> For the complete documentation index, see [llms.txt](https://docs.coti.io/coti-documentation/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.coti.io/coti-documentation/privacy-on-demand/contract-patterns-checklist.md).

# Contract patterns checklist

Use this checklist before deploying a production PoD contract. For conceptual background, see [Architecture and main components](/coti-documentation/privacy-on-demand/architecture-and-components.md) and [Async private operations](/coti-documentation/privacy-on-demand/async-private-operations.md).

## Recommended contract shape

1. Inherit from `PodLib` (primitive path) or `InboxUser` (custom path).
2. Add a network preset mixin such as [`PodUserSepolia`](https://github.com/coti-io/coti-contracts/blob/main/contracts/pod/mpc/PodUserSepolia.sol) or [`PodUserFuji`](https://github.com/coti-io/coti-contracts/blob/main/contracts/pod/mpc/PodUserFuji.sol) — presets auto-wire inbox and COTI routing in their constructor.
3. Accept private values as `it*` on EVM entrypoints.
4. Submit via `PodLib` helpers (e.g. `add64`, `gt256`) or `IInbox.sendTwoWayMessage{value: ...}(..., callbackFeeLocalWei)`.
5. Store `requestId` correlation state.
6. Implement success and error callbacks with `onlyInbox`.
7. Decode callback data into `ct*` and persist.
8. Expose status/result readers for async UX.

Preset wiring (`PodLib` + `PodUserSepolia` / `PodUserFuji`): [Reference: PodLib primitives](/coti-documentation/privacy-on-demand/reference-podlib-and-primitives.md).

## Fee and gas checklist

* Expose **payable** entrypoints when using two-way sends; forward **`msg.value`** as the total fee and pass explicit **`callbackFeeLocalWei`**.
* Call **`calculateTwoWayFeeRequiredInLocalToken`** on the deployed Inbox before production sends (see [How do PoA fees work?](/coti-documentation/privacy-on-demand/how-poa-fees-work.md)).
* Test **too-low total** and **too-low callback** reverts in addition to the happy path.

## Critical security controls

* Access-control any method that changes routing or configuration.
* Gate any call path that invokes `configureCoti(...)`.
* Keep callback handlers `onlyInbox`.
* In custom COTI flows, verify `inboxMsgSender()` matches the expected peer contract.
* Avoid external calls before state updates inside callback handlers.

## ABI correctness checklist

* Selector in `MpcAbiCodec.create(...)` matches the COTI-side target function.
* `.addArgument(...)` order matches COTI parameter order.
* Callback `abi.decode(...)` tuple matches COTI `abi.encode(...)` exactly.
* Add regression tests when callback tuple schema changes.

## Async robustness checklist

* Request starts as `PENDING` and transitions to a terminal state.
* Unknown or duplicate callback handling is defined.
* Errors are observable (`requestId`, error code, message).
* Frontend can query status by request ID (see [`PodRequest`](/coti-documentation/privacy-on-demand/typescript-pod-sdk.md)).

## Privacy and type checklist

* No plaintext sensitive values in EVM storage.
* No `gt*` in EVM public interfaces.
* `ct*` used for user-decryptable outputs.
* Type width consistency preserved through the full flow (see [Reference: data types](/coti-documentation/privacy-on-demand/reference-data-types.md)).

## Observability checklist

* Emit request-created event with indexed `requestId`.
* Emit completion and failure events.
* Index `MessageSent` on the Inbox for off-chain correlation (compact event carries `requestId` in indexed topics).

## Testing checklist

* Success callback updates expected state.
* Non-Inbox callback call reverts.
* Error callback updates failure state.
* Request ID mapping is deterministic.
* Frontend integration decrypts returned ciphertext correctly.
* Fee sufficiency: success with estimated fees plus buffer; reverts when under-funded.

## See also

* [Reference: examples and contracts](/coti-documentation/privacy-on-demand/reference-examples-and-contracts.md)
* [Contract patterns in custom logic tutorial](/coti-documentation/privacy-on-demand/tutorial-custom-logic.md)
* [Cookbook: private investor allocations](/coti-documentation/privacy-on-demand/cookbook-private-investor-allocations.md)


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