Looking at my prospective solo 401k contribution: increasing from $2999 to $3001 my refund rises by $1, but at $3000 it drops by $214. Seems like a bug.

[dmertz]

Do you have Marketplace insurance?  If so, the result is not implausible due to the convoluted nature of the calculations.

[davidmoler]

I do have Marketplace insurance. I know there was a discontinuity with taxes (when crossing 400% FPL), but my understanding is that it was fixed thru tax year 2025. Maybe there are other impacts I'm not aware of.

What throws me is that it isn't just a discontinuous jump ($2999 to $3000), but that it jumps immediately back to the original trend ($3000 to $3001). I also see discontinuity at $1 (jumps $207), $6000 (drops by $221), $6001 (jumps $222), $8999 (drops $227), $9000 (jumps $228). Aside from these jumps, my refund is perfectly linear with my solo 401(k) contribution up to $10,000 (from the ~30 values I've tried). 

[dmertz · Accepted solution]

It likely has to do with whether the iterative calculation for PTC and the self-employed health insurance deduction described in IRS Pub 974 converges to a stable result.  The deduction for self-employed retirement contributions changes AGI, affecting the iterative PTC calculation.  From $2,999 and $3,001 the calculation apparently converges to a stable result while for $3,000 it does not and TurboTax defaults to a more conservative result that lowers your refund.