diff --git a/internal/playevents/writer.go b/internal/playevents/writer.go
index e8b73bb8..14350f48 100644
--- a/internal/playevents/writer.go
+++ b/internal/playevents/writer.go
@@ -9,6 +9,7 @@ package playevents
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"log/slog"
 	"time"
@@ -19,6 +20,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/playsessions"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/recommendation"
 )
 
 type Writer struct {
@@ -87,6 +89,11 @@ func (w *Writer) RecordPlayStarted(
 		}
 		out.PlayEventID = ev.ID
 		out.SessionID = sessionID
+
+		// Capture session vector for the just-inserted row.
+		if err := w.captureSessionVector(ctx, q, ev.ID, sessionID, at); err != nil {
+			return err
+		}
 		return nil
 	})
 	return out, err
@@ -223,6 +230,9 @@ func (w *Writer) RecordSyntheticCompletedPlay(
 		if err != nil {
 			return err
 		}
+		if err := w.captureSessionVector(ctx, q, ev.ID, sessionID, at); err != nil {
+			return err
+		}
 		ratio := 1.0
 		dur := track.DurationMs
 		_, err = q.UpdatePlayEventEnded(ctx, dbq.UpdatePlayEventEndedParams{
@@ -236,6 +246,36 @@ func (w *Writer) RecordSyntheticCompletedPlay(
 	})
 }
 
+// captureSessionVector queries the user's prior plays in the given session
+// (before `at`), builds the session vector, and UPDATEs the just-inserted
+// play_event with it. Runs inside the caller's transaction (q is the
+// transaction-bound *dbq.Queries). Errors here propagate — vector capture
+// is best-effort, but DB errors should fail the transaction.
+func (w *Writer) captureSessionVector(
+	ctx context.Context,
+	q *dbq.Queries,
+	playEventID, sessionID pgtype.UUID,
+	at time.Time,
+) error {
+	priorTracks, err := q.ListRecentSessionTracks(ctx, dbq.ListRecentSessionTracksParams{
+		SessionID: sessionID,
+		StartedAt: pgtype.Timestamptz{Time: at, Valid: true},
+		Limit:     5,
+	})
+	if err != nil {
+		return err
+	}
+	vec := recommendation.BuildSessionVector(priorTracks)
+	vecJSON, err := json.Marshal(vec)
+	if err != nil {
+		return err
+	}
+	return q.UpdatePlayEventVector(ctx, dbq.UpdatePlayEventVectorParams{
+		ID:                  playEventID,
+		SessionVectorAtPlay: vecJSON,
+	})
+}
+
 // autoClosePriorOpen closes any open (ended_at IS NULL) play_event for the
 // user. Sets ended_at = at, duration_played_ms = min(at - started_at, track
 // duration), was_skipped = true. Skip rule is NOT applied — auto-closed
diff --git a/internal/playevents/writer_test.go b/internal/playevents/writer_test.go
index 4d300a5c..5786e826 100644
--- a/internal/playevents/writer_test.go
+++ b/internal/playevents/writer_test.go
@@ -2,6 +2,7 @@ package playevents
 
 import (
 	"context"
+	"encoding/json"
 	"io"
 	"log/slog"
 	"os"
@@ -234,3 +235,85 @@ func TestRecordSyntheticCompletedPlay_WritesBothRows(t *testing.T) {
 		t.Errorf("closed play_events count = %d, want 1", count)
 	}
 }
+
+func TestRecordPlayStarted_PersistsSessionVector_Seed(t *testing.T) {
+	f := newFixture(t, 200_000)
+	now := time.Now().UTC()
+	res, err := f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", now)
+	if err != nil {
+		t.Fatalf("RecordPlayStarted: %v", err)
+	}
+	got, err := f.q.GetPlayEventByID(context.Background(), res.PlayEventID)
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if got.SessionVectorAtPlay == nil {
+		t.Fatalf("session_vector_at_play is NULL")
+	}
+	var vec map[string]any
+	if err := json.Unmarshal(got.SessionVectorAtPlay, &vec); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if seed, _ := vec["seed"].(bool); !seed {
+		t.Errorf("seed = %v, want true (first play in session)", vec["seed"])
+	}
+	if artists, _ := vec["artists"].([]any); len(artists) != 0 {
+		t.Errorf("artists not empty: %v", artists)
+	}
+}
+
+func TestRecordPlayStarted_PersistsSessionVector_Populated(t *testing.T) {
+	f := newFixture(t, 200_000)
+	// Seed 4 prior plays with the same track (reusing the fixture's single
+	// track is fine — vector dedup means we'll see 1 artist regardless).
+	t0 := time.Now().UTC().Add(-1 * time.Hour)
+	for i := 0; i < 4; i++ {
+		at := t0.Add(time.Duration(i) * time.Minute)
+		_, _ = f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", at)
+	}
+	// 5th play — should see vector with 4 prior tracks.
+	at := t0.Add(10 * time.Minute)
+	res, err := f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", at)
+	if err != nil {
+		t.Fatalf("RecordPlayStarted: %v", err)
+	}
+	got, _ := f.q.GetPlayEventByID(context.Background(), res.PlayEventID)
+	if got.SessionVectorAtPlay == nil {
+		t.Fatal("session_vector_at_play is NULL")
+	}
+	var vec map[string]any
+	_ = json.Unmarshal(got.SessionVectorAtPlay, &vec)
+	if seed, _ := vec["seed"].(bool); seed {
+		t.Errorf("seed = true after 4 prior tracks, want false")
+	}
+	recentIDs, _ := vec["recent_track_ids"].([]any)
+	if len(recentIDs) != 4 {
+		t.Errorf("recent_track_ids len = %d, want 4", len(recentIDs))
+	}
+}
+
+func TestRecordPlayStarted_VectorScopedToSession(t *testing.T) {
+	f := newFixture(t, 200_000)
+	// Play 1: at t=0.
+	t0 := time.Now().UTC().Add(-2 * time.Hour)
+	_, _ = f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", t0)
+	// Play 2: t = t0 + 31 minutes (exceeds 30-min session timeout).
+	t1 := t0.Add(31 * time.Minute)
+	res, err := f.w.RecordPlayStarted(context.Background(), f.user, f.track, "c", t1)
+	if err != nil {
+		t.Fatalf("RecordPlayStarted: %v", err)
+	}
+	got, _ := f.q.GetPlayEventByID(context.Background(), res.PlayEventID)
+	if got.SessionVectorAtPlay == nil {
+		t.Fatal("vector NULL")
+	}
+	var vec map[string]any
+	_ = json.Unmarshal(got.SessionVectorAtPlay, &vec)
+	if seed, _ := vec["seed"].(bool); !seed {
+		t.Errorf("seed = false in fresh session, want true")
+	}
+	recentIDs, _ := vec["recent_track_ids"].([]any)
+	if len(recentIDs) != 0 {
+		t.Errorf("recent_track_ids has %d entries from prior session, want 0", len(recentIDs))
+	}
+}