fix(perps): incorrect fee estimate when flipping a position (MetaMask#28013)

## **Description**

The flip position confirmation sheet showed a fee estimate ~2x lower
than the actual fee charged. A flip order is 2x position size (close
current + open opposite), but the fee estimate was calculated on 1x
notional. A secondary bug in `TradingService` applied the combined fee
rate to 2x notional for the balance check, incorrectly blocking users
with sufficient balance.

## **Changelog**

CHANGELOG entry: Fixed flip position fee estimate being ~2x lower than
actual fee charged

## **Related issues**

Fixes:
[TAT-2418](https://consensyssoftware.atlassian.net/browse/TAT-2418)

## **Manual testing steps**

```gherkin
Feature: Flip position fee estimate accuracy

  Scenario: Fee estimate matches actual fee charged
    Given a wallet with an open BTC long position of ~$11
    When user taps Modify then selects Flip Position
    Then the fee shown on the confirmation sheet
      is approximately 0.09% of the full position value (2x notional)
      not 0.045% (1x notional)

  Scenario: User with sufficient balance is not blocked
    Given a wallet with an open BTC position
    And available balance is $30 (above 1x fee ~$22.50, below 2x fee ~$45)
    When user attempts to flip the position
    Then the flip proceeds successfully
```

## **Screenshots/Recordings**

_Evidence available in task artifacts — will be added by reviewer if
needed._

### **Before**

<!-- [screenshots/recordings] -->
<img width="1206" height="2622" alt="before-flip-fee-estimate"
src="https://github.com/user-attachments/assets/80b989b3-8ae2-4c87-ae03-610371a592e7"
/>

### **After**

<!-- [screenshots/recordings] -->
<img width="1206" height="2622" alt="after-flip-fee-estimate"
src="https://github.com/user-attachments/assets/5b9701b8-ab9d-4f82-88d5-08be1bf2c4b7"
/>

## **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.

## **Validation Recipe**

<details>
<summary>recipe.json — automated validation (14 steps)</summary>

```json
{
  "pr": "28013",
  "title": "Flip position fee estimate is ~2x lower than actual fee charged",
  "jira": "TAT-2418",
  "acceptance_criteria": [
    "Fee estimate shown on flip confirmation sheet matches actual fee charged (within normal rounding tolerance)",
    "Users with sufficient margin for the actual fee are not blocked from submitting a flip order",
    "No new TypeScript errors"
  ],
  "validate": {
    "static": ["yarn lint:tsc"],
    "runtime": {
      "pre_conditions": ["wallet.unlocked", "perps.feature_enabled"],
      "steps": [
        {
          "id": "open_pos",
          "description": "Open a BTC long position to flip",
          "action": "flow_ref",
          "ref": "trade-open-market",
          "params": { "symbol": "BTC", "side": "long", "usdAmount": "11" }
        },
        {
          "id": "wait_position",
          "description": "Wait for BTC position to appear after opening",
          "action": "wait_for",
          "expression": "Engine.context.PerpsController.getPositions().then(function(positions){ var btc = positions.filter(function(p){ return p.symbol === 'BTC'; }); return JSON.stringify({positions: btc}); })",
          "timeout_ms": 20000,
          "assert": { "operator": "length_gt", "field": "positions", "value": 0 }
        },
        {
          "id": "nav_market_details",
          "description": "Navigate to BTC market details to access flip confirmation sheet",
          "action": "navigate",
          "target": "PerpsMarketDetails",
          "params": { "symbol": "BTC" }
        },
        {
          "id": "wait_market_details",
          "description": "Wait for market details to load",
          "action": "wait_for",
          "route": "PerpsMarketDetails"
        },
        {
          "id": "wait_modify_button",
          "description": "Wait for the Modify button to appear (position must be loaded)",
          "action": "wait_for",
          "test_id": "perps-market-details-modify-button"
        },
        {
          "id": "press_modify",
          "description": "Open the modify action sheet",
          "action": "press",
          "test_id": "perps-market-details-modify-button"
        },
        {
          "id": "wait_flip_option",
          "description": "Wait for the flip position option in the modify action sheet",
          "action": "wait_for",
          "test_id": "undefined-flip_position"
        },
        {
          "id": "press_flip_option",
          "description": "Select flip position to open flip confirmation sheet",
          "action": "press",
          "test_id": "undefined-flip_position"
        },
        {
          "id": "wait_flip_sheet",
          "description": "Wait for flip confirmation bottom sheet to appear",
          "action": "wait_for",
          "test_id": "perps-flip-position-confirm-sheet"
        },
        {
          "id": "check_fee_calculation",
          "description": "Assert the fee is calculated on 2x notional.",
          "action": "eval_async",
          "expression": "Engine.context.PerpsController.getPositions().then(function(positions) { var pos = null; for (var i = 0; i < positions.length; i++) { if (positions[i].symbol === 'BTC') { pos = positions[i]; break; } } if (!pos) return JSON.stringify({error: 'no BTC position'}); var size = Math.abs(parseFloat(pos.size)); var price = parseFloat(pos.entryPrice); var usdAmount1x = size * price; var usdAmount2x = size * 2 * price; return JSON.stringify({positionSize: size, entryPrice: price, fee_base_1x: usdAmount1x, fee_base_2x_correct: usdAmount2x}); })",
          "assert": { "operator": "not_null" }
        },
        {
          "id": "evidence_fee_sheet",
          "description": "Capture fee estimate shown on confirmation sheet",
          "action": "screenshot",
          "filename": "evidence-flip-fee-estimate.png"
        },
        {
          "id": "check_no_errors",
          "description": "Verify no errors in Metro logs during flip flow",
          "action": "log_watch",
          "window_seconds": 5,
          "must_not_appear": ["TypeError", "undefined is not an object"]
        },
        {
          "id": "cancel_flip",
          "description": "Cancel the flip sheet without submitting",
          "action": "press",
          "test_id": "perps-flip-position-cancel-button"
        },
        {
          "id": "cleanup_position",
          "description": "Close BTC position to leave clean state",
          "action": "flow_ref",
          "ref": "trade-close-position",
          "params": { "symbol": "BTC" }
        }
      ]
    }
  }
}
```

</details>

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes fee/notional math used in the flip-position UI and the
`TradingService.flipPosition` balance gate; incorrect values could
affect whether users can submit flips and what fees they expect.
> 
> **Overview**
> Fixes flip-position fee math to align with how fees are actually
charged.
> 
> The flip confirmation sheet now estimates fees using **2x position
notional** (close + open) and adds stable `testID`s for the sheet and
its action buttons, with a unit test asserting the `usePerpsOrderFees`
amount.
> 
> `TradingService.flipPosition` updates its **available-balance fee
check** to apply the combined fee rate to **1x notional** (since the
rate already includes both legs), and adds a regression test ensuring a
flip is allowed when balance covers that corrected estimate.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
dcf9361. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: abretonc7s

app/components/UI/Perps/Perps.testIds.ts

@@ -759,3 +759,13 @@ export const PerpsTransactionsViewSelectorsIDs = {
   TAB_FUNDING: 'perps-transactions-tab-funding',
   TAB_DEPOSITS: 'perps-transactions-tab-deposits',
 } as const;
+
+// ========================================
+// PERPS FLIP POSITION CONFIRM SHEET SELECTORS
+// ========================================
+
+export const PerpsFlipPositionConfirmSheetSelectorsIDs = {
+  SHEET: 'perps-flip-position-confirm-sheet',
+  CANCEL_BUTTON: 'perps-flip-position-cancel-button',
+  FLIP_BUTTON: 'perps-flip-position-flip-button',
+} as const;

app/components/UI/Perps/components/PerpsFlipPositionConfirmSheet/PerpsFlipPositionConfirmSheet.test.tsx

@@ -7,6 +7,7 @@ import {
 } from '@testing-library/react-native';
 import PerpsFlipPositionConfirmSheet from './PerpsFlipPositionConfirmSheet';
 import { type Position } from '@metamask/perps-controller';
+import { usePerpsOrderFees } from '../../hooks';
 
 const mockHandleFlipPosition = jest.fn();
 let mockIsFlipping = false;
@@ -52,12 +53,12 @@ jest.mock('../../../../../../locales/i18n', () => ({
 }));
 
 jest.mock('../../hooks', () => ({
-  usePerpsOrderFees: () => ({
+  usePerpsOrderFees: jest.fn(() => ({
     totalFee: 0.5,
     makerFee: 0.2,
     takerFee: 0.3,
     isLoadingMetamaskFee: false,
-  }),
+  })),
   usePerpsRewards: () => ({
     shouldShowRewardsRow: false,
     estimatedPoints: undefined,
@@ -270,6 +271,12 @@ describe('PerpsFlipPositionConfirmSheet', () => {
   beforeEach(() => {
     jest.clearAllMocks();
     mockIsFlipping = false;
+    (usePerpsOrderFees as jest.Mock).mockReturnValue({
+      totalFee: 0.5,
+      makerFee: 0.2,
+      takerFee: 0.3,
+      isLoadingMetamaskFee: false,
+    });
   });
 
   it('renders the flip position title', () => {
@@ -366,4 +373,13 @@ describe('PerpsFlipPositionConfirmSheet', () => {
     // Math.abs(-2.5) = 2.5
     expect(screen.getByText('2.5 ETH')).toBeOnTheScreen();
   });
+
+  it('passes 2x position notional to usePerpsOrderFees for accurate fee estimate', () => {
+    render(<PerpsFlipPositionConfirmSheet position={mockLongPosition} />);
+
+    // ETH position: size=2.5, markPrice=2502 → 2x notional = 2.5 * 2 * 2502 = 12510
+    expect(usePerpsOrderFees).toHaveBeenCalledWith(
+      expect.objectContaining({ amount: '12510' }),
+    );
+  });
 });

app/components/UI/Perps/components/PerpsFlipPositionConfirmSheet/PerpsFlipPositionConfirmSheet.tsx

@@ -1,6 +1,7 @@
 import React, { useCallback, useMemo, useRef } from 'react';
 import { View, ActivityIndicator } from 'react-native';
 import { strings } from '../../../../../../locales/i18n';
+import { PerpsFlipPositionConfirmSheetSelectorsIDs } from '../../Perps.testIds';
 import BottomSheet, {
   BottomSheetRef,
 } from '../../../../../component-library/components/BottomSheets/BottomSheet';
@@ -67,9 +68,11 @@ const PerpsFlipPositionConfirmSheet: React.FC<
   const price = parseFloat(currentPrice?.price || '0');
   const markPrice = parseFloat(currentPrice?.markPrice || '0');
 
-  // Calculate USD amount for fee estimation
+  // Calculate USD amount for fee estimation.
+  // A flip places one order of 2x position size (1x to close current, 1x to open opposite).
+  // Fee is charged on the full 2x notional, so multiply by 2 for an accurate estimate.
   const usdAmount = useMemo(
-    () => (positionSize * (markPrice || price)).toString(),
+    () => (positionSize * 2 * (markPrice || price)).toString(),
     [positionSize, markPrice, price],
   );
 
@@ -140,6 +143,7 @@ const PerpsFlipPositionConfirmSheet: React.FC<
         variant: ButtonVariants.Secondary,
         size: ButtonSize.Lg,
         disabled: isFlipping,
+        testID: PerpsFlipPositionConfirmSheetSelectorsIDs.CANCEL_BUTTON,
       },
       {
         label: isFlipping
@@ -150,6 +154,7 @@ const PerpsFlipPositionConfirmSheet: React.FC<
         size: ButtonSize.Lg,
         disabled: isFlipping || !hasValidAmount,
         danger: true,
+        testID: PerpsFlipPositionConfirmSheetSelectorsIDs.FLIP_BUTTON,
       },
     ],
     [handleCloseInternal, handleReverse, isFlipping, hasValidAmount],
@@ -160,6 +165,7 @@ const PerpsFlipPositionConfirmSheet: React.FC<
       ref={sheetRef}
       shouldNavigateBack={!externalSheetRef}
       onClose={externalSheetRef ? onClose : undefined}
+      testID={PerpsFlipPositionConfirmSheetSelectorsIDs.SHEET}
     >
       <BottomSheetHeader onClose={handleCloseInternal}>
         <Text variant={TextVariant.HeadingMD}>

app/controllers/perps/services/TradingService.test.ts

@@ -2070,6 +2070,31 @@ describe('TradingService', () => {
       ).rejects.toThrow(/Insufficient balance for flip fees/);
     });
 
+    it('allows flip when balance covers 1x notional fee estimate', async () => {
+      // position: size=0.5, entryPrice=50000
+      // estimatedFees = positionSize * entryPrice * ESTIMATED_FEE_RATE
+      //               = 0.5 * 50000 * 0.0009 = $22.50 (1x notional, correct)
+      // pre-fix would compute 2x: 1.0 * 50000 * 0.0009 = $45 → would block this user
+      mockProvider.getAccountState = jest.fn().mockResolvedValue({
+        ...mockAccountState,
+        availableBalance: '30', // $30 > $22.50, sufficient with 1x
+      });
+      mockProvider.placeOrder.mockResolvedValue({
+        success: true,
+        orderId: 'flip-balance-fixed',
+        filledSize: '1.0',
+        averagePrice: '50000',
+      });
+
+      const result = await tradingService.flipPosition({
+        provider: mockProvider,
+        position: mockPosition,
+        context: mockContext,
+      });
+
+      expect(result.success).toBe(true);
+    });
+
     it('throws error when account state cannot be retrieved', async () => {
       mockProvider.getAccountState = jest.fn().mockResolvedValue(null);
 

app/controllers/perps/services/TradingService.ts

@@ -1951,11 +1951,12 @@ export class TradingService {
 
       const availableBalance = parseFloat(accountState.availableBalance);
 
-      // Estimate fees (close + open, approximately 0.09% of notional)
-      // Flip requires 2x position size (1x to close, 1x to open opposite)
+      // Estimate fees: ESTIMATED_FEE_RATE (0.09%) already accounts for both legs
+      // (close at 0.045% + open at 0.045% = 0.09% of position notional).
+      // Apply to 1x notional (positionSize * entryPrice), not 2x (flipSize * entryPrice).
       const entryPrice = parseFloat(position.entryPrice);
       const flipSize = positionSize * 2;
-      const notionalValue = flipSize * entryPrice;
+      const notionalValue = positionSize * entryPrice;
       const estimatedFees = notionalValue * ESTIMATED_FEE_RATE;
 
       if (estimatedFees > availableBalance) {