From 847a8df7dc764e4d6208ca365cf17fcde058a68e Mon Sep 17 00:00:00 2001
From: Danswar <48102227+Danswar@users.noreply.github.com>
Date: Mon, 20 Jul 2026 10:34:20 -0300
Subject: [PATCH] fix(bitcoin): clamp send fee rate to the node's minimum relay
 fee (#4289)

estimateSmartFee can return an estimate below the node's own minimum
relay fee during periods of very low mempool activity, since Bitcoin
Core's historical-inclusion estimator isn't bounded by the live
relay-policy floor. Once the configured safety multiplier is applied,
the resulting fee rate could still round to a value under 1 sat/vB,
causing the node to reject the broadcast with "Fee rate is lower than
the minimum fee rate setting".

Clamp the value returned by getSendFeeRate() to a 1 sat/vB floor
before it is handed to the send RPC. The unclamped estimate from
getRecommendedFeeRate() is left untouched since it's also used for
quoting/display. This is fixed in the shared base class, so it
applies to both Bitcoin and Firo payouts.
---
 .../__tests__/bitcoin-fee.service.spec.ts     | 46 +++++++++++++++++++
 .../services/bitcoin-based-fee.service.ts     |  8 +++-
 2 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/src/integration/blockchain/bitcoin/services/__tests__/bitcoin-fee.service.spec.ts b/src/integration/blockchain/bitcoin/services/__tests__/bitcoin-fee.service.spec.ts
index dddb323452..2ef1755f0d 100644
--- a/src/integration/blockchain/bitcoin/services/__tests__/bitcoin-fee.service.spec.ts
+++ b/src/integration/blockchain/bitcoin/services/__tests__/bitcoin-fee.service.spec.ts
@@ -126,6 +126,52 @@ describe('BitcoinFeeService', () => {
     });
   });
 
+  // --- getSendFeeRate() minimum fee rate floor Tests --- //
+
+  describe('getSendFeeRate() minimum fee rate floor', () => {
+    const feeConfig = { allowUnconfirmedUtxos: false, cpfpFeeMultiplier: 2.0, defaultFeeMultiplier: 1.5 };
+
+    beforeEach(() => {
+      Object.defineProperty(service, 'feeConfig', { get: () => feeConfig });
+    });
+
+    it('should leave a fee rate comfortably above the minimum relay fee unaffected', async () => {
+      mockClient.estimateSmartFee.mockResolvedValueOnce(10); // 10 * defaultFeeMultiplier (1.5) = 15
+
+      const result = await service.getSendFeeRate();
+
+      expect(result).toBe(15);
+    });
+
+    it('should clamp a below-floor estimate to the minimum relay fee', async () => {
+      // Reproduces a production incident: estimateSmartFee returned 0.664 sat/vB during a
+      // period of very low mempool activity. Even after the default safety multiplier
+      // (0.664 * 1.5 = 0.996), the result still undercut the node's 1 sat/vB minimum relay
+      // fee, so the broadcast was rejected. The clamp must raise this to exactly 1.
+      mockClient.estimateSmartFee.mockResolvedValueOnce(0.664);
+
+      const result = await service.getSendFeeRate();
+
+      expect(result).toBe(1);
+    });
+
+    it('should act as a floor and not override values already above the minimum', async () => {
+      mockClient.estimateSmartFee.mockResolvedValueOnce(0.7); // 0.7 * 1.5 = 1.05
+
+      const result = await service.getSendFeeRate();
+
+      expect(result).toBe(1.05);
+    });
+
+    it('should not clamp the underlying recommended fee rate, only the send fee rate', async () => {
+      mockClient.estimateSmartFee.mockResolvedValueOnce(0.664);
+
+      const recommended = await service.getRecommendedFeeRate();
+
+      expect(recommended).toBe(0.664);
+    });
+  });
+
   // --- getTxFeeRate() Tests --- //
 
   describe('getTxFeeRate()', () => {
diff --git a/src/integration/blockchain/bitcoin/services/bitcoin-based-fee.service.ts b/src/integration/blockchain/bitcoin/services/bitcoin-based-fee.service.ts
index f6ae0f5750..8bf81c9c79 100644
--- a/src/integration/blockchain/bitcoin/services/bitcoin-based-fee.service.ts
+++ b/src/integration/blockchain/bitcoin/services/bitcoin-based-fee.service.ts
@@ -16,6 +16,9 @@ export interface FeeConfig {
   defaultFeeMultiplier: number;
 }
 
+// Node's own minimum relay fee floor (sat/vB); broadcasts below this are rejected outright.
+const MIN_FEE_RATE_SAT_VB = 1;
+
 export abstract class BitcoinBasedFeeService {
   private readonly logger = new DfxLogger(BitcoinBasedFeeService);
 
@@ -92,6 +95,9 @@ export abstract class BitcoinBasedFeeService {
 
     // Bitcoin Core's send RPC parses fee_rate with decimals=3; un-rounded floating-point
     // products (e.g. 1.935 * 2 = 3.8699999999999997) are rejected with "Invalid amount".
-    return Util.round(baseRate * multiplier, 3);
+    // estimateSmartFee can also return an estimate below the node's own minimum relay fee
+    // during periods of very low mempool activity, so clamp the value that is actually
+    // broadcast to that floor; the raw estimate from getRecommendedFeeRate() stays unclamped.
+    return Math.max(Util.round(baseRate * multiplier, 3), MIN_FEE_RATE_SAT_VB);
   }
 }