diff --git a/internal/recommendation/sessionvector.go b/internal/recommendation/sessionvector.go
new file mode 100644
index 00000000..94cd3154
--- /dev/null
+++ b/internal/recommendation/sessionvector.go
@@ -0,0 +1,43 @@
+package recommendation
+
+import (
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+type SessionVector struct {
+	Seed           bool           `json:"seed"`
+	Artists        []string       `json:"artists"`
+	Tags           map[string]int `json:"tags"`
+	RecentTrackIDs []string       `json:"recent_track_ids"`
+}
+
+func BuildSessionVector(priorTracks []dbq.Track) SessionVector {
+	v := SessionVector{
+		Seed:           len(priorTracks) < 3,
+		Artists:        []string{},
+		Tags:           map[string]int{},
+		RecentTrackIDs: []string{},
+	}
+	seen := map[string]bool{}
+	for _, t := range priorTracks {
+		artistID := uuidString(t.ArtistID)
+		if !seen[artistID] {
+			seen[artistID] = true
+			v.Artists = append(v.Artists, artistID)
+		}
+		if t.Genre != nil && *t.Genre != "" {
+			v.Tags[*t.Genre]++
+		}
+		v.RecentTrackIDs = append(v.RecentTrackIDs, uuidString(t.ID))
+	}
+	return v
+}
+
+func uuidString(u pgtype.UUID) string {
+	if !u.Valid {
+		return ""
+	}
+	return u.String()
+}
diff --git a/internal/recommendation/sessionvector_test.go b/internal/recommendation/sessionvector_test.go
new file mode 100644
index 00000000..745b3bf9
--- /dev/null
+++ b/internal/recommendation/sessionvector_test.go
@@ -0,0 +1,151 @@
+package recommendation
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+func track(artistID pgtype.UUID, genre string) dbq.Track {
+	t := dbq.Track{
+		ArtistID: artistID,
+	}
+	if genre != "" {
+		g := genre
+		t.Genre = &g
+	}
+	return t
+}
+
+func uuid(s string) pgtype.UUID {
+	var u pgtype.UUID
+	_ = u.Scan(s)
+	return u
+}
+
+func TestBuildSessionVector_Empty_IsSeed(t *testing.T) {
+	v := BuildSessionVector(nil)
+	if !v.Seed {
+		t.Errorf("Seed = false, want true for empty input")
+	}
+	if len(v.Artists) != 0 || len(v.Tags) != 0 || len(v.RecentTrackIDs) != 0 {
+		t.Errorf("non-empty collections: %+v", v)
+	}
+}
+
+func TestBuildSessionVector_OneTrack_IsSeed(t *testing.T) {
+	a1 := uuid("11111111-1111-1111-1111-111111111111")
+	tr := track(a1, "rock")
+	tr.ID = uuid("22222222-2222-2222-2222-222222222222")
+	v := BuildSessionVector([]dbq.Track{tr})
+	if !v.Seed {
+		t.Errorf("1 track: Seed=false, want true (< 3)")
+	}
+	if len(v.Artists) != 1 {
+		t.Errorf("Artists: %+v", v.Artists)
+	}
+	if v.Tags["rock"] != 1 {
+		t.Errorf("Tags: %+v", v.Tags)
+	}
+	if len(v.RecentTrackIDs) != 1 {
+		t.Errorf("RecentTrackIDs: %+v", v.RecentTrackIDs)
+	}
+}
+
+func TestBuildSessionVector_TwoTracks_IsSeed(t *testing.T) {
+	a1 := uuid("11111111-1111-1111-1111-111111111111")
+	v := BuildSessionVector([]dbq.Track{track(a1, "rock"), track(a1, "jazz")})
+	if !v.Seed {
+		t.Errorf("2 tracks: Seed=false, want true (< 3)")
+	}
+}
+
+func TestBuildSessionVector_ThreeTracks_NotSeed(t *testing.T) {
+	a1 := uuid("11111111-1111-1111-1111-111111111111")
+	v := BuildSessionVector([]dbq.Track{
+		track(a1, "rock"), track(a1, "rock"), track(a1, "rock"),
+	})
+	if v.Seed {
+		t.Errorf("3 tracks: Seed=true, want false")
+	}
+	// All same artist → 1 entry.
+	if len(v.Artists) != 1 {
+		t.Errorf("Artists len = %d, want 1 (dedup)", len(v.Artists))
+	}
+	// All same genre → count 3.
+	if v.Tags["rock"] != 3 {
+		t.Errorf("Tags['rock'] = %d, want 3", v.Tags["rock"])
+	}
+}
+
+func TestBuildSessionVector_DistinctArtistsAndGenres(t *testing.T) {
+	a1 := uuid("11111111-1111-1111-1111-111111111111")
+	a2 := uuid("22222222-2222-2222-2222-222222222222")
+	a3 := uuid("33333333-3333-3333-3333-333333333333")
+	v := BuildSessionVector([]dbq.Track{
+		track(a1, "rock"),
+		track(a2, "jazz"),
+		track(a3, "rock"),
+		track(a1, "experimental"),
+	})
+	// 4 tracks → not seed.
+	if v.Seed {
+		t.Errorf("Seed=true, want false")
+	}
+	// Distinct artists: 3 (a1 dedup'd).
+	if len(v.Artists) != 3 {
+		t.Errorf("Artists len = %d, want 3", len(v.Artists))
+	}
+	// Tag counts.
+	if v.Tags["rock"] != 2 || v.Tags["jazz"] != 1 || v.Tags["experimental"] != 1 {
+		t.Errorf("Tags = %+v", v.Tags)
+	}
+	if len(v.RecentTrackIDs) != 4 {
+		t.Errorf("RecentTrackIDs len = %d", len(v.RecentTrackIDs))
+	}
+}
+
+func TestBuildSessionVector_NilGenre_NotIndexed(t *testing.T) {
+	a1 := uuid("11111111-1111-1111-1111-111111111111")
+	v := BuildSessionVector([]dbq.Track{
+		track(a1, ""), // empty genre via the helper (nil pointer)
+		track(a1, "rock"),
+		track(a1, ""),
+	})
+	if v.Tags["rock"] != 1 {
+		t.Errorf("Tags['rock'] = %d, want 1", v.Tags["rock"])
+	}
+	if _, exists := v.Tags[""]; exists {
+		t.Errorf("Tags has empty-string entry: %+v", v.Tags)
+	}
+}
+
+func TestBuildSessionVector_JSONRoundTrip(t *testing.T) {
+	a1 := uuid("11111111-1111-1111-1111-111111111111")
+	v := BuildSessionVector([]dbq.Track{
+		track(a1, "rock"), track(a1, "jazz"), track(a1, "rock"),
+	})
+	data, err := json.Marshal(v)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	var got SessionVector
+	if err := json.Unmarshal(data, &got); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if got.Seed != v.Seed {
+		t.Errorf("Seed: %v != %v", got.Seed, v.Seed)
+	}
+	if got.Tags["rock"] != v.Tags["rock"] {
+		t.Errorf("Tags: %+v != %+v", got.Tags, v.Tags)
+	}
+	if len(got.Artists) != len(v.Artists) {
+		t.Errorf("Artists: len %d != %d", len(got.Artists), len(v.Artists))
+	}
+	if len(got.RecentTrackIDs) != len(v.RecentTrackIDs) {
+		t.Errorf("RecentTrackIDs: len %d != %d", len(got.RecentTrackIDs), len(v.RecentTrackIDs))
+	}
+}