From 1b635766572aded2d54fe49a3cde95d99fa786b6 Mon Sep 17 00:00:00 2001
From: Vincent Gao <gaobing1230@gmail.com>
Date: Tue, 30 Jun 2026 09:42:33 +0200
Subject: [PATCH] Don't mutate caller's registers list in
 convert_from_registers

The STRING/BITS branch of convert_from_registers reversed the registers
list in place when word_order="little", corrupting the caller's input so
a second identical call decoded garbage (non-deterministic output). The
numeric branch already avoids this by reversing a slice copy; reverse a
copy here too so decoding is side-effect free.
---
 pymodbus/client/mixin.py   |  4 +++-
 test/client/test_client.py | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/pymodbus/client/mixin.py b/pymodbus/client/mixin.py
index a5c9d46d7..60859ada1 100644
--- a/pymodbus/client/mixin.py
+++ b/pymodbus/client/mixin.py
@@ -927,7 +927,9 @@ def convert_from_registers(  # noqa: C901
         if not (data_len := data_type.value[1]):
             byte_list = bytearray()
             if word_order == "little":
-                registers.reverse()
+                # Reverse a copy; the numeric branch below also works on a
+                # slice copy, so decoding must not mutate the caller's list.
+                registers = list(reversed(registers))
             for x in registers:
                 byte_list.extend(int.to_bytes(x, 2, "big"))
             if data_type == cls.DATATYPE.STRING:
diff --git a/test/client/test_client.py b/test/client/test_client.py
index faf791521..1b33a4dd0 100755
--- a/test/client/test_client.py
+++ b/test/client/test_client.py
@@ -366,6 +366,41 @@ def test_client_mixin_convert_1234(self, datatype, registers, value):
         assert result == value
         assert regs == registers
 
+    @pytest.mark.parametrize("word_order", ["big", "little"])
+    @pytest.mark.parametrize(
+        ("datatype", "value"),
+        [
+            (ModbusClientMixin.DATATYPE.STRING, "Hello"),
+            (ModbusClientMixin.DATATYPE.STRING, "abcdefghij"),
+            (ModbusClientMixin.DATATYPE.BITS, [True, False, True] + [False] * 17),
+            (ModbusClientMixin.DATATYPE.INT32, 123456),
+            (ModbusClientMixin.DATATYPE.INT32, [1, 2, 3, 4]),
+        ],
+    )
+    def test_client_mixin_convert_from_registers_no_mutation(
+        self, datatype, value, word_order
+    ):
+        """convert_from_registers must not mutate input and stay deterministic.
+
+        Regression: for STRING/BITS with word_order="little" the registers list
+        was reversed in place, corrupting the caller's data so a second identical
+        call decoded garbage (non-deterministic output).
+        """
+        registers = ModbusClientMixin.convert_to_registers(
+            value, datatype, word_order=word_order
+        )
+        snapshot = list(registers)
+        first = ModbusClientMixin.convert_from_registers(
+            registers, datatype, word_order=word_order
+        )
+        # Decoding must leave the caller's list untouched ...
+        assert registers == snapshot
+        # ... so repeating the identical call yields the identical result.
+        second = ModbusClientMixin.convert_from_registers(
+            registers, datatype, word_order=word_order
+        )
+        assert first == second
+
     def test_client_mixin_convert_fail(self):
         """Test convert fail."""
         with pytest.raises(TypeError):