From 673c54061c5bb957023e5aa9d042ab857843dc6b Mon Sep 17 00:00:00 2001
From: William Morriss <wjmelements@gmail.com>
Date: Thu, 7 May 2026 19:41:54 -0500
Subject: [PATCH] feat(eth-rpc): implement eth_baseFee returning next block
 base fee

Assisted-by: Claude:claude-sonnet-4-6
---
 .../Modules/Eth/EthRpcModuleTests.GasPrice.cs | 46 +++++++++++++++++++
 .../Modules/Eth/EthRpcModule.cs               | 17 +++++++
 .../Modules/Eth/IEthRpcModule.cs              |  6 +++
 3 files changed, 69 insertions(+)

diff --git a/src/Nethermind/Nethermind.JsonRpc.Test/Modules/Eth/EthRpcModuleTests.GasPrice.cs b/src/Nethermind/Nethermind.JsonRpc.Test/Modules/Eth/EthRpcModuleTests.GasPrice.cs
index a18bbf283ba..b15f468b183 100644
--- a/src/Nethermind/Nethermind.JsonRpc.Test/Modules/Eth/EthRpcModuleTests.GasPrice.cs
+++ b/src/Nethermind/Nethermind.JsonRpc.Test/Modules/Eth/EthRpcModuleTests.GasPrice.cs
@@ -52,6 +52,52 @@ public async Task<string> Eth_blobBaseFee_ShouldGiveCorrectResult(ulong? excessB
         return await ctx.Test.TestEthRpc("eth_blobBaseFee");
     }
 
+    [TestCaseSource(nameof(GetBaseFeeTestCases))]
+    public async Task<string> Eth_baseFee_ShouldGiveCorrectResult(UInt256 baseFeePerGas, long gasLimit, long gasUsed, bool londonEnabled)
+    {
+        ISpecProvider specProvider = londonEnabled
+            ? new TestSpecProvider(London.Instance)
+            : GetSpecProviderWithEip1559EnabledAs(false);
+        using Context ctx = await Context.Create(specProvider);
+        Block[] blocks = [
+            Build.A.Block.WithNumber(0).WithBaseFeePerGas(baseFeePerGas).WithGasLimit(gasLimit).WithGasUsed(gasUsed).TestObject,
+        ];
+        BlockTree blockTree = Build.A.BlockTree(blocks[0]).WithBlocks(blocks).TestObject;
+        ctx.Test = await TestRpcBlockchain.ForTest(SealEngineType.NethDev).WithBlockFinder(blockTree).Build(specProvider);
+
+        return await ctx.Test.TestEthRpc("eth_baseFee");
+    }
+
+    public static IEnumerable<TestCaseData> GetBaseFeeTestCases
+    {
+        get
+        {
+            static string Success(UInt256 result) => $"{{\"jsonrpc\":\"2.0\",\"result\":\"{result.ToHexString(true)}\",\"id\":67}}";
+            const string NullResult = "{\"jsonrpc\":\"2.0\",\"result\":null,\"id\":67}";
+
+            yield return new TestCaseData(UInt256.Zero, 30_000_000L, 0L, false)
+            {
+                TestName = "Pre-London block returns null",
+                ExpectedResult = NullResult
+            };
+            yield return new TestCaseData((UInt256)1_000_000_000, 30_000_000L, 15_000_000L, true)
+            {
+                TestName = "Block at gas target returns same base fee",
+                ExpectedResult = Success(1_000_000_000)
+            };
+            yield return new TestCaseData((UInt256)1_000_000_000, 30_000_000L, 30_000_000L, true)
+            {
+                TestName = "Block over gas target increases base fee by 12.5%",
+                ExpectedResult = Success(1_125_000_000)
+            };
+            yield return new TestCaseData((UInt256)1_000_000_000, 30_000_000L, 0L, true)
+            {
+                TestName = "Block under gas target decreases base fee by 12.5%",
+                ExpectedResult = Success(875_000_000)
+            };
+        }
+    }
+
     [TestCase(true, "0x3")] //Gas Prices: 1,2,3,3,4,5 | Max Index: 5 | 60th Percentile: 5 * (3/5) = 3 | Result: 3 (0x3)
     [TestCase(false, "0x2")] //Gas Prices: 0,1,1,2,2,3 | Max Index: 5 | 60th Percentile: 5 * (3/5) = 3 | Result: 2 (0x2)
     public async Task Eth_gasPrice_BlocksAvailableLessThanBlocksToCheckWith1559Tx_ShouldGiveCorrectResult(bool eip1559Enabled, string expected)
diff --git a/src/Nethermind/Nethermind.JsonRpc/Modules/Eth/EthRpcModule.cs b/src/Nethermind/Nethermind.JsonRpc/Modules/Eth/EthRpcModule.cs
index d8e98d7959a..9e6d60732ff 100644
--- a/src/Nethermind/Nethermind.JsonRpc/Modules/Eth/EthRpcModule.cs
+++ b/src/Nethermind/Nethermind.JsonRpc/Modules/Eth/EthRpcModule.cs
@@ -121,6 +121,23 @@ public ResultWrapper<string> eth_protocolVersion()
         return ResultWrapper<UInt256?>.Success(feePerBlobGas);
     }
 
+    public ResultWrapper<UInt256?> eth_baseFee()
+    {
+        BlockHeader? head = _blockFinder.Head?.Header;
+        if (head is null)
+        {
+            return ResultWrapper<UInt256?>.Success(null);
+        }
+
+        IEip1559Spec specFor1559 = _specProvider.GetSpecFor1559(head.Number + 1);
+        if (!specFor1559.IsEip1559Enabled)
+        {
+            return ResultWrapper<UInt256?>.Success(null);
+        }
+
+        return ResultWrapper<UInt256?>.Success(BaseFeeCalculator.Calculate(head, specFor1559));
+    }
+
     public ResultWrapper<UInt256?> eth_maxPriorityFeePerGas()
     {
         UInt256 gasPriceWithBaseFee = _gasPriceOracle.GetMaxPriorityGasFeeEstimate();
diff --git a/src/Nethermind/Nethermind.JsonRpc/Modules/Eth/IEthRpcModule.cs b/src/Nethermind/Nethermind.JsonRpc/Modules/Eth/IEthRpcModule.cs
index cf75526583c..6bbd60429c8 100644
--- a/src/Nethermind/Nethermind.JsonRpc/Modules/Eth/IEthRpcModule.cs
+++ b/src/Nethermind/Nethermind.JsonRpc/Modules/Eth/IEthRpcModule.cs
@@ -70,6 +70,12 @@ public interface IEthRpcModule : IRpcModule
             ExampleResponse = "0x1")]
         ResultWrapper<UInt256?> eth_blobBaseFee();
 
+        [JsonRpcMethod(IsImplemented = true,
+            Description = "Returns the base fee of the next block in wei",
+            IsSharable = true,
+            ExampleResponse = "0x3b9aca00")]
+        ResultWrapper<UInt256?> eth_baseFee();
+
         [JsonRpcMethod(IsImplemented = false,
             Description = "Returns accounts",
             IsSharable = true,