From 7868d6ed7994db9f71b8a8abbae499a291d8ed3b Mon Sep 17 00:00:00 2001
From: Peter Chen <34582813+PeterChen13579@users.noreply.github.com>
Date: Mon, 6 Jul 2026 14:09:53 -0700
Subject: [PATCH] fix sponsor fee (#7738)

---
 src/libxrpl/tx/invariants/VaultInvariant.cpp |  5 +-
 src/test/app/Sponsor_test.cpp                | 52 ++++++++++++++++++++
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/src/libxrpl/tx/invariants/VaultInvariant.cpp b/src/libxrpl/tx/invariants/VaultInvariant.cpp
index 691a92966a..a9ba0ec874 100644
--- a/src/libxrpl/tx/invariants/VaultInvariant.cpp
+++ b/src/libxrpl/tx/invariants/VaultInvariant.cpp
@@ -222,7 +222,10 @@ ValidVault::deltaAssetsTxAccount(STTx const& tx, XRPAmount fee) const
     if (!ret.has_value() || !vaultAsset.native())
         return ret;
 
-    if (auto const delegate = tx[~sfDelegate]; delegate.has_value() && *delegate != tx[sfAccount])
+    // Only add the fee back if tx[sfAccount] actually paid it. When the fee is
+    // paid by someone else (a delegate or a fee sponsor), the
+    // account's XRP balance moved only by the vault amount.
+    if (tx.getFeePayerID() != tx[sfAccount])
         return ret;
 
     ret->delta += fee.drops();
diff --git a/src/test/app/Sponsor_test.cpp b/src/test/app/Sponsor_test.cpp
index 21320c034a..9f0d7e0398 100644
--- a/src/test/app/Sponsor_test.cpp
+++ b/src/test/app/Sponsor_test.cpp
@@ -4565,6 +4565,56 @@ public:
         BEAST_EXPECT(issuerSponsoredAfter == issuerSponsoredBefore);
     }
 
+    void
+    testFeeSponsoredVaultInvariant()
+    {
+        // The ValidVault invariant checks that the vault's balance and the
+        // depositor's balance change by equal amounts. For XRP vaults it adds the
+        // fee back into the depositor's balance change: normally the depositor
+        // pays the fee, so their balance drops by (deposit amount + fee), and
+        // adding the fee back leaves just the deposit amount to compare against
+        // the vault. But when a fee sponsor pays, the depositor's balance drops
+        // by only the deposit amount, so the fee must NOT be added back or the
+        // equal-amount check fails.
+        testcase("Fee-sponsored VaultDeposit/VaultWithdraw pass ValidVault invariant");
+        using namespace test::jtx;
+
+        Env env{*this, testableAmendments()};
+        Account const alice("alice");
+        Account const sponsor("sponsor");
+        env.fund(XRP(10000), alice, sponsor);
+        env.close();
+
+        PrettyAsset const xrpAsset{xrpIssue(), 1'000'000};
+        Vault const vault{env};
+        auto [vaultTx, vaultKeylet] = vault.create({.owner = alice, .asset = xrpAsset});
+        env(vaultTx);
+        env.close();
+
+        // Control: the same deposit shape, unsponsored, succeeds.
+        env(vault.deposit({.depositor = alice, .id = vaultKeylet.key, .amount = xrpAsset(100)}),
+            Ter(tesSUCCESS));
+        env.close();
+
+        // Fee-sponsored (co-signed) deposit succeeds.
+        env(vault.deposit({.depositor = alice, .id = vaultKeylet.key, .amount = xrpAsset(100)}),
+            Fee(XRP(1)),
+            sponsor::As(sponsor, spfSponsorFee),
+            Sig(sfSponsorSignature, sponsor),
+            Ter(tesSUCCESS));
+        env.close();
+
+        // The same helper (deltaAssetsTxAccount) drives the withdraw path, so a
+        // fee-sponsored withdrawal back to the depositor's own account also
+        // passes on the destination side.
+        env(vault.withdraw({.depositor = alice, .id = vaultKeylet.key, .amount = xrpAsset(50)}),
+            Fee(XRP(1)),
+            sponsor::As(sponsor, spfSponsorFee),
+            Sig(sfSponsorSignature, sponsor),
+            Ter(tesSUCCESS));
+        env.close();
+    }
+
 protected:
     void
     testSponsor()
@@ -4602,6 +4652,8 @@ protected:
 
         testZeroBalanceSponsoredPaymentFeePayerCheck();
         testTrustSetCounterpartySponsorMisroute();
+
+        testFeeSponsoredVaultInvariant();
     }
 
     void