Account Constraints Deep Dive

Anchor's account constraints are the guard rails of Solana development. They validate accounts before your instruction logic runs, preventing entire classes of bugs.

Essential Constraints

#[account(init, payer = X, space = N)]

Creates a new account. The payer funds the rent-exempt deposit. space sets the data size in bytes. Can only be used once per account.

#[account(mut)]

Marks an account as mutable. Required for any account whose data or lamport balance changes. Forgetting mut gives you:

error: A mut constraint was expected but not found.

Signer<'info>

The account must have signed the transaction. Used for authorization — proving the caller owns the account. Signers automatically have their signature verified by the runtime.

Program<'info, System>

References a program account. System is the System Program (11111111...), which handles account creation and SOL transfers.

Account Types

Common Errors

Missing system_program

If your instruction creates an account (init), you must include the System Program:

pub system_program: Program<'info, System>,

Without it, the compiler error is:

error[E0277]: the trait bound `Initialize<'_>: anchor_lang::Accounts<'_>` is not satisfied

This error is cryptic because Anchor generates the account validation code via macros. The fix is always: add the missing account.

Missing mut on payer

The payer's balance decreases (pays rent), so it must be #[account(mut)]:

#[account(mut)]
pub user: Signer<'info>,

Wrong space calculation

If space is too small, the account can't hold your data. If too large, the payer overpays for rent. Always calculate: 8 (discriminator) + field sizes.

In the Next Challenge

You'll encounter a deliberate compiler error — a program with a missing account. Your job is to read the error output and fix it.