diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index 6aa34b2..41ec6ec 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -113,6 +113,43 @@ sensors (e.g. a serial/analog EGT/oil-PSI module) can feed extra channels later.
 - **PID discovery scan** in GUI (the brute-scan, auto-add hits).
 - Units toggle, night theme, big-touch cab mode.
 
+## Cross-platform support
+
+Target: **Windows, macOS, Linux** from one codebase. The whole stack is
+portable — `PySide6`, `pyqtgraph`, `numpy`, `pyserial` all ship wheels for
+all three (incl. Apple Silicon). `obdcore` is pure Python/IO with no
+OS-specific calls; the only platform code is the terminal dashboard's
+ANSI/key handling, guarded `os.name == "nt"` (Windows) vs `termios` (POSIX =
+macOS + Linux). Rules to keep it that way:
+
+- **Serial enumeration** goes through `serial.tools.list_ports` only — never
+  assume `COMx`. Port names differ per OS (`COM5` / `/dev/cu.usbserial-*` /
+  `/dev/ttyUSB0`); the GUI shows a dropdown from `find_ports()`, no typing.
+- **Paths**: `pathlib`/caller-supplied only; no hardcoded separators or drives.
+- **No shelling out** to OS tools in core; no `os.system`. (The Windows VT
+  enable via `ctypes.windll` is the one exception, fully guarded.)
+- **Config/recordings** live under a per-OS app-data dir
+  (`platformdirs`) rather than next to the script.
+
+Three seams that are inherently per-OS (not code we can unify):
+
+1. **CH340 USB driver** (the adapter, not our app):
+   - **Windows** — install WCH `CH341SER`.
+   - **macOS** — install the WCH `CH34xVCPDriver` (Mac App Store / WCH);
+     recent macOS bundles a CH34x driver but clones often need WCH's.
+     Port shows as `/dev/cu.wchusbserial*` or `/dev/cu.usbserial*`.
+   - **Linux** — `ch341` is in the kernel; zero install. Just add the user to
+     the `dialout` group for `/dev/ttyUSB0` access.
+2. **Packaging** — PyInstaller is per-OS (no cross-compile): build the `.exe`
+   on Windows, the `.app`/`.dmg` on macOS, an ELF/AppImage on Linux. We have
+   all three hosts available, or do it in CI. (Briefcase is the alternative if
+   we want macOS notarization/signing handled.)
+3. **macOS Gatekeeper / Windows SmartScreen** — unsigned builds warn on first
+   run; optional code-signing/notarization later (P5).
+
+Primary dev/use target is the Windows laptop; macOS/Linux are first-class for
+development (build the GUI against `MockLink` on any box) and supported targets.
+
 ## Roadmap
 
 - **P0 — core (this commit):** `obdcore` package + tests + this doc. *Next:*
@@ -130,5 +167,8 @@ sensors (e.g. a serial/analog EGT/oil-PSI module) can feed extra channels later.
 ## Dependencies
 
 - Runtime (core): `pyserial`.
-- GUI: `PySide6`, `pyqtgraph`, `numpy`.
+- GUI: `PySide6`, `pyqtgraph`, `numpy`, `platformdirs` (per-OS config/recording dirs).
 - Dev: `pytest`, `pyinstaller`.
+
+All are cross-platform with wheels for Windows / macOS (incl. Apple Silicon) /
+Linux, so `pip install` is the same everywhere.
diff --git a/README.md b/README.md
index ea2351a..95119f3 100644
--- a/README.md
+++ b/README.md
@@ -16,11 +16,22 @@ reading/clearing codes and the basics, not Ford-enhanced diesel PIDs (see Scope
 - Key **live values** (coolant, IAT, MAP, module voltage, RPM, load, throttle) + battery voltage
 - 6.0 Power Stroke **no-start triage** checklist (FICM, ICP, cam/crank, batteries, fuel)
 
-## Setup (Windows)
+## Setup
 
-1. Install the CH340 driver (WCH `CH341SER`) so the adapter appears as
-   `USB-SERIAL CH340 (COMx)` in Device Manager → Ports.
-2. Install Python from <https://www.python.org/downloads/> — tick **Add Python to PATH**.
+Runs on **Windows, macOS, and Linux** (Python + pyserial). The only per-OS
+difference is the CH340 USB driver:
+
+- **Windows** — install WCH `CH341SER`; adapter shows as `USB-SERIAL CH340 (COMx)`
+  in Device Manager → Ports. Install Python from <https://www.python.org/downloads/>
+  (tick **Add Python to PATH**), or just double-click `RUN_OBD.bat`.
+- **macOS** — install WCH `CH34xVCPDriver` (Mac App Store or wch.cn). Port appears
+  as `/dev/cu.wchusbserial*`. `pip install pyserial`.
+- **Linux** — `ch341` driver is built into the kernel (no install). Port is
+  `/dev/ttyUSB0`; add yourself to the `dialout` group for access
+  (`sudo usermod -aG dialout $USER`, then re-login). `pip install pyserial`.
+
+The tool auto-detects the port on all three; pass it explicitly if needed
+(`COM5`, `/dev/cu.usbserial-1420`, `/dev/ttyUSB0`).
 
 ## Usage