Wash-Sale Swaps: What Counts as 'Substantially Identical'

Chris did everything the tax-loss harvesting articles told him to. In March 2026, when the S&P 500 fell almost 5% in a single month and the VIX spiked into the 30s, he sold his $80,000 stake in Vanguard's S&P 500 fund, ticker VOO, at a $9,400 loss. He wanted the deduction. Then, so he wouldn't miss the bounce, he immediately bought $80,000 of the iShares S&P 500 fund, ticker IVV. Same 500 companies, different logo. He figured that was the whole trick.

The IRS disagreed. His $9,400 loss was disallowed the moment the second trade cleared.

Chris ran into the wash-sale rule, and specifically into the two words that decide every harvest: "substantially identical." Most people who harvest losses know the rule exists. Far fewer know what actually counts as a violation, which is the only thing that matters when you go to buy your replacement fund. Get that one choice right and the loss is yours. Get it wrong and you did all the work for nothing.

This post is about that single decision: what you're allowed to buy after you sell. If you want the mechanics of the 61-day window or the special way the rule turns lethal inside an IRA, the two companion posts below cover them. Here we stay on the swap itself, because that's where real money gets lost.

Why the swap is the whole ballgame

Quick refresher, then we move on. The wash-sale rule sits in Section 1091 of the tax code. It says you can't claim a loss if you buy the same or a "substantially identical" security within 30 days before or after the sale, a 61-day window once you count the day of the sale. The purpose is to stop you from booking a tax loss while never really leaving your position.

The catch is that the IRS has never defined "substantially identical" for mutual funds and ETFs. Publication 550 states the principle and stops there. For individual stocks it's obvious: Apple shares are substantially identical to other Apple shares. For funds, where thousands of products track overlapping baskets of the same companies, the line is blurry, and the IRS has never sued to draw it. So the rule lives on professional consensus rather than a bright statutory line, which is exactly why so many harvests go wrong at the replacement step.

And 2026 has handed people plenty of reasons to harvest. The S&P 500 fell 4.33% in the first quarter, most of it in a brutal March, and dispersion under the surface was worse than the index let on. More than half the companies in the index dropped at least 5% at some point early in the year. Parametric Portfolio Associates, which runs automated harvesting for clients, reported booking more than $3.9 billion in losses across roughly 360,000 trades in Q1 alone. Plenty of ordinary investors are sitting on individual positions that are down even while their account balance looks fine. That is precisely the setup where a careless swap costs you.

Same index, different label: the trap Chris fell into

Here's the mistake Chris made, and it's the most common one: he assumed two funds from two different companies couldn't be "identical."

They track the same index. VOO and IVV both follow the S&P 500. So do SPLG from State Street, FXAIX from Fidelity, and SWPPX from Schwab. Different tickers, different fee schedules, different fund companies with different boards. Underneath, they hold the same 500 stocks in nearly the same weights, rebalanced on the same schedule by the same index committee. When one moves 0.4% on a Tuesday, the others move about 0.4% too.

You will sometimes hear the counterargument, usually on investing forums: VOO and IVV are run by unrelated companies with unrelated boards and different expense ratios, so how can they be "identical"? It's a real argument. The trouble is that no court and no IRS ruling has ever blessed it, and a tax adviser who has to defend your position would have a hard time explaining why two funds tracking the identical index aren't substantially identical. When the safe alternative is just as easy and costs you nothing, being the test case is a bad trade. Practitioners treat same-index pairs, VOO to IVV, QQQ to QQQM, as off limits. So should you.

The share-class twin nobody warns you about

There's a version of this trap that's even sneakier, because the two funds don't just track the same index. They are literally the same fund.

Take VTSAX, Vanguard's Total Stock Market mutual fund, and VTI, Vanguard's Total Stock Market ETF. They look like two products. They are not. VTI is an ETF share class of the exact same underlying fund as VTSAX, holding one identical portfolio. Swapping VTSAX for VTI to dodge a wash sale is like selling your car and buying it back with a different license plate. Same problem exists for any mutual-fund-and-ETF pair that share one portfolio. If the two tickers point at the same basket of holdings managed as one fund, they aren't cousins. They're the same security wearing two name tags, and the harvest dies.

What a safe swap actually looks like

The reliable standard is simple to state: buy a fund that tracks a different index from a different provider. Different index means a different committee picking different constituents by different rules. Different provider means no shared portfolio hiding underneath.

Total-market funds make this concrete. VTI tracks the CRSP US Total Market Index. ITOT tracks the S&P Total Market Index. SCHB tracks the Dow Jones US Broad Stock Market Index. All three own essentially the whole US market and perform within a whisker of each other, yet each follows a different index built by a different company. That difference is the legal daylight you need.

Notice the gray-area row. VTI to ITOT swaps one total-market fund for another, but the two indexes are close enough in construction that some cautious planners avoid the pairing, since "different index" starts to feel thin when both aim to own the entire market. If you want to sleep well, pair funds that differ in a way you can explain in one sentence: an S&P 500 fund swapped for a total-market fund, or a total-market fund swapped for one built by a different index provider on a different methodology. The bigger the honest difference, the smaller the argument you'd ever have to make.

One more thing the swap buys you: you stay invested. If the market rebounds during your 61-day window, your replacement fund rides the rebound right along with the one you sold, because it holds nearly the same companies. You capture the tax loss without ever stepping out of the market. That's the entire point, and it's why "just sit in cash for 31 days" is the amateur move. You take on a month of market risk for no reason.

The pro move: buy the replacement first

Here's a small sequencing trick that professional harvesters use, and it fixes two problems at once.

Instead of selling and then scrambling to buy a replacement, buy the replacement fund first, with new cash if you have it, or by placing both orders the same day. Once the replacement is in place, sell the losing position. You're continuously invested with no gap, and you remove the temptation to "just rebuy the original once the dust settles," which is how people wander back into a wash sale three weeks later.

Whatever you buy, remember the window doesn't stop at your own brokerage account. A purchase of the same or substantially identical fund by your spouse, or inside your IRA, or through an automatic contribution or dividend reinvestment you forgot was running, can all trip the rule. That cross-account reach, especially the IRA version that erases the loss permanently rather than just delaying it, is the subject of the companion post below. Before you harvest, it's worth turning off automatic buys and dividend reinvestment on the fund you're selling, everywhere you hold it, for the window.

And the loss you're protecting is worth protecting. It offsets your capital gains dollar for dollar, and up to $3,000 of leftover loss each year comes off your ordinary income, with the rest carrying forward indefinitely. Chris's $9,400, had it survived, would have wiped out $9,400 of the gains he'd taken earlier in the year, saving him roughly $1,400 at a 15% long-term rate. Instead it bought him a lesson.

The Bottom Line

The wash-sale rule doesn't care how you feel about two funds being "basically different companies." It cares whether they're substantially identical, and for index funds that comes down to the index and the provider. Before your next harvest:

• Check the index, not the ticker. Two funds tracking the same index (VOO and IVV, both S&P 500) are a wash sale waiting to happen, no matter how different the fund companies are. Look up what index your replacement actually follows.

• Swap to a clearly different index from a different provider. An S&P 500 fund into a total-market fund, or VTI into SCHB, keeps you invested without buying something identical. When in doubt, pick the pair whose difference you can explain in one sentence.

• Never swap between a mutual fund and its own ETF share class. VTSAX and VTI are the same fund. So are other paired mutual-fund and ETF versions of one portfolio.

• Buy the replacement first, then sell the loser, and freeze automatic buys and dividend reinvestment on the harvested fund across every account, including your spouse's and your IRA, for the full 61 days.