feat: strictly block legacy agent instantiation under controller V2 build tag

cmd/ax/internal/cliutil/cliutil_harness.go

@@ -35,6 +35,15 @@ type ExecHandler = controller2.ExecHandler
 func NewControllerFromConfig(ctx context.Context, cfg *config.Config) (*controller2.Controller, error) {
 	reg := controller2.NewRegistry()
 	reg.RegisterHarness("antigravity", harness.NewAntigravityHarness(""))
+
+	// Register legacy agents so that the V2 registry can detect and block them with a clear validation error
+	for _, agentCfg := range cfg.Registry.RemoteAgents {
+		_ = reg.RegisterRemote(ctx, agentCfg)
+	}
+	for _, agentCfg := range cfg.Registry.ColabAgents {
+		_ = reg.RegisterColab(agentCfg)
+	}
+
 	return controller2.New(ctx, controller2.Config{
 		Registry: reg,
 		EventLogBuilder: func() (executor.EventLog, error) {

internal/controller2/controller.go

@@ -19,10 +19,8 @@ package controller2
 import (
 	"context"
 	"fmt"
-	"log"
 
 	"github.com/google/ax/internal/controller/executor"
-	"github.com/google/ax/internal/harness/harnesstest"
 	"github.com/google/ax/proto"
 	"github.com/google/uuid"
 )
@@ -75,9 +73,7 @@ func (d *Controller) Exec(ctx context.Context, req *proto.ExecRequest, handler E
 	// Adding harness registration support temporarily.
 	h, err := d.registry.Harness(req.AgentId)
 	if err != nil {
-		// Fallback to test harness
-		log.Printf("WARNING: harness %s not found in registry, falling back to test harness: %v", req.AgentId, err)
-		h = harnesstest.New()
+		return fmt.Errorf("harness/agent %q not found in registry: %w", req.AgentId, err)
 	}
 	exec, err := h.Start(ctx, req.ConversationId)
 	if err != nil {

internal/controller2/controller_test.go

@@ -41,6 +41,7 @@ func TestController2_ExecHelloWorld(t *testing.T) {
 
 	log := &executortest.MemoryEventLog{}
 	reg := NewRegistry()
+	reg.RegisterHarness("", harnesstest.New())
 	c, err := New(ctx, Config{
 		Registry: reg,
 		EventLogBuilder: func() (executor.EventLog, error) {
@@ -193,17 +194,8 @@ func TestController2_ExecRuntimeFallback(t *testing.T) {
 		Inputs:         inputs,
 		AgentId:        "antigravity",
 	}, handler)
-	if err != nil {
-		t.Fatalf("Controller2.Exec failed: %v", err)
-	}
-
-	if len(outputs) != 1 {
-		t.Fatalf("expected exactly 1 output message, got %d", len(outputs))
-	}
-
-	gotText := outputs[0].GetContent().GetText().GetText()
-	if gotText != "Hello world" {
-		t.Errorf("expected 'Hello world' output text response due to runtime fallback, got %q", gotText)
+	if err == nil {
+		t.Fatal("expected error for unregistered agent lookup under V2, got nil")
 	}
 }
 

internal/controller2/registry.go

@@ -253,7 +253,10 @@ func (r *Registry) Harness(id string) (harness.Harness, error) {
 	defer r.mu.RUnlock()
 	h, ok := r.harnesses[id]
 	if !ok {
-		return nil, fmt.Errorf("harness %s not found", id)
+		if _, exists := r.agents[id]; exists {
+			return nil, fmt.Errorf("agent %q is registered under legacy agent config but has no V2 harness interface implementation", id)
+		}
+		return nil, fmt.Errorf("harness %q not found (local V2 execution only)", id)
 	}
 	return h, nil
 }

internal/controller2/registry_test.go

@@ -73,3 +73,26 @@ func TestRegistry_GracefulShutdown(t *testing.T) {
 	// We are testing for absence of panic/deadlock here.
 	_ = r.Close()
 }
+
+func TestRegistry_LegacyAgentValidation(t *testing.T) {
+	r := NewRegistry()
+
+	err := r.RegisterRemote(context.Background(), config.RemoteAgentConfig{
+		ID:      "legacy-remote-agent",
+		Name:    "Legacy Agent",
+		Address: "localhost:8494",
+	})
+	if err != nil {
+		t.Fatalf("failed to register remote agent: %v", err)
+	}
+
+	_, err = r.Harness("legacy-remote-agent")
+	if err == nil {
+		t.Fatal("expected error for legacy agent lookup in V2 mode, got nil")
+	}
+
+	expectedErr := `agent "legacy-remote-agent" is registered under legacy agent config but has no V2 harness interface implementation`
+	if err.Error() != expectedErr {
+		t.Errorf("expected error message %q, got %q", expectedErr, err.Error())
+	}
+}