Cross-Account Tax-Loss Harvesting, Explained
The most valuable move an investing agent can make is also the hardest to do by hand, because the tax rule that governs it ignores the walls between your brokers.
Why harvesting is easy in one account and brutal across many
Tax-loss harvesting is simple to describe. You sell an investment that is down, book the capital loss, and use it to offset gains elsewhere or up to 3,000 dollars of ordinary income per year. Then you buy something similar so you stay invested. The IRS lets losses roll forward indefinitely, so a loss you bank today can shelter a gain years from now. Done well and reinvested consistently, Vanguard's 2024 research put the added after-tax return, what it calls TLH alpha, at roughly 0.47 to 1.27 percent per year. On a 500,000 dollar portfolio that is between 2,350 and 6,350 dollars of tax value in a single year.
The problem is that almost nobody holds their money in one account. You have a taxable brokerage, an IRA or two, maybe an old 401k, a crypto exchange, and a joint account with a partner. The wash-sale rule that governs harvesting was written to look across all of those at once. Human beings and most software look at one account at a time. That gap is where the opportunity lives, and where the danger hides.
How the wash-sale rule actually works
The wash-sale rule, codified in Internal Revenue Code Section 1091, disallows a loss if you buy a substantially identical security within 30 days before or after the sale. Count the sale day plus 30 days on each side and you get a 61-day window you have to keep clear. Trip it in a normal taxable account and the loss is not gone forever. It gets added to the cost basis of the replacement shares, so you recover it later. Annoying, but survivable.
The teeth are in two details most people miss. First, substantially identical is broader than identical. Selling one S&P 500 fund and buying a different provider's S&P 500 fund can count, because you are tracking the same index. Second, and this is the one that quietly destroys value, the rule spans every account you control, including your IRA and your spouse's accounts. Fidelity, Schwab, and Empower all state this plainly in their own guidance.
The IRA trap that turns a paper loss into a permanent one
Here is the scenario that costs real money. You sell a stock at a loss in your taxable brokerage to harvest it. Within 30 days, a fund inside your IRA buys the same stock, or you buy it there yourself because it looked cheap. That is a wash sale across accounts, and the loss in your taxable account is disallowed.
In a normal taxable-to-taxable wash sale you would at least recover the loss through a higher basis on the replacement shares. But an IRA does not track cost basis the way a taxable account does, and its distributions are taxed as ordinary income regardless of basis. So the disallowed loss has nowhere to attach. It is not deferred. It is permanently destroyed. You did the work of harvesting and ended up worse than if you had done nothing. This is the single strongest argument for coordinating harvests across every account at once, rather than one broker at a time.
Why coordination is the hard part, not the selling
Selling a loser is trivial. The difficulty is the bookkeeping before and after the sale, across institutions that do not talk to each other. To harvest safely you need a live, unified view of every lot in every account, the ability to check any proposed sale against buys in all other accounts over a rolling 61-day window, awareness of automatic reinvestment and recurring buys that could trip the rule on their own, and a replacement security that keeps your market exposure without being substantially identical.
No single broker can do this for you, because no single broker can see your other brokers. A spreadsheet can hold the data but cannot watch it in real time. This is exactly the kind of task that rewards one intelligence with full visibility over many narrow tools with partial visibility. The value is not in the trade. It is in the reconciliation that makes the trade safe.
From robo-advisor to agent: why the model finally changed
Legacy robo-advisors were built for a narrower job. Fortune described them in 2026 as a generic, incremental feature at best, sorting people into roughly 20 pre-built ETF baskets from a questionnaire and rebalancing on static rules. Some offer tax-loss harvesting, but typically only inside the accounts they custody. They do not see the held-away IRA or the crypto exchange that can silently trigger a wash sale, so their harvesting is blind to the risk that matters most.
The frontier, mapped by the World Economic Forum and by academic work such as the 2025 arXiv paper Robo-Advisors Beyond Automation, is a shift from chatbot to copilot to autonomous agent that can reason, plan, and act across accounts. Deloitte projects that generative AI could become retail investors' leading source of investment advice around 2027, reaching about 78 percent adoption by 2028. The mechanism behind that projection is not a smarter chatbot. It is an agent that holds your whole financial picture in view and can act on it.
What a single agent does differently, step by step
An investing agent treats harvesting as one connected problem. It connects every account, banks, brokerages, and crypto, so it sees your whole net worth instead of one silo. It continuously scans lots for unrealized losses worth capturing. Before proposing a sale it checks the full 61-day window across all connected accounts, including automatic reinvestments and a linked partner's accounts, so it will not hand you an IRA-trap loss. It then selects a replacement that preserves your exposure without being substantially identical, and it explains, in plain English, the dollar value of the harvest and any gain it offsets.
This is the pattern Tengu uses: find the opportunity across accounts, explain it, propose the move, and route it to your broker the moment you approve, where routing is supported. It is non-custodial and consent-first. You keep your own accounts and broker, you approve every move, and you hold a kill switch. Nothing moves without you. If you want more automation, you can set an agent to harvest on its own inside limits you define, a maximum per trade, a drawdown ceiling, a defined universe, still under the kill switch you hold.
The honest counterweight: autonomy adds risk, and design has to answer it
More autonomy is not free. The same research that is bullish on agents is candid about the new failure modes: hallucinated reasoning, over-trading, unclear accountability, regulatory uncertainty, and the plain question of whether you should trust a system to touch your money. A serious treatment of this topic names those risks rather than waving them away.
Consent-first design answers most of them. Requiring human approval on every proposed trade caps the blast radius of a bad idea. Hard limits on trade size, drawdown, and universe bound what an autonomous agent can do even when it acts alone. Plain-English explanations with citations let you audit the reasoning before you approve, which is the opposite of a black box. A kill switch you hold means you can stop everything instantly. None of this promises a return, and none of it is individualized tax advice. It is a structure that lets you capture a well-understood tax benefit while keeping a human firmly in the loop.
Key takeaways
- Tax-loss harvesting can add roughly 0.47 to 1.27 percent of after-tax return per year when reinvested consistently, per Vanguard's 2024 research.
- The wash-sale rule spans every account you control, including your IRA and your spouse's accounts, not just the broker where you sold.
- A wash sale where the replacement shares land in an IRA can destroy the loss permanently, because an IRA has no cost basis for it to attach to.
- Coordinating a harvest across brokers, banks, and crypto is the hard part, and no single broker can see your other accounts.
- This is why the job fits one AI agent with full visibility that finds, explains, proposes, and routes each move only after you approve.
Frequently asked questions
What is cross-account tax-loss harvesting?
It is harvesting investment losses while checking every account you own at once, taxable, IRA, and crypto, so the sale actually banks the loss instead of triggering a wash sale elsewhere. Harvesting inside a single broker ignores the accounts most likely to void the loss.
Does the wash-sale rule apply across different brokers and IRAs?
Yes. Under IRC Section 1091 the rule looks across all accounts you control, including IRAs and a spouse's accounts. Buying a substantially identical security in any of them within 30 days before or after a loss sale can disallow the loss.
Why can a wash sale in an IRA be permanent?
In a taxable account a disallowed loss is added to the replacement shares' cost basis, so you recover it later. An IRA does not track basis that way and its distributions are taxed as ordinary income, so a loss disallowed by an IRA purchase has nowhere to attach and is lost for good.
How much can tax-loss harvesting actually save?
Vanguard's 2024 research estimated an added after-tax return of about 0.47 to 1.27 percent per year for investors who harvest and reinvest consistently. The exact benefit depends on your tax rate, your gains, and your discipline, so treat it as a range, not a promise.
Does an AI agent move my money on its own?
Not unless you tell it to. A consent-first agent like Tengu finds and explains an opportunity, proposes the trade, and routes it to your broker only after you approve, where supported. You keep your accounts, set limits for any autonomous mode, and hold a kill switch.