diff --git a/backend/app/services/espidf_compiler.py b/backend/app/services/espidf_compiler.py
index 65d6ba47..3cbe8856 100644
--- a/backend/app/services/espidf_compiler.py
+++ b/backend/app/services/espidf_compiler.py
@@ -720,6 +720,23 @@ class ESPIDFCompiler:
         props_name = self._parse_library_properties(lib_root).get('name', '')
         return bool(props_name) and self._norm_lib_name(props_name) in allowed_norm
 
+    def _find_manifest_library_for_header(
+        self, header: str, libs_dir: Path, allowed_norm: set[str]
+    ) -> Path | None:
+        """Like _find_library_for_header, but returns the source root of the
+        first library that provides `header` AND is in the project manifest.
+        Returns None when no DECLARED library provides the header — so a stray
+        same-named lib in the shared dir is never picked up."""
+        for lib_dir in sorted(libs_dir.iterdir()):
+            if not lib_dir.is_dir():
+                continue
+            for src_root in (lib_dir, lib_dir / 'src'):
+                if (src_root / header).exists() and self._library_in_manifest(
+                    lib_dir, allowed_norm
+                ):
+                    return src_root
+        return None
+
     def _resolve_library_components(
         self,
         ext_headers: list[str],
@@ -801,11 +818,22 @@ class ESPIDFCompiler:
                 )
                 continue
 
-            src_root = (
-                self._find_library_for_header(header, arduino_libs)
-                if arduino_libs and arduino_libs.is_dir()
-                else None
-            )
+            # 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
+            # ship the same header name (e.g. DHT118266, DHT_sensor_library,
+            # servodht11 all have DHT.h); the legacy first-match would pick a
+            # stray. The manifest both picks the right lib AND excludes
+            # undeclared ones (another user's install, a clash). No manifest =
+            # legacy first-match (unchanged).
+            if not (arduino_libs and arduino_libs.is_dir()):
+                src_root = None
+            elif allowed_norm is not None:
+                src_root = self._find_manifest_library_for_header(
+                    header, arduino_libs, allowed_norm
+                )
+            else:
+                src_root = self._find_library_for_header(header, arduino_libs)
 
             # Architecture guard. A user lib that resolves the header but
             # whose library.properties declares architectures= without
@@ -820,20 +848,6 @@ class ESPIDFCompiler:
                     )
                     src_root = None
 
-            # P2 manifest scope. A user-installed library is merged only if it's
-            # in the project's declared manifest. Anything else from the shared
-            # dir is out of scope — drop it (the header then falls through to the
-            # core check, or to the "not found" path so install-on-missing can
-            # offer to add it to the manifest). Core/bundled libs aren't gated.
-            if src_root is not None and allowed_norm is not None:
-                _lr = src_root.parent if src_root.name == 'src' else src_root
-                if not self._library_in_manifest(_lr, allowed_norm):
-                    logger.info(
-                        f'[espidf] <{header}> resolves to "{_lr.name}" but it is not in '
-                        f'the project library manifest — not merging (scope)'
-                    )
-                    src_root = None
-
             # Tracks the "resolved to a core lib that's already compiled into
             # the arduino-esp32 component" case, so we don't fall through to
             # the scary "not found — build may fail" warning below for a
diff --git a/test/backend/unit/test_espidf_core_first.py b/test/backend/unit/test_espidf_core_first.py
index 9a01ac4e..74f70bfe 100644
--- a/test/backend/unit/test_espidf_core_first.py
+++ b/test/backend/unit/test_espidf_core_first.py
@@ -102,6 +102,11 @@ class TestManifestScope(unittest.TestCase):
             "name=DHT sensor library\n")
         _mk(self.ulibs / "DHT_sensor_library" / "DHT.h")
         _mk(self.ulibs / "DHT_sensor_library" / "DHT.cpp")
+        # A STRAY lib that also ships DHT.h and sorts BEFORE the manifest lib
+        # (real case: DHT118266 sorts before DHT_sensor_library). The manifest
+        # must still resolve DHT.h to the declared lib, not this stray.
+        _mk(self.ulibs / "AAAA_strayDHT" / "DHT.h")
+        _mk(self.ulibs / "AAAA_strayDHT" / "stray_marker.cpp")
         # Out-of-manifest lib (e.g. another user's install / a clash).
         _mk(self.ulibs / "RandomOtherLib" / "Foo.h")
         _mk(self.ulibs / "RandomOtherLib" / "Foo.cpp")
@@ -126,6 +131,11 @@ class TestManifestScope(unittest.TestCase):
         _, hdr2comp = self._resolve(["DHT.h", "Foo.h"], {"DHT sensor library"})
         self.assertEqual(hdr2comp.get("DHT.h"), "user_libs_all")  # declared → merged
         self.assertNotIn("Foo.h", hdr2comp)                       # undeclared → dropped
+        merged = self.out / "user_libs_all"
+        copied = [p.name for p in merged.rglob("*")] if merged.exists() else []
+        # The DECLARED DHT lib was merged, not the stray that sorts first.
+        self.assertIn("DHT.cpp", copied)
+        self.assertNotIn("stray_marker.cpp", copied)
 
     def test_none_manifest_is_scan_all(self):
         # No manifest → legacy behaviour: both resolve.