From 0954f71c168a83a843d4a673dda8fed84f8713a0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Luis=20Tani=C3=A7a?= <matallui@gmail.com>
Date: Fri, 7 Nov 2025 18:23:41 -0700
Subject: [PATCH] fix(predict): cap slippage to avoid sharePrice > 1 (#22354)

## **Description**

This PR fixes a bug in the Polymarket prediction provider where orders
could fail due to an invalid share price calculation. The issue occurred
when slippage was applied to `minAmountReceived`, causing the calculated
share price to exceed 1 (which is mathematically impossible for
prediction market shares).

The fix introduces a cap on `minAmountWithSlippage` to ensure it never
falls below `maxAmountSpent + tickSize`, preventing the share price from
exceeding 1 while still accounting for slippage to reduce order failure
rates.

## **Changelog**

CHANGELOG entry: null

## **Related issues**

Fixes:

## **Manual testing steps**

```gherkin
Feature: Polymarket prediction order execution

  Scenario: user creates a prediction market order with slippage
    Given the user is on the Predict feature
    And a Polymarket prediction market is available

    When user creates an order with slippage applied
    Then the order should execute successfully without sharePrice > 1 errors
    And the minAmountReceived should be capped appropriately
```

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

<!-- Orders could fail with sharePrice > 1 errors -->

### **After**

<!-- Orders execute successfully with proper slippage capping -->

## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I've included tests if applicable
- [x] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adjusts slippage handling in `PolymarketProvider.placeOrder` by
flooring BUY `minAmountWithSlippage` at `maxAmountSpent + tickSize`
before rounding to avoid invalid share prices.
>
> - **Predict/Polymarket**:
>   - Update `placeOrder` slippage logic:
>     - Compute `_minWithSlippage = minAmountReceived * (1 - slippage)`.
> - For `BUY`, floor to `maxAmountSpent + tickSize` before rounding via
`roundOrderAmount`.
>     - Use the rounded value to derive `takerAmount`.
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
dc8a157e44c5f3074e9daebb749580c19e83c3d0. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
---
 .../polymarket/PolymarketProvider.ts          | 20 +++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/app/components/UI/Predict/providers/polymarket/PolymarketProvider.ts b/app/components/UI/Predict/providers/polymarket/PolymarketProvider.ts
index 788db1b8deb..09a49af9e84 100644
--- a/app/components/UI/Predict/providers/polymarket/PolymarketProvider.ts
+++ b/app/components/UI/Predict/providers/polymarket/PolymarketProvider.ts
@@ -588,11 +588,27 @@ export class PolymarketProvider implements PredictProvider {
         throw new Error('Maker address not found');
       }
 
-      // Introduce slippage into minAmountReceived to reduce failure rate
+      /*
+       * Introduce slippage into minAmountReceived to reduce failure rate.
+       */
       const roundConfig = ROUNDING_CONFIG[tickSize.toString() as TickSize];
       const decimals = roundConfig.amount ?? 4;
+
+      let _minWithSlippage = minAmountReceived * (1 - slippage);
+      /*
+       * For BUY orders, the minAmountWithSlippage needs to be capped at
+       * maxAmountSpent + tickSize, otherwise, the order will fail due to
+       * sharePrice being >= 1 (which is impossible).
+       */
+      if (side === Side.BUY) {
+        _minWithSlippage = Math.max(
+          _minWithSlippage,
+          maxAmountSpent + tickSize,
+        );
+      }
+
       const minAmountWithSlippage = roundOrderAmount({
-        amount: minAmountReceived * (1 - slippage),
+        amount: _minWithSlippage,
         decimals,
       });