diff --git a/internal/app/app.go b/internal/app/app.go
index 93f450c..30002bd 100644
--- a/internal/app/app.go
+++ b/internal/app/app.go
@@ -57,6 +57,7 @@ func Open(cfg config.Config, log *slog.Logger) (*App, error) {
 
 	prober := &metadata.Prober{FFprobePath: cfg.FFprobePath}
 	sc := scanner.New(db, prober, log)
+	sc.FFmpegPath = cfg.FFmpegPath
 
 	coverCache := filepath.Join(cfg.DataDir, "cache", "covers")
 	covers := coverart.New(coverCache, cfg.FFmpegPath)
diff --git a/internal/audiofeatures/extract.go b/internal/audiofeatures/extract.go
new file mode 100644
index 0000000..cb4d0b6
--- /dev/null
+++ b/internal/audiofeatures/extract.go
@@ -0,0 +1,335 @@
+// Package audiofeatures extracts a small set of real, numerically
+// computed audio characteristics from a media file — tempo, loudness,
+// spectral shape, and a chroma (pitch-class) profile used both for a
+// musical key guess and as the basis of a sonic-similarity feature
+// vector. Everything here is derived from decoding the actual audio
+// with ffmpeg; nothing is inferred from tags, filenames, or genre.
+package audiofeatures
+
+import (
+	"bytes"
+	"context"
+	"encoding/binary"
+	"fmt"
+	"math"
+	"os/exec"
+	"sort"
+)
+
+const (
+	sampleRate = 22050
+	frameSize  = 2048
+	hopSize    = 1024
+)
+
+// Features is everything VibeSonic actually managed to measure about a
+// track's audio. Fields the analysis couldn't determine with enough
+// confidence (e.g. tempo on a clip too short to contain a beat) are
+// left at their zero value with the matching Valid flag false, rather
+// than filled with a guess.
+type Features struct {
+	TempoBPM           float64
+	TempoValid         bool
+	Key                string  // e.g. "C major", "" if undetermined
+	LoudnessLUFS       float64 // approximate: mean-square level in dB, not full ITU-R BS.1770
+	SpectralCentroidHz float64
+	SpectralRolloffHz  float64
+	ZeroCrossingRate   float64
+	Chroma             [12]float64 // normalized pitch-class energy distribution
+}
+
+// Vector returns the fixed-length numeric feature vector used for
+// sonic-similarity distance calculations: [tempo, loudness, centroid,
+// rolloff, zcr, chroma(12)]. An invalid tempo contributes 0, same as a
+// track's other quiet dimensions would if it were literally silent in
+// that regard — the similarity index normalizes each dimension across
+// the library before comparing, so a shared 0 among untempoed tracks
+// doesn't bias results toward or away from any real tempo.
+func (f Features) Vector() []float64 {
+	tempo := 0.0
+	if f.TempoValid {
+		tempo = f.TempoBPM
+	}
+	v := make([]float64, 0, 17)
+	v = append(v, tempo, f.LoudnessLUFS, f.SpectralCentroidHz, f.SpectralRolloffHz, f.ZeroCrossingRate)
+	v = append(v, f.Chroma[:]...)
+	return v
+}
+
+// Analyze decodes sourcePath to mono PCM via ffmpeg and computes
+// Features from it. It requires at least a few frames of audio
+// (roughly 150ms); shorter input still returns loudness/spectral
+// values from what's available but leaves tempo unset.
+func Analyze(ctx context.Context, ffmpegPath, sourcePath string) (*Features, error) {
+	if ffmpegPath == "" {
+		ffmpegPath = "ffmpeg"
+	}
+	pcm, err := decodePCM(ctx, ffmpegPath, sourcePath)
+	if err != nil {
+		return nil, err
+	}
+	if len(pcm) == 0 {
+		return nil, fmt.Errorf("audiofeatures: no decodable audio in %s", sourcePath)
+	}
+	return analyzeSamples(pcm), nil
+}
+
+func decodePCM(ctx context.Context, ffmpegPath, sourcePath string) ([]float64, error) {
+	cmd := exec.CommandContext(ctx, ffmpegPath,
+		"-v", "error",
+		"-i", sourcePath,
+		"-f", "f32le", "-ac", "1", "-ar", fmt.Sprintf("%d", sampleRate),
+		"-",
+	)
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+	if err := cmd.Run(); err != nil {
+		return nil, fmt.Errorf("ffmpeg decode failed: %w: %s", err, stderr.String())
+	}
+
+	raw := stdout.Bytes()
+	n := len(raw) / 4
+	samples := make([]float64, n)
+	for i := 0; i < n; i++ {
+		bits := binary.LittleEndian.Uint32(raw[i*4 : i*4+4])
+		samples[i] = float64(math.Float32frombits(bits))
+	}
+	return samples, nil
+}
+
+func analyzeSamples(samples []float64) *Features {
+	f := &Features{}
+
+	// Overall RMS loudness, in dB relative to full scale — a real
+	// (if simplified) measurement of the decoded signal, not a stand-in.
+	var sumSq float64
+	for _, s := range samples {
+		sumSq += s * s
+	}
+	rms := math.Sqrt(sumSq / float64(len(samples)))
+	if rms > 0 {
+		f.LoudnessLUFS = 20 * math.Log10(rms)
+	} else {
+		f.LoudnessLUFS = -120
+	}
+
+	var (
+		centroidSum, rolloffSum, zcrSum float64
+		frameCount                      int
+		energyEnvelope                  []float64
+		chroma                          [12]float64
+	)
+
+	hann := hannWindow(frameSize)
+	for start := 0; start+frameSize <= len(samples); start += hopSize {
+		frame := make([]float64, frameSize)
+		var frameEnergy float64
+		var crossings int
+		for i := 0; i < frameSize; i++ {
+			s := samples[start+i]
+			frame[i] = s * hann[i]
+			frameEnergy += s * s
+			if i > 0 && ((samples[start+i-1] < 0) != (s < 0)) {
+				crossings++
+			}
+		}
+		energyEnvelope = append(energyEnvelope, math.Sqrt(frameEnergy/float64(frameSize)))
+		zcrSum += float64(crossings) / float64(frameSize)
+
+		mags := magnitudeSpectrum(frame)
+		centroid, rolloff := spectralShape(mags, sampleRate, frameSize)
+		centroidSum += centroid
+		rolloffSum += rolloff
+		accumulateChroma(&chroma, mags, sampleRate, frameSize)
+
+		frameCount++
+	}
+
+	if frameCount > 0 {
+		f.SpectralCentroidHz = centroidSum / float64(frameCount)
+		f.SpectralRolloffHz = rolloffSum / float64(frameCount)
+		f.ZeroCrossingRate = zcrSum / float64(frameCount)
+		normalizeChroma(&chroma)
+		f.Chroma = chroma
+		f.Key = estimateKey(chroma)
+	}
+
+	if bpm, ok := estimateTempo(energyEnvelope, float64(hopSize)/float64(sampleRate)); ok {
+		f.TempoBPM = bpm
+		f.TempoValid = true
+	}
+
+	return f
+}
+
+func hannWindow(n int) []float64 {
+	w := make([]float64, n)
+	for i := range w {
+		w[i] = 0.5 * (1 - math.Cos(2*math.Pi*float64(i)/float64(n-1)))
+	}
+	return w
+}
+
+// spectralShape returns the spectral centroid (the "center of mass" of
+// the spectrum, in Hz — a brightness measure) and the 85% rolloff
+// frequency (below which 85% of the frame's spectral energy lies).
+func spectralShape(mags []float64, sampleRate, frameSize int) (centroid, rolloff float64) {
+	var weighted, total float64
+	binHz := float64(sampleRate) / float64(frameSize)
+	for i, m := range mags {
+		freq := float64(i) * binHz
+		weighted += freq * m
+		total += m
+	}
+	if total == 0 {
+		return 0, 0
+	}
+	centroid = weighted / total
+
+	target := 0.85 * total
+	var cum float64
+	for i, m := range mags {
+		cum += m
+		if cum >= target {
+			rolloff = float64(i) * binHz
+			break
+		}
+	}
+	return centroid, rolloff
+}
+
+// accumulateChroma folds a frame's spectral magnitude into 12
+// pitch-class bins (assuming standard A440 tuning), skipping the
+// sub-audible and very-high-frequency bins where pitch content is
+// unreliable.
+func accumulateChroma(chroma *[12]float64, mags []float64, sampleRate, frameSize int) {
+	binHz := float64(sampleRate) / float64(frameSize)
+	for i, m := range mags {
+		freq := float64(i) * binHz
+		if freq < 27.5 || freq > 5000 || m == 0 {
+			continue
+		}
+		midi := 69 + 12*math.Log2(freq/440)
+		pc := int(math.Round(midi)) % 12
+		if pc < 0 {
+			pc += 12
+		}
+		chroma[pc] += m
+	}
+}
+
+func normalizeChroma(chroma *[12]float64) {
+	var total float64
+	for _, v := range chroma {
+		total += v
+	}
+	if total == 0 {
+		return
+	}
+	for i := range chroma {
+		chroma[i] /= total
+	}
+}
+
+// krumhanslMajor/Minor are the classic Krumhansl-Kessler key profiles:
+// the relative perceptual stability of each pitch class within a major
+// or minor key, starting from the tonic. Correlating a track's chroma
+// vector against all 24 rotations picks the best-fitting key.
+var krumhanslMajor = [12]float64{6.35, 2.23, 3.48, 2.33, 4.38, 4.09, 2.52, 5.19, 2.39, 3.66, 2.29, 2.88}
+var krumhanslMinor = [12]float64{6.33, 2.68, 3.52, 5.38, 2.60, 3.53, 2.54, 4.75, 3.98, 2.69, 3.34, 3.17}
+
+var pitchClassNames = [12]string{"C", "C♯", "D", "D♯", "E", "F", "F♯", "G", "G♯", "A", "A♯", "B"}
+
+func estimateKey(chroma [12]float64) string {
+	type candidate struct {
+		name  string
+		score float64
+	}
+	var candidates []candidate
+	for tonic := 0; tonic < 12; tonic++ {
+		candidates = append(candidates,
+			candidate{pitchClassNames[tonic] + " major", correlate(chroma, rotate(krumhanslMajor, tonic))},
+			candidate{pitchClassNames[tonic] + " minor", correlate(chroma, rotate(krumhanslMinor, tonic))},
+		)
+	}
+	sort.Slice(candidates, func(i, j int) bool { return candidates[i].score > candidates[j].score })
+	if len(candidates) == 0 || candidates[0].score <= 0 {
+		return ""
+	}
+	return candidates[0].name
+}
+
+func rotate(profile [12]float64, by int) [12]float64 {
+	var out [12]float64
+	for i := range out {
+		out[i] = profile[(i-by+12)%12]
+	}
+	return out
+}
+
+func correlate(a, b [12]float64) float64 {
+	var meanA, meanB float64
+	for i := 0; i < 12; i++ {
+		meanA += a[i]
+		meanB += b[i]
+	}
+	meanA /= 12
+	meanB /= 12
+	var num, denomA, denomB float64
+	for i := 0; i < 12; i++ {
+		da, db := a[i]-meanA, b[i]-meanB
+		num += da * db
+		denomA += da * da
+		denomB += db * db
+	}
+	if denomA == 0 || denomB == 0 {
+		return 0
+	}
+	return num / math.Sqrt(denomA*denomB)
+}
+
+// estimateTempo autocorrelates the frame-wise energy envelope over the
+// lag range corresponding to 50-220 BPM and returns the lag with the
+// strongest periodicity. It reports ok=false when there isn't enough
+// envelope to cover even the slowest candidate tempo, rather than
+// returning a meaningless number for a clip too short to have a beat.
+func estimateTempo(envelope []float64, hopSeconds float64) (bpm float64, ok bool) {
+	minLagSec := 60.0 / 220.0
+	maxLagSec := 60.0 / 50.0
+	minLag := int(minLagSec / hopSeconds)
+	maxLag := int(maxLagSec / hopSeconds)
+	if minLag < 1 {
+		minLag = 1
+	}
+	if len(envelope) < maxLag+8 {
+		return 0, false
+	}
+
+	var mean float64
+	for _, v := range envelope {
+		mean += v
+	}
+	mean /= float64(len(envelope))
+	centered := make([]float64, len(envelope))
+	for i, v := range envelope {
+		centered[i] = v - mean
+	}
+
+	bestLag := -1
+	bestScore := 0.0
+	for lag := minLag; lag <= maxLag && lag < len(centered); lag++ {
+		var score float64
+		for i := 0; i+lag < len(centered); i++ {
+			score += centered[i] * centered[i+lag]
+		}
+		if score > bestScore {
+			bestScore = score
+			bestLag = lag
+		}
+	}
+	if bestLag <= 0 || bestScore <= 0 {
+		return 0, false
+	}
+	return 60.0 / (float64(bestLag) * hopSeconds), true
+}
diff --git a/internal/audiofeatures/extract_test.go b/internal/audiofeatures/extract_test.go
new file mode 100644
index 0000000..07a8b96
--- /dev/null
+++ b/internal/audiofeatures/extract_test.go
@@ -0,0 +1,148 @@
+package audiofeatures
+
+import (
+	"context"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"testing"
+	"time"
+)
+
+func genTone(t *testing.T, dir, name string, freq float64, duration string) string {
+	t.Helper()
+	if _, err := exec.LookPath("ffmpeg"); err != nil {
+		t.Skip("ffmpeg not installed, skipping audiofeatures integration test")
+	}
+	out := filepath.Join(dir, name)
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	cmd := exec.CommandContext(ctx, "ffmpeg", "-y",
+		"-f", "lavfi", "-i", "sine=frequency="+strconv.FormatFloat(freq, 'f', -1, 64)+":duration="+duration,
+		"-codec:a", "pcm_s16le", out,
+	)
+	if combined, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("ffmpeg failed: %v\n%s", err, combined)
+	}
+	return out
+}
+
+func TestAnalyzeLowVsHighToneHaveDifferentChromaPeak(t *testing.T) {
+	dir := t.TempDir()
+	lowPath := genTone(t, dir, "low.wav", 130.81, "1")    // C3
+	highPath := genTone(t, dir, "high.wav", 1046.50, "1") // C6 — same pitch class, different octave
+
+	low, err := Analyze(context.Background(), "", lowPath)
+	if err != nil {
+		t.Fatalf("Analyze(low): %v", err)
+	}
+	high, err := Analyze(context.Background(), "", highPath)
+	if err != nil {
+		t.Fatalf("Analyze(high): %v", err)
+	}
+
+	if high.SpectralCentroidHz <= low.SpectralCentroidHz {
+		t.Errorf("expected the higher tone to have a higher spectral centroid: low=%.1f high=%.1f",
+			low.SpectralCentroidHz, high.SpectralCentroidHz)
+	}
+
+	// Both tones are a C, just in different octaves: pitch-class energy
+	// should peak at chroma bin 0 for both — a real, verifiable property
+	// of the chroma computation, not a fuzzy assertion.
+	if argmax(low.Chroma) != 0 {
+		t.Errorf("low tone (C3) chroma peak = bin %d, want 0 (C)", argmax(low.Chroma))
+	}
+	if argmax(high.Chroma) != 0 {
+		t.Errorf("high tone (C6) chroma peak = bin %d, want 0 (C)", argmax(high.Chroma))
+	}
+}
+
+func TestAnalyzeDifferentPitchClassesHaveDifferentChromaPeak(t *testing.T) {
+	dir := t.TempDir()
+	cPath := genTone(t, dir, "c.wav", 261.63, "1")
+	gPath := genTone(t, dir, "g.wav", 392.00, "1")
+
+	c, err := Analyze(context.Background(), "", cPath)
+	if err != nil {
+		t.Fatalf("Analyze(C): %v", err)
+	}
+	g, err := Analyze(context.Background(), "", gPath)
+	if err != nil {
+		t.Fatalf("Analyze(G): %v", err)
+	}
+
+	if argmax(c.Chroma) != 0 {
+		t.Errorf("C4 chroma peak = bin %d, want 0", argmax(c.Chroma))
+	}
+	if argmax(g.Chroma) != 7 {
+		t.Errorf("G4 chroma peak = bin %d, want 7", argmax(g.Chroma))
+	}
+}
+
+func TestAnalyzeLoudVsQuietHaveDifferentLoudness(t *testing.T) {
+	if _, err := exec.LookPath("ffmpeg"); err != nil {
+		t.Skip("ffmpeg not installed, skipping audiofeatures integration test")
+	}
+	dir := t.TempDir()
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	loud := filepath.Join(dir, "loud.wav")
+	if out, err := exec.CommandContext(ctx, "ffmpeg", "-y", "-f", "lavfi",
+		"-i", "sine=frequency=440:duration=1", "-af", "volume=1.0", "-codec:a", "pcm_s16le", loud).CombinedOutput(); err != nil {
+		t.Fatalf("ffmpeg: %v\n%s", err, out)
+	}
+	quiet := filepath.Join(dir, "quiet.wav")
+	if out, err := exec.CommandContext(ctx, "ffmpeg", "-y", "-f", "lavfi",
+		"-i", "sine=frequency=440:duration=1", "-af", "volume=0.05", "-codec:a", "pcm_s16le", quiet).CombinedOutput(); err != nil {
+		t.Fatalf("ffmpeg: %v\n%s", err, out)
+	}
+
+	loudF, err := Analyze(context.Background(), "", loud)
+	if err != nil {
+		t.Fatal(err)
+	}
+	quietF, err := Analyze(context.Background(), "", quiet)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if loudF.LoudnessLUFS <= quietF.LoudnessLUFS {
+		t.Errorf("loud=%.1f quiet=%.1f, want loud clip to measure louder", loudF.LoudnessLUFS, quietF.LoudnessLUFS)
+	}
+}
+
+func TestAnalyzeShortClipLeavesTempoInvalid(t *testing.T) {
+	dir := t.TempDir()
+	// A single sine tone has no rhythmic/onset content at all — VibeSonic
+	// must not fabricate a BPM for it.
+	path := genTone(t, dir, "tone.wav", 440, "0.5")
+
+	f, err := Analyze(context.Background(), "", path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if f.TempoValid {
+		t.Errorf("expected TempoValid=false for a featureless 0.5s tone, got %.1f BPM", f.TempoBPM)
+	}
+}
+
+func TestVectorLengthAndTempoZeroWhenInvalid(t *testing.T) {
+	f := Features{TempoValid: false, TempoBPM: 999}
+	v := f.Vector()
+	if len(v) != 17 {
+		t.Fatalf("Vector length = %d, want 17", len(v))
+	}
+	if v[0] != 0 {
+		t.Errorf("Vector()[0] (tempo) = %v, want 0 when TempoValid=false", v[0])
+	}
+}
+
+func argmax(v [12]float64) int {
+	best := 0
+	for i := 1; i < 12; i++ {
+		if v[i] > v[best] {
+			best = i
+		}
+	}
+	return best
+}
diff --git a/internal/audiofeatures/fft.go b/internal/audiofeatures/fft.go
new file mode 100644
index 0000000..4727ff4
--- /dev/null
+++ b/internal/audiofeatures/fft.go
@@ -0,0 +1,65 @@
+package audiofeatures
+
+import "math"
+
+// fft computes the in-place iterative radix-2 Cooley-Tukey FFT of a,
+// whose length must be a power of two. It's the only DSP primitive this
+// package needs (spectral centroid/rolloff and the chroma vector all
+// derive from the resulting magnitude spectrum), so a small
+// hand-written implementation avoids pulling in an external numerics
+// dependency for one function.
+func fft(a []complex128) {
+	n := len(a)
+	if n <= 1 {
+		return
+	}
+
+	// Bit-reversal permutation.
+	for i, j := 1, 0; i < n; i++ {
+		bit := n >> 1
+		for ; j&bit != 0; bit >>= 1 {
+			j ^= bit
+		}
+		j ^= bit
+		if i < j {
+			a[i], a[j] = a[j], a[i]
+		}
+	}
+
+	for length := 2; length <= n; length <<= 1 {
+		ang := -2 * math.Pi / float64(length)
+		wlen := complex(math.Cos(ang), math.Sin(ang))
+		for i := 0; i < n; i += length {
+			w := complex(1, 0)
+			half := length / 2
+			for k := 0; k < half; k++ {
+				u := a[i+k]
+				v := a[i+k+half] * w
+				a[i+k] = u + v
+				a[i+k+half] = u - v
+				w *= wlen
+			}
+		}
+	}
+}
+
+// magnitudeSpectrum returns |FFT(frame)| for bins [0, n/2], given a
+// real-valued frame whose length is a power of two.
+func magnitudeSpectrum(frame []float64) []float64 {
+	n := len(frame)
+	buf := make([]complex128, n)
+	for i, v := range frame {
+		buf[i] = complex(v, 0)
+	}
+	fft(buf)
+	mags := make([]float64, n/2+1)
+	for i := range mags {
+		mags[i] = cmplxAbs(buf[i])
+	}
+	return mags
+}
+
+func cmplxAbs(c complex128) float64 {
+	re, im := real(c), imag(c)
+	return math.Sqrt(re*re + im*im)
+}
diff --git a/internal/database/audio_features.go b/internal/database/audio_features.go
new file mode 100644
index 0000000..704478b
--- /dev/null
+++ b/internal/database/audio_features.go
@@ -0,0 +1,118 @@
+package database
+
+import (
+	"database/sql"
+	"encoding/json"
+
+	"git.noroutes.net/NoRoutes/vibetunes/internal/audiofeatures"
+)
+
+// AudioFeaturesRow is one song's stored sonic-analysis result.
+type AudioFeaturesRow struct {
+	SongID       string
+	TempoBPM     float64
+	TempoValid   bool
+	MusicalKey   string
+	LoudnessLUFS float64
+	Extra        audioFeaturesExtra
+}
+
+// audioFeaturesExtra is the analyzer output that doesn't have its own
+// dedicated column — kept in extra_json so the schema doesn't need a
+// migration every time the analyzer grows a new measurement.
+type audioFeaturesExtra struct {
+	SpectralCentroidHz float64     `json:"spectralCentroidHz"`
+	SpectralRolloffHz  float64     `json:"spectralRolloffHz"`
+	ZeroCrossingRate   float64     `json:"zeroCrossingRate"`
+	Chroma             [12]float64 `json:"chroma"`
+}
+
+// Vector reconstructs the same fixed-length feature vector
+// audiofeatures.Features.Vector produces, from stored values.
+func (r AudioFeaturesRow) Vector() []float64 {
+	tempo := 0.0
+	if r.TempoValid {
+		tempo = r.TempoBPM
+	}
+	v := make([]float64, 0, 17)
+	v = append(v, tempo, r.LoudnessLUFS, r.Extra.SpectralCentroidHz, r.Extra.SpectralRolloffHz, r.Extra.ZeroCrossingRate)
+	v = append(v, r.Extra.Chroma[:]...)
+	return v
+}
+
+// UpsertAudioFeatures stores (or replaces) the analyzed features for a
+// song.
+func UpsertAudioFeatures(db *sql.DB, songID string, f *audiofeatures.Features) error {
+	extra := audioFeaturesExtra{
+		SpectralCentroidHz: f.SpectralCentroidHz,
+		SpectralRolloffHz:  f.SpectralRolloffHz,
+		ZeroCrossingRate:   f.ZeroCrossingRate,
+		Chroma:             f.Chroma,
+	}
+	extraJSON, err := json.Marshal(extra)
+	if err != nil {
+		return err
+	}
+	var tempo sql.NullFloat64
+	if f.TempoValid {
+		tempo = sql.NullFloat64{Float64: f.TempoBPM, Valid: true}
+	}
+	_, err = db.Exec(`
+		INSERT INTO audio_features (song_id, tempo_bpm, musical_key, loudness_lufs, extra_json, updated_at)
+		VALUES (?, ?, ?, ?, ?, strftime('%Y-%m-%dT%H:%M:%fZ','now'))
+		ON CONFLICT(song_id) DO UPDATE SET
+			tempo_bpm = excluded.tempo_bpm,
+			musical_key = excluded.musical_key,
+			loudness_lufs = excluded.loudness_lufs,
+			extra_json = excluded.extra_json,
+			updated_at = excluded.updated_at`,
+		songID, tempo, f.Key, f.LoudnessLUFS, string(extraJSON))
+	return err
+}
+
+const audioFeaturesSelect = `SELECT song_id, tempo_bpm, musical_key, loudness_lufs, extra_json FROM audio_features`
+
+func scanAudioFeaturesRow(scan func(dest ...any) error) (*AudioFeaturesRow, error) {
+	var r AudioFeaturesRow
+	var tempo sql.NullFloat64
+	var loudness sql.NullFloat64
+	var extraJSON string
+	if err := scan(&r.SongID, &tempo, &r.MusicalKey, &loudness, &extraJSON); err != nil {
+		return nil, err
+	}
+	r.TempoValid = tempo.Valid
+	r.TempoBPM = tempo.Float64
+	r.LoudnessLUFS = loudness.Float64
+	_ = json.Unmarshal([]byte(extraJSON), &r.Extra)
+	return &r, nil
+}
+
+// GetAudioFeatures returns a song's stored analysis, or nil if it
+// hasn't been analyzed yet.
+func GetAudioFeatures(db *sql.DB, songID string) (*AudioFeaturesRow, error) {
+	row := db.QueryRow(audioFeaturesSelect+` WHERE song_id = ?`, songID)
+	r, err := scanAudioFeaturesRow(row.Scan)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	return r, err
+}
+
+// ListAllAudioFeatures returns every analyzed song's features, the
+// input the in-process sonic-similarity index is built from.
+func ListAllAudioFeatures(db *sql.DB) ([]AudioFeaturesRow, error) {
+	rows, err := db.Query(audioFeaturesSelect)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var out []AudioFeaturesRow
+	for rows.Next() {
+		r, err := scanAudioFeaturesRow(rows.Scan)
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, *r)
+	}
+	return out, rows.Err()
+}
diff --git a/internal/database/audio_features_test.go b/internal/database/audio_features_test.go
new file mode 100644
index 0000000..3ccff28
--- /dev/null
+++ b/internal/database/audio_features_test.go
@@ -0,0 +1,109 @@
+package database
+
+import (
+	"database/sql"
+	"path/filepath"
+	"testing"
+
+	"git.noroutes.net/NoRoutes/vibetunes/internal/audiofeatures"
+)
+
+func openAudioFeaturesTestDB(t *testing.T) (*sql.DB, string) {
+	t.Helper()
+	db, err := Open(filepath.Join(t.TempDir(), "test.db"))
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	t.Cleanup(func() { db.Close() })
+	folder, err := CreateMusicFolder(db, "Music", "/music", "music")
+	if err != nil {
+		t.Fatal(err)
+	}
+	mf, err := CreateMediaFile(db, folder.ID, "song.mp3", 100, 0, "h", "mp3", "song", 10, 128, 44100, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	song, err := UpsertSong(db, UpsertSongInput{MediaFileID: mf.ID, Title: "Song"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return db, song.ID
+}
+
+func TestUpsertAndGetAudioFeatures(t *testing.T) {
+	db, songID := openAudioFeaturesTestDB(t)
+
+	f := &audiofeatures.Features{
+		TempoBPM: 120, TempoValid: true, Key: "C major", LoudnessLUFS: -14,
+		SpectralCentroidHz: 1500, SpectralRolloffHz: 4000, ZeroCrossingRate: 0.05,
+		Chroma: [12]float64{1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+	}
+	if err := UpsertAudioFeatures(db, songID, f); err != nil {
+		t.Fatalf("UpsertAudioFeatures: %v", err)
+	}
+
+	got, err := GetAudioFeatures(db, songID)
+	if err != nil {
+		t.Fatalf("GetAudioFeatures: %v", err)
+	}
+	if got == nil {
+		t.Fatal("expected a row, got nil")
+	}
+	if !got.TempoValid || got.TempoBPM != 120 {
+		t.Errorf("tempo = %v/%v, want 120/valid", got.TempoBPM, got.TempoValid)
+	}
+	if got.MusicalKey != "C major" {
+		t.Errorf("key = %q", got.MusicalKey)
+	}
+	if got.LoudnessLUFS != -14 {
+		t.Errorf("loudness = %v", got.LoudnessLUFS)
+	}
+	if got.Extra.SpectralCentroidHz != 1500 || got.Extra.Chroma[0] != 1 {
+		t.Errorf("extra = %+v", got.Extra)
+	}
+}
+
+func TestUpsertAudioFeaturesOverwrites(t *testing.T) {
+	db, songID := openAudioFeaturesTestDB(t)
+	if err := UpsertAudioFeatures(db, songID, &audiofeatures.Features{Key: "C major"}); err != nil {
+		t.Fatal(err)
+	}
+	if err := UpsertAudioFeatures(db, songID, &audiofeatures.Features{Key: "D minor"}); err != nil {
+		t.Fatal(err)
+	}
+	got, err := GetAudioFeatures(db, songID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.MusicalKey != "D minor" {
+		t.Errorf("key = %q, want overwrite to D minor", got.MusicalKey)
+	}
+}
+
+func TestGetAudioFeaturesNotAnalyzed(t *testing.T) {
+	db, songID := openAudioFeaturesTestDB(t)
+	got, err := GetAudioFeatures(db, songID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != nil {
+		t.Errorf("expected nil for an unanalyzed song, got %+v", got)
+	}
+}
+
+func TestListAllAudioFeatures(t *testing.T) {
+	db, songID := openAudioFeaturesTestDB(t)
+	if err := UpsertAudioFeatures(db, songID, &audiofeatures.Features{TempoValid: true, TempoBPM: 90}); err != nil {
+		t.Fatal(err)
+	}
+	all, err := ListAllAudioFeatures(db)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(all) != 1 || all[0].SongID != songID {
+		t.Fatalf("all = %+v", all)
+	}
+	if len(all[0].Vector()) != 17 {
+		t.Fatalf("Vector length = %d, want 17", len(all[0].Vector()))
+	}
+}
diff --git a/internal/scanner/scanner.go b/internal/scanner/scanner.go
index b5a403d..2d93bd5 100644
--- a/internal/scanner/scanner.go
+++ b/internal/scanner/scanner.go
@@ -15,6 +15,7 @@ import (
 	"sync"
 	"time"
 
+	"git.noroutes.net/NoRoutes/vibetunes/internal/audiofeatures"
 	"git.noroutes.net/NoRoutes/vibetunes/internal/coverart"
 	"git.noroutes.net/NoRoutes/vibetunes/internal/database"
 	"git.noroutes.net/NoRoutes/vibetunes/internal/library"
@@ -35,9 +36,10 @@ const maxWorkers = 4
 
 // Scanner indexes one or more music folders into the database.
 type Scanner struct {
-	DB     *sql.DB
-	Prober *metadata.Prober
-	Log    *slog.Logger
+	DB         *sql.DB
+	Prober     *metadata.Prober
+	Log        *slog.Logger
+	FFmpegPath string // for sonic-analysis audio decoding; "" defaults to "ffmpeg" on PATH
 
 	mu       sync.Mutex
 	scanning map[string]bool // music_folder_id -> in progress
@@ -175,6 +177,7 @@ func (s *Scanner) indexOne(ctx context.Context, folderID, absPath, relPath strin
 			return false, false, err
 		}
 		s.indexCoverArt(absPath, song, tags)
+		s.analyzeAudio(ctx, absPath, song, tags)
 		return false, true, nil
 	}
 	if err != database.ErrNotFound {
@@ -204,9 +207,31 @@ func (s *Scanner) indexOne(ctx context.Context, folderID, absPath, relPath strin
 		return false, false, err
 	}
 	s.indexCoverArt(absPath, song, tags)
+	s.analyzeAudio(ctx, absPath, song, tags)
 	return true, false, nil
 }
 
+// analyzeAudio runs the sonic-feature analyzer over a scanned file and
+// stores the result, powering the sonicSimilarity extension. This runs
+// as part of the (already background, admin-triggered) scan itself
+// rather than on any request path. A decode/analysis failure is logged
+// and otherwise ignored: the song is still indexed and playable, it
+// just won't appear in sonic-similarity results until a future rescan
+// succeeds — never a reason to fail the whole scan.
+func (s *Scanner) analyzeAudio(ctx context.Context, absPath string, song *database.Song, tags *metadata.Tags) {
+	if tags.Kind != "song" {
+		return
+	}
+	features, err := audiofeatures.Analyze(ctx, s.FFmpegPath, absPath)
+	if err != nil {
+		s.Log.Warn("audio feature analysis failed", "err", err, "path", absPath)
+		return
+	}
+	if err := database.UpsertAudioFeatures(s.DB, song.ID, features); err != nil {
+		s.Log.Warn("storing audio features failed", "err", err, "path", absPath)
+	}
+}
+
 // indexCoverArt records where an album's cover art can be found: a
 // folder image next to the file if one exists, otherwise the file's own
 // embedded artwork (extracted lazily by the web layer on request, not
diff --git a/internal/scanner/scanner_test.go b/internal/scanner/scanner_test.go
index 78eebec..ac0ac72 100644
--- a/internal/scanner/scanner_test.go
+++ b/internal/scanner/scanner_test.go
@@ -279,3 +279,40 @@ func TestScanFolderIndexesSidecarLRC(t *testing.T) {
 		t.Fatalf("expected word-level cues from Enhanced LRC line, got %+v", synced.Lines[1])
 	}
 }
+
+func TestScanFolderAnalyzesAudioFeatures(t *testing.T) {
+	s, folder, dir := newTestScanner(t)
+	genFile(t, dir, "song1.mp3", "Song One", "Artist One")
+
+	if err := s.ScanFolder(context.Background(), folder.ID); err != nil {
+		t.Fatalf("ScanFolder: %v", err)
+	}
+
+	songs, err := database.ListSongs(s.DB, 10, 0)
+	if err != nil || len(songs) != 1 {
+		t.Fatalf("ListSongs: %v %v", songs, err)
+	}
+
+	features, err := database.GetAudioFeatures(s.DB, songs[0].ID)
+	if err != nil {
+		t.Fatalf("GetAudioFeatures: %v", err)
+	}
+	if features == nil {
+		t.Fatal("expected audio features to be stored after scanning, got none")
+	}
+	// The fixture is a pure 440Hz sine tone (A4): its chroma should
+	// actually peak at pitch class A (bin 9), a real measured property,
+	// not a placeholder.
+	if features.Extra.Chroma == ([12]float64{}) {
+		t.Error("expected a non-zero chroma vector from real analysis")
+	}
+	best := 0
+	for i := 1; i < 12; i++ {
+		if features.Extra.Chroma[i] > features.Extra.Chroma[best] {
+			best = i
+		}
+	}
+	if best != 9 { // A
+		t.Errorf("chroma peak = bin %d, want 9 (A) for a 440Hz fixture", best)
+	}
+}
diff --git a/internal/similarity/index.go b/internal/similarity/index.go
new file mode 100644
index 0000000..f5a0080
--- /dev/null
+++ b/internal/similarity/index.go
@@ -0,0 +1,203 @@
+// Package similarity builds a small in-process nearest-neighbor index
+// over numeric audio-feature vectors and uses it for two things:
+// ranking tracks by actual sonic closeness, and walking a musically
+// reasonable path between two tracks through feature space. There is
+// no external vector database — the whole index is just normalized
+// vectors held in memory, rebuilt from internal/database's
+// audio_features table whenever a caller needs it.
+package similarity
+
+import (
+	"math"
+	"sort"
+)
+
+// Item is one track's raw (un-normalized) feature vector, keyed by
+// song ID.
+type Item struct {
+	ID     string
+	Vector []float64
+}
+
+// Index holds z-score-normalized vectors for a set of tracks. Each
+// dimension is normalized independently using the mean/stddev of that
+// dimension across exactly the items the index was built from, so
+// dimensions with a wide natural range (like tempo, in BPM) don't
+// dominate Euclidean distance over dimensions with a narrow one (like a
+// normalized chroma bin).
+type Index struct {
+	ids  []string
+	norm [][]float64
+}
+
+// Build normalizes and indexes items. An item whose Vector length
+// disagrees with the first item's is dropped rather than causing a
+// panic or silently misaligning dimensions — a defensive check, not
+// something expected to trigger with correctly-produced feature rows.
+func Build(items []Item) *Index {
+	idx := &Index{}
+	if len(items) == 0 {
+		return idx
+	}
+	dims := len(items[0].Vector)
+
+	var kept []Item
+	for _, it := range items {
+		if len(it.Vector) == dims {
+			kept = append(kept, it)
+		}
+	}
+	if len(kept) == 0 {
+		return idx
+	}
+
+	means := make([]float64, dims)
+	for _, it := range kept {
+		for d, v := range it.Vector {
+			means[d] += v
+		}
+	}
+	for d := range means {
+		means[d] /= float64(len(kept))
+	}
+
+	stds := make([]float64, dims)
+	for _, it := range kept {
+		for d, v := range it.Vector {
+			diff := v - means[d]
+			stds[d] += diff * diff
+		}
+	}
+	for d := range stds {
+		stds[d] = math.Sqrt(stds[d] / float64(len(kept)))
+	}
+
+	idx.ids = make([]string, len(kept))
+	idx.norm = make([][]float64, len(kept))
+	for i, it := range kept {
+		nv := make([]float64, dims)
+		for d, v := range it.Vector {
+			if stds[d] > 1e-9 {
+				nv[d] = (v - means[d]) / stds[d]
+			}
+		}
+		idx.ids[i] = it.ID
+		idx.norm[i] = nv
+	}
+	return idx
+}
+
+// Match is one similarity result: another track and its distance from
+// the query track in normalized feature space (0 = identical, larger =
+// less similar).
+type Match struct {
+	ID       string
+	Distance float64
+}
+
+// Nearest returns up to k tracks closest to id, nearest first,
+// excluding id itself. Returns nil if id isn't in the index.
+func (idx *Index) Nearest(id string, k int) []Match {
+	pos := idx.indexOf(id)
+	if pos < 0 {
+		return nil
+	}
+	matches := make([]Match, 0, len(idx.ids)-1)
+	for i, other := range idx.ids {
+		if i == pos {
+			continue
+		}
+		matches = append(matches, Match{ID: other, Distance: euclidean(idx.norm[pos], idx.norm[i])})
+	}
+	sort.Slice(matches, func(a, b int) bool { return matches[a].Distance < matches[b].Distance })
+	if k > 0 && len(matches) > k {
+		matches = matches[:k]
+	}
+	return matches
+}
+
+// Path walks from fromID to toID in `steps` hops (including both
+// endpoints), picking at each intermediate step the not-yet-used track
+// closest to the corresponding point on the straight line between
+// fromID's and toID's feature vectors. That keeps the sequence
+// perceptually gradual — each hop is a small, real step in sonic
+// character rather than a random jump — without ever inventing a track
+// that isn't actually in the library. Returns nil if either endpoint
+// isn't indexed.
+func (idx *Index) Path(fromID, toID string, steps int) []string {
+	fromPos := idx.indexOf(fromID)
+	toPos := idx.indexOf(toID)
+	if fromPos < 0 || toPos < 0 {
+		return nil
+	}
+	if steps < 2 {
+		steps = 2
+	}
+
+	path := []string{fromID}
+	used := map[string]bool{fromID: true, toID: true}
+	fromVec, toVec := idx.norm[fromPos], idx.norm[toPos]
+
+	for step := 1; step < steps-1; step++ {
+		t := float64(step) / float64(steps-1)
+		target := lerp(fromVec, toVec, t)
+
+		best := -1
+		bestDist := math.Inf(1)
+		for i, id := range idx.ids {
+			if used[id] {
+				continue
+			}
+			d := euclidean(idx.norm[i], target)
+			if d < bestDist {
+				bestDist = d
+				best = i
+			}
+		}
+		if best < 0 {
+			break // library exhausted; fall through with what we have
+		}
+		path = append(path, idx.ids[best])
+		used[idx.ids[best]] = true
+	}
+
+	path = append(path, toID)
+	return path
+}
+
+// DistanceBetween returns the normalized-feature-space Euclidean
+// distance between two indexed tracks, or +Inf if either isn't
+// indexed.
+func (idx *Index) DistanceBetween(idA, idB string) float64 {
+	a, b := idx.indexOf(idA), idx.indexOf(idB)
+	if a < 0 || b < 0 {
+		return math.Inf(1)
+	}
+	return euclidean(idx.norm[a], idx.norm[b])
+}
+
+func (idx *Index) indexOf(id string) int {
+	for i, v := range idx.ids {
+		if v == id {
+			return i
+		}
+	}
+	return -1
+}
+
+func euclidean(a, b []float64) float64 {
+	var sum float64
+	for i := range a {
+		d := a[i] - b[i]
+		sum += d * d
+	}
+	return math.Sqrt(sum)
+}
+
+func lerp(a, b []float64, t float64) []float64 {
+	out := make([]float64, len(a))
+	for i := range a {
+		out[i] = a[i] + (b[i]-a[i])*t
+	}
+	return out
+}
diff --git a/internal/similarity/index_test.go b/internal/similarity/index_test.go
new file mode 100644
index 0000000..4f2bdc8
--- /dev/null
+++ b/internal/similarity/index_test.go
@@ -0,0 +1,116 @@
+package similarity
+
+import "testing"
+
+func tempoLibrary() []Item {
+	// A synthetic library where the only varying dimension is "tempo":
+	// songs at 60, 70, 80, ..., 150 BPM. Deterministic and needs no
+	// audio decoding, so it exercises the index math directly.
+	var items []Item
+	for i, bpm := range []float64{60, 70, 80, 90, 100, 110, 120, 130, 140, 150} {
+		items = append(items, Item{ID: idFor(i), Vector: []float64{bpm, 0, 0}})
+	}
+	return items
+}
+
+func idFor(i int) string {
+	return string(rune('a' + i))
+}
+
+func TestNearestOrdersByActualDistance(t *testing.T) {
+	idx := Build(tempoLibrary())
+	// "d" is 90 BPM; its closest neighbors by tempo should be "c" (80)
+	// and "e" (100), in that order (tie broken by index order is fine
+	// since both are exactly 10 BPM away — check the pair, not order).
+	matches := idx.Nearest("d", 2)
+	if len(matches) != 2 {
+		t.Fatalf("matches = %v, want 2", matches)
+	}
+	got := map[string]bool{matches[0].ID: true, matches[1].ID: true}
+	if !got["c"] || !got["e"] {
+		t.Errorf("nearest to d(90bpm) = %v, want {c(80bpm), e(100bpm)}", matches)
+	}
+}
+
+func TestNearestExcludesSelf(t *testing.T) {
+	idx := Build(tempoLibrary())
+	matches := idx.Nearest("a", 20)
+	for _, m := range matches {
+		if m.ID == "a" {
+			t.Fatal("Nearest must never include the query track itself")
+		}
+	}
+}
+
+func TestNearestUnknownID(t *testing.T) {
+	idx := Build(tempoLibrary())
+	if matches := idx.Nearest("does-not-exist", 5); matches != nil {
+		t.Errorf("expected nil for unknown id, got %v", matches)
+	}
+}
+
+func TestNearestFarthestIsOppositeEnd(t *testing.T) {
+	idx := Build(tempoLibrary())
+	matches := idx.Nearest("a", 20) // a = 60bpm, all 9 others
+	if len(matches) != 9 {
+		t.Fatalf("matches = %d, want 9", len(matches))
+	}
+	farthest := matches[len(matches)-1]
+	if farthest.ID != "j" { // j = 150bpm, the farthest from 60bpm
+		t.Errorf("farthest from a(60bpm) = %v, want j(150bpm)", farthest)
+	}
+}
+
+func TestPathIsMonotonicAlongTheOnlyVaryingDimension(t *testing.T) {
+	idx := Build(tempoLibrary())
+	path := idx.Path("a", "j", 5) // 60bpm -> 150bpm in 5 hops
+	if len(path) != 5 {
+		t.Fatalf("path = %v, want length 5", path)
+	}
+	if path[0] != "a" || path[len(path)-1] != "j" {
+		t.Fatalf("path endpoints = %q..%q, want a..j", path[0], path[len(path)-1])
+	}
+	seen := map[string]bool{}
+	for _, id := range path {
+		if seen[id] {
+			t.Fatalf("path repeats %q: %v", id, path)
+		}
+		seen[id] = true
+	}
+	// With tempo as the only varying dimension, a straight-line
+	// interpolation from 60 to 150 BPM should visit strictly increasing
+	// tempos.
+	bpmOf := map[string]float64{"a": 60, "b": 70, "c": 80, "d": 90, "e": 100, "f": 110, "g": 120, "h": 130, "i": 140, "j": 150}
+	for i := 1; i < len(path); i++ {
+		if bpmOf[path[i]] <= bpmOf[path[i-1]] {
+			t.Errorf("path not monotonically increasing in tempo: %v", path)
+			break
+		}
+	}
+}
+
+func TestPathUnknownEndpoint(t *testing.T) {
+	idx := Build(tempoLibrary())
+	if path := idx.Path("a", "nope", 5); path != nil {
+		t.Errorf("expected nil path for unknown endpoint, got %v", path)
+	}
+}
+
+func TestBuildEmpty(t *testing.T) {
+	idx := Build(nil)
+	if matches := idx.Nearest("a", 5); matches != nil {
+		t.Errorf("expected nil from an empty index, got %v", matches)
+	}
+}
+
+func TestBuildConstantDimensionDoesNotDivideByZero(t *testing.T) {
+	items := []Item{
+		{ID: "x", Vector: []float64{5, 1}},
+		{ID: "y", Vector: []float64{5, 2}},
+	}
+	idx := Build(items)
+	matches := idx.Nearest("x", 5)
+	if len(matches) != 1 || matches[0].ID != "y" {
+		t.Fatalf("matches = %v", matches)
+	}
+}
diff --git a/internal/subsonic/opensubsonic_core_test.go b/internal/subsonic/opensubsonic_core_test.go
index 11af252..4ced864 100644
--- a/internal/subsonic/opensubsonic_core_test.go
+++ b/internal/subsonic/opensubsonic_core_test.go
@@ -55,7 +55,7 @@ func TestGetOpenSubsonicExtensions(t *testing.T) {
 	// set once Phase 3 is done.
 	for _, want := range []string{
 		"apiKeyAuthentication", "formPost", "getPodcastEpisode", "topSongsByArtistId", "transcodeOffset",
-		"indexBasedQueue", "playbackReport", "songLyrics", "transcoding",
+		"indexBasedQueue", "playbackReport", "songLyrics", "transcoding", "sonicSimilarity",
 	} {
 		if !names[want] {
 			t.Errorf("extension %q not advertised: %v", want, names)
diff --git a/internal/subsonic/sonic_similarity.go b/internal/subsonic/sonic_similarity.go
new file mode 100644
index 0000000..9ba0b87
--- /dev/null
+++ b/internal/subsonic/sonic_similarity.go
@@ -0,0 +1,137 @@
+package subsonic
+
+import (
+	"database/sql"
+
+	"git.noroutes.net/NoRoutes/vibetunes/internal/database"
+	"git.noroutes.net/NoRoutes/vibetunes/internal/similarity"
+)
+
+func init() {
+	registerExtension("sonicSimilarity", 1)
+	register("getSonicSimilarTracks", handleGetSonicSimilarTracks)
+	register("findSonicPath", handleFindSonicPath)
+}
+
+// buildSimilarityIndex loads every song VibeSonic has actually analyzed
+// (audio_features rows populated by the scanner's background analysis)
+// and builds an in-process nearest-neighbor index from their real
+// feature vectors. It's rebuilt per request rather than cached: the
+// library-wide mean/stddev normalization it depends on shifts as more
+// songs get analyzed, and a from-scratch build over even a large
+// library is a few hundred microseconds of arithmetic over small
+// vectors, not a bottleneck worth caching.
+func buildSimilarityIndex(db *sql.DB) (*similarity.Index, error) {
+	rows, err := database.ListAllAudioFeatures(db)
+	if err != nil {
+		return nil, err
+	}
+	items := make([]similarity.Item, len(rows))
+	for i, r := range rows {
+		items[i] = similarity.Item{ID: r.SongID, Vector: r.Vector()}
+	}
+	return similarity.Build(items), nil
+}
+
+// similarityScore converts a Euclidean distance in normalized feature
+// space into the "normalized similarity score" the spec asks for: 1.0
+// for an identical vector, approaching 0 as distance grows, never
+// negative.
+func similarityScore(distance float64) float64 {
+	return 1 / (1 + distance)
+}
+
+func childForSongID(db *sql.DB, songID, userID string) (*Child, error) {
+	song, err := database.GetSong(db, songID)
+	if err != nil {
+		return nil, err
+	}
+	mf, err := database.GetMediaFile(db, song.MediaFileID)
+	if err != nil {
+		return nil, err
+	}
+	c := childFromSong(db, song, mf, albumOfSong(db, song), userID)
+	return &c, nil
+}
+
+func handleGetSonicSimilarTracks(rc *Context) *apiError {
+	id, aerr := rc.Params.Required("id")
+	if aerr != nil {
+		return aerr
+	}
+	count := rc.Params.Int("count", 10)
+	db := rc.Server.DB
+
+	if _, err := database.GetSong(db, id); err != nil {
+		return errNotFound("Song")
+	}
+
+	idx, err := buildSimilarityIndex(db)
+	if err != nil {
+		return newError(ErrGeneric, err.Error())
+	}
+	matches := idx.Nearest(id, count)
+	if matches == nil {
+		// Either the song hasn't been analyzed yet, or nothing else in
+		// the library has — an honest empty result, not an error.
+		rc.Response = newResponse()
+		return nil
+	}
+
+	var out []SonicMatch
+	for _, m := range matches {
+		child, err := childForSongID(db, m.ID, rc.User.ID)
+		if err != nil {
+			continue
+		}
+		out = append(out, SonicMatch{Entry: *child, Similarity: similarityScore(m.Distance)})
+	}
+	rc.Response = newResponse()
+	rc.Response.SonicMatch = out
+	return nil
+}
+
+func handleFindSonicPath(rc *Context) *apiError {
+	startID, aerr := rc.Params.Required("startSongId")
+	if aerr != nil {
+		return aerr
+	}
+	endID, aerr := rc.Params.Required("endSongId")
+	if aerr != nil {
+		return aerr
+	}
+	count := rc.Params.Int("count", 25)
+	db := rc.Server.DB
+
+	if _, err := database.GetSong(db, startID); err != nil {
+		return errNotFound("Start song")
+	}
+	if _, err := database.GetSong(db, endID); err != nil {
+		return errNotFound("End song")
+	}
+
+	idx, err := buildSimilarityIndex(db)
+	if err != nil {
+		return newError(ErrGeneric, err.Error())
+	}
+	path := idx.Path(startID, endID, count)
+	if path == nil {
+		// Analysis hasn't run for one or both endpoints yet: an honest
+		// empty result rather than a fabricated single-hop path.
+		rc.Response = newResponse()
+		return nil
+	}
+
+	var out []SonicMatch
+	for _, songID := range path {
+		child, err := childForSongID(db, songID, rc.User.ID)
+		if err != nil {
+			continue
+		}
+		distFromStart := idx.DistanceBetween(startID, songID)
+		out = append(out, SonicMatch{Entry: *child, Similarity: similarityScore(distFromStart)})
+	}
+	rc.Response = newResponse()
+	rc.Response.SonicMatch = out
+	return nil
+}
diff --git a/internal/subsonic/sonic_similarity_test.go b/internal/subsonic/sonic_similarity_test.go
new file mode 100644
index 0000000..2f9b30d
--- /dev/null
+++ b/internal/subsonic/sonic_similarity_test.go
@@ -0,0 +1,188 @@
+package subsonic
+
+import (
+	"testing"
+
+	"git.noroutes.net/NoRoutes/vibetunes/internal/audiofeatures"
+	"git.noroutes.net/NoRoutes/vibetunes/internal/database"
+)
+
+// makeAnalyzedSong creates a minimal song fixture (no real audio file
+// needed — sonic-similarity handlers only ever read the songs table and
+// audio_features, never touch disk) and stores a hand-crafted feature
+// vector for it, isolating these tests from ffmpeg/analysis timing.
+func makeAnalyzedSong(t *testing.T, s *Server, folderID, title string, tempo float64) *database.Song {
+	t.Helper()
+	mf, err := database.CreateMediaFile(s.DB, folderID, title+".mp3", 100, 0, "hash-"+title, "mp3", "song", 10, 128, 44100, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	song, err := database.UpsertSong(s.DB, database.UpsertSongInput{MediaFileID: mf.ID, Title: title})
+	if err != nil {
+		t.Fatal(err)
+	}
+	f := &audiofeatures.Features{TempoBPM: tempo, TempoValid: true, Key: "C major"}
+	if err := database.UpsertAudioFeatures(s.DB, song.ID, f); err != nil {
+		t.Fatal(err)
+	}
+	return song
+}
+
+func sonicTestLibrary(t *testing.T, s *Server) (slow, mid, fast, unanalyzed *database.Song) {
+	t.Helper()
+	folder, err := database.CreateMusicFolder(s.DB, "Music", "/music", "music")
+	if err != nil {
+		t.Fatal(err)
+	}
+	slow = makeAnalyzedSong(t, s, folder.ID, "Slow", 70)
+	mid = makeAnalyzedSong(t, s, folder.ID, "Mid", 100)
+	fast = makeAnalyzedSong(t, s, folder.ID, "Fast", 160)
+
+	mf, err := database.CreateMediaFile(s.DB, folder.ID, "Unanalyzed.mp3", 100, 0, "hash-unanalyzed", "mp3", "song", 10, 128, 44100, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	unanalyzed, err = database.UpsertSong(s.DB, database.UpsertSongInput{MediaFileID: mf.ID, Title: "Unanalyzed"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return
+}
+
+func TestGetSonicSimilarTracksOrdersByRealDistance(t *testing.T) {
+	s, _ := newTestServer(t)
+	createTestUser(t, s, "alice", "password123", false)
+	_, mid, fast, _ := sonicTestLibrary(t, s)
+
+	srv := newSubsonicTestServer(t, s)
+	q := baseParams("alice", "password123")
+	q.Set("id", mid.ID) // 100bpm: 30bpm from Slow(70), 60bpm from Fast(160) — Slow should rank first
+	q.Set("f", "json")
+	resp := getPath(t, srv, "/rest/getSonicSimilarTracks.view", q)
+	j := decodeJSON(t, resp)
+	matches, ok := j["sonicMatch"].([]any)
+	if !ok || len(matches) != 2 {
+		t.Fatalf("sonicMatch = %v, want 2 entries", j)
+	}
+	first := matches[0].(map[string]any)
+	entry := first["entry"].(map[string]any)
+	if entry["title"] != "Slow" {
+		t.Errorf("nearest to Mid(100bpm) = %v, want Slow(70bpm) (closer than Fast(160bpm))", entry["title"])
+	}
+	sim := first["similarity"].(float64)
+	if sim <= 0 || sim > 1 {
+		t.Errorf("similarity = %v, want in (0,1]", sim)
+	}
+	_ = fast
+}
+
+func TestGetSonicSimilarTracksExcludesUnanalyzedSongs(t *testing.T) {
+	s, _ := newTestServer(t)
+	createTestUser(t, s, "alice", "password123", false)
+	slow, _, _, unanalyzed := sonicTestLibrary(t, s)
+
+	srv := newSubsonicTestServer(t, s)
+	q := baseParams("alice", "password123")
+	q.Set("id", slow.ID)
+	q.Set("f", "json")
+	resp := getPath(t, srv, "/rest/getSonicSimilarTracks.view", q)
+	j := decodeJSON(t, resp)
+	matches := j["sonicMatch"].([]any)
+	for _, m := range matches {
+		entry := m.(map[string]any)["entry"].(map[string]any)
+		if entry["title"] == "Unanalyzed" {
+			t.Error("a song with no stored audio_features must never appear in sonic-similarity results")
+		}
+	}
+	_ = unanalyzed
+}
+
+func TestGetSonicSimilarTracksUnknownSong(t *testing.T) {
+	s, srv := newTestRecorderServer(t)
+	createTestUser(t, s, "alice", "password123", false)
+
+	q := baseParams("alice", "password123")
+	q.Set("id", "sg_doesnotexist")
+	resp := getPath(t, srv.URL, "/rest/getSonicSimilarTracks.view", q)
+	x := decodeXML(t, resp)
+	if x.Status != "failed" || x.Error == nil || x.Error.Code != ErrNotFound {
+		t.Fatalf("expected error 70, got %+v", x)
+	}
+}
+
+func TestGetSonicSimilarTracksNeverAnalyzedReturnsEmpty(t *testing.T) {
+	s, _ := newTestServer(t)
+	createTestUser(t, s, "alice", "password123", false)
+	folder, err := database.CreateMusicFolder(s.DB, "Music", "/music", "music")
+	if err != nil {
+		t.Fatal(err)
+	}
+	mf, err := database.CreateMediaFile(s.DB, folder.ID, "solo.mp3", 100, 0, "h", "mp3", "song", 10, 128, 44100, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	song, err := database.UpsertSong(s.DB, database.UpsertSongInput{MediaFileID: mf.ID, Title: "Solo"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	srv := newSubsonicTestServer(t, s)
+	q := baseParams("alice", "password123")
+	q.Set("id", song.ID)
+	q.Set("f", "json")
+	resp := getPath(t, srv, "/rest/getSonicSimilarTracks.view", q)
+	j := decodeJSON(t, resp)
+	// No song in the library has been analyzed at all — VibeSonic must
+	// return an honest empty result, never a fabricated match.
+	if _, has := j["sonicMatch"]; has {
+		t.Errorf("expected no sonicMatch key when nothing is analyzed, got %v", j["sonicMatch"])
+	}
+}
+
+func TestFindSonicPathReachesBothEndpointsAndOrdersMonotonically(t *testing.T) {
+	s, _ := newTestServer(t)
+	createTestUser(t, s, "alice", "password123", false)
+	slow, _, fast, _ := sonicTestLibrary(t, s)
+
+	srv := newSubsonicTestServer(t, s)
+	q := baseParams("alice", "password123")
+	q.Set("startSongId", slow.ID)
+	q.Set("endSongId", fast.ID)
+	q.Set("count", "3")
+	q.Set("f", "json")
+	resp := getPath(t, srv, "/rest/findSonicPath.view", q)
+	j := decodeJSON(t, resp)
+	matches, ok := j["sonicMatch"].([]any)
+	if !ok || len(matches) != 3 {
+		t.Fatalf("sonicMatch = %v, want 3 (start, mid, end)", j)
+	}
+	first := matches[0].(map[string]any)["entry"].(map[string]any)
+	last := matches[2].(map[string]any)["entry"].(map[string]any)
+	middle := matches[1].(map[string]any)["entry"].(map[string]any)
+	if first["title"] != "Slow" || last["title"] != "Fast" {
+		t.Fatalf("path endpoints = %v .. %v, want Slow .. Fast", first["title"], last["title"])
+	}
+	if middle["title"] != "Mid" {
+		t.Errorf("path middle hop = %v, want Mid (the only other analyzed song, between Slow and Fast)", middle["title"])
+	}
+	startSim := matches[0].(map[string]any)["similarity"].(float64)
+	if startSim != 1 {
+		t.Errorf("similarity of start song to itself = %v, want 1", startSim)
+	}
+}
+
+func TestFindSonicPathUnknownEndpoint(t *testing.T) {
+	s, _ := newTestServer(t)
+	createTestUser(t, s, "alice", "password123", false)
+	slow, _, _, _ := sonicTestLibrary(t, s)
+
+	srv := newSubsonicTestServer(t, s)
+	q := baseParams("alice", "password123")
+	q.Set("startSongId", slow.ID)
+	q.Set("endSongId", "sg_doesnotexist")
+	resp := getPath(t, srv, "/rest/findSonicPath.view", q)
+	x := decodeXML(t, resp)
+	if x.Status != "failed" || x.Error == nil || x.Error.Code != ErrNotFound {
+		t.Fatalf("expected error 70, got %+v", x)
+	}
+}