Skip to content
Web3 Academy

Bad debt

Reading depth

Each view includes the earlier layers; the complete engineer or auditor page is shown by default.

Bad debt is borrower debt that remains after recoverable collateral and ordinary repayment sources are insufficient or unavailable.

Split balance category cue

Bad debt is an unpaid borrower obligation that remains when recoverable collateral and ordinary repayment sources cannot satisfy it.

Why it exists

Section titled “Why it exists”

Calling every debt balance collectible can overstate pool assets and supplier claim value. Bad-debt recognition makes a shortfall explicit and forces the system to name which reserve, backstop, supplier, or other claim class loses.

Traditional-finance analogy

Section titled “Traditional-finance analogy”

A nonperforming loan and credit write-off are useful analogies. DeFi shortfalls can appear abruptly after price gaps, oracle failures, liquidation congestion, thin liquidity, token restrictions, or contract bugs. Loss recognition and allocation can be automatic rather than decided by an institution after review.

Step-by-step shortfall lifecycle

Section titled “Step-by-step shortfall lifecycle”
  1. Debt grows or recoverable collateral value falls until the position becomes liquidatable.
  2. A liquidator attempts to repay debt and seize or sell collateral under the protocol rules.
  3. Price gaps, market impact, costs, transfer limits, or execution failure prevent full recovery.
  4. Collateral becomes exhausted, inaccessible, or uneconomic to realize.
  5. The residual obligation is recognized as bad debt rather than a collectible pool asset.
  6. A funded reserve, backstop, junior claim, or supplier claim absorbs the accounting loss in the documented order.

A reference-value estimate is:

badDebtValueUsd
= max(debtValueUsd - recoverableCollateralValueUsd, 0)

Recoverable value is not merely oracle value. It may subtract liquidation bonus, market impact, fees, transfer limits, delay, and other realization costs. All terms must use the same valuation time and unit.

Four-flow view

Section titled “Four-flow view”

Capital flow covers attempted repayment, collateral realization, and any funded reserve or backstop transfer. Claim flow writes down the pool’s borrower receivable and the value supporting supplier claims. Return flow previously carried borrower interest toward suppliers, but uncollectible accrued interest must not remain recorded as income. Risk flow propagates the residual shortfall through the documented reserve, backstop, junior, and supplier priority.

Balance-sheet recognition

Section titled “Balance-sheet recognition”

Writing off borrower debt is not loss elimination. Another account must be charged by the same amount under the selected valuation and priority model.

Return source and loss allocation

Section titled “Return source and loss allocation”

Past borrower interest may have increased supplier claims, but uncollectible principal and interest can reverse economic value. Reserves, insurance funds, staked backstops, junior tranches, governance treasuries, or suppliers bear loss only in the documented order and only to the extent they are funded.

Engineer or auditor lens

Section titled “Engineer or auditor lens”

Reconcile debt, collateral, prices, recovery costs, reserves, supplier claims, and write-off state. Test collateral exhaustion, zero recovery, partial recovery, rounding, repeated realization, recovery after write-off, reserve depletion, loss socialization, claim-priority boundaries, and governance intervention. No write-off should make liabilities disappear without a matching loss account.

Morpho’s bad-debt guide documents one market model in which realized bad debt reduces supplied assets. Aave’s Umbrella documentation describes one asset-specific staked backstop. Both are protocol-specific examples reviewed 2026-08-09, not promises that every loss is covered.

Common misunderstandings

Section titled “Common misunderstandings”
  • “Bad debt means the protocol was never overcollateralized.” A later price, oracle, liquidity, execution, or code failure can consume the initial buffer.
  • “A write-off removes the loss.” It recognizes and reallocates the loss.
  • “Insurance means suppliers cannot lose.” Coverage depends on funded assets, scope, priority, triggers, and execution.

Return to the lending-pool balance sheet.

Machine-readable model

Key equations

Canonical expressions come from the structured concept record. KaTeX renders the notation, while the plain-text expression and variable table keep its meaning and units inspectable without JavaScript. Read the narrative above for the model's domain, assumptions, and rounding rules.

  1. Equation 1

    Plain-text equation: badDebtValue = max(debtValue - recoverableCollateralValue, 0)

Assurance contract

Security properties

These structured statements define desired behavior. Their stable IDs can bind tests, invariants, specifications, audit findings, or proof results without turning descriptive review advice into an assurance claim.

Desired · not evaluated: No test, audit, or proof result is implied until scoped evidence is linked to this property.

  1. SP-01Desired · not evaluated

    Bad debt uses reconciled debt, recoverable collateral, liquidation costs, and valuation units

  2. SP-02Desired · not evaluated

    Write-off cannot erase debt without an equal, explicit charge to reserves, backstops, claims, or another loss account

  3. SP-03Desired · not evaluated

    Loss allocation follows documented priority and cannot exceed each funded balance or claim class

  4. SP-04Desired · not evaluated

    Recovery after write-off is accounted for without duplicating assets or reversing unrelated losses

Knowledge check

Quiz

Answer in your own words, then open the model answer.

What problem does Bad debt exist to address?

Model answer

Naming bad debt makes lending losses visible so a protocol can reconcile supplier claims, reserves, backstops, write-offs, and any socialized shortfall.