A calc your compliance officer will sign.

Every published HMRC worked example. Zero pence tolerance.

The HMRC PTM (pension) and IPTM (bond) manuals publish worked examples. We extract each one to a JSON corpus and run it against the engine on every commit. The tolerance is zero pence— not a rounding band, not “close enough”, literal byte-for-byte equality with the HMRC-published numbers.

Every published HMRC worked example runs as a test at zero-pence tolerance.

Method: PTM 057100 (pension) · IPTM 3830 (bond) · IPTM3820 / HMRC Agent Update 83 (2021) (TSR PSA recalc)

Twelve months on, against the same versioned config.

Every calculation persists: the full inputs, the engine version, the tax-year config version, the date, the inputs in pence-integer form, the result, and every intermediate step. Twelve months later you can pull the row, feed the inputs back into the engine at the recorded version, and produce a byte-identical result.

The compliance annex PDF stamps the tool version, config version, and calculation date on the footer of every page. Signed results are immutable and replayable.

AI extracts the figures. Code calculates the tax. Never the other way around.

LLMs read the documents — P60, P11D, pension annual statements, chargeable-event certificates — and surface the figures for your review. Every extracted figure is shown on screen with its source. You approve before any calc runs.

Once approved, the figures feed into a pure-TypeScript calc engine with no network calls, no randomness, no LLM access, no hidden state. Same inputs in, same output out — every time, on any machine, in any tax year. The architectural rule is enforced by the codebase, not by intention.

This matters because LLM-generated arithmetic is not safe to put in front of a compliance officer. We never ship it.

Sub-processors (Anthropic, Supabase, Vercel, Stripe, Resend, Sentry) and the 30-day abuse-retention window on the extraction side: /privacy §4 →

The post-2021/22 top-slicing rule. Done right.

“Errors still exist in HMRC's tax calculations.”

AccountingWEB →·Tax Adviser Magazine: Learn to slice correctly →

IPTM3820 / HMRC Agent Update 83 (2021) changed top-slicing relief for chargeable-event gains arising on or after 6 April 2021: the post-2021/22 PSA/SRB recalculation that many tools miss. ParaplanAI applies this rule by default and tests it against the HMRC IPTM corpus on every commit.

HMRC's own published calculator still applies the pre-2021/22 method. Several widely-used third-party tools follow the same pattern; we're publishing a side-by-side comparison vs Quilter's Chargeable Event Gain XLSM and Absolute Topslicer on this page later in May 2026. The result of getting this wrong is a misstated tax liability — sometimes by tens of thousands of pounds when the gain pushes income through the PSA-zero threshold.

ParaplanAI applies the post-2021/22 rule by default. Every step of the PSA / SRB recalculation appears on the compliance annex with its IPTM 3820 citation. The codebase pins this as a load-bearing differentiator and tests it against the HMRC IPTM corpus on every commit.

Source PDFs gone in 24 hours. Audit trail kept for six years.

The replayability story has a counterpart: data minimisation. We hold the smallest set of fields a defensible calculation needs and purge raw source documents on a 24-hour horizon — an hourly cron, not a manual policy. The structured figures lifted from each document live on in the case record so the audit trail still resolves; the PDF bytes don't need to.

See the data-minimisation page →

Where HMRC is ambiguous, here's what we apply.

Where HMRC guidance is ambiguous, we record the interpretation we apply and which way any error leans — available on request.

Wherever a default could turn out to be the wrong reading, we hold the conservative direction — the one that never silently understates a tax charge or over-relieves. Firms should confirm acceptance in writing before adopting the output.

HMRC PTM / IPTM · IPTM3820 (HMRC Agent Update 83, 2021) · Questions? info@paraplanai.co.uk →