feat(plugins): add similar songs retrieval functions and improve duration consistency (#4933)

* feat: add duration filtering for similar songs matching

Signed-off-by: Deluan <deluan@navidrome.org>

* test: refactor expectations for similar songs in provider matching tests

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(plugins): add functions to retrieve similar songs by track, album, and artist

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(plugins): support uint32 in ndpgen

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(plugins): update duration field to use seconds as float instead of milliseconds as uint32

Signed-off-by: Deluan <deluan@navidrome.org>

* fix: add helper functions for Rust's skip_serializing_if with numeric types

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(provider): enhance track matching logic to fallback to title match when duration-filtered tracks fail

---------

Signed-off-by: Deluan <deluan@navidrome.org>

adapters/deezer/deezer.go

@@ -135,8 +135,9 @@ func (s *deezerAgent) GetArtistTopSongs(ctx context.Context, _, artistName, _ st
 
 	res := slice.Map(tracks, func(r Track) agents.Song {
 		return agents.Song{
-			Name:  r.Title,
-			Album: r.Album.Title,
+			Name:     r.Title,
+			Album:    r.Album.Title,
+			Duration: uint32(r.Duration * 1000), // Convert seconds to milliseconds
 		}
 	})
 	return res, nil

core/agents/interfaces.go

@@ -40,6 +40,7 @@ type Song struct {
 	ArtistMBID string
 	Album      string
 	AlbumMBID  string
+	Duration   uint32 // Duration in milliseconds, 0 means unknown
 }
 
 var (

core/external/provider_matching.go

@@ -3,6 +3,7 @@ package external
 import (
 	"context"
 	"fmt"
+	"math"
 
 	"github.com/Masterminds/squirrel"
 	"github.com/navidrome/navidrome/conf"
@@ -12,6 +13,11 @@ import (
 	"github.com/xrash/smetrics"
 )
 
+// durationToleranceSec is the maximum allowed difference in seconds when
+// matching tracks by duration. A tolerance of 3 seconds accounts for minor
+// encoding differences between sources.
+const durationToleranceSec = 3
+
 // matchSongsToLibrary matches agent song results to local library tracks using a multi-phase
 // matching algorithm that prioritizes accuracy over recall.
 //
@@ -175,6 +181,7 @@ type songQuery struct {
 	artistMBID string // MusicBrainz Artist ID (optional, for higher specificity matching)
 	album      string // Sanitized album name (optional, for specificity scoring)
 	albumMBID  string // MusicBrainz Album ID (optional, for highest specificity matching)
+	durationMs uint32 // Duration in milliseconds (0 means unknown, skip duration filtering)
 }
 
 // matchScore combines title/album similarity with metadata specificity for ranking matches
@@ -282,12 +289,50 @@ func (e *provider) loadTracksByTitleAndArtist(ctx context.Context, songs []agent
 	return matches, nil
 }
 
+// durationMatches checks if a track's duration is within tolerance of the target duration.
+// Returns true if durationMs is 0 (unknown) or if the difference is within durationToleranceSec.
+func durationMatches(durationMs uint32, mediaFileDurationSec float32) bool {
+	if durationMs <= 0 {
+		return true // Unknown duration matches anything
+	}
+	durationSec := float64(durationMs) / 1000.0
+	diff := math.Abs(durationSec - float64(mediaFileDurationSec))
+	return diff <= durationToleranceSec
+}
+
 // findBestMatch finds the best matching track using combined title/album similarity and specificity scoring.
+// When duration is known (durationMs > 0), it acts as a top-priority filter:
+// - First, only tracks with matching duration (±3 seconds) are considered
+// - If no title match is found among duration-filtered tracks, falls back to matching all tracks
 // A track must meet the threshold for title similarity, then the best match is chosen by:
 // 1. Highest title similarity
 // 2. Highest specificity level
 // 3. Highest album similarity (as final tiebreaker)
 func (e *provider) findBestMatch(q songQuery, tracks model.MediaFiles, threshold float64) (model.MediaFile, bool) {
+	// If duration is known, try to find matches among duration-filtered tracks first
+	if q.durationMs > 0 {
+		var durationFiltered model.MediaFiles
+		for _, mf := range tracks {
+			if durationMatches(q.durationMs, mf.Duration) {
+				durationFiltered = append(durationFiltered, mf)
+			}
+		}
+		// If we have duration-filtered candidates, try matching those first
+		if len(durationFiltered) > 0 {
+			if mf, found := findBestMatchInTracks(q, durationFiltered, threshold); found {
+				return mf, true
+			}
+		}
+		// Fall through to try all tracks if no duration-filtered match found
+	}
+
+	return findBestMatchInTracks(q, tracks, threshold)
+}
+
+// findBestMatchInTracks performs the core matching logic on a set of tracks.
+// It finds the track with the best combined score based on title similarity,
+// specificity level, and album similarity.
+func findBestMatchInTracks(q songQuery, tracks model.MediaFiles, threshold float64) (model.MediaFile, bool) {
 	var bestMatch model.MediaFile
 	bestScore := matchScore{titleSimilarity: -1}
 	found := false
@@ -338,6 +383,7 @@ func (e *provider) buildTitleQueries(songs []agents.Song, idMatches, mbidMatches
 			artistMBID: s.ArtistMBID,
 			album:      str.SanitizeFieldForSorting(s.Album),
 			albumMBID:  s.AlbumMBID,
+			durationMs: s.Duration,
 		})
 	}
 	return queries

core/external/provider_matching_test.go

@@ -41,6 +41,26 @@ var _ = Describe("Provider - Song Matching", func() {
 		provider = NewProvider(ds, agentsCombined)
 	})
 
+	// Shared helper for tests that only need artist track queries (no ID/MBID matching)
+	setupSimilarSongsExpectations := func(returnedSongs []agents.Song, artistTracks model.MediaFiles) {
+		agentsCombined.On("GetSimilarSongsByTrack", mock.Anything, mock.Anything, mock.Anything, mock.Anything, mock.Anything, mock.Anything).
+			Return(returnedSongs, nil).Once()
+
+		// loadTracksByTitleAndArtist - queries by artist name
+		mediaFileRepo.On("GetAll", mock.MatchedBy(func(opt model.QueryOptions) bool {
+			and, ok := opt.Filters.(squirrel.And)
+			if !ok || len(and) < 2 {
+				return false
+			}
+			eq, hasEq := and[0].(squirrel.Eq)
+			if !hasEq {
+				return false
+			}
+			_, hasArtist := eq["order_artist_name"]
+			return hasArtist
+		})).Return(artistTracks, nil).Maybe()
+	}
+
 	Describe("matchSongsToLibrary priority matching", func() {
 		var track model.MediaFile
 
@@ -261,26 +281,6 @@ var _ = Describe("Provider - Song Matching", func() {
 			mediaFileRepo.On("Get", "track-1").Return(&track, nil).Once()
 		})
 
-		setupFuzzyExpectations := func(returnedSongs []agents.Song, artistTracks model.MediaFiles) {
-			agentsCombined.On("GetSimilarSongsByTrack", mock.Anything, mock.Anything, mock.Anything, mock.Anything, mock.Anything, mock.Anything).
-				Return(returnedSongs, nil).Once()
-
-			// loadTracksByTitleAndArtist now queries by artist in a single pass
-			// Note: loadTracksByID and loadTracksByMBID return early when no IDs/MBIDs
-			mediaFileRepo.On("GetAll", mock.MatchedBy(func(opt model.QueryOptions) bool {
-				and, ok := opt.Filters.(squirrel.And)
-				if !ok || len(and) < 2 {
-					return false
-				}
-				eq, hasEq := and[0].(squirrel.Eq)
-				if !hasEq {
-					return false
-				}
-				_, hasArtist := eq["order_artist_name"]
-				return hasArtist
-			})).Return(artistTracks, nil).Maybe()
-		}
-
 		Context("with default threshold (85%)", func() {
 			It("matches songs with remastered suffix", func() {
 				conf.Server.SimilarSongsMatchThreshold = 85
@@ -294,7 +294,7 @@ var _ = Describe("Provider - Song Matching", func() {
 					{ID: "remastered", Title: "Paranoid Android - Remastered", Artist: "Radiohead"},
 				}
 
-				setupFuzzyExpectations(returnedSongs, artistTracks)
+				setupSimilarSongsExpectations(returnedSongs, artistTracks)
 
 				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
 
@@ -313,7 +313,7 @@ var _ = Describe("Provider - Song Matching", func() {
 					{ID: "live", Title: "Bohemian Rhapsody (Live)", Artist: "Queen"},
 				}
 
-				setupFuzzyExpectations(returnedSongs, artistTracks)
+				setupSimilarSongsExpectations(returnedSongs, artistTracks)
 
 				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
 
@@ -334,7 +334,7 @@ var _ = Describe("Provider - Song Matching", func() {
 					{ID: "different2", Title: "Here Comes The Sun", Artist: "The Beatles"},
 				}
 
-				setupFuzzyExpectations(returnedSongs, artistTracks)
+				setupSimilarSongsExpectations(returnedSongs, artistTracks)
 
 				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
 
@@ -355,7 +355,7 @@ var _ = Describe("Provider - Song Matching", func() {
 					{ID: "remastered", Title: "Paranoid Android - Remastered", Artist: "Radiohead"},
 				}
 
-				setupFuzzyExpectations(returnedSongs, artistTracks)
+				setupSimilarSongsExpectations(returnedSongs, artistTracks)
 
 				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
 
@@ -375,7 +375,7 @@ var _ = Describe("Provider - Song Matching", func() {
 					{ID: "extended", Title: "Song (Extended Mix)", Artist: "Artist"},
 				}
 
-				setupFuzzyExpectations(returnedSongs, artistTracks)
+				setupSimilarSongsExpectations(returnedSongs, artistTracks)
 
 				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
 
@@ -401,7 +401,7 @@ var _ = Describe("Provider - Song Matching", func() {
 					ID: "wrong", Title: "Bohemian Rhapsody", Artist: "Queen", Album: "Greatest Hits",
 				}
 
-				setupFuzzyExpectations(returnedSongs, model.MediaFiles{wrongMatch, correctMatch})
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{wrongMatch, correctMatch})
 
 				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
 
@@ -424,7 +424,7 @@ var _ = Describe("Provider - Song Matching", func() {
 					ID: "wrong", Title: "Enjoy the Silence", Artist: "Depeche Mode", Album: "101",
 				}
 
-				setupFuzzyExpectations(returnedSongs, model.MediaFiles{wrongMatch, correctMatch})
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{wrongMatch, correctMatch})
 
 				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
 
@@ -446,7 +446,7 @@ var _ = Describe("Provider - Song Matching", func() {
 					ID: "fuzzy", Title: "Enjoy the Silence", Artist: "Depeche Mode", Album: "Violator (Deluxe Edition)",
 				}
 
-				setupFuzzyExpectations(returnedSongs, model.MediaFiles{fuzzyMatch, exactMatch})
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{fuzzyMatch, exactMatch})
 
 				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
 
@@ -457,4 +457,171 @@ var _ = Describe("Provider - Song Matching", func() {
 			})
 		})
 	})
+
+	Describe("Duration filtering", func() {
+		var track model.MediaFile
+
+		BeforeEach(func() {
+			DeferCleanup(configtest.SetupConfig())
+			conf.Server.SimilarSongsMatchThreshold = 100 // Exact title match for predictable tests
+
+			track = model.MediaFile{ID: "track-1", Title: "Test Track", Artist: "Test Artist"}
+
+			// Setup for GetEntityByID to return the track
+			artistRepo.On("Get", "track-1").Return(nil, model.ErrNotFound).Once()
+			albumRepo.On("Get", "track-1").Return(nil, model.ErrNotFound).Once()
+			mediaFileRepo.On("Get", "track-1").Return(&track, nil).Once()
+		})
+
+		Context("when agent provides duration", func() {
+			It("prefers tracks with matching duration", func() {
+				// Agent returns song with duration 180000ms (180 seconds)
+				returnedSongs := []agents.Song{
+					{Name: "Similar Song", Artist: "Test Artist", Duration: 180000},
+				}
+				// Library has two versions: one matching duration, one not
+				correctMatch := model.MediaFile{
+					ID: "correct", Title: "Similar Song", Artist: "Test Artist", Duration: 180.0,
+				}
+				wrongDuration := model.MediaFile{
+					ID: "wrong", Title: "Similar Song", Artist: "Test Artist", Duration: 240.0,
+				}
+
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{wrongDuration, correctMatch})
+
+				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
+
+				Expect(err).ToNot(HaveOccurred())
+				Expect(songs).To(HaveLen(1))
+				Expect(songs[0].ID).To(Equal("correct"))
+			})
+
+			It("matches within 3-second tolerance", func() {
+				// Agent returns song with duration 180000ms (180 seconds)
+				returnedSongs := []agents.Song{
+					{Name: "Similar Song", Artist: "Test Artist", Duration: 180000},
+				}
+				// Library has track with 182 seconds (within tolerance)
+				withinTolerance := model.MediaFile{
+					ID: "within-tolerance", Title: "Similar Song", Artist: "Test Artist", Duration: 182.5,
+				}
+
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{withinTolerance})
+
+				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
+
+				Expect(err).ToNot(HaveOccurred())
+				Expect(songs).To(HaveLen(1))
+				Expect(songs[0].ID).To(Equal("within-tolerance"))
+			})
+
+			It("excludes tracks outside 3-second tolerance when other matches exist", func() {
+				// Agent returns song with duration 180000ms (180 seconds)
+				returnedSongs := []agents.Song{
+					{Name: "Similar Song", Artist: "Test Artist", Duration: 180000},
+				}
+				// Library has one within tolerance, one outside
+				withinTolerance := model.MediaFile{
+					ID: "within", Title: "Similar Song", Artist: "Test Artist", Duration: 181.0,
+				}
+				outsideTolerance := model.MediaFile{
+					ID: "outside", Title: "Similar Song", Artist: "Test Artist", Duration: 190.0,
+				}
+
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{outsideTolerance, withinTolerance})
+
+				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
+
+				Expect(err).ToNot(HaveOccurred())
+				Expect(songs).To(HaveLen(1))
+				Expect(songs[0].ID).To(Equal("within"))
+			})
+
+			It("falls back to normal matching when no duration matches", func() {
+				// Agent returns song with duration 180000ms
+				returnedSongs := []agents.Song{
+					{Name: "Similar Song", Artist: "Test Artist", Duration: 180000},
+				}
+				// Library only has tracks with very different duration
+				differentDuration := model.MediaFile{
+					ID: "different", Title: "Similar Song", Artist: "Test Artist", Duration: 300.0,
+				}
+
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{differentDuration})
+
+				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
+
+				Expect(err).ToNot(HaveOccurred())
+				// Should fall back and return the track despite duration mismatch
+				Expect(songs).To(HaveLen(1))
+				Expect(songs[0].ID).To(Equal("different"))
+			})
+
+			It("falls back to title match when duration-filtered tracks fail title threshold", func() {
+				// Agent returns "Similar Song" with duration 180000ms
+				returnedSongs := []agents.Song{
+					{Name: "Similar Song", Artist: "Test Artist", Duration: 180000},
+				}
+				// Library has:
+				// - differentTitle: matches duration but has different title (won't pass title threshold)
+				// - correctTitle: doesn't match duration but has correct title (should be found via fallback)
+				differentTitle := model.MediaFile{
+					ID: "wrong-title", Title: "Different Song", Artist: "Test Artist", Duration: 180.0,
+				}
+				correctTitle := model.MediaFile{
+					ID: "correct-title", Title: "Similar Song", Artist: "Test Artist", Duration: 300.0,
+				}
+
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{differentTitle, correctTitle})
+
+				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
+
+				Expect(err).ToNot(HaveOccurred())
+				// Should fall back to all tracks and find the title match
+				Expect(songs).To(HaveLen(1))
+				Expect(songs[0].ID).To(Equal("correct-title"))
+			})
+		})
+
+		Context("when agent does not provide duration", func() {
+			It("matches without duration filtering (duration=0)", func() {
+				// Agent returns song without duration
+				returnedSongs := []agents.Song{
+					{Name: "Similar Song", Artist: "Test Artist", Duration: 0},
+				}
+				// Library tracks with various durations should all be candidates
+				anyTrack := model.MediaFile{
+					ID: "any", Title: "Similar Song", Artist: "Test Artist", Duration: 999.0,
+				}
+
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{anyTrack})
+
+				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
+
+				Expect(err).ToNot(HaveOccurred())
+				Expect(songs).To(HaveLen(1))
+				Expect(songs[0].ID).To(Equal("any"))
+			})
+		})
+
+		Context("edge cases", func() {
+			It("handles very short songs with duration tolerance", func() {
+				// 30-second song with 1-second difference (within 3-second tolerance)
+				returnedSongs := []agents.Song{
+					{Name: "Short Song", Artist: "Test Artist", Duration: 30000},
+				}
+				shortTrack := model.MediaFile{
+					ID: "short", Title: "Short Song", Artist: "Test Artist", Duration: 31.0,
+				}
+
+				setupSimilarSongsExpectations(returnedSongs, model.MediaFiles{shortTrack})
+
+				songs, err := provider.SimilarSongs(ctx, "track-1", 5)
+
+				Expect(err).ToNot(HaveOccurred())
+				Expect(songs).To(HaveLen(1))
+				Expect(songs[0].ID).To(Equal("short"))
+			})
+		})
+	})
 })

plugins/capabilities/metadata_agent.go

@@ -40,6 +40,18 @@ type MetadataAgent interface {
 	// GetAlbumImages retrieves images for an album.
 	//nd:export name=nd_get_album_images
 	GetAlbumImages(AlbumRequest) (*AlbumImagesResponse, error)
+
+	// GetSimilarSongsByTrack retrieves songs similar to a specific track.
+	//nd:export name=nd_get_similar_songs_by_track
+	GetSimilarSongsByTrack(SimilarSongsByTrackRequest) (*SimilarSongsResponse, error)
+
+	// GetSimilarSongsByAlbum retrieves songs similar to tracks on an album.
+	//nd:export name=nd_get_similar_songs_by_album
+	GetSimilarSongsByAlbum(SimilarSongsByAlbumRequest) (*SimilarSongsResponse, error)
+
+	// GetSimilarSongsByArtist retrieves songs similar to an artist's catalog.
+	//nd:export name=nd_get_similar_songs_by_artist
+	GetSimilarSongsByArtist(SimilarSongsByArtistRequest) (*SimilarSongsResponse, error)
 }
 
 // ArtistMBIDRequest is the request for GetArtistMBID.
@@ -122,7 +134,7 @@ type TopSongsRequest struct {
 	Count int32 `json:"count"`
 }
 
-// SongRef is a reference to a song with name and optional MBID.
+// SongRef is a reference to a song with metadata for matching.
 type SongRef struct {
 	// ID is the internal Navidrome mediafile ID (if known).
 	ID string `json:"id,omitempty"`
@@ -130,6 +142,16 @@ type SongRef struct {
 	Name string `json:"name"`
 	// MBID is the MusicBrainz ID for the song.
 	MBID string `json:"mbid,omitempty"`
+	// Artist is the artist name.
+	Artist string `json:"artist,omitempty"`
+	// ArtistMBID is the MusicBrainz artist ID.
+	ArtistMBID string `json:"artistMbid,omitempty"`
+	// Album is the album name.
+	Album string `json:"album,omitempty"`
+	// AlbumMBID is the MusicBrainz release ID.
+	AlbumMBID string `json:"albumMbid,omitempty"`
+	// Duration is the song duration in seconds.
+	Duration float32 `json:"duration,omitempty"`
 }
 
 // TopSongsResponse is the response for GetArtistTopSongs.
@@ -165,3 +187,49 @@ type AlbumImagesResponse struct {
 	// Images is the list of album images.
 	Images []ImageInfo `json:"images"`
 }
+
+// SimilarSongsByTrackRequest is the request for GetSimilarSongsByTrack.
+type SimilarSongsByTrackRequest struct {
+	// ID is the internal Navidrome mediafile ID.
+	ID string `json:"id"`
+	// Name is the track title.
+	Name string `json:"name"`
+	// Artist is the artist name.
+	Artist string `json:"artist"`
+	// MBID is the MusicBrainz recording ID (if known).
+	MBID string `json:"mbid,omitempty"`
+	// Count is the maximum number of similar songs to return.
+	Count int32 `json:"count"`
+}
+
+// SimilarSongsByAlbumRequest is the request for GetSimilarSongsByAlbum.
+type SimilarSongsByAlbumRequest struct {
+	// ID is the internal Navidrome album ID.
+	ID string `json:"id"`
+	// Name is the album name.
+	Name string `json:"name"`
+	// Artist is the album artist name.
+	Artist string `json:"artist"`
+	// MBID is the MusicBrainz release ID (if known).
+	MBID string `json:"mbid,omitempty"`
+	// Count is the maximum number of similar songs to return.
+	Count int32 `json:"count"`
+}
+
+// SimilarSongsByArtistRequest is the request for GetSimilarSongsByArtist.
+type SimilarSongsByArtistRequest struct {
+	// ID is the internal Navidrome artist ID.
+	ID string `json:"id"`
+	// Name is the artist name.
+	Name string `json:"name"`
+	// MBID is the MusicBrainz artist ID (if known).
+	MBID string `json:"mbid,omitempty"`
+	// Count is the maximum number of similar songs to return.
+	Count int32 `json:"count"`
+}
+
+// SimilarSongsResponse is the response for GetSimilarSongsBy* functions.
+type SimilarSongsResponse struct {
+	// Songs is the list of similar songs.
+	Songs []SongRef `json:"songs"`
+}

plugins/capabilities/metadata_agent.yaml

@@ -64,6 +64,30 @@ exports:
         output:
             $ref: '#/components/schemas/AlbumImagesResponse'
             contentType: application/json
+    nd_get_similar_songs_by_track:
+        description: GetSimilarSongsByTrack retrieves songs similar to a specific track.
+        input:
+            $ref: '#/components/schemas/SimilarSongsByTrackRequest'
+            contentType: application/json
+        output:
+            $ref: '#/components/schemas/SimilarSongsResponse'
+            contentType: application/json
+    nd_get_similar_songs_by_album:
+        description: GetSimilarSongsByAlbum retrieves songs similar to tracks on an album.
+        input:
+            $ref: '#/components/schemas/SimilarSongsByAlbumRequest'
+            contentType: application/json
+        output:
+            $ref: '#/components/schemas/SimilarSongsResponse'
+            contentType: application/json
+    nd_get_similar_songs_by_artist:
+        description: GetSimilarSongsByArtist retrieves songs similar to an artist's catalog.
+        input:
+            $ref: '#/components/schemas/SimilarSongsByArtistRequest'
+            contentType: application/json
+        output:
+            $ref: '#/components/schemas/SimilarSongsResponse'
+            contentType: application/json
 components:
     schemas:
         AlbumImagesResponse:
@@ -229,8 +253,86 @@ components:
                         $ref: '#/components/schemas/ArtistRef'
             required:
                 - artists
+        SimilarSongsByAlbumRequest:
+            description: SimilarSongsByAlbumRequest is the request for GetSimilarSongsByAlbum.
+            properties:
+                id:
+                    type: string
+                    description: ID is the internal Navidrome album ID.
+                name:
+                    type: string
+                    description: Name is the album name.
+                artist:
+                    type: string
+                    description: Artist is the album artist name.
+                mbid:
+                    type: string
+                    description: MBID is the MusicBrainz release ID (if known).
+                count:
+                    type: integer
+                    format: int32
+                    description: Count is the maximum number of similar songs to return.
+            required:
+                - id
+                - name
+                - artist
+                - count
+        SimilarSongsByArtistRequest:
+            description: SimilarSongsByArtistRequest is the request for GetSimilarSongsByArtist.
+            properties:
+                id:
+                    type: string
+                    description: ID is the internal Navidrome artist ID.
+                name:
+                    type: string
+                    description: Name is the artist name.
+                mbid:
+                    type: string
+                    description: MBID is the MusicBrainz artist ID (if known).
+                count:
+                    type: integer
+                    format: int32
+                    description: Count is the maximum number of similar songs to return.
+            required:
+                - id
+                - name
+                - count
+        SimilarSongsByTrackRequest:
+            description: SimilarSongsByTrackRequest is the request for GetSimilarSongsByTrack.
+            properties:
+                id:
+                    type: string
+                    description: ID is the internal Navidrome mediafile ID.
+                name:
+                    type: string
+                    description: Name is the track title.
+                artist:
+                    type: string
+                    description: Artist is the artist name.
+                mbid:
+                    type: string
+                    description: MBID is the MusicBrainz recording ID (if known).
+                count:
+                    type: integer
+                    format: int32
+                    description: Count is the maximum number of similar songs to return.
+            required:
+                - id
+                - name
+                - artist
+                - count
+        SimilarSongsResponse:
+            description: SimilarSongsResponse is the response for GetSimilarSongsBy* functions.
+            properties:
+                songs:
+                    type: array
+                    description: Songs is the list of similar songs.
+                    items:
+                        $ref: '#/components/schemas/SongRef'
+            required:
+                - songs
         SongRef:
-            description: SongRef is a reference to a song with name and optional MBID.
+            description: SongRef is a reference to a song with metadata for matching.
             properties:
                 id:
                     type: string
@@ -241,6 +343,22 @@ components:
                 mbid:
                     type: string
                     description: MBID is the MusicBrainz ID for the song.
+                artist:
+                    type: string
+                    description: Artist is the artist name.
+                artistMbid:
+                    type: string
+                    description: ArtistMBID is the MusicBrainz artist ID.
+                album:
+                    type: string
+                    description: Album is the album name.
+                albumMbid:
+                    type: string
+                    description: AlbumMBID is the MusicBrainz release ID.
+                duration:
+                    type: number
+                    format: float
+                    description: Duration is the song duration in seconds.
             required:
                 - name
         TopSongsRequest:

plugins/cmd/ndpgen/internal/generator.go

@@ -568,6 +568,18 @@ func skipSerializingFunc(goType string) string {
 		return "String::is_empty"
 	case "bool":
 		return "std::ops::Not::not"
+	case "int32":
+		return "is_zero_i32"
+	case "uint32":
+		return "is_zero_u32"
+	case "int64":
+		return "is_zero_i64"
+	case "uint64":
+		return "is_zero_u64"
+	case "float32":
+		return "is_zero_f32"
+	case "float64":
+		return "is_zero_f64"
 	default:
 		return "Option::is_none"
 	}

plugins/cmd/ndpgen/internal/generator_test.go

@@ -1234,6 +1234,37 @@ type OnInitOutput struct {
 })
 
 var _ = Describe("Rust Generation", func() {
+	Describe("skipSerializingFunc", func() {
+		It("should return Option::is_none for pointer, slice, and map types", func() {
+			Expect(skipSerializingFunc("*string")).To(Equal("Option::is_none"))
+			Expect(skipSerializingFunc("*MyStruct")).To(Equal("Option::is_none"))
+			Expect(skipSerializingFunc("[]string")).To(Equal("Option::is_none"))
+			Expect(skipSerializingFunc("[]int32")).To(Equal("Option::is_none"))
+			Expect(skipSerializingFunc("map[string]int")).To(Equal("Option::is_none"))
+		})
+
+		It("should return String::is_empty for string type", func() {
+			Expect(skipSerializingFunc("string")).To(Equal("String::is_empty"))
+		})
+
+		It("should return std::ops::Not::not for bool type", func() {
+			Expect(skipSerializingFunc("bool")).To(Equal("std::ops::Not::not"))
+		})
+
+		It("should return is_zero_* functions for numeric types", func() {
+			Expect(skipSerializingFunc("int32")).To(Equal("is_zero_i32"))
+			Expect(skipSerializingFunc("uint32")).To(Equal("is_zero_u32"))
+			Expect(skipSerializingFunc("int64")).To(Equal("is_zero_i64"))
+			Expect(skipSerializingFunc("uint64")).To(Equal("is_zero_u64"))
+			Expect(skipSerializingFunc("float32")).To(Equal("is_zero_f32"))
+			Expect(skipSerializingFunc("float64")).To(Equal("is_zero_f64"))
+		})
+
+		It("should return Option::is_none for unknown types", func() {
+			Expect(skipSerializingFunc("CustomType")).To(Equal("Option::is_none"))
+		})
+	})
+
 	Describe("rustOutputType", func() {
 		It("should convert Go primitives to Rust primitives", func() {
 			Expect(rustOutputType("bool")).To(Equal("bool"))

plugins/cmd/ndpgen/internal/templates/capability.rs.tmpl

@@ -7,6 +7,20 @@ use serde::{Deserialize, Serialize};
 {{- if hasHashMap .Capability}}
 use std::collections::HashMap;
 {{- end}}
+
+// Helper functions for skip_serializing_if with numeric types
+#[allow(dead_code)]
+fn is_zero_i32(value: &i32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u32(value: &u32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_i64(value: &i64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u64(value: &u64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_f32(value: &f32) -> bool { *value == 0.0 }
+#[allow(dead_code)]
+fn is_zero_f64(value: &f64) -> bool { *value == 0.0 }
 {{- end}}
 
 {{- /* Generate type alias definitions */ -}}

plugins/cmd/ndpgen/internal/types.go

@@ -466,9 +466,7 @@ func RustDefaultValue(goType string) string {
 	switch goType {
 	case "string":
 		return `String::new()`
-	case "int", "int32":
-		return "0"
-	case "int64":
+	case "int", "int32", "int64", "uint", "uint32", "uint64":
 		return "0"
 	case "float32", "float64":
 		return "0.0"
@@ -602,6 +600,10 @@ func ToRustTypeWithStructs(goType string, knownStructs map[string]bool) string {
 		return "i32"
 	case "int64":
 		return "i64"
+	case "uint", "uint32":
+		return "u32"
+	case "uint64":
+		return "u64"
 	case "float32":
 		return "f32"
 	case "float64":

plugins/cmd/ndpgen/internal/xtp_schema.go

@@ -106,7 +106,7 @@ func buildExport(export Export) xtpExport {
 // isPrimitiveGoType returns true if the Go type is a primitive type.
 func isPrimitiveGoType(goType string) bool {
 	switch goType {
-	case "bool", "string", "int", "int32", "int64", "float32", "float64", "[]byte":
+	case "bool", "string", "int", "int32", "int64", "uint", "uint32", "uint64", "float32", "float64", "[]byte":
 		return true
 	}
 	return false
@@ -302,6 +302,12 @@ func goTypeToXTPTypeAndFormat(goType string) (typ, format string) {
 		return "integer", "int32"
 	case "int64":
 		return "integer", "int64"
+	case "uint", "uint32":
+		// XTP schema doesn't support unsigned formats; use int64 to hold full uint32 range
+		return "integer", "int64"
+	case "uint64":
+		// XTP schema doesn't support unsigned formats; use int64 (may lose precision for large values)
+		return "integer", "int64"
 	case "float32":
 		return "number", "float"
 	case "float64":

plugins/metadata_agent.go

@@ -14,14 +14,17 @@ const CapabilityMetadataAgent Capability = "MetadataAgent"
 
 // Export function names (snake_case as per design)
 const (
-	FuncGetArtistMBID      = "nd_get_artist_mbid"
-	FuncGetArtistURL       = "nd_get_artist_url"
-	FuncGetArtistBiography = "nd_get_artist_biography"
-	FuncGetSimilarArtists  = "nd_get_similar_artists"
-	FuncGetArtistImages    = "nd_get_artist_images"
-	FuncGetArtistTopSongs  = "nd_get_artist_top_songs"
-	FuncGetAlbumInfo       = "nd_get_album_info"
-	FuncGetAlbumImages     = "nd_get_album_images"
+	FuncGetArtistMBID           = "nd_get_artist_mbid"
+	FuncGetArtistURL            = "nd_get_artist_url"
+	FuncGetArtistBiography      = "nd_get_artist_biography"
+	FuncGetSimilarArtists       = "nd_get_similar_artists"
+	FuncGetArtistImages         = "nd_get_artist_images"
+	FuncGetArtistTopSongs       = "nd_get_artist_top_songs"
+	FuncGetAlbumInfo            = "nd_get_album_info"
+	FuncGetAlbumImages          = "nd_get_album_images"
+	FuncGetSimilarSongsByTrack  = "nd_get_similar_songs_by_track"
+	FuncGetSimilarSongsByAlbum  = "nd_get_similar_songs_by_album"
+	FuncGetSimilarSongsByArtist = "nd_get_similar_songs_by_artist"
 )
 
 func init() {
@@ -35,6 +38,9 @@ func init() {
 		FuncGetArtistTopSongs,
 		FuncGetAlbumInfo,
 		FuncGetAlbumImages,
+		FuncGetSimilarSongsByTrack,
+		FuncGetSimilarSongsByAlbum,
+		FuncGetSimilarSongsByArtist,
 	)
 }
 
@@ -147,12 +153,7 @@ func (a *MetadataAgent) GetArtistTopSongs(ctx context.Context, id, artistName, m
 		return nil, agents.ErrNotFound
 	}
 
-	songs := make([]agents.Song, len(result.Songs))
-	for i, s := range result.Songs {
-		songs[i] = agents.Song{ID: s.ID, Name: s.Name, MBID: s.MBID}
-	}
-
-	return songs, nil
+	return songRefsToAgentSongs(result.Songs), nil
 }
 
 // GetAlbumInfo retrieves album information
@@ -195,15 +196,62 @@ func (a *MetadataAgent) GetAlbumImages(ctx context.Context, name, artist, mbid s
 	return images, nil
 }
 
+func callSimilarSongsPluginFunction[T any](ctx context.Context, plugin *plugin, funcName string, input T) ([]agents.Song, error) {
+	result, err := callPluginFunction[T, *capabilities.SimilarSongsResponse](ctx, plugin, funcName, input)
+	if err != nil {
+		return nil, err
+	}
+	if result == nil || len(result.Songs) == 0 {
+		return nil, agents.ErrNotFound
+	}
+	return songRefsToAgentSongs(result.Songs), nil
+}
+
+// GetSimilarSongsByTrack retrieves songs similar to a specific track
+func (a *MetadataAgent) GetSimilarSongsByTrack(ctx context.Context, id, name, artist, mbid string, count int) ([]agents.Song, error) {
+	return callSimilarSongsPluginFunction[capabilities.SimilarSongsByTrackRequest](ctx, a.plugin, FuncGetSimilarSongsByTrack, capabilities.SimilarSongsByTrackRequest{ID: id, Name: name, Artist: artist, MBID: mbid, Count: int32(count)})
+}
+
+// GetSimilarSongsByAlbum retrieves songs similar to tracks on an album
+func (a *MetadataAgent) GetSimilarSongsByAlbum(ctx context.Context, id, name, artist, mbid string, count int) ([]agents.Song, error) {
+	return callSimilarSongsPluginFunction[capabilities.SimilarSongsByAlbumRequest](ctx, a.plugin, FuncGetSimilarSongsByAlbum, capabilities.SimilarSongsByAlbumRequest{ID: id, Name: name, Artist: artist, MBID: mbid, Count: int32(count)})
+}
+
+// GetSimilarSongsByArtist retrieves songs similar to an artist's catalog
+func (a *MetadataAgent) GetSimilarSongsByArtist(ctx context.Context, id, name, mbid string, count int) ([]agents.Song, error) {
+	return callSimilarSongsPluginFunction[capabilities.SimilarSongsByArtistRequest](ctx, a.plugin, FuncGetSimilarSongsByArtist, capabilities.SimilarSongsByArtistRequest{ID: id, Name: name, MBID: mbid, Count: int32(count)})
+}
+
+// songRefsToAgentSongs converts a slice of SongRef to agents.Song
+func songRefsToAgentSongs(refs []capabilities.SongRef) []agents.Song {
+	songs := make([]agents.Song, len(refs))
+	for i, s := range refs {
+		songs[i] = agents.Song{
+			ID:         s.ID,
+			Name:       s.Name,
+			MBID:       s.MBID,
+			Artist:     s.Artist,
+			ArtistMBID: s.ArtistMBID,
+			Album:      s.Album,
+			AlbumMBID:  s.AlbumMBID,
+			Duration:   uint32(s.Duration * 1000),
+		}
+	}
+	return songs
+}
+
 // Verify interface implementations at compile time
 var (
-	_ agents.Interface                = (*MetadataAgent)(nil)
-	_ agents.ArtistMBIDRetriever      = (*MetadataAgent)(nil)
-	_ agents.ArtistURLRetriever       = (*MetadataAgent)(nil)
-	_ agents.ArtistBiographyRetriever = (*MetadataAgent)(nil)
-	_ agents.ArtistSimilarRetriever   = (*MetadataAgent)(nil)
-	_ agents.ArtistImageRetriever     = (*MetadataAgent)(nil)
-	_ agents.ArtistTopSongsRetriever  = (*MetadataAgent)(nil)
-	_ agents.AlbumInfoRetriever       = (*MetadataAgent)(nil)
-	_ agents.AlbumImageRetriever      = (*MetadataAgent)(nil)
+	_ agents.Interface                     = (*MetadataAgent)(nil)
+	_ agents.ArtistMBIDRetriever           = (*MetadataAgent)(nil)
+	_ agents.ArtistURLRetriever            = (*MetadataAgent)(nil)
+	_ agents.ArtistBiographyRetriever      = (*MetadataAgent)(nil)
+	_ agents.ArtistSimilarRetriever        = (*MetadataAgent)(nil)
+	_ agents.ArtistImageRetriever          = (*MetadataAgent)(nil)
+	_ agents.ArtistTopSongsRetriever       = (*MetadataAgent)(nil)
+	_ agents.AlbumInfoRetriever            = (*MetadataAgent)(nil)
+	_ agents.AlbumImageRetriever           = (*MetadataAgent)(nil)
+	_ agents.SimilarSongsByTrackRetriever  = (*MetadataAgent)(nil)
+	_ agents.SimilarSongsByAlbumRetriever  = (*MetadataAgent)(nil)
+	_ agents.SimilarSongsByArtistRetriever = (*MetadataAgent)(nil)
 )

plugins/metadata_agent_test.go

@@ -108,6 +108,37 @@ var _ = Describe("MetadataAgent", Ordered, func() {
 			Expect(images[0].Size).To(Equal(500))
 		})
 	})
+
+	Describe("GetSimilarSongsByTrack", func() {
+		It("returns similar songs from the plugin", func() {
+			retriever := agent.(agents.SimilarSongsByTrackRetriever)
+			songs, err := retriever.GetSimilarSongsByTrack(GinkgoT().Context(), "track-1", "Yesterday", "The Beatles", "some-mbid", 3)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(songs).To(HaveLen(3))
+			Expect(songs[0].Name).To(Equal("Similar to Yesterday #1"))
+			Expect(songs[0].Artist).To(Equal("The Beatles"))
+		})
+	})
+
+	Describe("GetSimilarSongsByAlbum", func() {
+		It("returns similar songs from the plugin", func() {
+			retriever := agent.(agents.SimilarSongsByAlbumRetriever)
+			songs, err := retriever.GetSimilarSongsByAlbum(GinkgoT().Context(), "album-1", "Abbey Road", "The Beatles", "album-mbid", 3)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(songs).To(HaveLen(3))
+			Expect(songs[0].Album).To(Equal("Abbey Road"))
+		})
+	})
+
+	Describe("GetSimilarSongsByArtist", func() {
+		It("returns similar songs from the plugin", func() {
+			retriever := agent.(agents.SimilarSongsByArtistRetriever)
+			songs, err := retriever.GetSimilarSongsByArtist(GinkgoT().Context(), "artist-1", "The Beatles", "some-mbid", 3)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(songs).To(HaveLen(3))
+			Expect(songs[0].Name).To(ContainSubstring("The Beatles Style Song"))
+		})
+	})
 })
 
 var _ = Describe("MetadataAgent error handling", Ordered, func() {
@@ -186,6 +217,27 @@ var _ = Describe("MetadataAgent error handling", Ordered, func() {
 		Expect(err).To(HaveOccurred())
 		Expect(err.Error()).To(ContainSubstring("simulated plugin error"))
 	})
+
+	It("returns error from GetSimilarSongsByTrack", func() {
+		retriever := errorAgent.(agents.SimilarSongsByTrackRetriever)
+		_, err := retriever.GetSimilarSongsByTrack(GinkgoT().Context(), "track-1", "Test", "Artist", "mbid", 5)
+		Expect(err).To(HaveOccurred())
+		Expect(err.Error()).To(ContainSubstring("simulated plugin error"))
+	})
+
+	It("returns error from GetSimilarSongsByAlbum", func() {
+		retriever := errorAgent.(agents.SimilarSongsByAlbumRetriever)
+		_, err := retriever.GetSimilarSongsByAlbum(GinkgoT().Context(), "album-1", "Album", "Artist", "mbid", 5)
+		Expect(err).To(HaveOccurred())
+		Expect(err.Error()).To(ContainSubstring("simulated plugin error"))
+	})
+
+	It("returns error from GetSimilarSongsByArtist", func() {
+		retriever := errorAgent.(agents.SimilarSongsByArtistRetriever)
+		_, err := retriever.GetSimilarSongsByArtist(GinkgoT().Context(), "artist-1", "Artist", "mbid", 5)
+		Expect(err).To(HaveOccurred())
+		Expect(err.Error()).To(ContainSubstring("simulated plugin error"))
+	})
 })
 
 var _ = Describe("MetadataAgent partial implementation", Ordered, func() {
@@ -255,6 +307,23 @@ var _ = Describe("MetadataAgent partial implementation", Ordered, func() {
 		retriever := partialAgent.(agents.AlbumImageRetriever)
 		_, err := retriever.GetAlbumImages(GinkgoT().Context(), "Album", "Artist", "mbid")
 		Expect(err).To(MatchError(errNotImplemented))
+	})
 
+	It("returns ErrNotFound for unimplemented method (GetSimilarSongsByTrack)", func() {
+		retriever := partialAgent.(agents.SimilarSongsByTrackRetriever)
+		_, err := retriever.GetSimilarSongsByTrack(GinkgoT().Context(), "track-1", "Test", "Artist", "mbid", 5)
+		Expect(err).To(MatchError(errNotImplemented))
+	})
+
+	It("returns ErrNotFound for unimplemented method (GetSimilarSongsByAlbum)", func() {
+		retriever := partialAgent.(agents.SimilarSongsByAlbumRetriever)
+		_, err := retriever.GetSimilarSongsByAlbum(GinkgoT().Context(), "album-1", "Album", "Artist", "mbid", 5)
+		Expect(err).To(MatchError(errNotImplemented))
+	})
+
+	It("returns ErrNotFound for unimplemented method (GetSimilarSongsByArtist)", func() {
+		retriever := partialAgent.(agents.SimilarSongsByArtistRetriever)
+		_, err := retriever.GetSimilarSongsByArtist(GinkgoT().Context(), "artist-1", "Artist", "mbid", 5)
+		Expect(err).To(MatchError(errNotImplemented))
 	})
 })

plugins/pdk/go/metadata/metadata.go

@@ -117,7 +117,53 @@ type SimilarArtistsResponse struct {
 	Artists []ArtistRef `json:"artists"`
 }
 
-// SongRef is a reference to a song with name and optional MBID.
+// SimilarSongsByAlbumRequest is the request for GetSimilarSongsByAlbum.
+type SimilarSongsByAlbumRequest struct {
+	// ID is the internal Navidrome album ID.
+	ID string `json:"id"`
+	// Name is the album name.
+	Name string `json:"name"`
+	// Artist is the album artist name.
+	Artist string `json:"artist"`
+	// MBID is the MusicBrainz release ID (if known).
+	MBID string `json:"mbid,omitempty"`
+	// Count is the maximum number of similar songs to return.
+	Count int32 `json:"count"`
+}
+
+// SimilarSongsByArtistRequest is the request for GetSimilarSongsByArtist.
+type SimilarSongsByArtistRequest struct {
+	// ID is the internal Navidrome artist ID.
+	ID string `json:"id"`
+	// Name is the artist name.
+	Name string `json:"name"`
+	// MBID is the MusicBrainz artist ID (if known).
+	MBID string `json:"mbid,omitempty"`
+	// Count is the maximum number of similar songs to return.
+	Count int32 `json:"count"`
+}
+
+// SimilarSongsByTrackRequest is the request for GetSimilarSongsByTrack.
+type SimilarSongsByTrackRequest struct {
+	// ID is the internal Navidrome mediafile ID.
+	ID string `json:"id"`
+	// Name is the track title.
+	Name string `json:"name"`
+	// Artist is the artist name.
+	Artist string `json:"artist"`
+	// MBID is the MusicBrainz recording ID (if known).
+	MBID string `json:"mbid,omitempty"`
+	// Count is the maximum number of similar songs to return.
+	Count int32 `json:"count"`
+}
+
+// SimilarSongsResponse is the response for GetSimilarSongsBy* functions.
+type SimilarSongsResponse struct {
+	// Songs is the list of similar songs.
+	Songs []SongRef `json:"songs"`
+}
+
+// SongRef is a reference to a song with metadata for matching.
 type SongRef struct {
 	// ID is the internal Navidrome mediafile ID (if known).
 	ID string `json:"id,omitempty"`
@@ -125,6 +171,16 @@ type SongRef struct {
 	Name string `json:"name"`
 	// MBID is the MusicBrainz ID for the song.
 	MBID string `json:"mbid,omitempty"`
+	// Artist is the artist name.
+	Artist string `json:"artist,omitempty"`
+	// ArtistMBID is the MusicBrainz artist ID.
+	ArtistMBID string `json:"artistMbid,omitempty"`
+	// Album is the album name.
+	Album string `json:"album,omitempty"`
+	// AlbumMBID is the MusicBrainz release ID.
+	AlbumMBID string `json:"albumMbid,omitempty"`
+	// Duration is the song duration in seconds.
+	Duration float32 `json:"duration,omitempty"`
 }
 
 // TopSongsRequest is the request for GetArtistTopSongs.
@@ -193,16 +249,34 @@ type AlbumInfoProvider interface {
 // AlbumImagesProvider provides the GetAlbumImages function.
 type AlbumImagesProvider interface {
 	GetAlbumImages(AlbumRequest) (*AlbumImagesResponse, error)
+}
+
+// SimilarSongsByTrackProvider provides the GetSimilarSongsByTrack function.
+type SimilarSongsByTrackProvider interface {
+	GetSimilarSongsByTrack(SimilarSongsByTrackRequest) (*SimilarSongsResponse, error)
+}
+
+// SimilarSongsByAlbumProvider provides the GetSimilarSongsByAlbum function.
+type SimilarSongsByAlbumProvider interface {
+	GetSimilarSongsByAlbum(SimilarSongsByAlbumRequest) (*SimilarSongsResponse, error)
+}
+
+// SimilarSongsByArtistProvider provides the GetSimilarSongsByArtist function.
+type SimilarSongsByArtistProvider interface {
+	GetSimilarSongsByArtist(SimilarSongsByArtistRequest) (*SimilarSongsResponse, error)
 } // Internal implementation holders
 var (
-	artistMBIDImpl      func(ArtistMBIDRequest) (*ArtistMBIDResponse, error)
-	artistURLImpl       func(ArtistRequest) (*ArtistURLResponse, error)
-	artistBiographyImpl func(ArtistRequest) (*ArtistBiographyResponse, error)
-	similarArtistsImpl  func(SimilarArtistsRequest) (*SimilarArtistsResponse, error)
-	artistImagesImpl    func(ArtistRequest) (*ArtistImagesResponse, error)
-	artistTopSongsImpl  func(TopSongsRequest) (*TopSongsResponse, error)
-	albumInfoImpl       func(AlbumRequest) (*AlbumInfoResponse, error)
-	albumImagesImpl     func(AlbumRequest) (*AlbumImagesResponse, error)
+	artistMBIDImpl           func(ArtistMBIDRequest) (*ArtistMBIDResponse, error)
+	artistURLImpl            func(ArtistRequest) (*ArtistURLResponse, error)
+	artistBiographyImpl      func(ArtistRequest) (*ArtistBiographyResponse, error)
+	similarArtistsImpl       func(SimilarArtistsRequest) (*SimilarArtistsResponse, error)
+	artistImagesImpl         func(ArtistRequest) (*ArtistImagesResponse, error)
+	artistTopSongsImpl       func(TopSongsRequest) (*TopSongsResponse, error)
+	albumInfoImpl            func(AlbumRequest) (*AlbumInfoResponse, error)
+	albumImagesImpl          func(AlbumRequest) (*AlbumImagesResponse, error)
+	similarSongsByTrackImpl  func(SimilarSongsByTrackRequest) (*SimilarSongsResponse, error)
+	similarSongsByAlbumImpl  func(SimilarSongsByAlbumRequest) (*SimilarSongsResponse, error)
+	similarSongsByArtistImpl func(SimilarSongsByArtistRequest) (*SimilarSongsResponse, error)
 )
 
 // Register registers a metadata implementation.
@@ -232,6 +306,15 @@ func Register(impl Metadata) {
 	if p, ok := impl.(AlbumImagesProvider); ok {
 		albumImagesImpl = p.GetAlbumImages
 	}
+	if p, ok := impl.(SimilarSongsByTrackProvider); ok {
+		similarSongsByTrackImpl = p.GetSimilarSongsByTrack
+	}
+	if p, ok := impl.(SimilarSongsByAlbumProvider); ok {
+		similarSongsByAlbumImpl = p.GetSimilarSongsByAlbum
+	}
+	if p, ok := impl.(SimilarSongsByArtistProvider); ok {
+		similarSongsByArtistImpl = p.GetSimilarSongsByArtist
+	}
 }
 
 // NotImplementedCode is the standard return code for unimplemented functions.
@@ -453,3 +536,84 @@ func _NdGetAlbumImages() int32 {
 
 	return 0
 }
+
+//go:wasmexport nd_get_similar_songs_by_track
+func _NdGetSimilarSongsByTrack() int32 {
+	if similarSongsByTrackImpl == nil {
+		// Return standard code - host will skip this plugin gracefully
+		return NotImplementedCode
+	}
+
+	var input SimilarSongsByTrackRequest
+	if err := pdk.InputJSON(&input); err != nil {
+		pdk.SetError(err)
+		return -1
+	}
+
+	output, err := similarSongsByTrackImpl(input)
+	if err != nil {
+		pdk.SetError(err)
+		return -1
+	}
+
+	if err := pdk.OutputJSON(output); err != nil {
+		pdk.SetError(err)
+		return -1
+	}
+
+	return 0
+}
+
+//go:wasmexport nd_get_similar_songs_by_album
+func _NdGetSimilarSongsByAlbum() int32 {
+	if similarSongsByAlbumImpl == nil {
+		// Return standard code - host will skip this plugin gracefully
+		return NotImplementedCode
+	}
+
+	var input SimilarSongsByAlbumRequest
+	if err := pdk.InputJSON(&input); err != nil {
+		pdk.SetError(err)
+		return -1
+	}
+
+	output, err := similarSongsByAlbumImpl(input)
+	if err != nil {
+		pdk.SetError(err)
+		return -1
+	}
+
+	if err := pdk.OutputJSON(output); err != nil {
+		pdk.SetError(err)
+		return -1
+	}
+
+	return 0
+}
+
+//go:wasmexport nd_get_similar_songs_by_artist
+func _NdGetSimilarSongsByArtist() int32 {
+	if similarSongsByArtistImpl == nil {
+		// Return standard code - host will skip this plugin gracefully
+		return NotImplementedCode
+	}
+
+	var input SimilarSongsByArtistRequest
+	if err := pdk.InputJSON(&input); err != nil {
+		pdk.SetError(err)
+		return -1
+	}
+
+	output, err := similarSongsByArtistImpl(input)
+	if err != nil {
+		pdk.SetError(err)
+		return -1
+	}
+
+	if err := pdk.OutputJSON(output); err != nil {
+		pdk.SetError(err)
+		return -1
+	}
+
+	return 0
+}

plugins/pdk/go/metadata/metadata_stub.go

@@ -114,7 +114,53 @@ type SimilarArtistsResponse struct {
 	Artists []ArtistRef `json:"artists"`
 }
 
-// SongRef is a reference to a song with name and optional MBID.
+// SimilarSongsByAlbumRequest is the request for GetSimilarSongsByAlbum.
+type SimilarSongsByAlbumRequest struct {
+	// ID is the internal Navidrome album ID.
+	ID string `json:"id"`
+	// Name is the album name.
+	Name string `json:"name"`
+	// Artist is the album artist name.
+	Artist string `json:"artist"`
+	// MBID is the MusicBrainz release ID (if known).
+	MBID string `json:"mbid,omitempty"`
+	// Count is the maximum number of similar songs to return.
+	Count int32 `json:"count"`
+}
+
+// SimilarSongsByArtistRequest is the request for GetSimilarSongsByArtist.
+type SimilarSongsByArtistRequest struct {
+	// ID is the internal Navidrome artist ID.
+	ID string `json:"id"`
+	// Name is the artist name.
+	Name string `json:"name"`
+	// MBID is the MusicBrainz artist ID (if known).
+	MBID string `json:"mbid,omitempty"`
+	// Count is the maximum number of similar songs to return.
+	Count int32 `json:"count"`
+}
+
+// SimilarSongsByTrackRequest is the request for GetSimilarSongsByTrack.
+type SimilarSongsByTrackRequest struct {
+	// ID is the internal Navidrome mediafile ID.
+	ID string `json:"id"`
+	// Name is the track title.
+	Name string `json:"name"`
+	// Artist is the artist name.
+	Artist string `json:"artist"`
+	// MBID is the MusicBrainz recording ID (if known).
+	MBID string `json:"mbid,omitempty"`
+	// Count is the maximum number of similar songs to return.
+	Count int32 `json:"count"`
+}
+
+// SimilarSongsResponse is the response for GetSimilarSongsBy* functions.
+type SimilarSongsResponse struct {
+	// Songs is the list of similar songs.
+	Songs []SongRef `json:"songs"`
+}
+
+// SongRef is a reference to a song with metadata for matching.
 type SongRef struct {
 	// ID is the internal Navidrome mediafile ID (if known).
 	ID string `json:"id,omitempty"`
@@ -122,6 +168,16 @@ type SongRef struct {
 	Name string `json:"name"`
 	// MBID is the MusicBrainz ID for the song.
 	MBID string `json:"mbid,omitempty"`
+	// Artist is the artist name.
+	Artist string `json:"artist,omitempty"`
+	// ArtistMBID is the MusicBrainz artist ID.
+	ArtistMBID string `json:"artistMbid,omitempty"`
+	// Album is the album name.
+	Album string `json:"album,omitempty"`
+	// AlbumMBID is the MusicBrainz release ID.
+	AlbumMBID string `json:"albumMbid,omitempty"`
+	// Duration is the song duration in seconds.
+	Duration float32 `json:"duration,omitempty"`
 }
 
 // TopSongsRequest is the request for GetArtistTopSongs.
@@ -192,6 +248,21 @@ type AlbumImagesProvider interface {
 	GetAlbumImages(AlbumRequest) (*AlbumImagesResponse, error)
 }
 
+// SimilarSongsByTrackProvider provides the GetSimilarSongsByTrack function.
+type SimilarSongsByTrackProvider interface {
+	GetSimilarSongsByTrack(SimilarSongsByTrackRequest) (*SimilarSongsResponse, error)
+}
+
+// SimilarSongsByAlbumProvider provides the GetSimilarSongsByAlbum function.
+type SimilarSongsByAlbumProvider interface {
+	GetSimilarSongsByAlbum(SimilarSongsByAlbumRequest) (*SimilarSongsResponse, error)
+}
+
+// SimilarSongsByArtistProvider provides the GetSimilarSongsByArtist function.
+type SimilarSongsByArtistProvider interface {
+	GetSimilarSongsByArtist(SimilarSongsByArtistRequest) (*SimilarSongsResponse, error)
+}
+
 // NotImplementedCode is the standard return code for unimplemented functions.
 const NotImplementedCode int32 = -2
 

plugins/pdk/rust/nd-pdk-capabilities/src/metadata.rs

@@ -4,6 +4,20 @@
 // It is intended for use in Navidrome plugins built with extism-pdk.
 
 use serde::{Deserialize, Serialize};
+
+// Helper functions for skip_serializing_if with numeric types
+#[allow(dead_code)]
+fn is_zero_i32(value: &i32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u32(value: &u32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_i64(value: &i64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u64(value: &u64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_f32(value: &f32) -> bool { *value == 0.0 }
+#[allow(dead_code)]
+fn is_zero_f64(value: &f64) -> bool { *value == 0.0 }
 /// AlbumImagesResponse is the response for GetAlbumImages.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
@@ -150,7 +164,72 @@ pub struct SimilarArtistsResponse {
     #[serde(default)]
     pub artists: Vec<ArtistRef>,
 }
-/// SongRef is a reference to a song with name and optional MBID.
+/// SimilarSongsByAlbumRequest is the request for GetSimilarSongsByAlbum.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SimilarSongsByAlbumRequest {
+    /// ID is the internal Navidrome album ID.
+    #[serde(default)]
+    pub id: String,
+    /// Name is the album name.
+    #[serde(default)]
+    pub name: String,
+    /// Artist is the album artist name.
+    #[serde(default)]
+    pub artist: String,
+    /// MBID is the MusicBrainz release ID (if known).
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    pub mbid: String,
+    /// Count is the maximum number of similar songs to return.
+    #[serde(default)]
+    pub count: i32,
+}
+/// SimilarSongsByArtistRequest is the request for GetSimilarSongsByArtist.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SimilarSongsByArtistRequest {
+    /// ID is the internal Navidrome artist ID.
+    #[serde(default)]
+    pub id: String,
+    /// Name is the artist name.
+    #[serde(default)]
+    pub name: String,
+    /// MBID is the MusicBrainz artist ID (if known).
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    pub mbid: String,
+    /// Count is the maximum number of similar songs to return.
+    #[serde(default)]
+    pub count: i32,
+}
+/// SimilarSongsByTrackRequest is the request for GetSimilarSongsByTrack.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SimilarSongsByTrackRequest {
+    /// ID is the internal Navidrome mediafile ID.
+    #[serde(default)]
+    pub id: String,
+    /// Name is the track title.
+    #[serde(default)]
+    pub name: String,
+    /// Artist is the artist name.
+    #[serde(default)]
+    pub artist: String,
+    /// MBID is the MusicBrainz recording ID (if known).
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    pub mbid: String,
+    /// Count is the maximum number of similar songs to return.
+    #[serde(default)]
+    pub count: i32,
+}
+/// SimilarSongsResponse is the response for GetSimilarSongsBy* functions.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SimilarSongsResponse {
+    /// Songs is the list of similar songs.
+    #[serde(default)]
+    pub songs: Vec<SongRef>,
+}
+/// SongRef is a reference to a song with metadata for matching.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct SongRef {
@@ -163,6 +242,21 @@ pub struct SongRef {
     /// MBID is the MusicBrainz ID for the song.
     #[serde(default, skip_serializing_if = "String::is_empty")]
     pub mbid: String,
+    /// Artist is the artist name.
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    pub artist: String,
+    /// ArtistMBID is the MusicBrainz artist ID.
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    pub artist_mbid: String,
+    /// Album is the album name.
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    pub album: String,
+    /// AlbumMBID is the MusicBrainz release ID.
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    pub album_mbid: String,
+    /// Duration is the song duration in seconds.
+    #[serde(default, skip_serializing_if = "is_zero_f32")]
+    pub duration: f32,
 }
 /// TopSongsRequest is the request for GetArtistTopSongs.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
@@ -377,3 +471,66 @@ macro_rules! register_metadata_album_images {
         }
     };
 }
+
+/// SimilarSongsByTrackProvider provides the GetSimilarSongsByTrack function.
+pub trait SimilarSongsByTrackProvider {
+    fn get_similar_songs_by_track(&self, req: SimilarSongsByTrackRequest) -> Result<SimilarSongsResponse, Error>;
+}
+
+/// Register the get_similar_songs_by_track export.
+/// This macro generates the WASM export function for this method.
+#[macro_export]
+macro_rules! register_metadata_similar_songs_by_track {
+    ($plugin_type:ty) => {
+        #[extism_pdk::plugin_fn]
+        pub fn nd_get_similar_songs_by_track(
+            req: extism_pdk::Json<$crate::metadata::SimilarSongsByTrackRequest>
+        ) -> extism_pdk::FnResult<extism_pdk::Json<$crate::metadata::SimilarSongsResponse>> {
+            let plugin = <$plugin_type>::default();
+            let result = $crate::metadata::SimilarSongsByTrackProvider::get_similar_songs_by_track(&plugin, req.into_inner())?;
+            Ok(extism_pdk::Json(result))
+        }
+    };
+}
+
+/// SimilarSongsByAlbumProvider provides the GetSimilarSongsByAlbum function.
+pub trait SimilarSongsByAlbumProvider {
+    fn get_similar_songs_by_album(&self, req: SimilarSongsByAlbumRequest) -> Result<SimilarSongsResponse, Error>;
+}
+
+/// Register the get_similar_songs_by_album export.
+/// This macro generates the WASM export function for this method.
+#[macro_export]
+macro_rules! register_metadata_similar_songs_by_album {
+    ($plugin_type:ty) => {
+        #[extism_pdk::plugin_fn]
+        pub fn nd_get_similar_songs_by_album(
+            req: extism_pdk::Json<$crate::metadata::SimilarSongsByAlbumRequest>
+        ) -> extism_pdk::FnResult<extism_pdk::Json<$crate::metadata::SimilarSongsResponse>> {
+            let plugin = <$plugin_type>::default();
+            let result = $crate::metadata::SimilarSongsByAlbumProvider::get_similar_songs_by_album(&plugin, req.into_inner())?;
+            Ok(extism_pdk::Json(result))
+        }
+    };
+}
+
+/// SimilarSongsByArtistProvider provides the GetSimilarSongsByArtist function.
+pub trait SimilarSongsByArtistProvider {
+    fn get_similar_songs_by_artist(&self, req: SimilarSongsByArtistRequest) -> Result<SimilarSongsResponse, Error>;
+}
+
+/// Register the get_similar_songs_by_artist export.
+/// This macro generates the WASM export function for this method.
+#[macro_export]
+macro_rules! register_metadata_similar_songs_by_artist {
+    ($plugin_type:ty) => {
+        #[extism_pdk::plugin_fn]
+        pub fn nd_get_similar_songs_by_artist(
+            req: extism_pdk::Json<$crate::metadata::SimilarSongsByArtistRequest>
+        ) -> extism_pdk::FnResult<extism_pdk::Json<$crate::metadata::SimilarSongsResponse>> {
+            let plugin = <$plugin_type>::default();
+            let result = $crate::metadata::SimilarSongsByArtistProvider::get_similar_songs_by_artist(&plugin, req.into_inner())?;
+            Ok(extism_pdk::Json(result))
+        }
+    };
+}

plugins/pdk/rust/nd-pdk-capabilities/src/scheduler.rs

@@ -4,6 +4,20 @@
 // It is intended for use in Navidrome plugins built with extism-pdk.
 
 use serde::{Deserialize, Serialize};
+
+// Helper functions for skip_serializing_if with numeric types
+#[allow(dead_code)]
+fn is_zero_i32(value: &i32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u32(value: &u32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_i64(value: &i64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u64(value: &u64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_f32(value: &f32) -> bool { *value == 0.0 }
+#[allow(dead_code)]
+fn is_zero_f64(value: &f64) -> bool { *value == 0.0 }
 /// SchedulerCallbackRequest is the request provided when a scheduled task fires.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]

plugins/pdk/rust/nd-pdk-capabilities/src/scrobbler.rs

@@ -4,6 +4,20 @@
 // It is intended for use in Navidrome plugins built with extism-pdk.
 
 use serde::{Deserialize, Serialize};
+
+// Helper functions for skip_serializing_if with numeric types
+#[allow(dead_code)]
+fn is_zero_i32(value: &i32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u32(value: &u32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_i64(value: &i64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u64(value: &u64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_f32(value: &f32) -> bool { *value == 0.0 }
+#[allow(dead_code)]
+fn is_zero_f64(value: &f64) -> bool { *value == 0.0 }
 /// ScrobblerError represents an error type for scrobbling operations.
 pub type ScrobblerError = &'static str;
 /// ScrobblerErrorNotAuthorized indicates the user is not authorized.

plugins/pdk/rust/nd-pdk-capabilities/src/websocket.rs

@@ -4,6 +4,20 @@
 // It is intended for use in Navidrome plugins built with extism-pdk.
 
 use serde::{Deserialize, Serialize};
+
+// Helper functions for skip_serializing_if with numeric types
+#[allow(dead_code)]
+fn is_zero_i32(value: &i32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u32(value: &u32) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_i64(value: &i64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_u64(value: &u64) -> bool { *value == 0 }
+#[allow(dead_code)]
+fn is_zero_f32(value: &f32) -> bool { *value == 0.0 }
+#[allow(dead_code)]
+fn is_zero_f64(value: &f64) -> bool { *value == 0.0 }
 /// OnBinaryMessageRequest is the request provided when a binary message is received.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]

plugins/testdata/test-metadata-agent/main.go

@@ -120,4 +120,64 @@ func (t *testMetadataAgent) GetAlbumImages(input metadata.AlbumRequest) (*metada
 	}, nil
 }
 
+func (t *testMetadataAgent) GetSimilarSongsByTrack(input metadata.SimilarSongsByTrackRequest) (*metadata.SimilarSongsResponse, error) {
+	if err := checkConfigError(); err != nil {
+		return nil, err
+	}
+	count := int(input.Count)
+	if count == 0 {
+		count = 5
+	}
+	songs := make([]metadata.SongRef, 0, count)
+	for i := range count {
+		songs = append(songs, metadata.SongRef{
+			ID:         "similar-track-id-" + strconv.Itoa(i+1),
+			Name:       "Similar to " + input.Name + " #" + strconv.Itoa(i+1),
+			MBID:       "similar-mbid-" + strconv.Itoa(i+1),
+			Artist:     input.Artist,
+			ArtistMBID: "artist-mbid-" + strconv.Itoa(i+1),
+		})
+	}
+	return &metadata.SimilarSongsResponse{Songs: songs}, nil
+}
+
+func (t *testMetadataAgent) GetSimilarSongsByAlbum(input metadata.SimilarSongsByAlbumRequest) (*metadata.SimilarSongsResponse, error) {
+	if err := checkConfigError(); err != nil {
+		return nil, err
+	}
+	count := int(input.Count)
+	if count == 0 {
+		count = 5
+	}
+	songs := make([]metadata.SongRef, 0, count)
+	for i := range count {
+		songs = append(songs, metadata.SongRef{
+			ID:     "album-similar-id-" + strconv.Itoa(i+1),
+			Name:   "Album Similar #" + strconv.Itoa(i+1),
+			Artist: input.Artist,
+			Album:  input.Name,
+		})
+	}
+	return &metadata.SimilarSongsResponse{Songs: songs}, nil
+}
+
+func (t *testMetadataAgent) GetSimilarSongsByArtist(input metadata.SimilarSongsByArtistRequest) (*metadata.SimilarSongsResponse, error) {
+	if err := checkConfigError(); err != nil {
+		return nil, err
+	}
+	count := int(input.Count)
+	if count == 0 {
+		count = 5
+	}
+	songs := make([]metadata.SongRef, 0, count)
+	for i := range count {
+		songs = append(songs, metadata.SongRef{
+			ID:     "artist-similar-id-" + strconv.Itoa(i+1),
+			Name:   input.Name + " Style Song #" + strconv.Itoa(i+1),
+			Artist: input.Name + " Similar Artist",
+		})
+	}
+	return &metadata.SimilarSongsResponse{Songs: songs}, nil
+}
+
 func main() {}