Short answer. The whole debt. Where the decedent's obligation was solidary with another debtor, the claim is filed against the estate as if the decedent were the only debtor — for the entire amount, not just a proportionate share of what is owed.
What the law says
Where the obligation of the decedent is solidary with another debtor, the claim shall be filed against the decedent as if he were the only debtor, without prejudice to the right of the estate to recover contribution from the other debtor.
Rule 86, Section 6 — Solidary obligation of decedent. Read the full provision →
A solidary debt is filed against the estate in full
The rule treats a claim against the estate of a solidary debtor the same way solidary liability works outside of estate settlement: the creditor may go after the whole obligation from any one solidary debtor. Filing the claim as if he were the only debtor means the estate has to answer for the entire debt, not merely the decedent's proportionate share, even though another debtor is equally liable.
The estate can still seek contribution from the co-debtor
This is not the estate's final word on the matter. The rule expressly preserves the right of the estate to recover contribution from the other debtor — after the estate pays the claim, it (or the heirs) can turn around and collect the surviving co-debtor's fair share from that co-debtor directly. The creditor's claim against the estate and the estate's own contribution claim against the co-debtor are two separate matters, so paying the whole debt to the creditor does not mean the estate absorbs the co-debtor's share permanently.
Joint obligations are treated differently
The rule draws a sharp line between solidary and joint obligations. In a joint obligation of the decedent, the claim shall be confined to the portion belonging to him — the creditor can only claim the decedent's own share from the estate, not the whole debt. Whether an obligation is solidary or merely joint therefore determines how much of the debt the estate is actually answerable for.
Why the distinction matters to a creditor
A creditor deciding how to file a claim against a deceased debtor's estate needs to know, first, whether the underlying obligation was solidary or joint. Filing for only a proportionate share of a genuinely solidary debt would leave money on the table that the estate was actually obligated to pay in full, while overclaiming against a merely joint obligation risks a claim the estate is not bound to satisfy. Reviewing the actual agreement or the source of the obligation, rather than assuming, is what settles which rule applies to any given debt.