▲ Cairn

A private, self-hosted financial dashboard for tracking net worth, modelling retirement, and getting rule-based financial insights. Built for UK users with full Scottish and rUK tax band support.

One stone at a time.

Your data never leaves your machine. Cairn is a single-user app designed to run on your home LAN in one Docker container — SQLite on a local volume, no third-party services, no telemetry, no accounts. The only outbound calls are an optional Claude API request for AI commentary (numbers only, never account names) and a Bank of England base-rate lookup.

Contents: Screenshots · Features · Quick Start · Architecture · API · Backup · Development · Security

Screenshots

Every figure below is fictional demo data — a made-up persona on a £72k Scottish salary.

Advisor — rule-based insights, filterable by severity and topic	Tax Year — ISA, LISA and pension allowances at a glance
Projections — ±1σ forecast envelope and retirement readiness	Marginal Rate — the 60% PA-taper zone and HICBC ramp, made visible
Accounts — every wrapper type, with contribution and return tracking	FIRE — Lean/Regular/Fat targets plus Coast FIRE
Salary Sacrifice — Scottish and rUK bands, NI saving, real cost	Goals — auto-tracked against net worth or an account type

Light theme is a click away:

Features

Net Worth Tracking

• Record monthly snapshots and chart progress over time
• Edit or delete historical snapshots
• Real vs Nominal toggle — restate the whole history in today's purchasing power, so a flat line means wealth only kept pace with inflation
• Estimated contributions overlay on the main net worth chart — dashed line showing cumulative investment contributions so the gap reveals market growth at a glance
• Stacked area chart showing net worth split by asset category (pensions, ISAs, GIA, property, cash, debts) over time
• Annual cashflow Sankey — visualises where your salary goes (tax/NI → sacrifice → take-home → ISA/savings/mortgage/spending), live with Scottish / rUK tax bands
• Net worth target line on the main chart
• Snapshot reminder banner — nudges you when the last snapshot is over 35 days old
• Automated monthly snapshot via cron job

Account Management

• Account types: Current Account, Savings, Cash ISA, Stocks & Shares ISA, Lifetime ISA (with £4k sub-allowance + 25% bonus tracking), General Investment Account (GIA) with unrealised-gain tracking for CGT planning, DC/Workplace Pension, SIPP, Defined Benefit / Final Salary Pension (CETV + guaranteed annual income), Property, Mortgage, Credit Card, Loan, Other
• Search, filter by type, and reorder accounts via drag-and-drop
• Track total_contributed for investment accounts to calculate real returns
• Portfolio Performance panel showing gain/loss and return % per account
• Bulk update screen — edit every account balance in one place and save them all (plus a snapshot) in one click; the monthly logging workflow

Retirement & Projections

• Compound growth modelling with real returns (adjustable nominal growth rate and inflation)
• Forecast fan chart — ±1σ envelope around the central projection (lognormal model, 15% equity-like volatility) so the range of outcomes is visible at a glance
• Drawdown simulator: project how long your pot lasts in retirement, with adjustable retirement age, spending, and State Pension inputs
• Defined Benefit pension income shown separately and factored into drawdown projections

FIRE Calculator

• Calculates your FIRE number (target pot = annual expenses ÷ SWR)
• Years-to-FIRE projection using current net worth and monthly savings
• Coast FIRE number: how much you need now to coast to FIRE without further contributions
• Three scenarios: Lean FIRE (5% SWR, 70% expenses), Regular FIRE (4%), Fat FIRE (3.5%, 130% expenses)

Financial Advisor

Rule-based insights engine — a pure function over your accounts, profile and snapshots, so every recommendation is auditable in frontend/src/advisor.js. Rules cover:

• ISA allowance usage and days remaining in the tax year
• LISA bonus headroom — flags missed 25% government bonus when under 50
• CGT allowance — bed-and-ISA prompt when GIA unrealised gains exceed £3k
• Pension headroom vs annual allowance
• Workplace match underused — flags when your contribution sits below the employer match threshold
• Annual Allowance taper — alerts high earners (>£200k) when the pension AA taper may be in play
• HICBC — High Income Child Benefit Charge taper warning with sacrifice-to-£60k recommendation
• Marriage Allowance — flags £252/yr opportunity when one partner is a non-taxpayer
• Salary sacrifice optimisation (with Scottish / rUK tax band awareness)
• Salary band cliff alerts — actionable warnings just above higher-rate and additional-rate thresholds
• Debt prioritisation (high-interest first)
• Emergency fund adequacy
• Fixed-savings maturity reminder — heads-up before fixed-rate bonds roll over to dismal rates
• Mortgage alerts and overpayment opportunity
• Variable mortgage drift vs BoE — flags tracker/SVR rates running materially above market margin
• Net worth velocity (month-on-month momentum)
• Pension carry-forward opportunity
• Insights are filterable by category (Savings, Debt, Pension, Tax, etc.)

Tax Year Summary

• Live tax-year panel showing:
  • ISA contributions vs £20,000 annual allowance
  • LISA sub-allowance (£4k) usage + expected government bonus, if you have one
  • Pension contributions (employee + employer) vs annual allowance
  • Personal allowance status with taper warning above £100,000 salary
  • Days remaining in the current tax year

Tools

• Salary Sacrifice Calculator — Scottish and rUK income tax bands, NI savings, effective cost, employer NI saving, take-home impact
• Bonus Optimiser — Model a one-off lump sum (annual bonus, RSU vest) as cash vs sacrifice-to-pension; shows marginal rate, take-home impact, and total value gained. Warns when the bonus pushes you into the PA taper or additional-rate band
• Marginal Rate Curve — plots your effective marginal tax + NI rate across £0–£200k, making the 60% PA-taper zone and HICBC ramp visible, with markers at your current and after-sacrifice salary
• IHT Estimator — Inheritance tax calculator using 2025/26 allowances. NRB + RNRB taper (>£2M), transferable allowances for married couples, reduced 36% rate for 10%+ charitable bequests
• Debt Payoff Planner — Avalanche vs snowball comparison with total interest saved
• Mortgage Scenarios — Model different rates, terms, and overpayment strategies
• Carry-Forward Pension Calculator — Auto-rolls to the current tax year using a built-in pension annual-allowance history (2014/15 onwards). Editable prior-year inputs persist locally per tax year; calculates unused allowance, total carry-forward headroom, and monthly contribution needed to fully utilise it this year
• Bank of England Base Rate — Live BoE base rate displayed for context

Goals Tracker

• Create financial goals with a name, target amount, target date, and emoji icon
• Link goals to auto-tracked values: overall net worth or a specific account type balance
• Progress bar, percentage complete, days remaining, and "ACHIEVED ✓" badge

AI Copilot

• Claude-powered natural language analysis of your financial position
• Sends only numerical summaries — no personal identifiers

Theme

• Dark theme (default) and light theme, toggled in the header and persisted in localStorage
• Keyboard shortcuts — vim-style g-prefix navigation (g s snapshot, g u update balances, g a advisor, …); press ? for the cheatsheet

Data & Auth

• Full JSON export and import for backup/migration
• Safe imports — every JSON import automatically writes a pre-import database backup first, and any failure rolls back atomically
• CSV snapshot import — paste historic net-worth data to bootstrap years of pre-Cairn history in one go
• Automated daily backups with rotation — SQLite .backup() writes to a configurable host path; keeps the last 14 by default
• Token-based authentication with hashed passwords (7-day session, designed for local network use)
• Login rate-limiting — 5 failed attempts per IP triggers a 15-minute lockout (SQLite-backed so it holds across gunicorn workers)
• Security headers — strict CSP, X-Frame-Options, X-Content-Type-Options, Referrer-Policy, Permissions-Policy on every response

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Quick Start (Docker)

1. Clone and configure

git clone https://github.com/existentia/Cairn.git cairn
cd cairn
cp .env.example .env

Edit .env:

ADMIN_USERNAME=username
ADMIN_PASSWORD=your-secure-password
ANTHROPIC_API_KEY=sk-ant-...   # Optional: enables AI Copilot
BACKUP_DIR=/app/data/backups   # Optional: where cron writes daily backups (default shown)
BACKUP_RETAIN_DAYS=14          # Optional: how many backups to keep (default 14)

Changing ADMIN_PASSWORD and restarting the container invalidates all existing browser sessions, forcing re-login.

2. Deploy with Docker Compose

docker compose up -d --build

Available at http://<your-server-ip>:8070. Log in with the ADMIN_USERNAME / ADMIN_PASSWORD you set above, then add your accounts and take a first snapshot.

3. Deploy via Portainer

1. Stacks → Add Stack → Repository (point to your fork or this repo)
2. Add environment variables: ADMIN_USERNAME, ADMIN_PASSWORD, optionally ANTHROPIC_API_KEY
3. Deploy the stack

Configuration reference

Variable	Required	Default	Purpose
ADMIN_USERNAME	yes	admin	Single-user login name
ADMIN_PASSWORD	yes	changeme	Login password; hashed at rest. Changing it invalidates all sessions
ANTHROPIC_API_KEY	no	(unset)	Enables the AI Copilot tab; returns 503 when unset
BACKUP_DIR	no	/app/data/backups	Where the daily backup cron writes SQLite copies
BACKUP_RETAIN_DAYS	no	14	How many daily backups to keep before rotating

---

Architecture

cairn/
├── docker-compose.yml
├── Dockerfile              # Multi-stage build (Node → Python)
├── entrypoint.sh           # Starts cron + gunicorn
├── backend/
│   ├── app.py              # Flask API + static serving
│   ├── db_auth.py          # Shared get_db / require_auth (used by blueprints)
│   ├── routes_tools.py     # Calculator endpoints blueprint (/api/tools/*)
│   ├── uk_tax.py           # UK tax constants + calc helpers (2025/26)
│   ├── snapshots.py        # Shared snapshot writer + idempotent backfill
│   ├── migrations.py       # Named schema/data migrations
│   ├── cron_snapshot.py    # Automated monthly snapshots
│   ├── cron_backup.py      # Automated daily SQLite backups with rotation
│   └── requirements.txt    # Flask + Anthropic SDK + Werkzeug + gunicorn
├── frontend/
│   ├── src/
│   │   ├── App.jsx         # Main dashboard UI + tab routing
│   │   ├── api.js          # API client
│   │   ├── advisor.js      # Financial insights engine (pure function)
│   │   ├── constants.js    # UK tax constants mirror (frontend twin of uk_tax.py)
│   │   ├── ui.jsx          # Shared design tokens (theme), formatters, primitives
│   │   ├── tools/          # Self-contained calculator components
│   │   │   ├── FIRECalculator.jsx
│   │   │   ├── DrawdownSimulator.jsx
│   │   │   ├── CarryForwardTool.jsx
│   │   │   ├── SalarySacrificeTool.jsx
│   │   │   ├── BonusOptimiser.jsx
│   │   │   ├── MarginalRateCurve.jsx
│   │   │   ├── IhtEstimator.jsx
│   │   │   ├── TaxYearDashboard.jsx
│   │   │   └── DebtPayoffTool.jsx
│   │   └── main.jsx
│   ├── index.html
│   ├── vite.config.js
│   └── package.json
└── data/                   # SQLite database (Docker volume)
    └── cairn.db

Database upgrades are handled by a named-migration framework in backend/migrations.py. On every container start, init_db() applies any not-yet-run migrations (recorded in a schema_migrations table) and runs an idempotent backfill that fills in missing per-category snapshot breakdowns. Existing deployments upgrade in place — no manual SQL.

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API Endpoints

Method	Path	Description
POST	/api/auth/login	Authenticate
GET	/api/auth/check	Verify token validity
POST	/api/auth/logout	Invalidate session
GET	/api/dashboard	All data in one call
GET/PUT	/api/profile	User profile (salary, age, etc.)
GET/POST	/api/accounts	List / create accounts
PUT/DELETE	/api/accounts/:id	Update / delete account
GET/POST	/api/snapshots	History / take snapshot
PUT/DELETE	/api/snapshots/:id	Edit / delete a snapshot
POST	/api/snapshots/import-csv	Bulk-import historic net worth from CSV
GET/PUT	/api/settings	Projection & tax assumptions
GET/POST	/api/goals	List / create goals
PUT/DELETE	/api/goals/:id	Update / delete a goal
POST	/api/ai/commentary	AI-powered financial analysis
POST	/api/tools/salary-sacrifice	Scottish / rUK salary sacrifice calculator
POST	/api/tools/bonus-optimiser	Lump sum: cash vs sacrifice-to-pension
POST	/api/tools/debt-payoff	Avalanche vs snowball debt comparison
POST	/api/tools/mortgage-scenarios	Mortgage scenario modelling
POST	/api/tools/iht-estimator	Inheritance tax estimate (NRB + RNRB)
GET	/api/rates/boe-base-rate	Bank of England base rate
GET	/api/export	Full data export (JSON)
POST	/api/import	Data import (JSON)

All /api/* routes except /api/auth/login require an Authorization: Bearer <token> header.

---

Backup & restore

A cron job inside the container writes a rotated SQLite backup daily (see BACKUP_DIR / BACKUP_RETAIN_DAYS above). For an ad-hoc copy:

docker cp cairn:/app/data/cairn.db ./backups/cairn-$(date +%Y%m%d).db

For a portable, human-readable backup, use Data → Export in the UI (JSON). Importing that file writes a pre-import database backup first and rolls back atomically if anything fails.

---

Development

# Backend (Flask, port 8000)
cd backend && pip install -r requirements.txt && FLASK_DEBUG=1 python app.py

# Frontend (Vite dev server, port 3000 — proxies /api → :8000)
cd frontend && npm install && npm run dev

# Production build of the SPA (output: frontend/dist/, served by Flask as static)
cd frontend && npm run build

There is no test suite, linter, or type checker configured — this is a personal project kept deliberately dependency-light. Schema changes go through the named-migration framework in backend/migrations.py; append a new (name, callable) entry to MIGRATIONS rather than editing existing ones. CLAUDE.md documents the architecture in more depth, including the mirrored UK-tax constants in backend/uk_tax.py / frontend/src/constants.js that must be updated together.

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Contributing

This is a personal project built around one person's finances, so the roadmap is idiosyncratic and I may not merge everything. That said, issues and PRs are welcome — especially bug reports, UK tax-year updates, and corrections to the tax logic. Fork it freely if you want to take it in your own direction.

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Security Notes

• Designed for local network use behind your home firewall
• Passwords are hashed at rest using Werkzeug's default (scrypt on modern Pythons); rotating ADMIN_PASSWORD and restarting wipes all existing sessions
• Bearer tokens live in SQLite with a 7-day expiry; expired rows are sample-purged on ~1% of authenticated requests
• Request bodies are capped at 5 MB
• If exposing externally, use a reverse proxy with HTTPS and consider Authelia/Authentik
• AI Copilot sends only numerical summaries to the Claude API — no personal identifiers

See SECURITY.md for the full threat model and how to report a vulnerability privately.

Disclaimer

General financial information only — not regulated advice. Cairn is not authorised by the FCA and nothing it produces is a personal recommendation. Tax rules are hand-coded for the 2025/26 UK tax year and may be incomplete, out of date, or simply wrong; projections use simplified models that ignore sequence-of-returns risk, fees, and most edge cases. Check anything important against HMRC guidance and consult an FCA-regulated adviser before acting.

Licence

MIT © 2026 Neil Johnson