test(agent): add regression test for invalid exec config fallback

pkg/agent/instance_test.go

@@ -246,3 +246,37 @@ func TestNewAgentInstance_AllowsMediaTempDirForReadListAndExec(t *testing.T) {
 		t.Fatalf("exec output missing media content: %s", execResult.ForLLM)
 	}
 }
+
+func TestNewAgentInstance_InvalidExecConfigDoesNotExit(t *testing.T) {
+	workspace := t.TempDir()
+
+	cfg := &config.Config{
+		Agents: config.AgentsConfig{
+			Defaults: config.AgentDefaults{
+				Workspace: workspace,
+				ModelName: "test-model",
+			},
+		},
+		Tools: config.ToolsConfig{
+			ReadFile: config.ReadFileToolConfig{Enabled: true},
+			Exec: config.ExecConfig{
+				ToolConfig:         config.ToolConfig{Enabled: true},
+				EnableDenyPatterns: true,
+				CustomDenyPatterns: []string{"[invalid-regex"},
+			},
+		},
+	}
+
+	agent := NewAgentInstance(nil, &cfg.Agents.Defaults, cfg, &mockProvider{})
+	if agent == nil {
+		t.Fatal("expected agent instance, got nil")
+	}
+
+	if _, ok := agent.Tools.Get("exec"); ok {
+		t.Fatal("exec tool should not be registered when exec config is invalid")
+	}
+
+	if _, ok := agent.Tools.Get("read_file"); !ok {
+		t.Fatal("read_file tool should still be registered")
+	}
+}