diff --git a/proxy/nested_payload.go b/proxy/nested_payload.go
new file mode 100644
index 00000000..737c460e
--- /dev/null
+++ b/proxy/nested_payload.go
@@ -0,0 +1,81 @@
+package proxy
+
+import (
+	"reflect"
+
+	"go.temporal.io/api/workflowservice/v1/workflowservicenexus"
+	"google.golang.org/protobuf/proto"
+)
+
+const (
+	// TemporalSystemNexusEndpoint is the well-known endpoint name for system Nexus operations.
+	TemporalSystemNexusEndpoint = "__temporal_system"
+)
+
+// SystemNexusOpKey identifies a system Nexus operation by its (endpoint, operation) pair.
+type SystemNexusOpKey struct {
+	Endpoint  string
+	Operation string
+}
+
+// SystemNexusOpTypes maps a system Nexus operation to the proto request and response
+// types whose bytes are serialized in NexusOperationScheduled.Input and
+// NexusOperationCompleted.Result.
+type SystemNexusOpTypes struct {
+	// NewRequest returns a fresh, zero-valued instance of the request proto.
+	NewRequest func() proto.Message
+	// NewResponse returns a fresh, zero-valued instance of the response proto.
+	NewResponse func() proto.Message
+}
+
+// SystemNexusOperations is the canonical registry of known system Nexus operations.
+// It is built dynamically from the generated workflowservicenexus package so that
+// new operations added to the proto definitions are picked up automatically without
+// requiring changes to this file or downstream consumers.
+var SystemNexusOperations = buildNexusServiceRegistry(
+	TemporalSystemNexusEndpoint,
+	workflowservicenexus.WorkflowService,
+)
+
+// buildNexusServiceRegistry uses reflection to iterate the fields of a generated
+// Nexus service struct (e.g. workflowservicenexus.WorkflowService) and builds a
+// registry entry for each field that implements the nexus.OperationReference
+// interface (i.e. has Name(), InputType(), and OutputType() methods).
+func buildNexusServiceRegistry(endpoint string, service any) map[SystemNexusOpKey]SystemNexusOpTypes {
+	registry := make(map[SystemNexusOpKey]SystemNexusOpTypes)
+	v := reflect.ValueOf(service)
+	for i := 0; i < v.NumField(); i++ {
+		field := v.Field(i)
+		nameMethod := field.MethodByName("Name")
+		if !nameMethod.IsValid() {
+			continue
+		}
+		inputTypeMethod := field.MethodByName("InputType")
+		outputTypeMethod := field.MethodByName("OutputType")
+		if !inputTypeMethod.IsValid() || !outputTypeMethod.IsValid() {
+			continue
+		}
+
+		name := nameMethod.Call(nil)[0].String()
+		inputType := inputTypeMethod.Call(nil)[0].Interface().(reflect.Type)
+		outputType := outputTypeMethod.Call(nil)[0].Interface().(reflect.Type)
+
+		registry[SystemNexusOpKey{Endpoint: endpoint, Operation: name}] = SystemNexusOpTypes{
+			NewRequest:  newProtoFactory(inputType),
+			NewResponse: newProtoFactory(outputType),
+		}
+	}
+	return registry
+}
+
+// newProtoFactory returns a function that creates a new zero-valued proto.Message
+// of the given type. The type should be a struct type (not a pointer); the returned
+// function allocates a new instance and returns a pointer to it as proto.Message.
+func newProtoFactory(t reflect.Type) func() proto.Message {
+	if t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+	return func() proto.Message {
+		return reflect.New(t).Interface().(proto.Message)
+	}
+}
diff --git a/proxy/nested_payload_test.go b/proxy/nested_payload_test.go
new file mode 100644
index 00000000..7c2db406
--- /dev/null
+++ b/proxy/nested_payload_test.go
@@ -0,0 +1,25 @@
+package proxy
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"go.temporal.io/api/workflowservice/v1"
+)
+
+func TestSystemNexusOperations_SignalWithStartRegistered(t *testing.T) {
+	key := SystemNexusOpKey{
+		Endpoint:  TemporalSystemNexusEndpoint,
+		Operation: "SignalWithStartWorkflowExecution",
+	}
+	types, ok := SystemNexusOperations[key]
+	require.True(t, ok, "SignalWithStartWorkflowExecution must be registered")
+
+	req := types.NewRequest()
+	_, ok = req.(*workflowservice.SignalWithStartWorkflowExecutionRequest)
+	require.True(t, ok, "request type must be SignalWithStartWorkflowExecutionRequest")
+
+	resp := types.NewResponse()
+	_, ok = resp.(*workflowservice.SignalWithStartWorkflowExecutionResponse)
+	require.True(t, ok, "response type must be SignalWithStartWorkflowExecutionResponse")
+}