I have read that HSA contributions and SEP contributions are adjustments that are supposed to be added back to arrive at MAGI.

Artificial intelligence strikes again?  I have read that people who don't cite their sources are predictably unreliable.  Here's what AI tells me:

When it comes to calculating Modified Adjusted Gross Income (MAGI), the specific deductions you need to add back to your Adjusted Gross Income (AGI) can vary depending on the purpose of the MAGI calculation. This is because MAGI is used for different tax benefits and programs, and each has its own specific set of rules for which adjustments are included.

Important note: It's important to be aware that the specific formula for calculating MAGI can vary depending on the particular tax credit, deduction, or program you're evaluating. It's always best to consult with a tax professional or utilize reliable tax software to accurately calculate your MAGI for your specific circumstances and intended purpose.