diff --git a/backend/app/services/espidf_compiler.py b/backend/app/services/espidf_compiler.py
index 8328e5b3..deaf8225 100644
--- a/backend/app/services/espidf_compiler.py
+++ b/backend/app/services/espidf_compiler.py
@@ -1520,6 +1520,27 @@ class ESPIDFCompiler:
         'WCharacter.h', 'binary.h', 'pins_arduino.h', 'Esp.h',
     })
 
+    # Config headers the PROJECT is meant to supply, never a library.
+    #
+    # LVGL's sources do `#include "lv_conf.h"` behind `#if !LV_CONF_SKIP`.
+    # The include scan is textual and does not evaluate that guard, so the
+    # header is queued even for a build that defines LV_CONF_SKIP=1 and will
+    # never read it. Resolving it against the shared library dir then finds
+    # whichever installed library vendors its own copy -- Adafruit LvGL Glue
+    # ships one written for LVGL v7 -- and merges that ENTIRE library. Its
+    # sources fail against the LVGL v8 that the sketch actually uses
+    # ("lv_fs_drv_t has no member named 'user_data'", "'file_t' was not
+    # declared"), so a sketch whose only LVGL include is <lvgl.h> dies with a
+    # wall of errors naming a library the user never asked for. Same failure
+    # shape as the Hash.h -> stemihexapod case above. Reported 2026-08-28.
+    #
+    # A project that DOES ship its own lv_conf.h is unaffected: sketch files
+    # are copied into the build's own source dir and found there, not through
+    # this library scan.
+    _PROJECT_SUPPLIED_HEADERS: frozenset[str] = frozenset({
+        'lv_conf.h', 'lv_drv_conf.h',
+    })
+
     # Basenames of C/C++ standard headers. A user library that ships a private
     # header with one of these names (e.g. LovyanGFX's src/lgfx/internal/
     # limits.h, algorithm.h, memory.h, alloca.h) must NOT put that directory on
@@ -1858,6 +1879,16 @@ class ESPIDFCompiler:
                 )
                 continue
 
+            # Project-supplied config headers (lv_conf.h ...). See
+            # _PROJECT_SUPPLIED_HEADERS: resolving these globally drags in
+            # whatever unrelated library happens to vendor a copy.
+            if header in self._PROJECT_SUPPLIED_HEADERS:
+                logger.info(
+                    f'[espidf] <{header}> is a project-supplied config header '
+                    f'— never resolving against user libraries'
+                )
+                continue
+
             # P2 manifest scope. When a manifest is supplied, resolve the header
             # to the DECLARED library that provides it — not the first-
             # alphabetical lib in the shared dir. Several installed libs may
diff --git a/test/backend/unit/test_espidf_library_ranking.py b/test/backend/unit/test_espidf_library_ranking.py
index d96be989..390a3f49 100644
--- a/test/backend/unit/test_espidf_library_ranking.py
+++ b/test/backend/unit/test_espidf_library_ranking.py
@@ -144,5 +144,72 @@ class TestMergedLibraryReport(unittest.TestCase):
         self.assertEqual(hdr2comp.get("A.h"), "user_libs_all")
 
 
+class TestProjectSuppliedHeaders(unittest.TestCase):
+    """A config header the PROJECT owns must never drag in a library.
+
+    The live failure (2026-08-28, an LVGL clock on a XIAO ESP32-S3): the
+    sketch included only <lvgl.h>, but LVGL's sources carry
+    `#include "lv_conf.h"` behind `#if !LV_CONF_SKIP`. The include scan is
+    textual, so lv_conf.h got queued even though the build defined
+    LV_CONF_SKIP=1, resolved to Adafruit LvGL Glue (which vendors its own
+    copy for LVGL v7), and merged that whole library. Its sources then failed
+    against LVGL v8 and the build died naming a library the user never asked
+    for.
+    """
+
+    def setUp(self) -> None:
+        self.tmp = Path(tempfile.mkdtemp())
+        self.libs = self.tmp / "libraries"
+        self.out = self.tmp / "project" / "user_libs"
+        self.out.mkdir(parents=True)
+        self.c = ESPIDFCompiler()
+        self.c.arduino_path = ""
+        self.c._core_headers_cache = None
+
+    def tearDown(self) -> None:
+        shutil.rmtree(self.tmp, ignore_errors=True)
+
+    def _resolve(self, headers, merged):
+        return self.c._resolve_library_components(
+            headers, arduino_libs=self.libs, esp32_libs=None,
+            arduino_comp_name="arduino-esp32", user_libs_dir=self.out,
+            merged_libs=merged,
+        )
+
+    def _make_lvgl_world(self) -> None:
+        # LVGL itself: lvgl.h pulls lv_conf.h the way the real sources do.
+        _mk(self.libs / "lvgl@8.3.11-0d103ab1" / "src" / "lvgl.h",
+            '#include "lv_conf.h"\n')
+        _mk(self.libs / "lvgl@8.3.11-0d103ab1" / "library.properties",
+            "name=lvgl\narchitectures=*\n")
+        # The unrelated library that happens to vendor an lv_conf.h.
+        _mk(self.libs / "adafruitlittlevglgluelibrary@2.1.7-896f753c" / "src" / "lv_conf.h")
+        _mk(self.libs / "adafruitlittlevglgluelibrary@2.1.7-896f753c" / "src"
+            / "Adafruit_LvGL_Glue.h")
+        _mk(self.libs / "adafruitlittlevglgluelibrary@2.1.7-896f753c" / "library.properties",
+            "name=Adafruit LvGL Glue Library\narchitectures=*\n")
+
+    def test_lv_conf_does_not_drag_in_the_glue_library(self):
+        self._make_lvgl_world()
+        merged: dict[str, str] = {}
+        self._resolve(["lvgl.h"], merged)
+        self.assertEqual(merged.get("lvgl.h"), "lvgl")
+        self.assertNotIn("lv_conf.h", merged)
+        self.assertNotIn(
+            "Adafruit LvGL Glue Library", merged.values(),
+            "a sketch that only includes <lvgl.h> must not merge the glue library",
+        )
+
+    def test_a_library_that_owns_lv_conf_is_still_reachable_by_its_own_header(self):
+        # The guard is scoped to the config header, not to the library: a
+        # project that really wants the glue can still include its header.
+        self._make_lvgl_world()
+        merged: dict[str, str] = {}
+        self._resolve(["Adafruit_LvGL_Glue.h"], merged)
+        self.assertEqual(
+            merged.get("Adafruit_LvGL_Glue.h"), "Adafruit LvGL Glue Library"
+        )
+
+
 if __name__ == "__main__":
     unittest.main()