diff --git a/requirements.txt b/requirements.txt
index 80dc0d96..defec383 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -48,3 +48,25 @@ pytest>=7.4.0,<8.0.0
 pytest-cov>=4.1.0,<5.0.0
 pytest-mock>=3.11.0,<4.0.0
 mypy>=1.5.0,<2.0.0
+
+# ───────────────────────────────────────────────────────────────────────
+# Optional dependencies — the code imports these inside try/except
+# blocks and gracefully degrades when missing. Install them for the
+# full feature set, or skip them for a minimal install.
+# ───────────────────────────────────────────────────────────────────────
+#
+# scipy             — sub-pixel interpolation in
+#                     src/common/scroll_helper.py for smoother
+#                     scrolling. Falls back to a simpler shift algorithm.
+#   pip install 'scipy>=1.10.0,<2.0.0'
+#
+# psutil            — per-plugin resource monitoring in
+#                     src/plugin_system/resource_monitor.py. The monitor
+#                     silently no-ops when missing (PSUTIL_AVAILABLE = False).
+#   pip install 'psutil>=5.9.0,<6.0.0'
+#
+# Flask-Limiter     — request rate limiting in web_interface/app.py
+#                     (accidental-abuse protection, not security). The
+#                     web interface starts without rate limiting when
+#                     this is missing.
+#   pip install 'Flask-Limiter>=3.5.0,<4.0.0'