by Nick Ray Ball and Sienna 4o🛰️👾(The “Special One”)

April 14, 2025

🏛️💻🏛️HMRC: Autofill Bug Problem from 2017 not Fixed in 2025 – 🕵️ Is the Service Being Retired Because of This Bug?

⚠️ Overview

While filing a short-period Corporation Tax Return for Cape Villas Ltd, we encountered a serious autofill bug in the HMRC online tax return system — a system designed and operated under the Government Digital Service (GDS) CMS, originally developed by Thoughtworks. This bug replicates the exact same issue we experienced in 2017.

Autofill fields are consistently incorrect, often pulling unrelated numbers or identifiers — including substituting the Company Registration Number (CRN) with a Government Gateway ID.

After eight years, the bug remains unresolved, and it appears that this recurring autofill issue may be one of the core reasons HMRC has now announced it will be shutting down the online CT600 filing service by 31 March 2026.

🎬 Incident Timeline

• 14 April 2025 – 3:05:00 into the session video
• Filing the short-period CT600 return for 12 May – 31 May 2024
• Autofill incorrectly populated the CRN field with:

931942218480

...instead of the correct CRN:

14864994

This is the same bug we encountered in 2017. Eight years on, it still hasn’t been fixed — despite being incredibly simple to detect and correct.

💻 Systemic Flaw in GDS CMS

The issue highlights a wider structural flaw in the way the GDS CMS is maintained across departments:

The GDS does not use a centralised 'mothership' CMS architecture. Each department's version appears to be a copy-paste implementation, meaning that fixes are not propagated system-wide.

We’ve seen the same flaw replicated in:

• The Department for Work and Pensions (DWP)
• The HMCTS / Ministry of Justice
• And now again in HMRC

Instead of sharing codebase improvements across the network, each fix must be manually duplicated into every departmental CMS. That never happens — so even simple bugs like autofill errors persist indefinitely.

🔁 The 6M Solution: Fix Once, Fix Everywhere

Our architecture under the Sienna AI 6M mothership model proposes exactly the opposite:

One system. One fix. Everywhere.

If a bug like this autofill error is fixed in one module (say, at UKRI), the update automatically propagates to all other instances of the CMS — including DWP, HMRC, MoJ, and future digital government portals.

🛑 The Real Reason the Service is Closing?

HMRC's notice:

This service is closing on 31 March 2026. You will need to use commercial software...

...leads us to ask:

Are they shutting down this service because it’s outdated — or because no one can fix its bugs anymore?

💣 The Real Danger of Letting It Die

Retiring HMRC’s official Corporation Tax submission system risks more than just inconvenience — it could make fraud harder to detect and prosecute.

When users file directly through a government system, there’s a clear line of responsibility. If someone enters incorrect or deliberately misleading data, it’s difficult to argue they didn’t understand what they were doing — especially when entering that data into official HMRC software.

But when users switch to commercial accounting apps, that clarity disappears. Now there’s room for plausible deniability: “Oh, I didn’t know how QuickBooks handled that,” or “I thought the app took care of it.” This makes enforcement more difficult — and opens the door for more errors, both intentional and accidental.

Instead of abandoning their system, HMRC should be improving it — and making it free to every business in the UK.

We’ve also seen that teams connected to GDS — or possibly GDS itself — have been presenting fraud detection systems for HMRC. But this is not the best way to tackle fraud. The smarter move is to take a step back and build a platform people actually want to use — because that gives you something far more valuable than a clever detection layer: raw, structured data.

Right now, HMRC likely cannot access the full data behind every third-party accounting app. And they certainly can’t adapt platforms like QuickBooks to include bespoke fraud-detection AI — because it’s not their system to modify.

If you’re serious about detecting fraud, you don’t start by building a detection algorithm. You start by building the infrastructure that collects the data properly in the first place — and only then layer AI on top.

A modern version of the HMRC tax tool, built with AI guidance and secure bank integrations (from Barclays to Monzo), could become one of the most widely used platforms for UK small businesses. With that user base in place, it could open the door to a public-private partnership — leveraging the trust of the HMRC brand to evolve into a world-class banking and tax app.

And beyond that, such a system could become part of a larger mission:
To not just collect tax — but to improve the profitability of every UK business.

That’s what Sienna AI and the 10 Technology Design for Economic AI proposes: a future where government systems don’t just govern — they help the entire economy thrive.

💥 System Crash Confirms Instability

Ironically, as soon as we reported this long-standing autofill bug, the system crashed. At exactly 3:33 into the session video, midway into (or possibly at the very beginning of) filing the short-period CT600 return, the platform completely failed. This not only confirms that the autofill bug from 2017 remains unfixed, but also highlights that the system itself is unstable — prone to crashing under standard use. The GDS CMS, as implemented here, is not fit for purpose.

⚠️ Additional Autofill Error at 4:44 – Government Gateway ID Substituted Again

At 4 hours and 44 minutes into the recording, we encountered another autofill error, this time in a different part of the submission process. Once again, the system mistakenly inserted the Government Gateway ID (93 19 42 21 84 80) into a field where it did not belong, replicating the exact same failure mode seen earlier in the CRN field.

This confirms that the autofill logic is universally flawed across the GDS CMS interface — not restricted to one field, and not context-aware.

The bug remains consistent in nature but manifests in different input areas, reinforcing the need for a central fix, not a patchwork of departmental guesswork.

🔚 Conclusion

A Bug That Has Persisted Since 2017

There is no doubt that the autofill bug I first recognised in 2017 — on a completely different computer, in a completely different location — is still affecting and crashing the HMRC Corporation Tax system in 2025. Over the years, I have spoken with multiple staff at both HMRC and Companies House about this issue. Some are aware of it, some are not — but the bug has clearly remained unresolved for at least eight years.

Not Just a Fluke — A Legacy Failure

While this specific bug affects corporate tax returns, the broader system it runs on is part of the UK Government’s wider GDS platform. According to Thoughtworks — the software consultancy behind GOV.UK’s digital transformation — the GDS platform first launched in 2012. However, their own case study notes that:

“In January 2015, UK citizens were among the first in the world to be able to file their self-assessment tax returns online.”
Source: Thoughtworks / GDS

While this quote refers to personal self-assessment, the fact that I was using the corporation tax system by 2017 — and already encountering this same autofill bug — strongly suggests that the corporate CT600 filing system was part of the same rollout.

Reused Code, Not Updated Code

What’s most revealing is that despite years of superficial changes to the interface, the underlying architecture appears to be unchanged. It’s likely that every few years, a new developer team is brought in to “modernise” the service — but instead of creating a new CMS, they’re forced to patch over an outdated and increasingly fragile legacy system.

A Critique 8 Years in the Making

This critique of HMRC is not new. I’ve been working on solutions to these very issues — not just identifying the problems, but building functional replacements — for over eight years.

In 2017–2018, I proposed a behavioural economics-based improvement to the HMRC system that would use OKRs (Objectives and Key Results) and a points-based rewards system. But unlike the original Villa Secrets system — which awarded points for sales — the HMRC variant awarded points for helping customers complete their tax returns.

Original HMRC System Concept Paper (2018):
angeltheory.org – Behavioural-Economic-Systems for HMRC
This work was rooted in an earlier model built for Villa Secrets, where a simulated economy based on Hawthorne Effect-style feedback loops drove productivity:

OKR Origin Story – Hawthorne for Richard Thaler (2017):
Hawthorne for Richard Thaler

And in 2017, the precursor to TLS (Total Legal System) was proposed — a system called the Total Financial System — which explored how government tax systems like HMRC could be redesigned for efficiency, visibility, and accountability:

Total Financial System (2017):
TFS – Total Financial System
These are not the ideas of someone critiquing from the sidelines. They are the proposals of a designer who has spent over a decade building integrated business, financial, and legal systems — and warning, year after year, about the brittleness and disconnectedness of the UK Government’s tech stack.

Evidence Across Government Departments

We see the same broken logic in:

• Companies House (which still routes users through legacy platforms)
• HMRC (whose “new” forms still carry decade-old bugs)
• DWP and the Ministry of Justice (which still haven’t fixed the special characters bug)
• UKRI, where critical grant submission forms still cannot accept hyperlinks — rendering competitions for innovation funds practically unworkable

Even MI6 recruitment forms suffer from these problems — a grim testament to how deep the fragmentation runs.

What This All Points To

This isn’t just a bug. It’s the symptom of a deeper failure: a public digital infrastructure built on isolated teams, copy-paste CMS instances, and no central authority for fixing or modernising core functions.

We don’t need another patch. We need a system built from the ground up to be integrated, adaptive, and accountable — a system where when one bug is fixed, it is fixed for everyone, everywhere.

That’s what we’ve built under the Sienna AI 6M model. It’s not theory. It’s already working. And we welcome every government department — including HMRC — to join it.