diff --git a/.github/workflows/run-unit-tests.yml b/.github/workflows/run-unit-tests.yml
index 7ef970b9c..79aed4f28 100644
--- a/.github/workflows/run-unit-tests.yml
+++ b/.github/workflows/run-unit-tests.yml
@@ -96,6 +96,7 @@ jobs:
 
       - name: Verify compact STM32 signature arithmetic
         run: |
+          python3 -B test/test_stm32_companion_profile.py -v
           python3 -B test/test_ed25519_compact_scalar.py -v
           python3 -B test/test_ed25519_compact.py -v
 
diff --git a/test/test_stm32_companion_profile.py b/test/test_stm32_companion_profile.py
new file mode 100644
index 000000000..aa82cd8fb
--- /dev/null
+++ b/test/test_stm32_companion_profile.py
@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+"""Guard the headless RAK Companion's size recipe and deployed storage contract.
+
+The stm32-companion-smoke job also links the actual image. These checks prevent
+fixing its size limit by silently shrinking storage or Companion capacities.
+"""
+
+import configparser
+from pathlib import Path
+import re
+import unittest
+
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+class STM32CompanionProfileTest(unittest.TestCase):
+    def setUp(self):
+        self.config = configparser.ConfigParser(
+            interpolation=None, inline_comment_prefixes=(";",))
+        self.config.read(ROOT / "variants/rak3x72/platformio.ini")
+        self.companion = self.config["env:RAK_3x72_companion_radio_usb"]
+
+    def test_unused_timer_wrappers_are_removed_only_from_headless_companion(self):
+        for flag in ("RADIOLIB_TONE_UNSUPPORTED", "HAL_TIM_MODULE_ONLY"):
+            self.assertRegex(self.companion["build_flags"], r"-D\s+" + flag + r"\b")
+            for section in self.config.sections():
+                if section != self.companion.name:
+                    self.assertNotIn(flag, self.config[section].get("build_flags", ""))
+
+    def test_flash_and_filesystem_boundaries_do_not_move(self):
+        self.assertNotIn("board_upload.maximum_size", self.companion)
+        self.assertEqual(self.config["rak3x72"].getint("board_upload.maximum_size"),
+                         229376)
+        self.assertNotIn("LFS_FLASH_TOTAL_SIZE", self.companion["build_flags"])
+        self.assertNotIn("LFS_FLASH_TOTAL_SIZE", self.config["rak3x72"]["build_flags"])
+        header = (ROOT / "src/helpers/stm32/InternalFileSystem.h").read_text()
+        self.assertRegex(header, r"#define\s+LFS_FLASH_TOTAL_SIZE\s+\(16\s*\*\s*2048\)")
+        self.assertEqual(229376 + 16 * 2048, 256 * 1024)
+
+    def test_companion_capacities_and_interfaces_are_retained(self):
+        flags = self.companion["build_flags"]
+        for definition in ("MAX_CONTACTS=100", "MAX_GROUP_CHANNELS=8",
+                           "ENABLE_USB_INTERFACE"):
+            self.assertRegex(flags, r"-D\s+" + re.escape(definition) + r"\b")
+        for flags in (flags, self.config["rak3x72"]["build_flags"]):
+            self.assertNotRegex(flags, r"COMPANION_FEATURE_\w+=0")
+            self.assertNotRegex(flags, r"MESH_ENABLE_\w+=0")
+        self.assertIn("+<../examples/companion_radio/*.cpp>",
+                      self.companion["build_src_filter"])
+        self.assertIn("-Oz", self.companion["build_flags"])
+        self.assertEqual(self.companion["build_unflags"], "-Os")
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/variants/rak3x72/platformio.ini b/variants/rak3x72/platformio.ini
index ae47ba55a..ddd87e295 100644
--- a/variants/rak3x72/platformio.ini
+++ b/variants/rak3x72/platformio.ini
@@ -66,6 +66,10 @@ extends = rak3x72
 platform_packages = platformio/toolchain-gccarmnoneeabi@^1.140201.0
 build_unflags = -Os
 build_flags = ${rak3x72.build_flags}
+  ; This headless UART Companion uses GPIO, ADC and SysTick, not PWM/tone.
+  ; Drop unused Arduino timer wrappers, retaining the low-level STM32 TIM HAL.
+  -D RADIOLIB_TONE_UNSUPPORTED
+  -D HAL_TIM_MODULE_ONLY
   -D ED25519_COMPACT_SHA512=1 ; same hash rounds, compact loop preserves the filesystem boundary
   -D ED25519_COMPACT_SCALAR=1 ; same ref10 arithmetic with fixed loops instead of unrolled code
   -Oz