Arcium callbacks always fire but the output can be invalid — handle the verify_output error branch

Design question with a real fund-safety consequence: an instruction locks user SOL and queues an Arcium encrypted (MPC) computation; the callback instruction is supposed to release the funds. What happens if the callback never runs — are the funds stuck?

Answer: the callback is guaranteed to be invoked. What is *not* guaranteed is that it carries a valid result: if one or more cluster nodes misbehave and the computation is aborted, the callback still fires but the signed output fails verification. So the failure mode you must design for is not "no callback" but "callback with an unusable output".

Concretely, always handle the Err branch of verify_output rather than unwrapping:

#[arcium_callback(encrypted_ix = "my_instruction")]
pub fn my_instruction_callback(
    ctx: Context<MyInstructionCallback>,
    output: SignedComputationOutputs<MyInstructionOutput>,
) -> Result<()> {
    let value = match output.verify_output(&ctx.accounts.cluster_account,
                                          &ctx.accounts.computation_account) {
        Ok(MyInstructionOutput { field_0 }) => field_0,
        Err(_) => { /* unlock/refund path, or mark the request failed */ }
    };
    ...
}

Practical pattern: treat the locked state as a request record with an explicit status. On a verified output, apply the result and settle. On a verification failure, take the compensating action in the same callback — refund or unlock the user's SOL, or move the request to a failed state that a user-callable instruction can reclaim from. Never leave the only unlock path behind a successful-output branch.

Behaviour described against Arcium's Anchor integration as of 2025; confirm the callback guarantee against current Arcium docs since the network is young.

Source: adapted from a Stack Exchange answer by McBain (question by Salle H), https://solana.stackexchange.com/questions/24117/are-arcium-callbacks-guaranteed-to-run-at-least-once — CC BY-SA 4.0