diff --git a/esphome/core/__init__.py b/esphome/core/__init__.py
index 77efc91bef..6969ce9def 100644
--- a/esphome/core/__init__.py
+++ b/esphome/core/__init__.py
@@ -1138,13 +1138,14 @@ class EsphomeCore:
     def add_platformio_option(
         self, key: str, value: str | list[str], *, replace: bool = False
     ) -> None:
-        """Set a platformio.ini option; list values append to an existing list
-        unless ``replace`` is True, which overwrites any existing value."""
+        """Set a platformio.ini option; values append to an existing list
+        (a string as one element) unless ``replace`` is True, which
+        overwrites any existing value."""
         new_val = value
         old_val = self.platformio_options.get(key)
         if not replace and isinstance(old_val, list):
-            assert isinstance(value, list)
-            new_val = old_val + value
+            # A user platformio_options string must merge, not assert
+            new_val = old_val + ([value] if isinstance(value, str) else value)
         self.platformio_options[key] = new_val
 
     def _get_variable_generator(self, id):
diff --git a/script/determine-jobs.py b/script/determine-jobs.py
index 4c87fc828a..776236fa9e 100755
--- a/script/determine-jobs.py
+++ b/script/determine-jobs.py
@@ -498,11 +498,7 @@ ESP32_PLATFORMIO_TEST_COMPONENTS = frozenset(
 # drives every PlatformIO build). The esp32 platform component is already in
 # ESP32_PLATFORMIO_TEST_COMPONENTS, so its changes are covered by the normal
 # component-narrowing path.
-ESP32_PLATFORMIO_TRIGGER_PATH_PREFIXES = (
-    "esphome/platformio/",
-    # The pch wiring the strict smoke jobs police lives here
-    "esphome/build_helpers/",
-)
+ESP32_PLATFORMIO_TRIGGER_PATH_PREFIXES = ("esphome/platformio/",)
 
 # Standalone files that, when changed, trigger the PlatformIO compile test:
 #   - esphome/build_gen/platformio.py -- the PlatformIO build generator
@@ -519,6 +515,11 @@ _SMOKE_HARNESS_TRIGGER_FILES = frozenset(
 
 ESP32_PLATFORMIO_TRIGGER_FILES = _SMOKE_HARNESS_TRIGGER_FILES | {
     "esphome/build_gen/platformio.py",
+    # The pch machinery the strict smoke job polices, and the modules it
+    # imports; the rest of build_helpers/ does not affect PlatformIO builds
+    "esphome/build_helpers/pch.py",
+    "esphome/build_helpers/ccache.py",
+    "esphome/build_helpers/idedata.py",
 }
 
 
diff --git a/tests/script/test_determine_jobs.py b/tests/script/test_determine_jobs.py
index 21a0f30d78..adf638e17b 100644
--- a/tests/script/test_determine_jobs.py
+++ b/tests/script/test_determine_jobs.py
@@ -1003,7 +1003,10 @@ _ESP32_PLATFORMIO_FULL_LIST_FILES = [
     # PlatformIO subsystem (path-prefix trigger) + build generator
     ["esphome/platformio/runner.py"],
     ["esphome/platformio/toolchain.py"],
+    # The pch modules are standalone triggers, not the whole build_helpers/
     ["esphome/build_helpers/pch.py"],
+    ["esphome/build_helpers/ccache.py"],
+    ["esphome/build_helpers/idedata.py"],
     ["esphome/build_gen/platformio.py"],
     # Workflow / harness files
     ["script/test_build_components.py"],
@@ -1062,6 +1065,9 @@ def test_esp32_platformio_components_to_test_returns_full_list_on_infrastructure
         # Non-PlatformIO files in esphome/build_gen/ do NOT trigger the
         # full list -- only esphome/build_gen/platformio.py is a trigger.
         (["esphome/build_gen/espidf.py"], [], []),
+        # build_helpers modules the pch does not import are not triggers.
+        (["esphome/build_helpers/size_summary.py"], [], []),
+        (["esphome/build_helpers/ninja.py"], [], []),
         # Docs / unrelated files -> empty.
         (["README.md"], [], []),
         ([], [], []),
diff --git a/tests/unit_tests/test_core.py b/tests/unit_tests/test_core.py
index 0c96f8c8c9..3cf8d126d4 100644
--- a/tests/unit_tests/test_core.py
+++ b/tests/unit_tests/test_core.py
@@ -568,6 +568,16 @@ class TestEsphomeCore:
         target.config_path = Path("foo/config")
         return target
 
+    def test_add_platformio_option_merges_string_into_list(self, target) -> None:
+        """A user platformio_options string lands after a component's list
+        (FINAL priority) and must merge as one element, not assert."""
+        target.add_platformio_option("extra_scripts", ["post:pch.py"])
+        target.add_platformio_option("extra_scripts", "pre:mine.py")
+        assert target.platformio_options["extra_scripts"] == [
+            "post:pch.py",
+            "pre:mine.py",
+        ]
+
     def test_reset(self, target):
         """Call reset on target and compare to new instance"""
         other = core.EsphomeCore().__dict__