diff --git a/HISTORY.rst b/HISTORY.rst
index e463857e..dafe0cae 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -15,6 +15,7 @@ PlatformIO Core 5
 - Improved checking of available Internet connection for IPv6-only workstations (`pull #4151 <https://github.com/platformio/platformio-core/pull/4151>`_)
 - Better detecting of default PlatformIO project directory on Linux OS (`pull #4158 <https://github.com/platformio/platformio-core/pull/4158>`_)
 - Respect disabling debugging server from "platformio.ini" passing an empty value to the `debug_server <https://docs.platformio.org/en/latest/projectconf/section_env_debug.html#debug-server>`__ option
+- Run library extra script only at a build process (`issue #3915 <https://github.com/platformio/platformio-core/issues/3915>`_)
 
 5.2.4 (2021-12-15)
 ~~~~~~~~~~~~~~~~~~
diff --git a/platformio/builder/tools/piolib.py b/platformio/builder/tools/piolib.py
index 3b57c1e7..f50724e7 100644
--- a/platformio/builder/tools/piolib.py
+++ b/platformio/builder/tools/piolib.py
@@ -285,15 +285,8 @@ class LibBuilderBase(object):
         return {}
 
     def process_extra_options(self):
-        with fs.cd(self.path):
-            self.env.ProcessFlags(self.build_flags)
-            if self.extra_script:
-                self.env.SConscriptChdir(1)
-                self.env.SConscript(
-                    os.path.abspath(self.extra_script),
-                    exports={"env": self.env, "pio_lib_builder": self},
-                )
-            self.env.ProcessUnFlags(self.build_unflags)
+        self.env.ProcessFlags(self.build_flags)
+        self.env.ProcessUnFlags(self.build_unflags)
 
     def process_dependencies(self):
         if not self.dependencies:
@@ -447,7 +440,15 @@ class LibBuilderBase(object):
         for lb, lb_search_files in lib_inc_map.items():
             self.depend_recursive(lb, lb_search_files)
 
-    def build(self):
+    def build(self):  # pylint: disable=too-many-branches
+        if self.extra_script and not self._is_built:
+            with fs.cd(self.path):
+                self.env.SConscriptChdir(1)
+                self.env.SConscript(
+                    os.path.abspath(self.extra_script),
+                    exports={"env": self.env, "pio_lib_builder": self},
+                )
+
         libs = []
         for lb in self._depbuilders:
             libs.extend(lb.build())
@@ -460,8 +461,8 @@ class LibBuilderBase(object):
 
         if self._is_built:
             return libs
-        self._is_built = True
 
+        self._is_built = True
         self.env.PrependUnique(CPPPATH=self.get_include_dirs())
 
         if self.lib_ldf_mode == "off":