Add proposal for adjusting rent

Author: joncinque

proposals/XXXX-rent-adjusted-delegations.md

@@ -0,0 +1,111 @@
+---
+simd: 'XXXX'
+title: Rent-Adjusted Stake Delegations
+authors:
+  - Jon C (Anza)
+category: Standard
+type: Core
+status: Review
+created: 2026-03-06
+feature: (fill in with feature key and github tracking issues once accepted)
+---
+
+## Summary
+
+A new calculation is proposed to adjust stake delegation amounts during the
+partitioned rewards payout system of
+[SIMD-118](https://github.com/solana-foundation/solana-improvement-documents/blob/main/proposals/0118-partitioned-epoch-reward-distribution.md),
+based on the Rent sysvar parameters at the beginning of that epoch.
+
+## Motivation
+
+If
+[SIMD-0438 (Rent Increase)](https://github.com/solana-foundation/solana-improvement-documents/blob/main/proposals/0438-rent-increase-safeguard.md)
+is enabled after any of the Rent decreases described in
+[SIMD-0437](https://github.com/solana-foundation/solana-improvement-documents/blob/main/proposals/0437-incremental-rent-reduction.md),
+a stake account created with lower Rent MAY have an inflated delegation amount,
+consisting of the delta between the previous (decreased) minimum balance and the
+new (increased) minimum balance.
+
+Although the potential divergence is on the order of 1/1000 of a SOL per stake
+account, an incorrect delegation amount gives validators an artificially higher
+stake weight, reflecting lamports that are not properly backed by real lamports
+in a stake account.
+
+## Detailed Design
+
+During the partitioned epoch rewards calculation outlined in
+[SIMD-118](https://github.com/solana-foundation/solana-improvement-documents/blob/main/proposals/0118-partitioned-epoch-reward-distribution.md),
+a stake's updated delegation MUST be calculated with the following formula:
+
+```
+max(min(delegation + stake_rewards, lamports + stake_rewards - rent_exempt_reserve), 0)
+```
+
+Where:
+* `delegation`: the account's pre-reward delegated lamport amount
+* `stake_rewards`: the account's calculated stake reward lamport amount for the
+  past epoch
+* `lamports`: the account's pre-reward lamports
+* `rent_exempt_reserve`: the minimum lamport balance required for the stake
+  account
+
+The distribution phase MUST set the `delegation.stake` field (offset `[156,164)`)
+in the stake account's data to the new delegation amount, expressed as a little-
+endian unsigned 64-bit integer.
+
+If the new delegation amount is 0, then `delegation.deactivation_epoch` (offset
+`[172,180)`) MUST be set to the rewarded epoch, expressed as a little-endian
+unsigned 64-bit integer.
+
+If the stake account does not have enough lamports to meet the minimum balance,
+no other change is required. The account will continue to exist as any other
+account that does not meet minimum balance requirements as described in
+[SIMD-0392](https://github.com/solana-foundation/solana-improvement-documents/blob/main/proposals/0392-relax-minimum-balance-check.md).
+
+The stake weight for the delegated vote account MUST take into account the new
+calculation.
+
+New entries in the Stake History sysvar MUST take into account the adjusted
+delegation amounts.
+
+During the implementation of block revenue distribution in
+[SIMD-0123](https://github.com/solana-foundation/solana-improvement-documents/blob/main/proposals/0123-block-revenue-distribution.md),
+block rewards MUST be used to cover the new required minimum balance, so
+the formula becomes:
+
+```
+max(min(delegation + stake_rewards, lamports + stake_rewards + block_rewards - rent_exempt_reserve), 0)
+```
+
+Where `block_rewards` represents the block rewards earned by the stake account
+in that epoch. All other variables are the same as before.
+
+## Alternatives Considered
+
+We could fix the minimum balance for stake accounts to the current minimum
+balance for 200 bytes. This approach breaks any existing on-chain programs or
+tooling that use the Rent sysvar to calculate the minimum balance of a stake
+account.
+
+We could explicitly tie this logic to
+[SIMD-0438](https://github.com/solana-foundation/solana-improvement-documents/blob/main/proposals/0438-rent-increase-safeguard.md)
+or any future proposal to increase rent. That approach would complicate a rent
+increase feature, and complicate any attempt for dynamic rent in the future.
+This proposal even works if Rent becomes dynamic, since delegation amounts will
+always be correct when new stake weights are calculated for the epoch.
+
+## Impact
+
+The biggest impact is that a delegation MAY decrease between epochs. Protocols
+MUST relax assumptions that delegation amounts only increase or stay the same.
+
+Protocols MUST allow for stake accounts to become inactive as a result of reward
+distribution, without an explicit call to `Deactivate` or `DeactivateDelinquent`.
+
+## Security Considerations
+
+Nothing to note in particular.
+
+In general, reward calculation and distribution is a complicated part of the
+protocol, and this change introduces more complexity to the calculation portion.