SIMD-0289: propose to reduce load data size limit to zero by default

proposals/0289-default-loaded-accounts-data-size-to-zero.md

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+---
+simd: '0289'
+title: Set default loaded_accounts_bytes to zero 
+authors: Tao Zhu (Anza)
+category: Standard
+type: Core
+status: Review
+created: 2025-05-27
+feature: 
+supersedes: 
+superseded-by:
+extends:
+---
+
+## Summary
+
+This SIMD proposes setting the default value for loaded accounts data size to
+**zero bytes** in the Solana runtime, replacing the current implicit default of
+`MAX_LOADED_ACCOUNTS_DATA_SIZE_BYTES` (64MB).
+
+## Motivation
+
+Currently, the Solana runtime allows transactions to implicitly load up to 64MB
+of account data, [code](https://github.com/anza-xyz/agave/blob/e9389a091f679c9e4595e4286091d1092f58f5dc/program-runtime/src/execution_budget.rs#L27-L30).
+
+This generous default was intended to avoid accidental transaction failures
+during development or early deployment phases. However, it introduces several
+downsides:
+
+- Reduces transparency in runtime constraints.
+
+- Enables unintentional or excessive resource usage.
+
+- Increases the risk of performance degradation or abuse.
+
+By reducing the default to zero, developers and operators are required to
+explicitly configure this budget, leading to safer, more predictable, and
+better-controlled execution environments.
+
+This aligns with broader Solana design goals to ensure deterministic resource
+consumption and to encourage clear contract behavior.
+
+## Alternatives Considered
+
+- Reduce the default to a arbitrary value (e.g., 8MB or 16MB) instead of 0.
+
+## New Terminology
+
+None
+
+## Detailed Design
+
+1. Introduce a New Default Constant:
+   Add a new constant:
+   `pub const DEFAULT_LOADED_ACCOUNTS_DATA_SIZE_BYTES: usize = 0`
+   Use this wherever a default loaded account data size is required.
+
+2. Preserve the Current Maximum:
+   Continue to enforce `MAX_LOADED_ACCOUNTS_DATA_SIZE_BYTES` (64MB) as the upper
+bound for explicitly configured limits.
+
+3. Gradual Ramp-Down Strategy:
+   Implement a phased reduction toward zero. For example, decrease
+`DEFAULT_LOADED_ACCOUNTS_DATA_SIZE_BYTES` by fixed increments (e.g., 8MB per
+epoch). This gives developers time to adapt.
+
+4. Final Enforcement:
+   Once the target default of zero is reached, completely remove the default.
+All accounts data size limits must then be explicitly defined by the
+transaction or runtime.
+
+## Impact
+
+This change is not backward-compatible by default. Workloads or tests that
+rely on the implicit 64MB limit may break.
+
+To mitigate the impact:
+
+- Issue deprecation warnings during the ramp-down phase.
+
+- Provide guidance and tools for setting explicit limits using the
+set_loaded_accounts_data_size_limit instruction.
+
+- Use feature gating to manage rollout and allow clusters to opt in progressively.
+
+
+## Security Considerations
+
+To maintain consensus integrity, both Agave and Firedancer clients must adopt
+this change in a coordinated fashion.