End-to-end CT600 filing
Prepare the return, validate against HMRC business rules, generate the iXBRL statutory accounts, and submit directly to the HMRC Transaction Engine. All in one open-source platform.
Finance & Compliance · production
AI-native Corporation Tax filing for UK businesses.
OpenCT prepares, validates, and files CT600 returns with HMRC — open source, self-hostable, and a fraction of the cost of commercial software. Built for UK SMEs with up to £250K revenue, accountants serving them, and software vendors who want to embed CT600 filing in their own products.
OpenCT has its own dedicated marketing landing page hosted on its product subdomain
at tax.openli.ai. Open it for the full product story, pricing tiers,
and direct sign-up.
v1.1.4 production
CT600 v3
iXBRL FRS 105 / 102
HMRC direct submission
AGPL-3.0
Whitelabel-ready
tax.openli.ai
Why OpenCT
UK Corporation Tax filing has been dominated by closed, expensive commercial software. HMRC’s own free CT600 service is closing on 31 March 2026. OpenCT fills that gap with an open-source, AI-native alternative built specifically for the UK SME market — and for the accountants and software vendors who serve them.
AI-assisted preparation
Describe your business in plain English. The AI agent prepares the CT600, the iXBRL accounts, and the tax computation. Built on the same OpenLI Codex foundation as every other OpenLI product.
Open source & self-hostable
AGPL-3.0 licensed. Auditable by your accountant or tax adviser. Self-hostable behind your own firewall. No vendor lock-in for your tax data.
Capabilities (v1.1.4 in production)
CT600 v3 XML generation
Schema-conformant CT600 v3 XML with GovTalkMessage envelope and IRmark digital signatures. Submits cleanly through the HMRC Transaction Engine.
iXBRL statutory accounts
FRS 105 micro-entity accounts, FRS 102 Section 1A small-company accounts, and dormant-company templates. All generated as compliant iXBRL ready for HMRC.
Tax computation engine
19% / 25% Corporation Tax rates with marginal relief, AIA, WDA, capital allowances, losses, and short accounting periods all handled correctly.
HMRC direct submission
Async submit, poll, retry, and structured error mapping against the HMRC Transaction Engine. No more screen-scraping or manual document upload.
Sage / Xero / QuickBooks import
Import your trial balance from Sage, Xero, QuickBooks or generic CSV. Auto-classification across 40+ standard account codes.
AI Chat filing interface
Conversational filing through Claude Agent SDK + OpenAI Codex SDK dual-runner. 5 specialised tax skills baked in.
Multi-tenant RBAC
Owner, admin, accountant, director, viewer roles. Multi-company per user (one accountant can file for many clients).
Whitelabel API
White-label REST API for accounting firms and software vendors who want to embed CT600 filing in their own platforms.
GDPR compliance
Article 15+20 data export, Article 17 right to erasure. Full GDPR compliance for tax data handling.
Open the live product
OpenCT’s full marketing landing page, pricing tiers, and direct sign-up are all
at tax.openli.ai.
FOUNDATION
Built on OpenLI Codex
OpenCT’s AI Chat filing interface, dual-runner architecture, and audit trail all come from the OpenLI Codex foundation. The same platform that powers our healthcare integration products powers our finance products too.