# Self-Trade Prevention

`D12-F03-A03` · Matching Engines and Venue Logic → Order Controls · archetype `record-transform` · difficulty 3/5 · verification **verified**

Full page: https://docs.thefintechbuilder.com/matching-engines-and-venue-logic/order-controls/self-trade-prevention/
Agent skill: `npx skills add IslamBaraka90/Fintech-Algorithms-Library` — https://docs.thefintechbuilder.com/guides/agent-skill/

## Install and import

```bash
npm install fintech-algorithms
```

```ts
import { selfTradePrevention } from "fintech-algorithms/matching-engines-and-venue-logic/order-controls/self-trade-prevention";
```

## Signature

```ts
selfTradePrevention(incoming_order, resting_orders, mode)
```

Stops a participant trading with itself — required by most venues, and the *mode* determines who loses their order. Cancel-newest, cancel-oldest and cancel-both give materially different outcomes for queue position.

## Parameters

| Name | Type | Required | Notes |
| --- | --- | --- | --- |
| `incoming_order` | `Order` | yes | The aggressing order with its participant identifier. |
| `resting_orders` | `Order[]` | yes | Resting orders that would otherwise match. |
| `mode` | `string` | yes | Prevention mode — cancel newest, cancel oldest, cancel both, or decrement. |

## Returns

`{ action, cancelled, remaining, mode, … }`

Which orders were cancelled and which survive, under the mode applied.

## Errors

- When the mode is unrecognised — throws

## Complexity

Time `O(resting)`, space `O(cancelled)`.

## Worked example

Captured by running this function on the input its own test provides. Real output of real code — but not asserted against a published figure.

### Input

`incoming_order`:

```json
{
  "order_id": "IN-1",
  "side": "buy",
  "price_atoms": 1000200,
  "quantity": 600,
  "participant_id": "P-1",
  "stp_group": "G-1"
}
```

`resting_orders`:

```json
[
  {
    "order_id": "R-EXT-1",
    "side": "sell",
    "price_atoms": 1000000,
    "quantity": 150,
    "participant_id": "P-EXT",
    "stp_group": "G-X",
    "sequence": 1
  },
  {
    "order_id": "R-SELF",
    "side": "sell",
    "price_atoms": 1000100,
    "quantity": 220,
    "participant_id": "P-1",
    "stp_group": "G-1",
    "sequence": 2
  },
  {
    "order_id": "R-EXT-2",
    "side": "sell",
    "price_atoms": 1000200,
    "quantity": 300,
    "participant_id": "P-EXT",
    "stp_group": "G-X",
    "sequence": 3
  }
]
```

`mode`:

```json
"cancel_newest"
```

### Call

```ts
selfTradePrevention(incoming_order, resting_orders, mode)
```

### Returns

object with 11 fields: mode, incoming_order_id, original_incoming_quantity, external_executed_quantity, prevented_self_quantity, canceled_incoming_quantity, canceled_resting_quantity, remaining_incoming_quantity, …

```json
{
  "mode": "cancel_newest",
  "incoming_order_id": "IN-1",
  "original_incoming_quantity": 600,
  "external_executed_quantity": 150,
  "prevented_self_quantity": 220,
  "canceled_incoming_quantity": 450,
  "canceled_resting_quantity": 0,
  "remaining_incoming_quantity": 0,
  "events": [
    {
      "action": "execute-external",
      "incoming_order_id": "IN-1",
      "resting_order_id": "R-EXT-1",
      "quantity": 150,
      "price_atoms": 1000000
    },
    {
      "action": "cancel-incoming",
      "incoming_order_id": "IN-1",
      "resting_order_id": "R-SELF",
      "prevented_quantity": 220
    }
  ],
  "resting_orders": [
    {
      "order_id": "R-SELF",
      "side": "sell",
      "price_atoms": 1000100,
      "quantity": 220,
      "participant_id": "P-1",
      "stp_group": "G-1",
      "sequence": 2
    },
    {
      "order_id": "R-EXT-2",
      "side": "sell",
      "price_atoms": 1000200,
      "quantity": 300,
      "participant_id": "P-EXT",
      "stp_group": "G-X",
      "sequence": 3
    }
  ],
  "state": "self-trade-prevented"
}
```

## Other exports

`tickSizeValidation`, `priceBandValidation`, `cancelOnDisconnect`, `fatFingerLimit`, `circuitBreakerTrigger`, `calculate`. Every module additionally exports `run` as an alias of its primary
function, and a `meta` object carrying its catalog id, domain, family, shape and article URL.

## Verification and provenance

Tier: **verified** (via D).

The worked example below is the figure published in this algorithm's article, replayed and asserted by the test suite on every build. The arithmetic cannot drift without the build failing.

Both tiers guarantee the signature. Full explanation: https://docs.thefintechbuilder.com/guides/verification/

Generated from the docs.json payload shipped inside fintech-algorithms@0.13.1.
The signature and parameter list are checked against the compiled implementation at build time,
so a description that contradicts the code fails the build rather than reaching this file.

## Links

- Article (how it works, step by step): https://thefintechbuilder.com/matching-engines-and-venue-logic/order-controls/self-trade-prevention/
- Implementation source: https://github.com/IslamBaraka90/Fintech-Algorithms-Library/blob/main/src/matching-engines-and-venue-logic/order-controls/self-trade-prevention/impl.ts
- Package on npm: https://www.npmjs.com/package/fintech-algorithms
- Domain index for agents: https://docs.thefintechbuilder.com/matching-engines-and-venue-logic/llms.txt