What a sinking fund is for
Most budget-breaking months are not emergencies. They are the car insurance renewal, the December gifts, the annual vet visit: costs you knew about but did not save for. A sinking fund spreads each one across the year. Twelve monthly transfers of $75 turn a $900 bill into a non-event.
The hard part is the number. Too low and the fund runs dry before the bill; too high and you are starving the rest of your budget. Your bank history already has the answer.
1. Find your irregular expenses
Connect checking, savings, and every card at bankbridge.money, add BankBridge to your agent (Claude, ChatGPT, Claude Code), and ask:
“Look at the last 24 months. Find expenses that do not happen every month: annual and semiannual bills, seasonal spending like December gifts and summer travel, and irregular costs like car repairs, medical, and vet bills. Group them into categories and show the total per category for each of the two years.”
The agent combines get_recurring_charges (for annual and semiannual bills), get_spending_summary by month (for seasonal spikes), and list_transactions with a minimum amount (for large one-off costs). Correct its groupings: it cannot know the $640 at a garage was a planned service rather than a breakdown, but you can.
2. Size each fund
Two methods, depending on the expense:
- Known bill, known date (insurance, registration, annual subscriptions): amount due divided by months until it is due. A $900 premium due in five months needs $180 a month now and $75 a month after the next renewal.
- Variable cost (repairs, medical, gifts): the higher of the two yearly totals divided by twelve. Using the higher year gives you a margin.
“For each category, suggest a monthly contribution. Use months until due for fixed bills and the higher yearly total divided by 12 for variable ones. Show the total monthly amount and what share of my average monthly income it is.”
If the total is more than you can set aside, decide which funds get priority. Fixed bills come first, because they will arrive either way.
3. Keep a ledger
BankBridge does not store anything, and one savings account holds many funds, so keep a ledger file the agent reads and updates: a Claude Project file or a sinking-funds.md in your Claude Code folder.
Sinking funds (savings ending 5512)
Fund Monthly Balance Next due
Car insurance $180 $360 Feb 2027 $900
Car maintenance $60 $420 as needed
Gifts + holidays $110 $990 Dec 2026
Annual renewals $45 $210 see renewals.md
Pet care $40 $280 as needed
Total $435 $2,260
Savings account balance should be >= $2,260.4. Update it monthly
“Update sinking-funds.md for September. Add each monthly contribution if my transfer to savings arrived. Subtract any spending this month that belongs to a fund, and list those charges. Then compare the ledger total to my savings balance.”
The agent checks for the transfer, finds fund-related charges, and reads your savings balance with list_accounts. If the balance is lower than the ledger, something was spent from savings that the ledger does not know about; the agent can list the withdrawals so you can assign them.
Common sizing mistakes
- Sizing from one year. A quiet year for repairs makes the fund too small. See why one period misleads.
- Missing annual subscriptions. Build a renewal calendar first; it feeds the annual renewals fund.
- Counting fund spending as overspending. When the insurance bill hits, your monthly spending spikes. Tell the agent to report fund-covered charges separately in your monthly review.
Ready? Connect your accounts ($5 per month per bank) and ask your agent for two years of irregular expenses.