A contribution you enter (TFSA, RRSP, FHSA or a child's RESP) is an intention, not a fact. If your take-home minus living costs minus housing does not leave the money, the contribution cannot happen in full.
What the engine does
Each month, what is left after spending joins your cash. Then every contribution, in the order your accounts are listed, takes what cash can give it — up to the amount you asked for, and never past the account's room. A contribution that cash cannot cover in full is funded in part, not borrowed, and the plan carries this warning.
Order matters when money is short: the first account listed gets its full ask before the second gets anything. Put the account you care most about first.
The room an account has is a separate limit; see over-contributions. To place the leftover automatically, see auto-allocation.
This is the same honesty rule as the shortfall: the engine reports a gap rather than quietly forgiving it. A projection that funds contributions out of nothing would show a net worth nobody can reach — and one that funded nothing until every contribution could be covered at once (what the engine did until 2026-09-15) left money sitting idle that could have been working.
What to do about it
Either lower the contributions, reorder the accounts so the important one is first, or look at what is consuming the cash — living expenses, loan payments, or a housing purchase whose carrying costs land before the plan can absorb them. A month that needs more than the plan has can instead draw savings and borrow; see borrowing when short. Click a year in the net worth chart to see that year's flows account by account.