FinLang: The financial rules engine.

The financial rules engine. A new language for money.

Transaction categorisation that explains itself.
Stop categorising transactions with brittle spreadsheets. Start with rules you can read, test, and audit.
International | 24K rows/sec | Self-hosted | EU AI Act Ready

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Early Access
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Browse Rulepacks

Deterministic

FinLang guarantees consistent, audit-grade outputs for every transaction. No randomness. No surprises.

Auditable

Every change is logged in JSON. Perfect traceability for teams and compliance workflows.

Fast

26,600 rows/sec on 5M×50 datasets. Performance that scales linearly.

View benchmarks

EU AI Act Ready

FinLang is engineered for transparency, explainability and auditability — core obligations of the EU AI Act (effective Aug 2026). Every decision is deterministic, logged, and reproducible.

Deterministic Logic

No black-box models — every rule is human-readable and predictable.

Full Audit Trail

Every classification produces a JSON diff for complete traceability.

Explainable Decisions

Outputs link directly to the exact rule that triggered them.

Learn about the EU AI Act →

Built for professionals. Trusted by developers.

FinLang uses a human-readable DSL for defining financial rules — clear, explicit, and fully auditable. It bridges simple, readable rules with enterprise-grade financial automation, bringing clarity to everything from personal finance to institutional reconciliation.

rule "Flag: Large Transaction (Debit)" {
  match:
  - amount in -1000000..-500
  set:
  - flags += "LargeTxn"
}

rule "Flag: Large Transaction (Credit)" {
  match:
  - amount in 500..1000000
  set:
  - flags += "LargeTxn"
}

rule "Subs: SPOTIFY" {
  match:
  - counterparty ~ "*SPOTIFY*"
  set:
  - category = "Review"
  - flags += "Subscription"
}

Product Overview

A short walkthrough demonstrating FinLang’s engine, rules, CLI, and audit trail.

Under the hood: FinLang ships with a fully auditable CLI, rule engine, and feedback learning loop.

Deterministic execution

finlang --input in.csv --output out.csv --rules rules.fin

Consistent, audit-grade outputs for every run.

JSON audit trail

finlang ... --audit results.json --audit-mode lite

Every change logged for compliance and reviews.

Adaptive rule discovery

finlang-discover --input categorized.csv --candidates out.csv

Learn from data to propose new rules safely.

Early Access Programme

To help shape the future of FinLang, we’re inviting a small number of organisations to join our Early Access programme where teams can help shape the FinLang DSL and contribute rulepack patterns.

WHAT YOU GET

✅6 months free Team licence
✅Priority support
✅Direct input on the roadmap
✅Early access to commercial rulepacks
✅Optional: co-authored case study

WHO IT'S FOR

•Fintech platforms
•Accounting & reconciliation vendors
•Banks and compliance teams
•Data teams building finance automation

WHAT WE ASK

→30-minute feedback session each month
→Real-world usage insights
→(Optional) workflow review

No IP transfer. No revenue sharing. Just early access in exchange for real-world feedback.

Apply for Early Access →

Licensing

Get started in 60 seconds

Install FinLang via pip, apply your first rules, and audit results instantly.

Standard

>pip install finlang

Fast I/O

>pip install "finlang[fastio]"

Verify

>finlang --help

Community

£0

AGPLv3 Open Source
Full FinLang engine
Self-hosted & Community support

View on GitHub

£1,200/yr

Commercial license (no AGPL obligations)
Up to 10 users
Email support (48-hour response)
All updates included

Buy Team License

Invoices & VAT handled by Lemon Squeezy.

Enterprise

Custom

Unlimited users
Priority SLA support
Private deployment
Custom rule packs

Contact Sales

Custom pricing & tailored onboarding available.

Rulepacks

Domain Knowledge, Downloadable.

Writing regex for banking data is a solved problem. Don’t waste weeks debugging edge cases. FinLang Rulepacks are verified, commercial-grade logic libraries that drop directly into your engine— giving you instant compliance, safety, and normalisation.

FinLang Banking Pack v1.0

A deterministic logic layer for UK & global banking transactions.

Stops “wallet funding” reading as “spending”.
Decodes UK rails (FPS, BGC, CHAPS).
Includes 92 verified, contract-tested rules.

Buy Banking Pack - £499

Instant download after purchase. Works with the open-source engine.