# Recurring Transactions — Implementation Plan

## Goal

Automatically detect recurring transactions (direct debits, subscriptions, standing orders) from imported and existing transaction history. Tag them, predict next payment dates, and surface them in a dedicated Subscriptions page. All auto-detected values are user-correctable.

---

## Scope

- Fixed-amount recurring detection only (variable-amount bills excluded — too unreliable)
- Detection runs automatically after every CSV import and on manual trigger
- Minimum 2 occurrences to flag as recurring
- Supported frequencies: weekly, fortnightly, monthly, quarterly, yearly
- User can manually mark or unmark any transaction as recurring
- New Subscriptions page in nav + recurring indicator in transaction list

---

## Detection Algorithm

### Description normalisation

UK bank CSV exports embed reference numbers, dates, and prefixes in descriptions that differ per occurrence but refer to the same payee. Normalise before grouping:

1. Lowercase the full string
2. Strip known UK bank prefixes: `direct debit`, `dd `, `so `, `standing order`, `bacs`, `faster payment`
3. Strip trailing reference numbers: sequences of 6+ digits at end of string
4. Strip embedded date patterns (e.g. `15apr`, `15/04`)
5. Collapse multiple spaces, trim

Examples:
```
"DIRECT DEBIT NETFLIX 00123456"  →  "netflix"
"DD SPOTIFY AB 987654"           →  "spotify ab"
"NETFLIX.COM"                     →  "netflix.com"
"COUNCIL TAX REF 20240415"       →  "council tax ref"
```

### Grouping and interval analysis

Group all non-deleted transactions for a user by `(normalised_description, exact_amount)`.

For each group with 2+ transactions:
1. Sort by date ascending
2. Calculate day-intervals between consecutive occurrences
3. Compute average interval
4. Classify against frequency table:

| Frequency | Avg days range | Per-interval tolerance |
|---|---|---|
| Weekly | 6–8 | ±2 days |
| Fortnightly | 13–15 | ±3 days |
| Monthly | 26–35 | ±5 days |
| Quarterly | 85–95 | ±10 days |
| Yearly | 355–375 | ±15 days |

5. Check all individual intervals fall within tolerance of the average
6. If matched: tag all transactions in the group

### Confidence score

`confidence = 1 - (std_dev_of_intervals / expected_interval)`

Capped at 1.0. Stored in `recurring_rule` for potential future use (e.g. sorting by confidence).

### Next expected date

- **Weekly/fortnightly**: last_date + N days
- **Monthly**: last_date + 1 month (using `dateutil.relativedelta` — handles month-end correctly)
- **Quarterly**: last_date + 3 months
- **Yearly**: last_date + 1 year

If next_expected is in the past (missed payment or detection ran late), advance by one frequency period until it's in the future.

---

## Data Model

No new tables required. Uses existing columns on `transactions`:

```
is_recurring: bool        — already exists
recurring_rule: JSONB     — already exists, was never populated
```

### `recurring_rule` JSONB shape

```json
{
  "frequency": "monthly",
  "typical_amount": -10.99,
  "typical_day": 15,
  "next_expected": "2026-05-15",
  "confidence": 0.94,
  "detected_at": "2026-04-23T10:00:00Z",
  "manually_set": false
}
```

`manually_set: true` when the user explicitly toggled it — detection will not overwrite manually-set entries on subsequent re-scans.

---

## Backend

### Service: `backend/app/services/recurring_service.py` (new)

```python
def normalise_description(raw: str) -> str
    # Strips prefixes, refs, dates, lowercases

def classify_frequency(avg_days: float) -> str | None
    # Returns "weekly" | "fortnightly" | "monthly" | "quarterly" | "yearly" | None

def next_expected_date(last_date: date, frequency: str) -> date
    # Advances last_date by one frequency period; repeats until future date

async def detect_recurring(db: AsyncSession, user_id: uuid.UUID) -> dict
    # Loads all transactions, runs grouping + interval analysis
    # Updates is_recurring and recurring_rule on matched transactions
    # Skips transactions where recurring_rule.manually_set == true
    # Returns {"newly_tagged": int, "total_recurring": int}
```

### Changes to `transaction_service.py`

- `import_csv()`: call `detect_recurring(db, user_id)` after flush, before return
- `_to_response()`: ensure `recurring_rule` is included in the response dict (it currently isn't)

### New endpoint: `POST /transactions/detect-recurring`

Manual trigger. Calls `detect_recurring()` and returns the counts. Rate-limited — no need to spam it.

### Changes to `GET /transactions` (existing)

No changes needed. `is_recurring` filter already works.

### New endpoint: `GET /subscriptions`

Returns a grouped summary for the Subscriptions page:

```json
{
  "total_monthly_equivalent": 87.43,
  "currency": "GBP",
  "subscriptions": [
    {
      "name": "Netflix",
      "amount": -10.99,
      "frequency": "monthly",
      "next_expected": "2026-05-03",
      "last_paid": "2026-04-03",
      "account_name": "Monzo",
      "account_id": "...",
      "transaction_ids": ["...", "..."],
      "confidence": 0.98,
      "manually_set": false
    }
  ]
}
```

Monthly equivalent conversion:
- Weekly × 52 / 12
- Fortnightly × 26 / 12
- Monthly × 1
- Quarterly / 3
- Yearly / 12

### Changes to `PUT /transactions/{id}` (existing)

Already accepts `is_recurring`. Extend to also accept `recurring_rule` patch so the frontend can write `manually_set: true` when the user toggles.

---

## Frontend

### Transaction list — recurring indicator

In `TransactionList.tsx`, next to the existing paperclip icon for attachments, add a `↻` (`RefreshCw`) icon for transactions where `is_recurring === true`. Small, muted, same style as the paperclip.

### Transaction detail drawer — manual toggle

In `TransactionDetailDrawer.tsx`, add a toggle in the detail panel:

```
[ ↻ ] Mark as recurring   [toggle]
```

When toggled on manually: sets `is_recurring=true`, writes `recurring_rule: { manually_set: true, frequency: null, ... }` — opens a small inline form to let the user pick the frequency and confirm the amount. When toggled off: sets `is_recurring=false`, clears `recurring_rule`.

### New page: `frontend/src/pages/subscriptions/SubscriptionsPage.tsx`

#### Header

```
Subscriptions & Standing Orders
Estimated monthly spend: £87.43        [ Re-scan ]
```

Re-scan button calls `POST /transactions/detect-recurring`, invalidates the subscriptions query.

#### Subscription cards / list

Each row:
```
[ icon ] Netflix                    £10.99 / month
         Monzo · last paid 3 Apr    Next: 3 May  [3 days]   [ ··· ]
```

Colour coding on the "Next" badge:
- Red: overdue (next_expected in the past)
- Amber: within 7 days
- Muted: more than 7 days away

The `···` menu: "Mark as not recurring" (unsets it with `manually_set: true`).

#### Sort options

- Next payment (default)
- Amount (high to low)
- Name (A–Z)

#### Empty state

If no recurring transactions detected yet: prompt to import a CSV or use Re-scan.

### Nav addition

Add `Subscriptions` to the sidebar and mobile nav, between Transactions and Reports. Icon: `RefreshCw` or `Repeat`.

### API client: `frontend/src/api/subscriptions.ts` (new)

```typescript
export interface Subscription { ... }
export interface SubscriptionsSummary { ... }

export const getSubscriptions = (): Promise<SubscriptionsSummary> => ...
export const triggerDetection = (): Promise<{ newly_tagged: number; total_recurring: number }> => ...
```

---

## Implementation Order

1. **`recurring_service.py`** — detection logic, fully unit-testable without DB
2. **Wire into `import_csv()`** — auto-detect after every import
3. **`POST /transactions/detect-recurring`** — manual trigger endpoint
4. **`GET /subscriptions`** — grouped summary endpoint
5. **Update `_to_response()`** — include `recurring_rule` in transaction response
6. **`SubscriptionsPage.tsx` + `api/subscriptions.ts`** — main UI
7. **Transaction list `↻` indicator** — small icon addition
8. **Transaction detail drawer toggle** — manual mark/unmark

---

## Testing Checkpoints

- Normalisation: verify `"DIRECT DEBIT NETFLIX 00123456"` → `"netflix"`
- Monthly detection: 3 transactions, same amount, ~30 days apart → detected
- Below threshold: 1 occurrence → not detected
- Tolerance: transactions on 1st, 31st, 2nd of following months → monthly detected (within ±5 days)
- `manually_set: true` entries are not overwritten on re-scan
- Price change: Netflix at £10.99 (3 entries) + £11.99 (1 entry) → old entries stay tagged, new one not auto-tagged
- Monthly equivalent: yearly £120 subscription → shows £10.00/month

---

## Notes & Decisions

- **No new DB migration required** — `is_recurring` and `recurring_rule` columns already exist from the initial schema.
- **Detection is idempotent** — running it twice produces the same result. Safe to call on every import.
- **`manually_set` flag protects user corrections** — if a user untags something, a subsequent CSV import will not re-tag it.
- **`recurring_rule` not currently returned by `_to_response()`** — this must be added before the frontend can read frequency/next_expected.
- **Variable-amount recurrings** (energy, credit card payments) are out of scope. User can manually tag these if desired.
- **2-occurrence minimum** is configurable as a constant in `recurring_service.py` (`MIN_OCCURRENCES = 2`).