diff --git a/tests/test_msg_ethereum_signtx.py b/tests/test_msg_ethereum_signtx.py
index 15a61d3f..f7534375 100644
--- a/tests/test_msg_ethereum_signtx.py
+++ b/tests/test_msg_ethereum_signtx.py
@@ -381,6 +381,60 @@ def test_ethereum_eip_1559(self):
             "67297089e0ba53c29dda1aafc23fce64a772c5433e127e5885edc03ece4670c9",
         )
 
+    def test_ethereum_eip_1559_multibyte_chain_id(self):
+        """EIP-1559 must hash the WHOLE chain_id, not just its low byte.
+
+        Regression for the multi-byte chain_id bug: the EIP-1559 hash step used
+        hash_rlp_field((uint8_t*)&chain_id, 1), which on little-endian ARM fed
+        only the least-significant byte into keccak. For Base (8453 = 0x2105)
+        that hashed 0x05, so the signature recovered to an unrelated address
+        with no funds. The RLP *length* was computed correctly from the full
+        value, and the legacy EIP-155 path was always correct — only the
+        EIP-1559 hash was wrong. Affected: Base (8453), Arbitrum (42161),
+        Avalanche (43114). Unaffected: ETH (1), OP (10), BSC (56), Polygon (137).
+
+        Asserting a golden r/s would need a device run to produce it. Instead
+        sign one identical transaction under two chain ids chosen so the BUGGY
+        firmware cannot tell them apart:
+
+            8453 = 0x2105   low byte 0x05, two-byte value
+            4357 = 0x1105   low byte 0x05, two-byte value
+
+        Same low byte AND same RLP length header, so the broken code hashes a
+        byte-identical pre-image for both. Signing is deterministic (RFC 6979),
+        so buggy firmware returns the SAME signature twice and this fails.
+        Correct firmware hashes 0x21 0x05 vs 0x11 0x05, which must differ.
+
+        Vault cannot reach this path: it forces legacy txs for chainId >= 256
+        (eip1559Ok = chainId < 256). The BEX builds type-2 with no such guard,
+        which is why the failures only ever showed up there.
+        """
+        self.requires_fullFeature()
+        self.requires_firmware("7.15.0")
+        self.setup_mnemonic_nopin_nopassphrase()
+
+        def sign(chain_id):
+            return self.client.ethereum_sign_tx(
+                n=[0x80000000 | 44, 0x80000000 | 60, 0x80000000, 0, 0],
+                nonce=0,
+                gas_limit=0x5ac3,
+                max_fee_per_gas=0x16854be509,
+                max_priority_fee_per_gas=0x540ae480,
+                to=binascii.unhexlify("fc0cc6e85dff3d75e3985e0cb83b090cfd498dd1"),
+                value=0x1550f7dca70000,
+                chain_id=chain_id,
+            )
+
+        _, base_r, base_s = sign(8453)
+        _, twin_r, twin_s = sign(4357)
+
+        self.assertNotEqual(
+            (binascii.hexlify(base_r), binascii.hexlify(base_s)),
+            (binascii.hexlify(twin_r), binascii.hexlify(twin_s)),
+            "chain_id 8453 and 4357 produced the same signature — only the low "
+            "byte of chain_id reached the EIP-1559 hash",
+        )
+
     def test_ethereum_signtx_nodata_eip_1559(self):
         self.requires_fullFeature()
         self.requires_firmware("7.2.1")