IR35 and fractional AI hires: what a hiring manager has to check
6 minute read. Updated 2026-08-08.
Since April 2021, if you are a medium or large private-sector business, you make the IR35 determination for a fractional AI hire, not the contractor and not their agency. For a role working 2 days a week, closely embedded in your team, the practical risk is that the arrangement looks like part-time employment. Assess control, substitution and mutuality of obligation honestly, document the reasoning, and decide before work starts, not after a tax enquiry.
The determination is yours to make, not theirs to claim
Since April 2021, medium and large private-sector clients make the IR35 determination for each engagement, not the contractor and not the agency that introduced them. If a hiring manager treats this as the specialist's paperwork to sort out, it is being handled the wrong way round from the outset.
Whether a business counts as medium or large is decided by standard company-size tests. If that is genuinely unclear, it is a question for finance or legal before the engagement starts, not something to assume either way.
Why a fractional AI lead carries more risk than a typical contractor
A developer brought in for a fixed-scope project, working from a spec and delivering a defined output, is comparatively easy to assess as outside IR35. A fractional AI lead working 2 days a week, attending the same standups as permanent staff, managing a junior data scientist, reporting into the CTO and carrying a title on the org chart, looks a great deal like a part-time employee in most of the ways a determination actually weighs.
That does not make the engagement automatically inside IR35. It means the assessment has to be made honestly against the working pattern as it actually runs, not against what the contract says on paper, because a contract describing independence while the day-to-day reality describes integration will not hold up if it is ever tested.
What actually gets assessed
3 things carry the most weight in practice: control, whether the client directs how and when the work gets done or only what the outcome should be; substitution, whether the specialist could send someone else to do the work and the client would genuinely accept that; and mutuality of obligation, whether the client is obliged to keep offering work and the specialist obliged to accept it, in the way an employer and employee are to each other.
For a fractional AI lead, control is usually the factor that tips the balance. An arrangement that genuinely amounts to "come in 2 days a week, on days we set, and do what your manager tells you" reads as employment whatever the invoice says. One that amounts to "deliver this roadmap, on your own schedule, reporting progress rather than taking instruction" reads differently.
What has to happen before work starts
The determination has to be made with reasonable care and recorded, ideally as a formal statement setting out the conclusion and the reasoning behind it, given to the specialist and to any agency involved. Doing this after the fact, once a question has already been raised, is the position nobody wants to find themselves defending.
For a 2-day-a-week arrangement specifically, the reasoning should address why that pattern does not amount to part-time employment in practice: what genuine autonomy the specialist has over how the work gets done, and what would actually happen if they were unavailable for a period, because an arrangement with no answer to that question defaults to looking like employment.
What it costs to get this wrong
If an engagement is determined inside IR35, the specialist is taxed broadly as an employee for that engagement, usually through deductions applied via an umbrella company or the client's own payroll. This does not make them an employee for any other purpose, but it substantially reduces what reaches them for the same day rate, and experienced specialists price accordingly once they know the determination in advance.
Getting the determination wrong the other way, calling something outside IR35 that a proper assessment would call inside, exposes the client to liability, not the contractor, for medium and large private-sector engagements. That liability sits with the buyer, which is exactly why the determination cannot be treated as a formality completed once and filed away.
The practical position for a hiring manager
Assume a fractional AI lead working 2 or more days a week, embedded in the team, managing others or attending the meetings a permanent lead would attend, carries a real risk of an inside determination unless the working pattern is deliberately structured otherwise. Do not assume outside IR35 simply because the invoice looks like every other contractor invoice already on file.
If genuine outside-IR35 status matters to how the engagement has been priced, structure it that way from the outset: defined deliverables rather than a headcount-shaped weekly schedule, real autonomy over method, and a substitution clause that would actually be honoured if it were ever invoked. If that structure does not fit how the role genuinely needs to work, price the engagement as inside IR35 from day one rather than discovering the gap later.
What to do about it
- The client makes the IR35 determination for medium and large private-sector engagements, not the contractor or the agency.
- Assess the actual working pattern, not the contract wording; a fractional AI lead embedded 2 days a week reads a lot like part-time employment.
- Weigh control, substitution and mutuality of obligation honestly, with control usually the deciding factor for embedded fractional roles.
- Record the determination and its reasoning before work starts, not after a query arises.
- If genuine outside-IR35 status matters, structure the engagement around deliverables and autonomy, not a fixed weekly schedule and line management.
Questions people also ask
Who is responsible for the IR35 determination on a fractional AI hire?
You are, if your business meets the medium or large threshold in the private sector. Since April 2021 that responsibility sits with the client, not the contractor and not any agency that introduced them. The determination has to be made with reasonable care and recorded before work starts, and it has to reflect the actual working pattern rather than what the contract says on paper.
Is a 2-day-a-week AI lead automatically inside IR35?
No, not automatically, but the working pattern raises the risk. Someone attending the same meetings as staff, taking direction on how and when to work, and managing a junior team member looks a lot like part-time employment when assessed against control and mutuality of obligation. Whether it is genuinely inside or outside depends on the actual arrangement, not the number of days, and has to be assessed honestly rather than assumed either way.
What happens if we determine an engagement inside IR35?
The specialist is taxed broadly as an employee for that engagement, typically through deductions applied via an umbrella company. It does not make them an employee for other purposes, such as notice or benefits, but it reduces what reaches them for the same day rate, and most specialists will price the engagement differently once they know the determination in advance.
What if we get the determination wrong?
For medium and large private-sector clients, the liability for an incorrect determination sits with the client, not the contractor. That is precisely why it needs proper assessment and a written record before work starts, rather than being treated as paperwork to complete once and forget. If the working pattern changes significantly during the engagement, the determination should be revisited rather than left as a one-off exercise.
Where the figures come from
Every rate and salary quoted in this article is a median or percentile of figures advertised in UK job postings over the six months to 8 August 2026. They are not rates paid, and the gap widens at the top of a range.
- IT Jobs Watch, UK contract rates, 6 months to 8 August 2026, read 2026-08-08.
- IT Jobs Watch, UK permanent salaries, 6 months to 8 August 2026, read 2026-08-08.