When you have someone else do something for you, use the causative construction: faire + an infinitive.
In the passé composé, the past participle usually agrees with a preceding direct object. However, there is a strict exception for the causative faire: when the past participle fait is followed by an infinitive, it never agrees.
| The dress that you had sewn is perfect. | La robe que tu as fait coudre est parfaite. |
| The skirts that they had lengthened are beautiful. | Les jupes qu'ils ont fait rallonger sont belles. |
| The dresses that you had tightened are perfect. | Les robes que vous avez fait resserrer sont parfaites. |
Notice how the preceding direct objects—la robe (feminine singular), les jupes (feminine plural), and les robes (feminine plural)—would normally force the past participle to change. Because fait is immediately followed by an infinitive (coudre, rallonger, resserrer), it remains invariable.
Reflexive Causatives
This absolute rule also applies to reflexive verbs. If you have something made or done for yourself, the past participle fait ignores the preceding reflexive pronoun and remains invariable.
| She had a new skirt sewn for herself. | Elle s'est fait coudre une nouvelle jupe. |
Even with a feminine subject (elle) and a preceding reflexive pronoun (s'), fait does not change to faite because it is followed by the infinitive coudre.