If I sold some bonds within my 401k and immediately purchased new bonds within that 401k, does that qualify as taxable income?
No. You don't report transactions inside a 401K or IRA. Only withdrawals and rollovers to another account like to an IRA. All the gains or losses stay inside the account. That's one of the benefits of having a 401K or IRA. They can grow a lot and you only report it when you take a distribution. But then it is taxed as ordinary income and not at capital gains rates.
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