diff --git a/internal/fs/seed_generate.go b/internal/fs/seed_generate.go
index 86f326942..ed5a628b1 100644
--- a/internal/fs/seed_generate.go
+++ b/internal/fs/seed_generate.go
@@ -102,6 +102,61 @@ type SeedArtifact struct {
 	Path     string `json:"path,omitempty"`
 }
 
+// DeriveSeedName derives a sensible default seed name from the source paths:
+// single selection uses the file name, multi selection uses the common base
+// name (ignoring extensions) when all files share one, otherwise the folder name.
+func DeriveSeedName(paths []string) string {
+	if len(paths) == 0 {
+		return "OpenList Seed"
+	}
+	if len(paths) == 1 {
+		return seedBaseName(paths[0])
+	}
+	// Common base name ignoring extensions (e.g. a.docx + a.exe -> "a").
+	commonBase := seedBaseName(paths[0])
+	for _, p := range paths[1:] {
+		if base := seedBaseName(p); base != commonBase {
+			commonBase = ""
+			break
+		}
+	}
+	if commonBase != "" {
+		return commonBase
+	}
+	// Fall back to the common parent directory name.
+	dir := commonParentDir(paths)
+	if base := stdpath.Base(dir); base != "" && base != "/" && base != "." {
+		return base
+	}
+	return "OpenList Seed"
+}
+
+// seedBaseName returns the file name without its extension.
+func seedBaseName(p string) string {
+	base := stdpath.Base(p)
+	return strings.TrimSuffix(base, stdpath.Ext(base))
+}
+
+// commonParentDir returns the longest common parent directory of the given paths.
+func commonParentDir(paths []string) string {
+	if len(paths) == 0 {
+		return "/"
+	}
+	parts := strings.Split(strings.Trim(stdpath.Dir(paths[0]), "/"), "/")
+	for _, p := range paths[1:] {
+		cur := strings.Split(strings.Trim(stdpath.Dir(p), "/"), "/")
+		n := 0
+		for n < len(parts) && n < len(cur) && parts[n] == cur[n] {
+			n++
+		}
+		parts = parts[:n]
+	}
+	if len(parts) == 0 {
+		return "/"
+	}
+	return "/" + strings.Join(parts, "/")
+}
+
 // NormalizeSeedFormats validates and deduplicates a list of seed format names.
 func NormalizeSeedFormats(rawFormats []string) ([]string, error) {
 	formats := append([]string(nil), rawFormats...)
@@ -277,11 +332,7 @@ func GenerateSeedArtifacts(ctx context.Context, user *model.User, params SeedGen
 	}
 	seedName := strings.TrimSpace(params.Name)
 	if seedName == "" {
-		if len(params.Paths) == 1 {
-			seedName = stdpath.Base(params.Paths[0])
-		} else {
-			seedName = "OpenList Seed"
-		}
+		seedName = DeriveSeedName(params.Paths)
 	}
 	seed := torrent.NewSeed(seedName, "OpenList", pieceSize)
 	seed.Comment = params.Comment
@@ -491,26 +542,6 @@ func GenerateSeedArtifacts(ctx context.Context, user *model.User, params SeedGen
 		}
 		seenFormats[format] = struct{}{}
 
-		// CAS is inherently a single-file container: emit one .cas per file
-		// instead of rejecting multi-file selection.
-		if format == "cas" {
-			for _, file := range seed.Files {
-				singleSeed := *seed
-				singleSeed.Files = []torrent.SeedFile{file}
-				data, err := EncodeGeneratedSeed(&singleSeed, format, nil)
-				if err != nil {
-					return nil, nil, fmt.Errorf("generate %s seed for %s: %w", format, file.Path, err)
-				}
-				fileName := stdpath.Base(file.Path) + ".cas"
-				artifact, err := writeArtifact(format, fileName, data)
-				if err != nil {
-					return nil, nil, err
-				}
-				artifacts = append(artifacts, artifact)
-			}
-			continue
-		}
-
 		data, err := EncodeGeneratedSeed(seed, format, globalHasher.GetPieceHashes())
 		if err != nil {
 			return nil, nil, fmt.Errorf("generate %s seed: %w", format, err)
diff --git a/pkg/torrent/torrent.go b/pkg/torrent/torrent.go
index 54a9bfea7..c7340ef9c 100644
--- a/pkg/torrent/torrent.go
+++ b/pkg/torrent/torrent.go
@@ -102,8 +102,8 @@ type SeedSource struct {
 	ShareID   string `json:"share_id,omitempty"`
 }
 
-// CASPayload exactly matches the five-field reference .cas JSON payload.
-type CASPayload struct {
+// CASFileEntry describes one file inside a multi-file .cas payload.
+type CASFileEntry struct {
 	Name       string `json:"name"`
 	Size       int64  `json:"size"`
 	MD5        string `json:"md5"`
@@ -111,6 +111,18 @@ type CASPayload struct {
 	CreateTime string `json:"create_time"`
 }
 
+// CASPayload matches the reference .cas JSON payload. The five legacy fields
+// describe a single file (byte-for-byte compatible with the reference project);
+// the optional "files" array extends it to multi-file seeds.
+type CASPayload struct {
+	Name       string         `json:"name"`
+	Size       int64          `json:"size"`
+	MD5        string         `json:"md5"`
+	SliceMD5   string         `json:"sliceMd5"`
+	CreateTime string         `json:"create_time"`
+	Files      []CASFileEntry `json:"files,omitempty"`
+}
+
 // ParseLimits controls resource use while parsing untrusted seeds.
 type ParseLimits struct {
 	MaxBytes int64
@@ -804,20 +816,13 @@ func DecodeOSS(data []byte, limits ParseLimits) (*Seed, error) {
 	return &seed, nil
 }
 
-// EncodeCAS writes the reference-compatible base64 encoded JSON payload.
-func EncodeCAS(seed *Seed) ([]byte, error) {
-	if err := ValidateSeed(seed, DefaultParseLimits()); err != nil {
-		return nil, err
-	}
-	if len(seed.Files) != 1 {
-		return nil, fmt.Errorf("CAS requires exactly one file")
-	}
-	file := seed.Files[0]
+// buildCASFileEntry computes the reference-compatible five fields for one file.
+func buildCASFileEntry(file SeedFile, pieceSize int64) (CASFileEntry, error) {
 	if file.Hashes.MD5 == "" {
-		return nil, fmt.Errorf("CAS requires a whole-file MD5")
+		return CASFileEntry{}, fmt.Errorf("CAS requires a whole-file MD5 for %s", file.Path)
 	}
 	sliceMD5 := strings.ToUpper(file.CASSliceMD5)
-	if sliceMD5 == "" && file.Hashes.Pieces != nil && len(file.Hashes.Pieces.MD5) > 0 && seed.PieceSize == DefaultPieceSize {
+	if sliceMD5 == "" && file.Hashes.Pieces != nil && len(file.Hashes.Pieces.MD5) > 0 && pieceSize == DefaultPieceSize {
 		pieces := upperStrings(file.Hashes.Pieces.MD5)
 		sliceMD5 = pieces[0]
 		if len(pieces) > 1 {
@@ -826,7 +831,7 @@ func EncodeCAS(seed *Seed) ([]byte, error) {
 	}
 	if sliceMD5 == "" {
 		if file.Size > DefaultPieceSize {
-			return nil, fmt.Errorf("CAS requires a legacy slice MD5 or complete 10 MiB MD5 pieces")
+			return CASFileEntry{}, fmt.Errorf("CAS requires a legacy slice MD5 or complete 10 MiB MD5 pieces for %s", file.Path)
 		}
 		sliceMD5 = strings.ToUpper(file.Hashes.MD5)
 	}
@@ -834,9 +839,44 @@ func EncodeCAS(seed *Seed) ([]byte, error) {
 	if createTime == "" {
 		createTime = fmt.Sprintf("%d", time.Now().Unix())
 	}
-	payload := CASPayload{
+	return CASFileEntry{
 		Name: path.Base(file.Path), Size: file.Size, MD5: strings.ToUpper(file.Hashes.MD5),
 		SliceMD5: sliceMD5, CreateTime: createTime,
+	}, nil
+}
+
+// EncodeCAS writes the reference-compatible base64 encoded JSON payload. A
+// single-file seed uses the legacy five fields; multiple files are stored in
+// the "files" array.
+func EncodeCAS(seed *Seed) ([]byte, error) {
+	if err := ValidateSeed(seed, DefaultParseLimits()); err != nil {
+		return nil, err
+	}
+	if len(seed.Files) == 0 {
+		return nil, fmt.Errorf("CAS requires at least one file")
+	}
+	var payload CASPayload
+	if len(seed.Files) == 1 {
+		entry, err := buildCASFileEntry(seed.Files[0], seed.PieceSize)
+		if err != nil {
+			return nil, err
+		}
+		payload = CASPayload{
+			Name: entry.Name, Size: entry.Size, MD5: entry.MD5,
+			SliceMD5: entry.SliceMD5, CreateTime: entry.CreateTime,
+		}
+	} else {
+		entries := make([]CASFileEntry, 0, len(seed.Files))
+		var totalSize int64
+		for _, file := range seed.Files {
+			entry, err := buildCASFileEntry(file, seed.PieceSize)
+			if err != nil {
+				return nil, err
+			}
+			entries = append(entries, entry)
+			totalSize += entry.Size
+		}
+		payload = CASPayload{Name: seed.Name, Size: totalSize, Files: entries}
 	}
 	content, err := json.Marshal(payload)
 	if err != nil {
@@ -864,6 +904,31 @@ func DecodeCAS(data []byte, limits ParseLimits) (*Seed, error) {
 	if err = json.Unmarshal(data, &payload); err != nil {
 		return nil, fmt.Errorf("decode CAS: %w", err)
 	}
+	seed := NewSeed(payload.Name, "OpenList CAS", DefaultPieceSize)
+	if len(payload.Files) > 0 {
+		// Multi-file extension.
+		if len(payload.Files) > limits.MaxFiles {
+			return nil, fmt.Errorf("CAS seed exceeds %d files", limits.MaxFiles)
+		}
+		for _, entry := range payload.Files {
+			if entry.Name == "" || entry.Size < 0 || !validHexHash(entry.MD5, 32) {
+				return nil, fmt.Errorf("invalid CAS file entry")
+			}
+			sliceMD5 := entry.SliceMD5
+			if sliceMD5 == "" {
+				sliceMD5 = entry.MD5
+			}
+			if !validHexHash(sliceMD5, 32) {
+				return nil, fmt.Errorf("invalid CAS sliceMd5")
+			}
+			seed.Files = append(seed.Files, SeedFile{
+				Path: entry.Name, Size: entry.Size, CASCreateTime: entry.CreateTime,
+				CASSliceMD5: strings.ToLower(sliceMD5),
+				Hashes:      SeedHashes{MD5: strings.ToLower(entry.MD5)},
+			})
+		}
+		return seed, ValidateSeed(seed, limits)
+	}
 	if payload.Name == "" || payload.Size < 0 || !validHexHash(payload.MD5, 32) {
 		return nil, fmt.Errorf("invalid CAS payload")
 	}
@@ -873,7 +938,6 @@ func DecodeCAS(data []byte, limits ParseLimits) (*Seed, error) {
 	if !validHexHash(payload.SliceMD5, 32) {
 		return nil, fmt.Errorf("invalid CAS sliceMd5")
 	}
-	seed := NewSeed(payload.Name, "OpenList CAS", DefaultPieceSize)
 	seed.Files = []SeedFile{{
 		Path: payload.Name, Size: payload.Size, CASCreateTime: payload.CreateTime,
 		CASSliceMD5: strings.ToLower(payload.SliceMD5),
@@ -961,10 +1025,7 @@ func DiagnoseConversion(seed *Seed, format string) []string {
 			}
 		}
 	case "cas":
-		if len(seed.Files) != 1 {
-			diagnostics = append(diagnostics, "CAS supports exactly one file")
-		} else {
-			file := seed.Files[0]
+		for _, file := range seed.Files {
 			if file.Hashes.MD5 == "" {
 				diagnostics = append(diagnostics, file.Path+": missing whole-file MD5")
 			}