Cosmos Agents offer a small series of missions, sorta like a mini arc, from 3 to 5 parts.
Most of the Cosmos Agents can only be completed once in the life of the character and can only be accessed with positive Faction standing. They should only be done when you’re ready and have the time to run them.
Most of the missions are basically a ‘Search & Retrieve’ mission for specific items found by hacking containers in specific Complexes along with looting NPC’s defending the sites.
When you talk to a Cosmos Agent, the mission offer will sit in your Journal for 7 days, that will allow you time to find the required items before actually accepting the mission.
If the mission is not accepted within 7 days, it becomes a fail.
If you decline the mission offer, it becomes a fail.
If you quit the mission, it becomes a fail.
So yeah, I suggest researching each Cosmos Agent first before talking to them, make a list of items required to complete their mission series, go in-game and collect those items, then talk to the Agent and accept/complete their mission series.