Can I contribute the full IRS deduction for 2023 before the medical plan ends July 31, 2023?

if you are cover by a HDHP on 12/1/2023, you can make a full years contribution to your HSA but then you have to continue coverage under a HDHP through 2024.   if not, your contribution/deduction is prorated by the the number of months covered by a HDHP so you cannot make a full year's contribution. 

If you are covered by an HDHP for 7 months, and one of those months is not December, then you can only contribute 7/12th the annual limit.

For 2023, single HDHP, that would be $3850÷12 = $320.83 per month x 7 = $2245.83.

For 2023, family HDHP, that would be $7750÷12 = $645.83 per month x 7 = $4520.83.

If you are age 50 or older, you can contribute an extra $1000÷12 x 7 = $583.33.

Note that the contribution don't have to be made before 7/31, you can contribute any time during the tax year, as long as you don't exceed your eligibility limit.