empty on single-user servers.
+		UserCoplay: 40,
 	}
 }
 

internal/recommendation/candidates.go

@@ -82,6 +82,10 @@ type CandidateSourceLimits struct {
 	// weighted taste-profile artists. 0 disables the arm (e.g. cold-start
 	// users have an empty profile, so it contributes nothing anyway).
 	TasteOverlap int
+	// UserCoplay (#1533): tracks by artists co-played across the instance
+	// with the seed's artist (source='user_cooccurrence'). Empty on
+	// single-user servers, so it contributes nothing there.
+	UserCoplay int
 }
 
 // DefaultCandidateSourceLimits returns the v1 hardcoded constants per spec.
@@ -93,6 +97,7 @@ func DefaultCandidateSourceLimits() CandidateSourceLimits {
 		LikesOverlap:  20,
 		RandomFill:    30,
 		TasteOverlap:  20,
+		UserCoplay:    20,
 	}
 }
 
@@ -125,6 +130,7 @@ func LoadCandidatesFromSimilarity(
 		Limit_4: int32(limits.LikesOverlap),
 		Limit_5: int32(limits.RandomFill),
 		Limit_6: int32(limits.TasteOverlap),
+		Limit_7: int32(limits.UserCoplay),
 	})
 	if err != nil {
 		return nil, err