Official data published by HMRC highlights a substantial compliance challenge for businesses relying on the Check Employment Status for Tax (CEST) tool. Approximately 22% of all assessments processed by the tool return an "unable to determine" outcome. For HR directors, procurement leads, and finance boards, this means that for roughly one in every five contractors evaluated, the government’s digital questionnaire fails to provide a definitive answer.
When an automated algorithm cannot reach a conclusion, the legal responsibility for assessing tax risk remains entirely with the hiring organisation. This article examines why CEST struggles with borderline cases and explains how businesses can safely navigate an undetermined outcome to maintain full compliance and demonstrate reasonable care.
Why does CEST Struggle with Borderline Decisions?
The high volume of inconclusive results from CEST stems directly from its structural limitations. The tool relies on binary logic that evaluates individual factors in isolation rather than analysing the commercial reality of an engagement as a whole.
CEST oversimplifies key status indicators such as substitution, control, and financial risk. Crucially, it lacks a comprehensive assessment of Mutuality of Obligation, a fundamental principle in employment status case law. When an engagement presents a nuanced mix of contractor autonomy and commercial collaboration, the algorithm stalls.
What is the Legal Reality of an Inconclusive Result?
An inconclusive outcome is a legitimate technical response within CEST, but it leaves businesses in a precarious position. An "unable to determine" result offers no protection and cannot serve as a valid audit trail.
For a hiring business, an undetermined status means choices must be made. Leaving the assessment incomplete or making an unreasoned guess breaches the statutory duty to exercise reasonable care under the off-payroll working rules.
How to Resolve a CEST 'Unable to Determine' Outcome
When a business receives an inconclusive outcome, it must treat the result as an immediate requirement for intervention. The situation demands a deeper, more comprehensive qualitative assessment than CEST.
Organisations generally have three pathways to resolve an undetermined status:
- Gather additional business evidence: Review the broader commercial context of the engagement. Collect proof of the contractor’s independent business infrastructure, financial risk, and level of integration to support the position. IR35 Shield’s core product does this by default.
- Review and adjust the engagement terms: If the working practices are genuinely ambiguous, assess whether the operational requirements allow for clearer contractual boundaries, such as a robust right of substitution or well-defined project deliverables.
- Engage a specialised expert review: Seek an independent, professional assessment. Status experts can interpret the nuances of evolving case law and deliver a definitive, legally defensible determination. Our experts at IR35 Shield are here to help with that too.
What does a CEST "unable to determine" result mean?
A CEST "unable to determine" result means that HMRC’s digital status tool cannot reach a definitive conclusion on whether an engagement falls inside or outside IR35. This is not a good outcome for a hiring business wishing to accurately determine the IR35 status of their contingent workforce.
Why does the HMRC CEST tool fail to determine IR35 status?
The CEST tool fails to determine status because it relies on a rigid format that cannot capture the subtle operational realities of complex engagements. Crucially, CEST does not fully assess Mutuality of Obligation and oversimplifies critical legal tests, making it ineffective at least 20% of the time.
Is an undetermined CEST result a compliance risk?
An undetermined CEST result poses a significant compliance risk if left unresolved. Because HMRC provides no protection or guidance for inconclusive outcomes, the hiring business remains fully liable for any potential back-dated tax, interest, and penalties until a valid, reasoned Status Determination Statement is produced.
What should a business do when CEST cannot determine IR35 status?
When CEST returns an inconclusive result, the hiring business must escalate the case to an independent IR35 assessment service. This will involve the service provider examining the actual day-to-day working practices, reviewing the contract construction, and applying established case law to form a definitive, well-reasoned conclusion. IR35 Shield provides the expert services your business needs to remain compliant.
Does relying on CEST satisfy the requirement for reasonable care?
Relying solely on CEST does not satisfy the legal requirement for reasonable care if the tool returns an "unable to determine" result and the business takes no further action. To demonstrate reasonable care under the off-payroll working rules, organisations must actively resolve any ambiguity through expert review and clearly document their rationale.
Achieving Certainty in IR35 Compliance
Relying on a tool that fails in one-fifth of all cases is an unsustainable compliance strategy for medium and large enterprises, recruitment agencies, and consultancies.
Independent status assessments and contract reviews conducted by seasoned status experts rarely result in an "undetermined" outcome. For corporate boards and compliance teams, moving away from CEST toward expert-led, collaborative compliance processes is the only way to minimise IR35 risk while maintaining access to vital flexible talent.
Partner with the IR35 experts at IR35 Shield
Do not let CEST paralyse your onboarding or expose your business to tax liabilities. At IR35 Shield, we provide the expert-led, legally robust status assessments your organisation needs to stay compliant and confident.
Our team is here to help your business every step of the way. If you need help with your IR35 processes, feel free to reach out. You can email us at info@ir35shield.co.uk or get in touch with an expert to find out how we can help.
