Thalys offers complimentary food and drinks to passengers travelling in their first class called Comfort 1. However while a proper and rather decent three courses meal is served at lunch and dinner times only a rather uninspiring cold snack is served on train services at other times during the day. There is quite a significant difference between the different meal options and it is not obvious what train will offer what type of service. Neither are you informed about it during the booking process on the Thalys webpage.
A dinner service from Paris to Brussels may include this menu:
While a snack service will consist of a small sandwich or a pastry with drinks.
However you can check in advance what type of meal will be offered on each train service and choose a train accordingly.
A good source is the Thalys printed timetable. This also seems to be available through the B-Europe web page as PDF. You can also easily find it by searching for “Thalys timetable” on Google.
This is what the timetable looks like. The image below the train number indicates what type of meal will be served on the first sector, the image further down shows what type of meal will be served on the second sector. A croissant indicates breakfast. The muffin (which rather looks like an ice cream cone) means a snack. The plate with a dome indicates a full three courses meal will be served.
It is also possible to access this information on the Thalys webpage through the timetables section. However on some trains there are two images, a snack and a meal or a snack and breakfast, and it is not always clear what meal will be served on which sector.