I've been away for a while, so maybe someone else posted this already. I think there's a bug in the way the Thraddish work.
*SPOILER ALERT*
In the normal course of the game, you make friends with the Thraddish and they give you the plans and captains so you can build their Torch ships. Then you steal their artifact and they break the treat, refusing to give you any more starship captains.
However, I managed to steal the artifact without talking to any Threaddish. As a result, the treaty has never been officaly broken and I can still build their ships.
I suspect this is a minor bug and they should break the treaty even if you don't talk with them face to face after stealing their artifact.