The IL-1040 Line 5 adjustment for taxable social security benefits is working correctly; because your federal 1040 adjusted gross income 11a amount only includes the 85% taxable amount from 1040 line...
See more...
The IL-1040 Line 5 adjustment for taxable social security benefits is working correctly; because your federal 1040 adjusted gross income 11a amount only includes the 85% taxable amount from 1040 line 6b, when you add back that 6b amount (per the state instructions) on line 5 of the state return, you are adding "everything" back.
And so there are no social security benefits remaining in the state income number, you don't have to worry about that.
Take a look at the instructions for Line 5 on the IL-1040 PDF I linked above, it reads:
"5 Social Security benefits and certain retirement plan income if included in Line 1. (Generally on fed. Forms 1040/1040-SR, Lines 4b, 5b, and 6b)."
If the state wanted you to add back the 100% line 5a amount and not the 85% taxable amount, they would be asking you to use federal lines 4a, 5a, and 6a amounts from Form 1040, rather than the taxable amounts from lines 4b, 5b, and 6b.
You can further confirm that the math works correctly (because sometimes the states get it wrong) by looking at how the numbers add up on your 1040, total to line 11a. Sum up your taxable income numbers starting with wages on line 1a, and include everything else as well as your taxable social security benefits on line 6b. You'll see that your AGI on line 11a is made up of the 85% taxable social security, and not the 100% value on line 6a. That's the number your IL-1040 starts with on line 1 of the state return. And so, everything gets added back in full for the state return.
See also:
2025 Form IL-1040 instructions
Illinois Dept of Revenue Publication 120 - Retirement Income