diff --git a/esphome/platformio/extra_script.py b/esphome/platformio/extra_script.py
index 726c3e04bb..ef75421671 100644
--- a/esphome/platformio/extra_script.py
+++ b/esphome/platformio/extra_script.py
@@ -35,7 +35,7 @@ import os
 from pathlib import Path
 from typing import TYPE_CHECKING
 
-from esphome.platformio.library import ensure_list
+from esphome.core import EsphomeError
 
 if TYPE_CHECKING:
     from esphome.platformio.library import ConvertedLibrary
@@ -89,9 +89,17 @@ def apply_extra_script(
     extra_flags = captured_as_build_flags(result, library_dir=source_path)
     if not extra_flags:
         return
-    flags = ensure_list(component.data.setdefault("build", {}).setdefault("flags", []))
-    flags.extend(extra_flags)
-    component.data["build"]["flags"] = flags
+    flags = component.data.setdefault("build", {}).setdefault("flags", [])
+    if isinstance(flags, str):
+        flags = [flags]
+    elif not isinstance(flags, list):
+        # A null/dict value coerced through a list wrapper would inject a
+        # non-string into the compiler command line; fail naming the library
+        raise EsphomeError(
+            f"Library {component.name} has a malformed build.flags "
+            f"({type(flags).__name__}); expected a string or list"
+        )
+    component.data["build"]["flags"] = [*flags, *extra_flags]
 
 
 # Keys we know how to translate back into ESPHome's build-flag pipeline.
diff --git a/tests/unit_tests/build_helpers/test_idedata.py b/tests/unit_tests/build_helpers/test_idedata.py
index 6c44192439..51f5b2384c 100644
--- a/tests/unit_tests/build_helpers/test_idedata.py
+++ b/tests/unit_tests/build_helpers/test_idedata.py
@@ -451,3 +451,17 @@ def test_load_or_build_idedata_never_caches_bad_compiler(tmp_path: Path) -> None
         )
     assert data["cxx_path"] == "/usr/bin/python3"
     assert not cache.exists()
+
+
+def test_load_or_build_idedata_cache_hit_skips_rebuild(tmp_path: Path) -> None:
+    """A valid cache newer than the compile DB is served without re-parsing."""
+    compile_commands = _write_compile_commands(tmp_path)
+    cache = tmp_path / "c.json"
+    cache.write_text(json.dumps({"cc_path": "/tools/gcc", "cached": True}))
+    os.utime(cache, (compile_commands.stat().st_mtime + 5,) * 2)
+    with patch.object(idedata, "idedata_from_build") as mock_build:
+        data = idedata.load_or_build_idedata(
+            compile_commands, tmp_path / "f.elf", cache
+        )
+    mock_build.assert_not_called()
+    assert data["cached"] is True