Tex. Prop. Code § 92.335

In an eviction suit, retaliation by the landlord under Section 92.331 is a defense and a rent deduction lawfully made by the tenant under this chapter is a defense for nonpayment of the rent to the extent allowed by this chapter. Other judicial actions under this chapter may not be joined with an eviction suit or asserted as a defense or crossclaim in an eviction suit.