diff --git a/kota_tech/serialio/manager.py b/kota_tech/serialio/manager.py
index 93095fd..18692a9 100644
--- a/kota_tech/serialio/manager.py
+++ b/kota_tech/serialio/manager.py
@@ -10,6 +10,7 @@ from __future__ import annotations
 
 import enum
 import queue
+import sys
 import threading
 import time
 from collections.abc import Callable
@@ -92,13 +93,21 @@ class SerialManager:
         ser = serial.serial_for_url(
             info.device, baudrate=self._baud, timeout=READ_TIMEOUT_S, do_not_open=True
         )
-        # Deassert both lines before open. On USB-Serial-JTAG, RTS=1/DTR=0
-        # holds the chip in reset and DTR=1 during reset selects download
-        # mode; pyserial would otherwise assert both on open (Windows).
-        ser.dtr = False
+        # On USB-Serial-JTAG, RTS=1/DTR=0 holds the chip in reset and DTR=1
+        # during reset selects download mode.
+        # Windows: pyserial applies both lines atomically at open, so
+        # deassert both beforehand (it would otherwise assert both).
+        # macOS/Linux: the OS asserts both lines on open and pyserial then
+        # writes DTR before RTS, passing through RTS=1/DTR=0 and resetting
+        # the box on every connect. Drop RTS first, then DTR.
+        posix = sys.platform != "win32"
+        if not posix:
+            ser.dtr = False
         ser.rts = False
         try:
             ser.open()
+            if posix:
+                ser.dtr = False
         except (serial.SerialException, OSError):
             return None
         self._target.device = info.device  # COM number / node may change
diff --git a/tests/test_manager.py b/tests/test_manager.py
index ad11fdf..8b832ba 100644
--- a/tests/test_manager.py
+++ b/tests/test_manager.py
@@ -86,6 +86,46 @@ def test_hard_reset_pushes_rts_via_dtr_write(monkeypatch):
     assert all(c[1] is False for c in ser.calls if c[0] == "dtr")
 
 
+class _PosixOpenSerial:
+    """pyserial on macOS/Linux: the OS asserts both lines on open, then
+    pyserial writes the requested DTR, then RTS. Records every line state."""
+
+    def __init__(self, *args, **kwargs):
+        self.port = args[0] if args else "fake"
+        self.dtr = self.rts = True  # pyserial defaults
+        self.states = []
+
+    def __setattr__(self, name, value):
+        object.__setattr__(self, name, value)
+        if name in ("dtr", "rts") and getattr(self, "is_open", False):
+            self.states.append((self.dtr, self.rts))
+
+    def open(self):
+        want_dtr, want_rts = self.dtr, self.rts
+        object.__setattr__(self, "dtr", True)
+        object.__setattr__(self, "rts", True)
+        self.is_open = True
+        self.states.append((True, True))
+        self.dtr = want_dtr
+        self.rts = want_rts
+
+
+def test_open_on_posix_never_passes_through_reset(monkeypatch):
+    monkeypatch.setattr(mgr_mod.sys, "platform", "darwin")
+    monkeypatch.setattr(mgr_mod, "find_port", lambda sn, dev: LOOP)
+    opened = []
+    monkeypatch.setattr(
+        mgr_mod.serial, "serial_for_url",
+        lambda *a, **k: opened.append(_PosixOpenSerial(*a, **k)) or opened[-1],
+    )
+    m = SerialManager(lambda ln: None, lambda s, d: None)
+    m._target = mgr_mod._Target(LOOP.device, None)
+    ser = m._open()
+    # (DTR, RTS) = (False, True) resets the chip on connect.
+    assert (False, True) not in ser.states
+    assert ser.states[-1] == (False, False)
+
+
 def test_waits_when_port_missing(monkeypatch):
     monkeypatch.setattr(mgr_mod, "find_port", lambda sn, dev: None)
     states, lines = [], []