Trust
Client files are processed on local hardware your practice controls. Your firm's rules decide what runs, the Ledger records what the AI did, and a named partner rules before anything leaves. This page sets out each control, gives its status and links to the full security documents.
Cast & Rule sends work to a cloud model only when your firm's rules allow it for that task and that person, after names and tax references are swapped for placeholders on your own machine. Tools outside Cast & Rule, such as a browser on a staff laptop, stay under your own policy and IT settings, and our AI-use policy template helps with those.
Yes. For any period, the no-cloud report lists every AI run, where it ran and what left the machine through Cast & Rule, and you check it without asking us. With cloud AI switched off, it shows no cloud calls at all.
Your firm decides what may run and for whom. The tasks themselves are built, signed and versioned by us, so neither your staff nor anyone else can quietly change what the AI is told to do; your practice adjusts only the settings we allow, and each change is recorded. We found no competitor offering tasks that are maintained centrally and extended for each firm (as of September 2026).
Nothing changes without you. Each task is tested against named model versions, and a new version installs only after you've tried it on your own machine and approved it. If a cloud model is withdrawn, the task carries on with the tested model on your hardware. We found no competitor offering upgrades that you test on your own machine and that install only when you approve them (as of September 2026).
No. Each client's records sit in their own encrypted store, with their own key, and each piece of work reads from one client only. Delete a client's key, including its backed-up copies, and their store can no longer be read.
Yes. The Ledger marks every change as made by a person or by the AI, with the task, its version, the model and the documents it changed. Each entry is locked to the one before it, so a later change shows up.
From the ground up: where the work lives, and what guards it at each layer.
Every piece waits at Awaiting partner until a named partner rules.
Client documents, prompts and answers stay on a machine in your practice's office. There are two ways to run it, and this is where the differences are set out.
Our service looks after updates, the task catalogue and health checks, and never receives client content. It sees the operational details of each run: who ran what, when, and its status. Results your rules allow off the machine are encrypted before they leave.
The whole system runs on your machine and keeps working if your internet goes down.
Two flows sit outside the machine whichever you choose: Xero, read through its own official connector, and remote support, where we reach your machine to help. Each is set out, with where the data goes, before it's used.
The detail behind this page, with the status of every control.
For partners and practice managers
A plain-English guide for partners and practice managers. It covers where client data lives, who can see it, what the AI can and can't do, the partner's ruling, the Ledger, a lost laptop, and how Cast & Rule helps you show you meet your ICAEW and UK GDPR duties.
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For iT leads, security reviewers and auditors
For IT leads and auditors: Cast & Rule's trust boundaries, data flows, identity and credential custody, least-privilege connectors, prompt-injection defences, egress control, the tamper-evident Ledger, encryption, key management, supply-chain integrity, what we build on, incident response, retention and erasure, and sub-processors, each with its status.
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For partners, practice managers and IT leads
Direct answers to twelve security questions about Cast & Rule: where client data is held, what leaves the practice, model training, sign-off, the Ledger, lost machines, prompt injection, sub-processors, retention and connections.
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| What it does | How it works | Status |
|---|---|---|
| Runs on your hardware | Client files are processed on local hardware the practice controls, on macOS. | Built |
| Signs every task | Our control service signs each task, and the agent checks that signature against the permissions your practice accepted. | Built |
| Shows later changes | The Ledger shows when a record it has already checked was changed later. | Built |
| Gives each machine its own key | Each machine creates its own identity key. The private key never appears in settings, on the network, in a log or in a database. | Built |
| Keeps documents from granting permissions | A document can't grant permissions. Every output still goes to a person to check, because hostile content can sway an answer. | Built |
| Revokes a lost machine | Revoking a lost machine blocks its later requests to our service. It doesn't erase the files on it. | Built |
| Holds work for the partner | Any action that sends, files, posts, pays or deletes in an outside system needs a partner's approval of that exact action. | In build |
| Encrypts results | Results your rules allow off the machine are encrypted before they leave: with keys we hold today, so we can read them, and next with a key only your practice holds. |
|
| Controls the agent's connections | Control and logging of every connection the agent makes. Other apps on the machine are outside their scope. | In build |
| Reads client systems | Read-only connections to Xero, Microsoft 365, Companies House and HMRC, one client per connection. | Planned |
| Keeps each client apart | Each client's records sit in their own encrypted Strongroom store, under their own key. | Planned |
| Reports what left | A report, per period, that shows what left the machine through Cast & Rule. | Planned |
| Publishes the paperwork | The list of suppliers who handle data, a diagram of where data flows, the full statement of what is never used for training, and the exact list of what the Ledger logs. | Planned |
Cast & Rule is built to help your firm meet these duties.
Confidentiality, including inside the firm. ICAEW issued a reminder on 28 July 2026.
Confidential client data must not go into public AI tools.
Limits on what each task can reach, an encrypted store for each client's records, a record of what the AI did and partner review.
Cast & Rule isn't an accountancy practice. Your firm keeps the client relationship, the professional judgement, the tax treatment and the sign-off. Directors still approve company accounts, and clients still approve their returns.
We set out the list of suppliers who handle data, with a diagram of where it flows. It covers hosting for our control service and a cloud-model route used only if your practice turns it on. Where a connection needs login details processed off the machine, we say where that data flows, and who is involved, before it's used.
On local hardware the practice controls. A cloud model runs only where your rules allow it for a task, after identifiers are replaced on your machine. The security architecture sets out where models sit and how they connect.
No tool does that for you. Cast & Rule is built to help with confidentiality under ICAEW Code section 114 and ICAEW's generative-AI guidance. The duties stay with your firm.
We set out the list of suppliers who handle data, with a diagram of where it flows. It covers hosting for our control service and a cloud-model route used only if your practice turns it on. Where a connection needs login details processed off the machine, we say where that data flows, and who is involved, before it's used.
One practice first
For a UK practice of 5 to 20 staff, with a partner who'll own the review. We run one workflow, measure it before and after, and agree in writing how it ends. Send an enquiry or email us today.