Where do I enter my annual college 529 contributions? Connecticut lets married couples deduct up to $10,000 of such contributions from AGI.

You'll deal with that on the state taxes when you get to it. There is nothing for 529 contributions on the federal side (that results in a deduction)

And only contributions to Connecticut's 529 plan (CHET) are deductible on the CT return.

I caught this after proceeding into the state taxes. Thank you both for the very prompt replies.

Contributions to a Connecticut 529 plan (CHET) of up to $5,000 per year by an individual, and up to $10,000 per year by a married couple filing jointly, are deductible in computing Connecticut taxable income, with a five-year carryforward of excess contributions.

To enter your 529 contributions on your CT return, please follow these steps:

1. Go back into your CT return.
2. If you have finished entering information in your return you will see the screen, Your 2017 Connecticut taxes are ready for us to check.  [See Screenshot #1]
3. Click on the Edit box next to Income.
4. If you're still entering information, continue through the screens until you reach Here's the income that Connecticut handles differently
5. On the screen, Here's the income that Connecticut handles differently, click on the Start box next to CHET contributions (in the Education section).  [Screenshot #2]
6. Continue through the screens, entering the requested information.
   

In the 2020 interview this was very hard to find. I almost forgot to deduct our $10000 --- since the place to enter it is under INCOME that Connecticut treats differently!  It's a deduction, not income! Even when I remembered that I should deduct it, I had to come here to see where TT asked me about (after wasting a lot of time searching for it under deductions).   This should be fixed.

this is a good point. thx! 

I am starting CT 529 (CHET) as a part-year CT resident. 

do I have the deduction of 5k/10k MFJ on CT state tax, as NR or part year resident?