HMRC CEST Tool - Why It's Not Enough

HMRC's Check Employment Status for Tax tool - universally known as CEST - was introduced in 2017 to coincide with the public sector off-payroll reforms. The government presented it as the definitive way for workers and clients to determine IR35 status. The reality is considerably more complicated, and relying on CEST alone carries real risk.

What CEST is

CEST is a web-based questionnaire - around 16 questions - that covers aspects of the working relationship including control, substitution, and financial risk. Based on the answers, it returns one of three results: “inside IR35,” “outside IR35,” or “unable to determine.”

HMRC states that it will stand behind a CEST result if the information entered was accurate and the tool was used in good faith. This sounds reassuring - but the protection is narrower than it appears.

Problem 1: CEST ignores mutuality of obligation

MOO is one of the foundational tests in employment status law, applied by every employment tribunal and affirmed by the Supreme Court in PGMOL v HMRC [2024]. CEST simply does not test it. HMRC's position - that MOO is present in every contract and therefore need not be tested - is at odds with decades of case law and was explicitly contradicted by the Supreme Court in PGMOL, which drew a clear distinction between basic MOO (present in any binding agreement) and the enhanced MOO that matters for employment status.

Problem 2: Outdated case law

CEST's algorithmic logic was built on HMRC's interpretation of employment status law as it stood when the tool was created in 2017. Since then, two landmark cases have materially changed the landscape:

• Atholl House v HMRC [2022]:The Court of Appeal confirmed that a substitution right that has never been exercised can still be given genuine weight if it reflects commercial reality. CEST's substitution questions do not fully reflect this nuance.
• PGMOL v HMRC [2024]: The Supreme Court clarified the MOO analysis, confirmed the importance of BOOA as a final-stage test, and set out the correct three-stage approach to employment status assessment. CEST incorporates none of this reasoning.

As of mid-2026, HMRC has not released a substantive update to CEST reflecting these rulings. A tool built on pre-2022 case law is applying a different legal framework from the one a tribunal would use today.

Problem 3: Binary output with no reasoning

Employment status is a judgment that weighs multiple factors, sometimes in tension with each other. A contractor might score well on substitution but poorly on control, or have strong BOOA indicators that partially offset weak MOO evidence. A thorough assessment explains how these factors balance.

CEST gives a single result - inside or outside - with a brief summary, but no factor-by-factor reasoning. You cannot tell which questions drove the outcome, which factors are borderline, or where you are vulnerable if HMRC investigates. This is useful for a quick indication but inadequate as a basis for confident tax decisions.

Problem 4: High “unable to determine” rate

CEST returns “unable to determine” in roughly one in five cases. This is not a neutral answer - it means HMRC's tool cannot help you, and you have no protection from HMRC standing behind the result. Contractors and clients who receive this outcome need a proper assessment regardless, which renders CEST's involvement largely wasted.

Problem 5: Protecting the result requires accurate entry

HMRC's guarantee to stand behind a CEST result is conditional on the information entered being accurate. In practice, this creates a significant trap. Answers to CEST questions often require legal judgment - whether supervision is “day-to-day”, whether control over method is “significant,” whether substitution is “genuinely available.” A worker who answers these questions differently from how a tribunal would characterise the facts loses HMRC's protection.

HMRC has investigated contractors and challenged inside-IR35 determinations even where CEST returned outside, where HMRC believed the working practices differed from the information entered. The tool does not verify what you enter.

What a proper assessment looks like

A thorough IR35 assessment should:

• Apply all the relevant status tests - including MOO post-PGMOL
• Consider both the written contract and the actual working practices
• Weigh factors against each other and explain the reasoning
• Reference current case law, including PGMOL [2024] and Atholl House [2022]
• Identify which factors are strong, which are weak, and where the risks lie
• Provide actionable suggestions for strengthening an outside-IR35 position

A single binary verdict - with no breakdown, no case law references, and no MOO analysis - is not adequate for a decision that can affect thousands of pounds of tax liability. The consequences of getting IR35 wrong are too significant to rely on a tool that the courts themselves do not apply.