Short answer. Article 281 requires probationary standards to be made known by the employer to the employee at the time of his engagement, but it does not itself say that must happen in writing. What matters is whether the standards were actually communicated to you when hired, not the format used.
What the law says
The services of an employee who has been engaged on a probationary basis may be terminated for a just cause or when he fails to qualify as a regular employee in accordance with reasonable standards made known by the employer to the employee at the time of his engagement.
Labor Code, Article 281 — Probationary Employment. Read the full provision →
What the article actually requires
Article 281 ties a probationary termination for failing to qualify to a specific condition: it must be in accordance with reasonable standards made known by the employer to the employee at the time of his engagement. Two things have to be true — the standards have to be reasonable, and they have to have been communicated to you at the start of your employment. The article's focus is on that act of communication happening at engagement, not on any particular medium for delivering it.
The article is silent on written versus verbal
Nothing in the text of Article 281 says the standards must be in writing to count. "Made known" describes an outcome — that you actually learned what was expected of you — rather than a specific method. On the bare wording, a verbal communication of your probationary standards at hiring is not automatically excluded from satisfying this requirement. Whether it actually did so in your case is a separate, factual question this article does not resolve for you in the abstract.
Why the practical stakes are still real
Even though the article does not demand a particular format, what was or was not communicated to you at engagement remains the thing the ground for termination depends on. If your employer later cannot show what standards were made known to you, or when, that affects whether they can rely on "failure to qualify" as a basis under this article — regardless of whether the underlying communication happened verbally or in writing.
The other limits Article 281 sets
The article also caps probationary employment at six (6) months from the date the employee started working, unless it is covered by an apprenticeship agreement stipulating a longer period, and adds that an employee who is allowed to work after a probationary period shall be considered a regular employee. So separately from the standards question, your probationary status itself has a time limit, and simply being kept on past it converts you to regular employment under this article.