From 84e84fbadf3c08e61a9b63ca15da945c94d662ac Mon Sep 17 00:00:00 2001
From: tequ <git@tequ.dev>
Date: Mon, 13 Apr 2026 14:03:37 +0900
Subject: [PATCH] fix  Sponsor able to provide sponsorship with just base
 reserve

---
 src/libxrpl/tx/Transactor.cpp | 10 +++++++++-
 src/test/app/Sponsor_test.cpp | 22 +++++++++++++++++++++-
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/src/libxrpl/tx/Transactor.cpp b/src/libxrpl/tx/Transactor.cpp
index b2e48f93a1..413a821354 100644
--- a/src/libxrpl/tx/Transactor.cpp
+++ b/src/libxrpl/tx/Transactor.cpp
@@ -479,7 +479,15 @@ Transactor::checkFee(PreclaimContext const& ctx, XRPAmount baseFee)
         if (payerSle->getType() != ltACCOUNT_ROOT)
             return tefINTERNAL;  // LCOV_EXCL_LINE
 
-        maxSpendable = payerSle->getFieldAmount(payer.balanceField).xrp();
+        if (payer.type == FeePayerType::SponsorCoSigned)
+        {
+            STAmount const sponsorReserve = calculateReserve(payerSle, ctx.view.fees());
+            maxSpendable = payerSle->getFieldAmount(sfBalance).xrp() - sponsorReserve.xrp();
+        }
+        else
+        {
+            maxSpendable = payerSle->getFieldAmount(payer.balanceField).xrp();
+        }
     }
 
     // NOTE: Because preclaim evaluates against a static readview, it
diff --git a/src/test/app/Sponsor_test.cpp b/src/test/app/Sponsor_test.cpp
index e1638de7f1..9c48f8ba25 100644
--- a/src/test/app/Sponsor_test.cpp
+++ b/src/test/app/Sponsor_test.cpp
@@ -1444,6 +1444,27 @@ public:
                 BEAST_EXPECT(env.balance(bob) == bobBalance);
                 BEAST_EXPECT(env.balance(sponsor) == sponsorBalance - feeAmt);
             }
+
+            {
+                // below reserve
+                adjustAccountXRPBalance(env, sponsor, env.current()->fees().reserve);
+                env.close();
+                auto const feeAmt = XRP(4);
+
+                env(noop(alice),
+                    fee(env.current()->fees().base),
+                    sponsor::as(sponsor, spfSponsorFee),
+                    sig(sfSponsorSignature, sponsor),
+                    ter(terINSUF_FEE_B));
+                env.close();
+
+                env(noop(alice),
+                    fee(XRP(10)),
+                    sponsor::as(sponsor, spfSponsorFee),
+                    sig(sfSponsorSignature, sponsor),
+                    ter(terINSUF_FEE_B));
+                env.close();
+            }
         }
 
         {
@@ -1595,7 +1616,6 @@ public:
 
                 BEAST_EXPECT(env.le(keylet::sponsor(sponsor, alice))->isFieldPresent(sfFeeAmount));
                 auto sponsorAvailableFee = sponsorFeeBalance(sponsor, alice);
-                printf("sponsorAvailableFee: %s\n", to_string(sponsorAvailableFee).c_str());
                 env(check::cancel(alice, uint256(1)),
                     fee(sponsorAvailableFee),
                     sponsor::as(sponsor, spfSponsorFee),