If you're a consultant with clients in six states, or a two-person LLC where one partner lives in Texas and the other in California, you've probably assumed that filing a federal partnership return and issuing K-1s covers the state side too. It doesn't. And right now, the rules that are supposed to tell states how much of your partnership's income they can tax are so inconsistent that the same dollar of income can legally be taxed twice — or, in some cases, not taxed at all.
That gap is exactly what the Multistate Tax Commission (MTC) is trying to close. On January 29, 2026, the MTC published a revised white paper, "State Tax Sourcing of Partnership Income Under the Pass-Through Tax System & the Blended Apportionment Method," proposing a uniform approach that states could adopt. It's not law anywhere yet. But if you run a multistate partnership or multi-member LLC, it's worth understanding now, before your state either adopts a version of it or keeps doing nothing and leaves you to sort out the mess yourself.
Why Partnership Income Is Harder to "Source" Than It Sounds
When a corporation earns income in multiple states, most states use a fairly standardized apportionment formula — typically based on the share of sales located in each state — to decide how much of that income they can tax. Partnerships and multi-member LLCs taxed as partnerships don't get that same clarity.
Two structural quirks make partnerships messier:
The aggregate vs. entity problem. Some states treat a partnership as an "aggregate" of its individual partners — meaning each partner is treated as if they personally earned their share of the business's income directly in each state where the partnership operates. Other states treat the partnership as its own taxable "entity" first, apportion income at the partnership level, and only then pass shares through to partners. Depending on which model a state uses, the same partner can end up with a different sourced amount of income in the same state.
Special allocations and related-party structures. Partnership agreements can allocate income and losses unevenly among partners (a "special allocation"), and many multistate partnerships route income through layers of related entities. The MTC's white paper notes that most states have never fully addressed how their sourcing rules apply once these structures are in play — and warns that the resulting gaps mean some partnership income currently isn't reliably taxed by any state at all, while other income risks being double-counted.
If you're a freelancer or small consultancy organized as a multi-member LLC, you may not have special allocations or exotic entity layers. But you're still caught in the base problem: very few states have clear guidance on where your distributive share of partnership income should even be sourced once you or your partnership operates across a state line.
What the "Blended Apportionment" Method Actually Proposes
The MTC's white paper centers on what it calls a "blended," or "factor flow-up," approach. In plain terms: instead of sourcing a partner's share of partnership income as a separate calculation, the method has the partner (if it's a corporation or another business entity) fold the partnership's apportionment factors — its sales, property, and payroll located in each state — directly into the partner's own apportionment factors. The partnership's business activity effectively "flows up" and blends into the partner's own state tax calculation, rather than being sourced independently.
The goal is consistency: instead of every state guessing at its own sourcing method (or having no method at all), a partner's share of a multistate partnership's activity would be measured the same way regardless of which states are involved.
That consistency comes at the price of complexity, which is exactly what the AICPA flagged in comments submitted on the earlier draft. The AICPA's main concerns, according to reporting on the comment letter:
- The white paper needs clearer "entry-level" rules spelling out which sourcing rules a partnership must apply in the first place before any blending happens.
- States should have to determine whether a partnership is engaged in an actual unitary business with its partners before assuming its income is even subject to apportionment — the draft, as written, risked assuming apportionment applies by default.
- Taxpayers should be allowed to use either a distributive-share-based or an item-based approach to calculating apportionment factors, rather than being locked into one method.
- Related-party transactions between a partnership and its partners shouldn't automatically trigger special sourcing treatment — only where there's actual evidence of tax avoidance.
- The paper should better explain why sourcing happens at the partner level rather than the partnership level, since that choice drives most of the downstream complexity.
None of this is finalized. The MTC's work group has continued meeting monthly through 2026, released revised combined model provisions in mid-April, and is still fielding feedback from the AICPA, energy-industry groups, and law firms as of this writing. Don't expect a state to adopt this model wholesale in the next filing season — but also don't assume it's purely academic. The MTC's model provisions have a track record of eventually showing up, in modified form, in individual states' tax codes.
A Simplified Example of Why This Matters
Say you and a business partner run a two-member consulting LLC. You're based in State A, your partner is based in State B, and the LLC also does project work with clients physically located in State C. Under a purely "aggregate" approach, each of you might be taxed in State C only on the portion of the partnership's State C activity attributable to your own distributive share — sourced independently of anything else going on in your individual tax situation. Under an "entity" approach, State C might instead look at the partnership's total apportionment factors (where its revenue, work hours, and any property are located) and pass a single sourced percentage through to both partners equally, regardless of who actually did the State C work.
Now add the blended, factor-flow-up method the MTC is proposing: if one of you also owns a separate business entity that holds the LLC interest, that entity would fold the LLC's own sales/property/payroll factors directly into its own apportionment calculation, rather than sourcing the LLC's income as a separate line item. Three plausible methods, three different tax outcomes, for the exact same underlying work. That's not a hypothetical edge case — it's the everyday reality for any partnership or multi-member LLC whose partners don't all live in the state where the business is headquartered.
The point isn't that one method is "correct." It's that until states converge on a single approach — whether it's the MTC's blended model or something else — the sourcing outcome for the same partnership can differ from state to state, and even a well-intentioned filer has no single settled answer to check their work against.
What This Means for You Right Now
Even before any state formally adopts a blended apportionment rule, three things are true today for anyone running a multistate partnership or multi-member LLC:
1. Sourcing uncertainty is already a real audit risk. If your state hasn't clearly said how it wants your distributive share sourced, that ambiguity cuts both ways — a state auditor can just as easily argue for the interpretation that maximizes your tax bill as you can argue for the one that minimizes it. Being able to show clean, well-documented records of exactly where your partnership's revenue, property, and labor were located each month is your best defense either way.
2. "No guidance" doesn't mean "no risk of double taxation." Because states differ on aggregate vs. entity treatment, a partner working across two states can genuinely end up with more than 100% of their distributive share taxed somewhere, with no federal mechanism forcing the states to reconcile it. Tracking your state-by-state activity granularly — not just at tax time, but as transactions happen — is the only way to catch this before it turns into an amended-return headache.
3. This is a compliance area where good bookkeeping is a tax strategy, not just an accounting formality. If your books can already answer "how much revenue, and from which clients, came from work performed in each state this quarter," you're in a dramatically stronger position than someone reconstructing that from bank statements every April. That's true whether your state ends up adopting blended apportionment, sticks with its current ad hoc approach, or does nothing for another five years.
A Practical Checklist for Multistate Partners
You don't need to wait for a state to formally adopt anything to reduce your exposure. A few concrete steps:
- Tag revenue by state of performance, not just by client. If you serve a client in California but did the work from Texas, note both — sourcing rules can turn on either fact depending on the state and the type of service.
- Track partner residency and any state-specific business locations separately from client billing addresses. They're not the same data point, and conflating them is one of the most common sourcing mistakes.
- Keep a running log of your partnership agreement's allocation provisions, especially if any special allocations exist — if a state ever asks why one partner's distributive share differs from a straight ownership-percentage split, you want that documentation ready, not reconstructed after the fact.
- Revisit your state filing footprint annually, not just at formation. A partnership that added a client or a remote partner in a new state mid-year can trigger a new filing obligation there even without opening an office.
- Loop in a CPA who actively tracks state pass-through developments, not just federal ones — state partnership sourcing rules change faster and less predictably than federal partnership tax law, and the MTC's project is only one of several moving pieces at the state level right now.
None of this requires waiting on the MTC, the AICPA, or any individual state legislature. It just requires treating "which state does this income belong to" as a question your books answer automatically, rather than one you reconstruct under deadline pressure.
Keep State-by-State Records Clean From Day One
Multistate partnership tax sourcing is exactly the kind of problem that's easy to ignore until an audit or an amended return forces the issue — and by then, reconstructing where your income actually originated can mean digging through a year of invoices and bank records. Beancount.io gives you plain-text, version-controlled accounting where every transaction can be tagged and tracked with the detail multistate compliance actually requires, so the records your CPA needs are already there when the rules finally catch up. Get started for free and see how transparent, auditable books simplify the state tax side of running a business across state lines.