Tex. Prop. Code § 94.256

In an eviction suit, retaliation by the landlord under Section 94.251 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, excluding an action that would be permitted under Chapter 24, may not be joined with an eviction suit or asserted as a defense or cross-claim in an eviction suit.