fix(a2a): reject ambiguous default agent selection (#79)

agent/execution/protocol/a2a/a2a_coverage_test.go

@@ -568,6 +568,21 @@ func TestHTTPServer_GetDefaultAgent_WithDefaultID(t *testing.T) {
 	assert.Equal(t, "default-agent", defaultAg.ID())
 }
 
+func TestHTTPServer_GetDefaultAgent_MultipleAgentsWithoutDefaultReturnsError(t *testing.T) {
+	server := NewHTTPServer(&ServerConfig{
+		BaseURL: "http://localhost:8080",
+		Logger:  zap.NewNop(),
+	})
+
+	_ = server.RegisterAgent(newMockAgent("agent-a", "Agent A"))
+	_ = server.RegisterAgent(newMockAgent("agent-b", "Agent B"))
+
+	ag, err := server.getDefaultAgent()
+	require.Error(t, err)
+	assert.Nil(t, ag)
+	assert.Contains(t, err.Error(), "multiple agents registered")
+}
+
 // --- AgentCard discovery with agent_id query param ---
 
 func TestHTTPServer_HandleAgentCardDiscovery_WithAgentID(t *testing.T) {

agent/execution/protocol/a2a/server_agent.go

@@ -2,6 +2,7 @@ package a2a
 
 import (
 	"fmt"
+	"sort"
 
 	"go.uber.org/zap"
 )
@@ -133,9 +134,19 @@ func (s *HTTPServer) getDefaultAgent() (Agent, error) {
 		}
 	}
 
-	// 返回第一个可用的代理
-	for _, ag := range s.agents {
-		return ag, nil
+	if len(s.agents) == 1 {
+		for _, ag := range s.agents {
+			return ag, nil
+		}
+	}
+
+	if len(s.agents) > 1 {
+		agentIDs := make([]string, 0, len(s.agents))
+		for id := range s.agents {
+			agentIDs = append(agentIDs, id)
+		}
+		sort.Strings(agentIDs)
+		return nil, fmt.Errorf("multiple agents registered (%d) but no default agent configured; set DefaultAgentID (available: %v)", len(agentIDs), agentIDs)
 	}
 
 	return nil, ErrAgentNotFound

agent/execution/protocol/a2a/server_test.go

@@ -123,6 +123,24 @@ func TestHTTPServer_HandleAgentCardDiscovery(t *testing.T) {
 	assert.Equal(t, "Test Agent", card.Name)
 }
 
+func TestHTTPServer_HandleAgentCardDiscovery_MultipleAgentsWithoutDefaultReturnsNotFound(t *testing.T) {
+	server := NewHTTPServer(&ServerConfig{
+		BaseURL: "http://localhost:8080",
+		Logger:  zap.NewNop(),
+	})
+
+	_ = server.RegisterAgent(newMockAgent("agent-a", "Agent A"))
+	_ = server.RegisterAgent(newMockAgent("agent-b", "Agent B"))
+
+	req := httptest.NewRequest(http.MethodGet, "/.well-known/agent.json", nil)
+	w := httptest.NewRecorder()
+
+	server.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusNotFound, w.Code)
+	assert.Contains(t, w.Body.String(), "multiple agents registered")
+}
+
 func TestHTTPServer_HandleSyncMessage(t *testing.T) {
 	server := NewHTTPServer(&ServerConfig{
 		BaseURL:        "http://localhost:8080",