Is it bad karma to think but not claim that one is enlightened, or just bad for your practice?

"Our faith that we are" is a direct disregard to Chan. Surangama Sutra supported the Chan school. Looking at Chan there is the question: What is Buddha? The answer follows in a modern sense: " about 5 five pounds of flour."

The faith part is only one pillar. No building can be constructed and to think that you have one is certainly bad karma. It's evidence of a incompetent. To claim it finished will result in great suffering.

Others here will bring about obtainment and this issue also as it's connected. The Zen reflection point, or another sobering point to keep with you is that enlightenment or not doesn't create absolution. You remain a victim of cause and effect. Now a nose arrives before you do so there's value to Chan (the school of sudden enlightenment.) Chan generally isn't considered an active school because of the teaching. If you are given it quickly and earned it why would there need to be a school for it? Karma isn't really a matter of white and black or the basics of justice. That's a western misunderstanding. It's a root of suffering in the senseless observation of other that is only a reflection of yourself that's meaning that was intended to be reflected.

It is delusion, with all resulting kamma. This case is addressed in MN1:

They perceive extinguishment as extinguishment. But then they identify with extinguishment, they identify regarding extinguishment, they identify as extinguishment, they identify that ‘extinguishment is mine’, they take pleasure in extinguishment. Why is that? Because they haven’t completely understood it, I say.

It may sound like progress, but it isn't. In fact, the Buddha would say the same about conquerers such as Alexander the Great, who grasped at earth:

“Take an uneducated ordinary person who has not seen the noble ones, and is neither skilled nor trained in the teaching of the noble ones. They’ve not seen good persons, and are neither skilled nor trained in the teaching of the good persons. They perceive earth as earth. But then they identify with earth, they identify regarding earth, they identify as earth, they identify that ‘earth is mine’, they take pleasure in earth. Why is that? Because they haven’t completely understood it, I say.

Constrast this with how the Buddha describes a Realized One. Notice that identify view is absent:

He directly knows extinguishment as extinguishment. But he doesn’t identify with extinguishment, he doesn’t identify regarding extinguishment, he doesn’t identify as extinguishment, he doesn’t identify that ‘extinguishment is mine’, he doesn’t take pleasure in extinguishment. Why is that? Because he has understood that relishing is the root of suffering, and that rebirth comes from continued existence; whoever has come to be gets old and dies. That’s why the Realized One—with the ending, fading away, cessation, giving up, and letting go of all cravings—has awakened to the supreme perfect Awakening, I say.”

Per MN1, even a trainee "shouldn’t identify as extinguishment, they shouldn’t identify that ‘extinguishment is mine’, they shouldn’t take pleasure in extinguishment. Why is that? So that they may completely understand it, I say."

Furthermore, per MN1, only "an uneducated ordinary person who has not seen the noble ones" would "identify as extinguishment."

Therefore, if we can agree that uneducated ordinary persons are entangled in suffering and kamma, we can infer that to think of oneself as enlightened is unskillful, uneducated and ordinary, with all corresponding kamma.

Other suttas reiterate this cautionary point about non-identification with extinguishment in different ways. For example, there is MN44:

“A mendicant who is entering such an attainment does not think: ‘I will enter the cessation of perception and feeling’ or ‘I am entering the cessation of perception and feeling’ or ‘I have entered the cessation of perception and feeling.’ Rather, their mind has been previously developed so as to lead to such a state.”

Both MN1 and MN44 stress non-identification even for those with experience of extinguishment such as stream-winners. In our practice, we should note that mind is included in good conduct (DN33). And therefore if we do not speak unskillfully, we should also avoid thinking unskillfully.

Three ways of performing good conduct: by body, speech, and mind.