grpctransport: Extensions field is %T, but must be %T in Ser

Error message

grpctransport: Extensions field is %T, but must be %T in ServerIdentifier

What it means

Builder.Build performs a type assertion si.Extensions.(ServerIdentifierExtension) (grpc_transport.go:96-98). If Extensions is set to any other type (a pointer, a string, a different struct), the assertion fails and this error reports both the actual type and the required type. The extension must be the concrete value type ServerIdentifierExtension, not a pointer.

Source

Thrown at internal/xds/clients/grpctransport/grpc_transport.go:98

		configs:     configs,
		connections: make(map[clients.ServerIdentifier]*grpc.ClientConn),
		refs:        make(map[clients.ServerIdentifier]int),
	}
}

// Build returns a gRPC-based clients.Transport.
//
// The Extension field of the ServerIdentifier must be a ServerIdentifierExtension.
func (b *Builder) Build(si clients.ServerIdentifier) (clients.Transport, error) {
	if si.ServerURI == "" {
		return nil, fmt.Errorf("grpctransport: ServerURI is not set in ServerIdentifier")
	}
	if si.Extensions == nil {
		return nil, fmt.Errorf("grpctransport: Extensions is not set in ServerIdentifier")
	}
	sce, ok := si.Extensions.(ServerIdentifierExtension)
	if !ok {
		return nil, fmt.Errorf("grpctransport: Extensions field is %T, but must be %T in ServerIdentifier", si.Extensions, ServerIdentifierExtension{})
	}

	config, ok := b.configs[sce.ConfigName]
	if !ok {
		return nil, fmt.Errorf("grpctransport: unknown config name %q specified in ServerIdentifierExtension", sce.ConfigName)
	}
	if config.Credentials == nil {
		return nil, fmt.Errorf("grpctransport: config %q has nil credentials bundle", sce.ConfigName)
	}

	b.mu.Lock()
	defer b.mu.Unlock()

	if cc, ok := b.connections[si]; ok {
		if logger.V(2) {
			logger.Infof("Reusing existing connection to the server for ServerIdentifier: %v", si)
		}
		b.refs[si]++

View on GitHub (pinned to 03255a9237)

Solutions

Set Extensions to a ServerIdentifierExtension value (not a pointer): grpctransport.ServerIdentifierExtension{ConfigName: "..."}.
Ensure no other type is assigned to the Extensions field.
Double-check that you are using the grpctransport package's extension type, not a similarly named one.

Example fix

// before:
//   ext := &grpctransport.ServerIdentifierExtension{ConfigName: "default"}
//   si.Extensions = ext // pointer — type assertion fails
// after:
//   si.Extensions = grpctransport.ServerIdentifierExtension{ConfigName: "default"}

Defensive patterns

Strategy: type-guard

Type guard

// Ensure Extensions is exactly ServerIdentifierExtension (value, not pointer).
func isServerIdentifierExtension(ext any) bool {
    _, ok := ext.(grpctransport.ServerIdentifierExtension)
    return ok
}
// usage:
//   if !isServerIdentifierExtension(si.Extensions) { return errors.New("bad ext type") }

Prevention

Assign the value type grpctransport.ServerIdentifierExtension{}, not a pointer.
Add a type-assertion guard in your ServerIdentifier factory.

When it happens

Trigger: Extensions is set to *ServerIdentifierExtension (pointer) instead of the value, or to a completely unrelated type. The type assertion in Build rejects it.

Common situations: Passing a pointer to the struct out of habit; using a custom extension type that embeds but is not exactly ServerIdentifierExtension; copy-paste from a different transport builder's extension type.

Related errors

AI-assisted analysis of grpc/grpc-go@03255a9237 (2026-08-07). Data as JSON: /api/errors/80e33bacef230204. Report an issue: GitHub.