If you "participated" in an employer's plan for even 1 day in 2025, then you must follow the deductibility rules for plan participants. Ultimately, it will come down to whether or not the employer checks the box for "retirement plan" in box 13 of your W-2. Did the company contribute to the 401k in 2025 based on the bonus (your voluntary deferrals, or their contribution, or both)?
https://www.irs.gov/retirement-plans/ira-deduction-limits
Separately, if you have W-2 box 1 income, that is considered compensation for working (including a bonus paid after you stopped working but was earned by working previously), and compensation is required for you to contribute to an IRA. So if you had no other job in 2025 and the bonus is your only compensation, you can use it to make an IRA contribution. The issue of whether or not it is deductible depends on your income, filing status, and whether you were covered by a retirement plan at work.
