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

Error message

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

What it means

Returned by (*Builder).Build when si.Extensions is populated but is not of type grpctransport.ServerIdentifierExtension (grpc_transport.go:98). The Builder uses a type assertion to read ConfigName, so any other concrete type (including a pointer *ServerIdentifierExtension) is rejected.

Solutions

Set Extensions to a value (not pointer): grpctransport.ServerIdentifierExtension{ConfigName: name}.
Ensure only one transport package's extension type is in use across the codebase.
The error message prints both the actual and expected types - read them to confirm the fix.

Example fix

// before
si.Extensions = &grpctransport.ServerIdentifierExtension{ConfigName: n}

// after
si.Extensions = grpctransport.ServerIdentifierExtension{ConfigName: n}

Defensive patterns

Strategy: type-guard

Type guard

func isGRPCExtension(si clients.ServerIdentifier) bool {
    _, ok := si.Extensions.(grpctransport.ServerIdentifierExtension)
    return ok
}

Try / catch

if _, ok := si.Extensions.(grpctransport.ServerIdentifierExtension); !ok {
    // fix the assignment: use value type, not pointer
}

Prevention

Pass ServerIdentifierExtension by value, not by pointer.
Add a helper that constructs the ServerIdentifier with the correct Extension type.
Write a test that type-asserts Extensions on every identifier your code produces.

When it happens

Trigger: Setting si.Extensions to a *ServerIdentifierExtension (pointer) instead of the value type, or to an unrelated struct/int. The type assertion sce, ok := si.Extensions.(ServerIdentifierExtension) fails.

Common situations: Caller writes Extensions: &grpctransport.ServerIdentifierExtension{...} (pointer); mixing extensions from a different transport implementation; copy-paste from an example that used a pointer form.

Related errors

AI-assisted analysis of grpc/grpc-go@0c51461d27 (2026-08-11). Data as JSON: /api/errors/80e33bacef230204. Report an issue: GitHub.

Appendix: 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 0c51461d27)