Lexiel vs ChatGPT for legal contracts: why generic AI is not enough

The problem: contracts need legal precision, not plausible text

ChatGPT and other generalist models generate convincing text. But in contract law, "convincing" is not enough: you need the exact rule, the correct ruling, and the deadline that applies in Spain today.

This article compares Lexiel and ChatGPT (GPT-4o) on five real contract analysis tasks. Every result was verified against BOE and CENDOJ.

The test: 5 contract analysis tasks

Task 1: Identify abusive clauses in a lease agreement

ChatGPT: Generic analysis mentioning Act 29/1994. No specific articles. Misses RDL 7/2019.

Lexiel: Clause void under Art. 19 LAU. Cites RDL 7/2019. Distinguishes residential vs. non-residential. Includes case law: SAP Madrid, Section 11, 14 February 2024.

Task 2: Calculate early termination compensation

ChatGPT: "Depends on what's in the contract." Vague and unusable.

Lexiel: Applies Art. 11 LAU. Explains penalty clause rules. Gives calculation framework with STS citation.

Task 3: Post-contractual non-compete clause

ChatGPT: Mentions adequate compensation. Misses Art. 21 ET limits.

Lexiel: Art. 21.2 ET: max 2 years for technical staff, 6 months for others. If any requirement missing, clause is void; not reducible.

Task 4: Arbitration clause in B2C contract

ChatGPT: "Valid if consumer consented.": INCORRECT under Spanish law.

Lexiel: Clause void. Art. 90.1 TRLGDCU. Cites CJEU 26 October 2006 (Mostaza Claro).

Task 5: Deadline to challenge a floor rate clause

ChatGPT: "Nullity action doesn't prescribe. You can claim." Incomplete.

Lexiel: Full map: nullity imprescriptible, restitution 5 years (Art. 1964 CC), full retroactivity from CJEU 21 December 2016.

Summary table

Conclusion

For general legal exploration, ChatGPT is useful. For real contracts under current Spanish law with verified case law, the difference is significant.

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