diff --git a/internal/subsonic/envelope.go b/internal/subsonic/envelope.go
new file mode 100644
index 00000000..3e2024c4
--- /dev/null
+++ b/internal/subsonic/envelope.go
@@ -0,0 +1,98 @@
+package subsonic
+
+import (
+	"encoding/json"
+	"encoding/xml"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+)
+
+// APIVersion is the Subsonic REST version we advertise. 1.16.1 is the last
+// "classic" level; OpenSubsonic extensions are surfaced via the openSubsonic
+// flag and the type/serverVersion fields.
+const (
+	APIVersion = "1.16.1"
+	ServerType = "minstrel"
+	XMLNS      = "http://subsonic.org/restapi"
+)
+
+// ServerVersion identifies this build. Overwritten by cmd/minstrel at startup.
+var ServerVersion = "dev"
+
+// Envelope holds the attributes present on every Subsonic response. Concrete
+// response types embed it and add endpoint-specific fields with json+xml tags.
+type Envelope struct {
+	XMLName       xml.Name `json:"-" xml:"subsonic-response"`
+	XMLNS         string   `json:"-" xml:"xmlns,attr"`
+	Status        string   `json:"status" xml:"status,attr"`
+	Version       string   `json:"version" xml:"version,attr"`
+	Type          string   `json:"type" xml:"type,attr"`
+	ServerVersion string   `json:"serverVersion" xml:"serverVersion,attr"`
+	OpenSubsonic  bool     `json:"openSubsonic" xml:"openSubsonic,attr"`
+	Error         *Error   `json:"error,omitempty" xml:"error,omitempty"`
+}
+
+// Error is the body of a failed response.
+type Error struct {
+	XMLName xml.Name `json:"-" xml:"error"`
+	Code    int      `json:"code" xml:"code,attr"`
+	Message string   `json:"message" xml:"message,attr"`
+}
+
+// NewEnvelope returns a populated header for the given status ("ok"|"failed").
+func NewEnvelope(status string) Envelope {
+	return Envelope{
+		XMLNS:         XMLNS,
+		Status:        status,
+		Version:       APIVersion,
+		Type:          ServerType,
+		ServerVersion: ServerVersion,
+		OpenSubsonic:  true,
+	}
+}
+
+// Write renders body in the format requested by the ?f= query parameter.
+// body must embed Envelope. Unknown formats fall back to JSON.
+func Write(w http.ResponseWriter, r *http.Request, body any) {
+	switch strings.ToLower(r.URL.Query().Get("f")) {
+	case "xml":
+		writeXML(w, body)
+	case "jsonp":
+		writeJSONP(w, body, r.URL.Query().Get("callback"))
+	default:
+		writeJSON(w, body)
+	}
+}
+
+// WriteFail emits an envelope-only failed response with the given error code.
+func WriteFail(w http.ResponseWriter, r *http.Request, code int, message string) {
+	env := NewEnvelope("failed")
+	env.Error = &Error{Code: code, Message: message}
+	Write(w, r, env)
+}
+
+func writeJSON(w http.ResponseWriter, body any) {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	_ = json.NewEncoder(w).Encode(map[string]any{"subsonic-response": body})
+}
+
+func writeJSONP(w http.ResponseWriter, body any, callback string) {
+	if callback == "" {
+		// Per spec the client must supply callback; fall back to plain JSON
+		// rather than producing invalid JavaScript.
+		writeJSON(w, body)
+		return
+	}
+	w.Header().Set("Content-Type", "application/javascript; charset=utf-8")
+	fmt.Fprintf(w, "%s(", callback)
+	_ = json.NewEncoder(w).Encode(map[string]any{"subsonic-response": body})
+	_, _ = io.WriteString(w, ")")
+}
+
+func writeXML(w http.ResponseWriter, body any) {
+	w.Header().Set("Content-Type", "application/xml; charset=utf-8")
+	_, _ = io.WriteString(w, xml.Header)
+	_ = xml.NewEncoder(w).Encode(body)
+}