As an adverb meaning "completely" or "very," tout is usually invariable (it doesn't change to match gender or number).
"Tout" with masculine adjectives
When modifying a masculine adjective (singular or plural), tout remains unchanged.
| This crab is completely red! | Ce crabe est tout rouge ! |
| These rocks are completely damp. | Ces rochers sont tout humides. |
The quirky exception: Feminine with a consonant
However, tout does change to agree in gender and number when placed directly before a feminine adjective starting with a consonant (or an aspirated h).
In these cases, tout becomes toute (singular) or toutes (plural).
| I am looking at this puddle; it is very small. | Je regarde cette flaque, elle est toute petite. |
| These shrimp are completely cold. | Ces crevettes sont toutes froides. |
| These seaweeds are completely green. | Ces algues sont toutes vertes. |
Feminine adjectives with a vowel or mute 'h'
If the feminine adjective starts with a vowel or a mute h, the exception is canceled! Because the final t in tout links smoothly to the following vowel, it returns to its normal state and stays invariable.
| The starfish is completely surprised by the crab. | L'étoile de mer est tout étonnée par le crabe. |
| This starfish is completely damp, but very pretty. | Cette étoile de mer est tout humide, mais très jolie. |
Side-by-side comparison
Here is how the rule applies to the same adjective, depending on whether the subject is masculine or feminine. Because fasciné(e) starts with a consonant, the feminine version triggers the special agreement exception.
| The boy is completely fascinated by the shrimp. | Le garçon est tout fasciné par les crevettes. |
| The girl is completely fascinated by the rocks. | La fille est toute fascinée par les rochers. |