diff --git a/test/unittests/statetest_loader_tx_test.cpp b/test/unittests/statetest_loader_tx_test.cpp
index efabfd3846..0dc739fdf8 100644
--- a/test/unittests/statetest_loader_tx_test.cpp
+++ b/test/unittests/statetest_loader_tx_test.cpp
@@ -74,6 +74,27 @@ TEST(statetest_loader, tx_create_legacy)
     EXPECT_EQ(tx.v, 1);
 }
 
+TEST(statetest_loader, tx_max_chain_id)
+{
+    // The maximum representable `chainId` (uint64 max = 0xffffffffffffffff) must be
+    // loaded without overflow or truncation.
+    constexpr std::string_view input = R"({
+        "input": "b0b1",
+        "gas": "0x9091",
+        "chainId": "0xffffffffffffffff",
+        "value": "0xe0e1",
+        "sender": "a0a1",
+        "gasPrice": "0x7071",
+        "nonce": "0",
+        "r": "0x1111111111111111111111111111111111111111111111111111111111111111",
+        "s": "0x2222222222222222222222222222222222222222222222222222222222222222",
+        "v": "1"
+    })";
+
+    const auto tx = test::from_json<state::Transaction>(json::json::parse(input));
+    EXPECT_EQ(tx.chain_id, std::numeric_limits<uint64_t>::max());
+}
+
 TEST(statetest_loader, tx_eip1559)
 {
     constexpr std::string_view input = R"({
diff --git a/test/unittests/tooling_t8n_test.cpp b/test/unittests/tooling_t8n_test.cpp
index 129f068e57..b45453716f 100644
--- a/test/unittests/tooling_t8n_test.cpp
+++ b/test/unittests/tooling_t8n_test.cpp
@@ -210,3 +210,41 @@ TEST(tooling_t8n, mismatched_tx_hash_throws)
 
     EXPECT_THROW(tooling::t8n(vm, args), std::logic_error);
 }
+
+TEST(tooling_t8n, max_chain_id)
+{
+    evmc::VM vm{evmc_create_evmone()};
+
+    // The maximum `chainId` (uint64 max = 0xffffffffffffffff) must be parsed and
+    // executed without overflow; regression test for `chainId` being loaded as
+    // `uint8_t`, which threw `from_json<uint8_t>: value > 0xFF`.
+    static constexpr auto TX_MAX_CHAIN_ID = R"([{
+        "to": null,
+        "input": "0x60015ff3",
+        "gas": "0x186a0",
+        "nonce": "0x0",
+        "value": "0x0",
+        "gasPrice": "0x32",
+        "chainId": "0xffffffffffffffff",
+        "sender": "0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b",
+        "v": "0x1b",
+        "r": "0x468a915f087692bb9be503831a3dfef2cf9c8dee26deb40ff2ec99e8d22665ae",
+        "s": "0x5cedae0810c3851ecd1004bfdbfe6ddc7753c2d665993bb01ce75af7857b13dc"
+    }])";
+
+    std::istringstream env{ENV_JSON};
+    std::istringstream alloc{ALLOC_JSON};
+    std::istringstream txs{TX_MAX_CHAIN_ID};
+    std::ostringstream out_result;
+
+    tooling::T8NArgs args;
+    args.rev = EVMC_SHANGHAI;
+    args.chain_id = std::numeric_limits<uint64_t>::max();
+    args.alloc = &alloc;
+    args.env = &env;
+    args.txs = &txs;
+    args.out_result = &out_result;
+
+    EXPECT_NO_THROW(tooling::t8n(vm, args));
+    EXPECT_THAT(out_result.str(), HasSubstr("\"transactionHash\""));
+}
diff --git a/test/utils/statetest_loader.cpp b/test/utils/statetest_loader.cpp
index b503f8d3ea..6ed2302696 100644
--- a/test/utils/statetest_loader.cpp
+++ b/test/utils/statetest_loader.cpp
@@ -375,7 +375,7 @@ static void from_json_tx_common(const json::json& j, state::Transaction& o)
     o.nonce = from_json<uint64_t>(j.at("nonce"));
 
     if (const auto chain_id_it = j.find("chainId"); chain_id_it != j.end())
-        o.chain_id = from_json<uint8_t>(*chain_id_it);
+        o.chain_id = from_json<uint64_t>(*chain_id_it);
     else
         o.chain_id = 1;