The system behind most sentencing and parole decisions
The most consequential algorithmic tool in the custodial system is one most of the public has never heard of. The Offender Assessment System, universally known as OASys, was developed by the Home Office following pilot studies between 1999 and 2001 and rolled out across the prison and probation services of England and Wales by 2005. It generates risk scores, drawing on both fixed historical factors and dynamic information gathered through offender interviews and self assessment questionnaires, that feed directly into decisions about sentencing, parole, and how an individual is managed in custody and under probation supervision. According to figures obtained by Statewatch through freedom of information requests, more than 1,300 OASys assessments are completed every day across the system, and the database held more than seven million risk scores as of the start of 2026.
OASys has been used for more than two decades, and campaigners and academics have raised concerns for much of that time about racial and class based bias in how its risk factors are constructed, the accuracy of its predictions, and the difficulty anyone assessed by it faces in meaningfully challenging a score whose underlying methodology is not fully public. It has been described in academic commentary as a black box system that has shaped criminal justice outcomes for over twenty years without the level of external scrutiny its influence would seem to warrant.
A second, separate system for prison security categorisation
A distinct algorithmic tool, the Digital Categorisation Service, is used specifically to inform how prisoners are security categorised, a decision that determines the conditions, restrictions and type of establishment someone is held in. Like OASys, it does not make the final decision by itself. It highlights risk information drawn from a prisoner's record and suggests an initial categorisation, which prison staff then review and may change. The distinction between a tool that suggests a categorisation for a human to confirm and one that determines the outcome outright matters enormously in practice, since it is the genuineness of that human review, rather than the mere presence of a human somewhere in the process, that determines whether meaningful oversight actually exists.
OASys is being replaced
His Majesty's Prison and Probation Service has confirmed it is developing a successor system, known as Assess Risks, Needs and Strengths, or ARNS, built in house by Ministry of Justice Digital in consultation with Capita, which provides technical support for the existing OASys system. An early prototype entered pilot testing in December 2024, and HM Inspectorate of Probation reported in April 2025 that a full national rollout was scheduled for completion by autumn 2026. The stated aims are more accurate, actuarial data informed risk assessment and faster, more consistent recording of the risk assessments, risk management plans and sentence plans that OASys currently produces.
Replacing a system that has operated for two decades is a significant undertaking in its own right, and the transition raises its own question distinct from whatever the merits or flaws of ARNS itself turn out to be: whether the criticisms levelled at OASys over accuracy, bias and contestability are genuinely addressed in the new system's design, or whether a new tool, built to do largely the same job with better underlying technology, ends up reproducing the same fundamental oversight gap under a different name.
Parliament has flagged the oversight gap directly
Written evidence submitted to a parliamentary committee in April 2026 raised concerns in strikingly direct terms about how algorithmic and AI tools in prisons and probation are currently scrutinised. It specifically named HM Inspectorate of Prisons, HM Inspectorate of Probation and the Prisons and Probation Ombudsman as bodies whose oversight of these tools needs strengthening, and criticised existing inspection practice for focusing on whether a required process was formally completed rather than examining how the underlying algorithmic assessment actually functioned, a distinction the evidence characterised as leaving genuine scrutiny of risk related tools largely confined to HMPPS itself. Given the department's own stated ambition to lead government adoption of AI more broadly, the submission argued this combination, extensive and growing algorithmic tool use alongside comparatively shallow independent scrutiny of how those tools actually work, warranted direct parliamentary attention.
What is not settled
There is no published, independent evaluation establishing how accurate OASys risk predictions have actually been over more than two decades of use, as distinct from the tool's own internal validation, nor is equivalent independent evidence yet available for its replacement, ARNS, which remains in the rollout stage. Detained and supervised individuals continue to have limited practical means of challenging a risk score that shapes decisions about their liberty, a constraint that applies with particular force in a custodial setting, where the person most affected by an algorithmic assessment typically has the least practical ability, of anyone touched by police or criminal justice AI, to contest it.
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