diff --git a/bin/optional-modules.py b/bin/optional-modules.py
new file mode 100644
index 0000000000..443a608084
--- /dev/null
+++ b/bin/optional-modules.py
@@ -0,0 +1,66 @@
+#!/usr/bin/env python3
+# trunk-ignore-all(ruff/F821)
+# trunk-ignore-all(flake8/F821): For SConstruct imports
+#
+# Registers optional modules dropped into src/modules/optional/.
+#
+# A module is a directory <Name>/ holding <Name>.h, which declares `void setup<Name>();`. This
+# writes $BUILD_DIR/OptionalModules.h with an include and a setup call for each one found;
+# Modules.cpp picks that header up through __has_include and calls OPTIONAL_MODULES_SETUP().
+#
+# src/modules/optional/ does not exist in a stock checkout, so a stock build generates a header that
+# defines nothing and registers nothing. Sources under the directory are compiled by the default
+# recursive build_src_filter, so dropping a module in needs no platformio.ini edit.
+import os
+import re
+
+Import("env")
+
+# The directory name becomes a C++ call, so it has to be a usable identifier.
+identifier = re.compile(r"^[A-Za-z_][A-Za-z0-9_]*$")
+
+optionalDir = os.path.join(env["PROJECT_DIR"], "src", "modules", "optional")
+
+names = []
+if os.path.isdir(optionalDir):
+    for entry in sorted(os.listdir(optionalDir)):
+        entryDir = os.path.join(optionalDir, entry)
+        if not os.path.isdir(entryDir):
+            continue
+        if not identifier.match(entry):
+            print(f"optional-modules: skipping {entry}/, setup{entry}() is not a valid identifier")
+        elif os.path.isfile(os.path.join(entryDir, entry + ".h")):
+            names.append(entry)
+        else:
+            print(f"optional-modules: skipping {entry}/, no {entry}.h")
+
+lines = ["// Generated by bin/optional-modules.py. Do not edit.", "#pragma once", ""]
+for name in names:
+    lines.append(f'#include "modules/optional/{name}/{name}.h"')
+if names:
+    lines.append("")
+    lines.append("#define OPTIONAL_MODULES_SETUP() \\")
+    lines.append("    do { \\")
+    for name in names:
+        lines.append(f"        setup{name}(); \\")
+    lines.append("    } while (0)")
+lines.append("")
+content = "\n".join(lines)
+
+buildDir = env.subst("$BUILD_DIR")
+os.makedirs(buildDir, exist_ok=True)
+header = os.path.join(buildDir, "OptionalModules.h")
+
+# Rewrite only on a change, so an unchanged set of modules does not keep rebuilding Modules.cpp.
+previous = None
+if os.path.isfile(header):
+    with open(header, encoding="utf-8") as f:
+        previous = f.read()
+if previous != content:
+    with open(header, "w", encoding="utf-8") as f:
+        f.write(content)
+
+env.Append(CPPPATH=[buildDir])
+
+if names:
+    print("optional-modules: " + ", ".join(names))
diff --git a/platformio.ini b/platformio.ini
index c819994102..64d0306aef 100644
--- a/platformio.ini
+++ b/platformio.ini
@@ -25,6 +25,7 @@ build_flags =
 test_build_src = true
 extra_scripts =
 	pre:bin/platformio-pre.py
+	pre:bin/optional-modules.py
 	bin/platformio-custom.py
 	post:extra_scripts/nrf54l15_linker.py
 ; note: we add src to our include search path so that lmic_project_config can override
diff --git a/src/modules/Modules.cpp b/src/modules/Modules.cpp
index d05ac35a34..1c32a0ab2e 100644
--- a/src/modules/Modules.cpp
+++ b/src/modules/Modules.cpp
@@ -34,6 +34,11 @@
 #if !MESHTASTIC_EXCLUDE_BEACON
 #include "modules/MeshBeaconModule.h"
 #endif
+// Generated by bin/optional-modules.py from whatever sits in src/modules/optional/. Absent from a
+// stock checkout, and empty unless a module has been dropped in.
+#if __has_include("OptionalModules.h")
+#include "OptionalModules.h"
+#endif
 #if !MESHTASTIC_EXCLUDE_GPS
 #include "modules/PositionModule.h"
 #endif
@@ -283,6 +288,11 @@ void setupModules()
 #endif
 #if defined(HAS_HARDWARE_WATCHDOG)
     watchdogThread = new WatchdogThread();
+#endif
+    // Anything dropped into src/modules/optional/. Undefined, so compiled away, unless a module is
+    // actually present.
+#ifdef OPTIONAL_MODULES_SETUP
+    OPTIONAL_MODULES_SETUP();
 #endif
     // NOTE! This module must be added LAST because it likes to check for replies from other modules and avoid sending extra
     // acks