diff --git a/platformio.ini b/platformio.ini
index c6fd1006..41287a9a 100644
--- a/platformio.ini
+++ b/platformio.ini
@@ -261,7 +261,9 @@ lib_ignore = iLabs Hearth
 
 [stm32_base]
 extends = arduino_base
-platform = ststm32
+; RadioLib's pinned STM32WL HAL uses the Arduino 2.x GPIO API. STM32 platform
+; 20 / Arduino 3 changes those signatures; keep release builds reproducible.
+platform = ststm32@19.4.0
 extra_scripts = post:arch/stm32/build_hex.py
   post:scripts/check_firmware_ram.py
 build_flags = ${arduino_base.build_flags}
diff --git a/test/test_stm32_float_conversion.py b/test/test_stm32_float_conversion.py
index ee3ff7d8..f8ab443b 100644
--- a/test/test_stm32_float_conversion.py
+++ b/test/test_stm32_float_conversion.py
@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 """Compile the real text helpers with STM32's utoa-only Arduino API."""
 from pathlib import Path
+import configparser
 import subprocess
 import tempfile
 import unittest
@@ -9,6 +10,11 @@ ROOT = Path(__file__).resolve().parents[1]
 
 
 class Stm32FloatConversionTest(unittest.TestCase):
+    def test_stm32_platform_keeps_the_supported_radiolib_gpio_api(self):
+        config = configparser.ConfigParser(interpolation=None, strict=False)
+        config.read(ROOT / "platformio.ini")
+        self.assertEqual(config["stm32_base"]["platform"], "ststm32@19.4.0")
+
     def test_signed_values_and_limits_without_ltoa(self):
         with tempfile.TemporaryDirectory() as directory:
             root = Path(directory)