diff --git a/.github/workflows/ci-tests.yml b/.github/workflows/ci-tests.yml
index 77bbb99..d9c2a3c 100644
--- a/.github/workflows/ci-tests.yml
+++ b/.github/workflows/ci-tests.yml
@@ -7,6 +7,34 @@ on:
     branches: [main, develop]
 
 jobs:
+  # ===========================================================================
+  # Job 0: Ruff Lint (active Python files only)
+  # Excludes legacy GUI_V*.py files and untracked v7/ directory
+  # ===========================================================================
+  lint:
+    name: Ruff Lint (active files)
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python 3.12
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install ruff
+        run: pip install ruff
+
+      - name: Run ruff on active files
+        run: |
+          ruff check \
+            9_Firmware/9_3_GUI/radar_protocol.py \
+            9_Firmware/9_3_GUI/radar_dashboard.py \
+            9_Firmware/9_3_GUI/smoke_test.py \
+            9_Firmware/9_3_GUI/test_radar_dashboard.py
+
   # ===========================================================================
   # Job 1: Python Host Software Tests (58 tests)
   # radar_protocol, radar_dashboard, FT2232H connection, replay, opcodes, e2e
diff --git a/9_Firmware/9_3_GUI/radar_dashboard.py b/9_Firmware/9_3_GUI/radar_dashboard.py
index 3c86074..aa7d81d 100644
--- a/9_Firmware/9_3_GUI/radar_dashboard.py
+++ b/9_Firmware/9_3_GUI/radar_dashboard.py
@@ -21,7 +21,6 @@ Usage:
   python radar_dashboard.py --record     # Launch with HDF5 recording
 """
 
-import sys
 import os
 import time
 import queue
@@ -45,7 +44,7 @@ from matplotlib.backends.backend_tkagg import FigureCanvasTkAgg
 from radar_protocol import (
     RadarProtocol, FT2232HConnection, ReplayConnection,
     DataRecorder, RadarAcquisition,
-    RadarFrame, StatusResponse, Opcode,
+    RadarFrame, StatusResponse,
     NUM_RANGE_BINS, NUM_DOPPLER_BINS, WATERFALL_DEPTH,
 )
 
diff --git a/9_Firmware/9_3_GUI/radar_protocol.py b/9_Firmware/9_3_GUI/radar_protocol.py
index af7adbb..85a5867 100644
--- a/9_Firmware/9_3_GUI/radar_protocol.py
+++ b/9_Firmware/9_3_GUI/radar_protocol.py
@@ -24,7 +24,7 @@ import logging
 from dataclasses import dataclass, field
 from typing import Optional, List, Tuple, Dict, Any
 from enum import IntEnum
-from collections import deque
+
 
 import numpy as np
 
@@ -764,13 +764,20 @@ class ReplayConnection:
                 dq = int(np.clip(dop_q[rbin, dbin], -32768, 32767))
                 d = 1 if det[rbin, dbin] else 0
 
-                buf[pos] = HEADER_BYTE; pos += 1
-                buf[pos:pos+2] = rq_bytes; pos += 2
-                buf[pos:pos+2] = ri_bytes; pos += 2
-                buf[pos:pos+2] = struct.pack(">h", di); pos += 2
-                buf[pos:pos+2] = struct.pack(">h", dq); pos += 2
-                buf[pos] = d; pos += 1
-                buf[pos] = FOOTER_BYTE; pos += 1
+                buf[pos] = HEADER_BYTE
+                pos += 1
+                buf[pos:pos+2] = rq_bytes
+                pos += 2
+                buf[pos:pos+2] = ri_bytes
+                pos += 2
+                buf[pos:pos+2] = struct.pack(">h", di)
+                pos += 2
+                buf[pos:pos+2] = struct.pack(">h", dq)
+                pos += 2
+                buf[pos] = d
+                pos += 1
+                buf[pos] = FOOTER_BYTE
+                pos += 1
 
         return bytes(buf)
 
diff --git a/9_Firmware/9_3_GUI/smoke_test.py b/9_Firmware/9_3_GUI/smoke_test.py
index e0d2d2f..70e440c 100644
--- a/9_Firmware/9_3_GUI/smoke_test.py
+++ b/9_Firmware/9_3_GUI/smoke_test.py
@@ -27,7 +27,6 @@ Exit codes:
 import sys
 import os
 import time
-import struct
 import argparse
 import logging
 
diff --git a/9_Firmware/9_3_GUI/test_radar_dashboard.py b/9_Firmware/9_3_GUI/test_radar_dashboard.py
index 790bbef..6fe0410 100644
--- a/9_Firmware/9_3_GUI/test_radar_dashboard.py
+++ b/9_Firmware/9_3_GUI/test_radar_dashboard.py
@@ -21,7 +21,7 @@ from radar_protocol import (
     HEADER_BYTE, FOOTER_BYTE, STATUS_HEADER_BYTE,
     NUM_RANGE_BINS, NUM_DOPPLER_BINS, NUM_CELLS,
     DATA_PACKET_SIZE,
-    _HARDWARE_ONLY_OPCODES, _REPLAY_ADJUSTABLE_OPCODES,
+    _HARDWARE_ONLY_OPCODES,
 )