feat: auto-fill engine/url from auth provider defaults

Each auth.Provider now declares DefaultServiceURL() so callers do not
need to know service-specific endpoints. ApplyAuthDefaults fills Engine
and URL from the provider before validation runs — explicit flags always
win, and providers with no default URL require -url to be set explicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/olcrtc/main.go

@@ -106,7 +106,15 @@ func runWithConfig(cfg config) error {
 		return runGen(cfg)
 	}
 
-	if err := session.Validate(toSessionConfig(cfg)); err != nil {
+	return runSessionMode(cfg)
+}
+
+func runSessionMode(cfg config) error {
+	scfg, err := session.ApplyAuthDefaults(toSessionConfig(cfg))
+	if err != nil {
+		return fmt.Errorf("validate config: %w", err)
+	}
+	if err := session.Validate(scfg); err != nil {
 		return fmt.Errorf("validate config: %w", err)
 	}
 
@@ -131,7 +139,7 @@ func runWithConfig(cfg config) error {
 
 	errCh := make(chan error, 1)
 	go func() {
-		errCh <- runSession(ctx, toSessionConfig(cfg))
+		errCh <- runSession(ctx, scfg)
 	}()
 
 	select {
@@ -145,7 +153,10 @@ func runWithConfig(cfg config) error {
 }
 
 func execGen(cfg config) error {
-	scfg := toSessionConfig(cfg)
+	scfg, err := session.ApplyAuthDefaults(toSessionConfig(cfg))
+	if err != nil {
+		return fmt.Errorf("validate gen config: %w", err)
+	}
 	if err := session.ValidateGen(scfg); err != nil {
 		return fmt.Errorf("validate gen config: %w", err)
 	}

internal/app/session/session.go

@@ -46,6 +46,8 @@ var (
 	// ErrAuthRequired indicates that no auth provider was selected.
 	ErrAuthRequired = errors.New(
 		"auth provider required (use -auth telemost, -auth jazz, -auth wbstream or -auth none)")
+	// ErrURLRequired indicates that -url must be provided when the auth provider has no default URL.
+	ErrURLRequired = errors.New("SFU URL required (use -url wss://...)")
 	// ErrUnsupportedCarrier indicates that carrier is not registered.
 	ErrUnsupportedCarrier = errors.New("unsupported carrier")
 	// ErrUnsupportedLink indicates that link is not registered.
@@ -151,6 +153,29 @@ func RegisterDefaults() {
 	transport.Register("vp8channel", vp8channel.New)
 }
 
+// ApplyAuthDefaults fills in Engine and URL from the auth provider when they are not set explicitly.
+// For -auth none the fields are left untouched (the caller supplies them directly).
+// Returns an error if the auth provider has no default URL and -url was not given.
+func ApplyAuthDefaults(cfg Config) (Config, error) {
+	if cfg.Auth == authNone || cfg.Auth == "" {
+		return cfg, nil
+	}
+	p, _ := auth.Get(cfg.Auth) // unknown auth is caught later by validateAuth
+	if p == nil {
+		return cfg, nil
+	}
+	if cfg.Engine == "" {
+		cfg.Engine = p.Engine()
+	}
+	if cfg.URL == "" {
+		cfg.URL = p.DefaultServiceURL()
+	}
+	if cfg.URL == "" {
+		return cfg, fmt.Errorf("%w: auth provider %q has no default URL", ErrURLRequired, cfg.Auth)
+	}
+	return cfg, nil
+}
+
 // Validate verifies that the runtime config refers to registered components and all required fields are present.
 func Validate(cfg Config) error {
 	if err := validateMode(cfg); err != nil {

internal/auth/auth.go

@@ -53,10 +53,12 @@ type Config struct {
 
 // Provider produces engine credentials.
 type Provider interface {
-	// Engine reports which engine this auth provider feeds. This is what lets
-	// a carrier resolve (auth, engine) pairs consistently — e.g. auth=jazz
-	// always pairs with engine=livekit.
+	// Engine reports which engine this auth provider feeds.
 	Engine() string
+	// DefaultServiceURL returns the well-known service URL for this provider
+	// (e.g. "https://stream.wb.ru"). Returns "" if no default exists — in that
+	// case the caller must supply -url explicitly.
+	DefaultServiceURL() string
 	// Issue obtains credentials for the given room.
 	Issue(ctx context.Context, cfg Config) (Credentials, error)
 }

internal/auth/salutejazz/salutejazz.go

@@ -14,6 +14,9 @@ type Provider struct{}
 // Engine reports which engine consumes credentials from this auth provider.
 func (Provider) Engine() string { return "salutejazz" }
 
+// DefaultServiceURL returns the SaluteJazz service URL.
+func (Provider) DefaultServiceURL() string { return "https://bk.salutejazz.ru" }
+
 // Issue runs the SaluteJazz API flow and returns engine credentials.
 //
 // cfg.RoomURL accepts either an empty value (a new room is created on the

internal/auth/telemost/telemost.go

@@ -16,6 +16,9 @@ type Provider struct{}
 // Engine reports which engine consumes credentials from this auth provider.
 func (Provider) Engine() string { return "goolom" }
 
+// DefaultServiceURL returns the Telemost conference base URL.
+func (Provider) DefaultServiceURL() string { return "https://telemost.yandex.ru" }
+
 // Issue fetches connection info for a Telemost room and returns engine credentials.
 //
 // cfg.RoomURL accepts either a full Telemost conference URL

internal/auth/wbstream/wbstream.go

@@ -13,6 +13,9 @@ type Provider struct{}
 // Engine reports which engine consumes credentials from this auth provider.
 func (Provider) Engine() string { return "livekit" }
 
+// DefaultServiceURL returns the WB Stream service URL.
+func (Provider) DefaultServiceURL() string { return "https://stream.wb.ru" }
+
 // Issue runs the WB Stream auth flow and returns LiveKit credentials.
 //
 // If cfg.RoomURL is empty or "any", a fresh room is created on the fly —

pkg/olcrtc/olcrtc_test.go

@@ -71,7 +71,8 @@ func registerStubEngineControlled(t *testing.T, name string, stub *stubSession)
 
 type stubAuth struct{ engineName string }
 
-func (a stubAuth) Engine() string { return a.engineName }
+func (a stubAuth) Engine() string          { return a.engineName }
+func (stubAuth) DefaultServiceURL() string { return "https://stub.example" }
 func (a stubAuth) Issue(_ context.Context, cfg auth.Config) (auth.Credentials, error) {
 	if cfg.RoomURL == "" {
 		return auth.Credentials{}, auth.ErrRoomIDRequired