diff --git a/.changeset/late-rings-pay.md b/.changeset/late-rings-pay.md
new file mode 100644
index 000000000..795e5de51
--- /dev/null
+++ b/.changeset/late-rings-pay.md
@@ -0,0 +1,5 @@
+---
+"@balancer/sdk": patch
+---
+
+Fix Permit2 spender for add liquidity unbalanced via swap to use UnbalancedAddViaSwapRouter, and sign the slippage-adjusted max adjustable amount.
diff --git a/src/entities/addLiquidityUnbalancedViaSwap/types.ts b/src/entities/addLiquidityUnbalancedViaSwap/types.ts
index 8ebb4aaf2..ec8585281 100644
--- a/src/entities/addLiquidityUnbalancedViaSwap/types.ts
+++ b/src/entities/addLiquidityUnbalancedViaSwap/types.ts
@@ -22,7 +22,7 @@ export type AddLiquidityUnbalancedViaSwapInput = {
  *
  * Use flow-specific fields for `buildCall` and Permit2:
  * - `pool` — router address argument (distinct from `poolId`)
- * - `expectedAdjustableAmountIn` — user budget / permit ceiling
+ * - `expectedAdjustableAmountIn` — user budget; Permit2 signs budget + slippage
  *
  * Generic `signAddLiquidityApproval` does not apply; use
  * `signAddLiquidityUnbalancedViaSwapApproval` instead.
diff --git a/src/entities/permit2Helper/index.ts b/src/entities/permit2Helper/index.ts
index 31292a0ab..790db768a 100644
--- a/src/entities/permit2Helper/index.ts
+++ b/src/entities/permit2Helper/index.ts
@@ -212,7 +212,12 @@ export class Permit2Helper {
             expiration?: number;
         },
     ): Promise<Permit2> {
-        const spender = AddressProvider.Router(input.chainId);
+        const spender = AddressProvider.UnbalancedAddViaSwapRouter(
+            input.chainId,
+        );
+        const maxAdjustableAmountIn = input.slippage.applyTo(
+            input.expectedAdjustableAmountIn.amount,
+        );
         const details: PermitDetails[] = [];
         details.push(
             await getDetails(
@@ -220,7 +225,7 @@ export class Permit2Helper {
                 input.expectedAdjustableAmountIn.token.address,
                 input.owner,
                 spender,
-                input.expectedAdjustableAmountIn.amount,
+                maxAdjustableAmountIn,
                 input.expiration,
                 input.nonce,
             ),
diff --git a/test/v3/addLiquidityUnbalancedViaSwap/addLiquidityUnbalancedViaSwap.test.ts b/test/v3/addLiquidityUnbalancedViaSwap/addLiquidityUnbalancedViaSwap.test.ts
index 8564c1231..98dc343eb 100644
--- a/test/v3/addLiquidityUnbalancedViaSwap/addLiquidityUnbalancedViaSwap.test.ts
+++ b/test/v3/addLiquidityUnbalancedViaSwap/addLiquidityUnbalancedViaSwap.test.ts
@@ -10,11 +10,21 @@ import { validateAddLiquidityUnbalancedViaSwapInput } from '@/entities/addLiquid
 import { AddressProvider } from '@/entities/inputValidator/utils/addressProvider';
 import { Token } from '@/entities/token';
 import { TokenAmount } from '@/entities/tokenAmount';
-import { Permit2Batch } from '@/entities/permit2Helper';
+import { Permit2Batch, Permit2Helper } from '@/entities/permit2Helper';
+import { PublicWalletClient } from '@/utils';
 import { TOKENS } from 'test/lib/utils/addresses';
 
 const chainId = ChainId.MAINNET;
 
+const testOwner = '0x1111111111111111111111111111111111111111' as Address;
+
+const createMockClient = (clientChainId: ChainId): PublicWalletClient =>
+    ({
+        getChainId: async () => clientChainId,
+        readContract: async () => [0n, 0n, 0] as const,
+        signTypedData: async () => `0x${'00'.repeat(65)}` as `0x${string}`,
+    }) as unknown as PublicWalletClient;
+
 const AAVE = TOKENS[chainId].AAVE;
 const WETH = TOKENS[chainId].WETH;
 const DAI = TOKENS[chainId].DAI;
@@ -433,4 +443,110 @@ describe('AddLiquidityUnbalancedViaSwap', () => {
             );
         });
     });
+
+    describe('signAddLiquidityUnbalancedViaSwapApproval', () => {
+        test('uses UnbalancedAddViaSwapRouter as permit2 spender', async () => {
+            const bptToken = new Token(chainId, mockPoolState.address, 18);
+            const exactToken = new Token(chainId, WETH.address, WETH.decimals);
+            const adjustableToken = new Token(
+                chainId,
+                AAVE.address,
+                AAVE.decimals,
+            );
+            const exactAmountIn = TokenAmount.fromRawAmount(exactToken, 0n);
+            const expectedAdjustableAmountIn = TokenAmount.fromRawAmount(
+                adjustableToken,
+                parseUnits('50', AAVE.decimals),
+            );
+
+            const buildCallInput = {
+                poolType: mockPoolState.type,
+                poolId: mockPoolState.id,
+                addLiquidityKind: AddLiquidityKind.UnbalancedViaSwap,
+                pool: mockPoolState.address,
+                bptOut: TokenAmount.fromRawAmount(
+                    bptToken,
+                    parseUnits('100', 18),
+                ),
+                exactAmountIn,
+                expectedAdjustableAmountIn,
+                amountsIn: [expectedAdjustableAmountIn, exactAmountIn],
+                tokenInIndex: 0,
+                chainId,
+                protocolVersion: 3 as const,
+                to: AddressProvider.UnbalancedAddViaSwapRouter(chainId),
+                addLiquidityUserData: '0x' as const,
+                swapUserData: '0x' as const,
+                slippage: Slippage.fromPercentage('1'),
+                deadline: maxUint256,
+            };
+
+            const permit2 =
+                await Permit2Helper.signAddLiquidityUnbalancedViaSwapApproval({
+                    ...buildCallInput,
+                    client: createMockClient(chainId),
+                    owner: testOwner,
+                });
+
+            expect(permit2.batch.spender).toEqual(
+                AddressProvider.UnbalancedAddViaSwapRouter(chainId),
+            );
+            expect(permit2.batch.spender).not.toEqual(
+                AddressProvider.Router(chainId),
+            );
+        });
+
+        test('applies slippage to permit amount', async () => {
+            const slippage = Slippage.fromPercentage('5');
+            const adjustableToken = new Token(
+                chainId,
+                AAVE.address,
+                AAVE.decimals,
+            );
+            const expectedAdjustableAmountIn = TokenAmount.fromRawAmount(
+                adjustableToken,
+                parseUnits('50', AAVE.decimals),
+            );
+            const bptToken = new Token(chainId, mockPoolState.address, 18);
+            const exactToken = new Token(chainId, WETH.address, WETH.decimals);
+            const exactAmountIn = TokenAmount.fromRawAmount(exactToken, 0n);
+
+            const buildCallInput = {
+                poolType: mockPoolState.type,
+                poolId: mockPoolState.id,
+                addLiquidityKind: AddLiquidityKind.UnbalancedViaSwap,
+                pool: mockPoolState.address,
+                bptOut: TokenAmount.fromRawAmount(
+                    bptToken,
+                    parseUnits('100', 18),
+                ),
+                exactAmountIn,
+                expectedAdjustableAmountIn,
+                amountsIn: [expectedAdjustableAmountIn, exactAmountIn],
+                tokenInIndex: 0,
+                chainId,
+                protocolVersion: 3 as const,
+                to: AddressProvider.UnbalancedAddViaSwapRouter(chainId),
+                addLiquidityUserData: '0x' as const,
+                swapUserData: '0x' as const,
+                slippage,
+                deadline: maxUint256,
+            };
+
+            const permit2 =
+                await Permit2Helper.signAddLiquidityUnbalancedViaSwapApproval({
+                    ...buildCallInput,
+                    client: createMockClient(chainId),
+                    owner: testOwner,
+                });
+
+            const maxAdjustableAmount = slippage.applyTo(
+                expectedAdjustableAmountIn.amount,
+            );
+            expect(permit2.batch.details[0].amount).toBe(maxAdjustableAmount);
+            expect(permit2.batch.details[0].amount).not.toBe(
+                expectedAdjustableAmountIn.amount,
+            );
+        });
+    });
 });