fix(realtime): resolve platform opus codec backend by alias (#9813)

The realtime WebRTC transport hardcoded loading the literal "opus" backend.
On darwin/arm64 the only installable opus codec is "metal-opus" (it shares
the gallery alias "opus"). Backend names resolve through the model loader's
external-backends map, and the "opus" alias key is only registered when a
user-path variant's alias was collected; a system-path metal-opus registers
only under its concrete name. As a result, with metal-opus installed the
realtime path still failed with "opus backend not available".

Resolve the opus codec from the set of currently loadable backends instead
of the hardcoded literal: an exact "opus" match wins (covers the plain
backend and the alias key), otherwise fall back to a platform-appropriate
"*opus*" codec, preferring the metal build on darwin/arm64. Behavior is
unchanged when a plain "opus" backend is present, and the same error is
surfaced when no opus codec is installed at all.

The selection logic is extracted into resolveOpusBackend and unit-tested.

Assisted-by: claude:claude-opus-4-8 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

core/http/endpoints/openai/realtime_webrtc.go

@@ -2,6 +2,8 @@ package openai
 
 import (
 	"net/http"
+	"runtime"
+	"strings"
 	"time"
 
 	"github.com/labstack/echo/v4"
@@ -11,6 +13,52 @@ import (
 	"github.com/pion/webrtc/v4"
 )
 
+// opusBackendName is the canonical gallery name/alias of the opus audio codec
+// backend that the realtime WebRTC transport needs.
+const opusBackendName = "opus"
+
+// resolveOpusBackend picks which installed opus-codec backend the realtime
+// WebRTC transport should load. The transport historically hardcoded the
+// literal "opus" backend name, but on darwin/arm64 the only installable opus
+// codec is "metal-opus" (it shares the gallery alias "opus"). When that
+// platform-specific variant is registered under its concrete directory name
+// rather than the "opus" alias key, loading the literal "opus" fails with
+// "opus backend not available" (issue #9813). Given the set of currently
+// loadable backend names, this returns the best opus codec to load for the
+// running platform, falling back to the literal name so the caller surfaces
+// the same error as before when no opus codec is installed at all.
+func resolveOpusBackend(installed []string, goos, goarch string) string {
+	// An exact match wins: this covers the plain "opus" backend as well as the
+	// "opus" alias key registered by gallery alias resolution for a
+	// user-installed platform variant.
+	for _, b := range installed {
+		if b == opusBackendName {
+			return opusBackendName
+		}
+	}
+
+	// No "opus" key is registered (e.g. a system-path metal-opus whose alias
+	// was never collected). Fall back to a platform-appropriate "*opus*" codec
+	// backend; on darwin/arm64 prefer the metal build.
+	var fallback string
+	for _, b := range installed {
+		if !strings.Contains(strings.ToLower(b), opusBackendName) {
+			continue
+		}
+		if goos == "darwin" && goarch == "arm64" && strings.Contains(strings.ToLower(b), "metal") {
+			return b
+		}
+		if fallback == "" {
+			fallback = b
+		}
+	}
+	if fallback != "" {
+		return fallback
+	}
+
+	return opusBackendName
+}
+
 // RealtimeCallRequest is the JSON body for POST /v1/realtime/calls.
 type RealtimeCallRequest struct {
 	SDP   string `json:"sdp"`
@@ -94,15 +142,25 @@ func RealtimeCalls(application *application.Application) echo.HandlerFunc {
 			}
 		}()
 
-		// Load the Opus backend
-		opusBackend, err := application.ModelLoader().Load(
-			model.WithBackendString("opus"),
+		// Load the Opus backend. The opus codec ships under different backend
+		// names per platform (e.g. "metal-opus" on darwin/arm64), so resolve the
+		// platform-appropriate variant from the installed backends instead of
+		// hardcoding the literal "opus" name (issue #9813).
+		ml := application.ModelLoader()
+		installed := make([]string, 0)
+		for name := range ml.GetAllExternalBackends(nil) {
+			installed = append(installed, name)
+		}
+		opusName := resolveOpusBackend(installed, runtime.GOOS, runtime.GOARCH)
+
+		opusBackend, err := ml.Load(
+			model.WithBackendString(opusName),
 			model.WithModelID("__opus_codec__"),
-			model.WithModel("opus"),
+			model.WithModel(opusName),
 		)
 		if err != nil {
 			pc.Close()
-			xlog.Error("failed to load opus backend", "error", err)
+			xlog.Error("failed to load opus backend", "error", err, "backend", opusName)
 			return c.JSON(http.StatusInternalServerError, map[string]string{"error": "opus backend not available"})
 		}
 

core/http/endpoints/openai/realtime_webrtc_opus_test.go

@@ -0,0 +1,32 @@
+package openai
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("resolveOpusBackend", func() {
+	It("prefers the exact opus backend when it is installed", func() {
+		Expect(resolveOpusBackend([]string{"opus", "metal-opus"}, "linux", "amd64")).To(Equal("opus"))
+	})
+
+	It("resolves to the opus alias key on linux", func() {
+		Expect(resolveOpusBackend([]string{"opus"}, "linux", "amd64")).To(Equal("opus"))
+	})
+
+	It("selects metal-opus on darwin/arm64 when no plain opus is installed", func() {
+		Expect(resolveOpusBackend([]string{"metal-opus"}, "darwin", "arm64")).To(Equal("metal-opus"))
+	})
+
+	It("selects metal-opus on darwin/arm64 even when other backends are present", func() {
+		Expect(resolveOpusBackend([]string{"silero-vad", "metal-opus", "whisper"}, "darwin", "arm64")).To(Equal("metal-opus"))
+	})
+
+	It("falls back to any opus codec backend when there is no exact match (non-darwin)", func() {
+		Expect(resolveOpusBackend([]string{"metal-opus"}, "linux", "amd64")).To(Equal("metal-opus"))
+	})
+
+	It("returns the literal opus name when no opus codec is installed", func() {
+		Expect(resolveOpusBackend([]string{"silero-vad", "whisper"}, "darwin", "arm64")).To(Equal("opus"))
+	})
+})