Does contributing to a retirement plan affect my QBI deduction?

It can, and the effect is a bit counterintuitive. Contributing to a SEP-IRA, Solo 401(k), or similar plan reduces your qualified business income, which shrinks the 20% QBI deduction slightly, so the two deductions partly offset each other. But retirement contributions also lower your total taxable income, which can pull you below a QBI phase-out threshold and preserve or restore eligibility, especially for service businesses near the limits. For most people the retirement deduction is the larger and more valuable tax break, so you rarely skip it just to protect QBI. The interaction is genuinely complex math. A CPA or the Self-Employed Hub at wealthserene.com/tools/self-employed-hub can model your specific numbers so you optimize both together.