diff --git a/backend/app/services/espidf_compiler.py b/backend/app/services/espidf_compiler.py
index b5fe2b9b..2dfe0134 100644
--- a/backend/app/services/espidf_compiler.py
+++ b/backend/app/services/espidf_compiler.py
@@ -693,6 +693,20 @@ class ESPIDFCompiler:
         'Udp.h', 'IPAddress.h', 'Client.h', 'Server.h', 'Stream.h',
         'Print.h', 'Printable.h', 'WiFiUdp.h', 'WiFiClient.h',
         'WiFiServer.h', 'WiFiType.h', 'esp_wifi.h',
+        # Platform headers that do NOT exist in the arduino-esp32 core, so
+        # the core-tree scan can't discover them, yet carry names generic
+        # enough that some random installed library owns a file by that
+        # name. Resolving one drags that whole library into user_libs_all
+        # and its sources then fail on THEIR own missing includes.
+        #
+        # Reported 2026-08-03 (nikas79): ESPAsyncWebServer includes <Hash.h>
+        # (an ESP8266-core header, guarded for that platform) and the global
+        # scan resolved it to `stemihexapod/src/Hash.h` — a hexapod-robot
+        # library — whose Serial.cpp / Hash.cpp / Server.cpp /
+        # BluetoothLowEnergy.cpp / ExpansionDriver.cpp then broke every
+        # ESP32 build with errors naming libraries the user never installed.
+        'Hash.h', 'Serial.h', 'HardwareSerial.h', 'WString.h',
+        'WCharacter.h', 'binary.h', 'pins_arduino.h', 'Esp.h',
     })
 
     # arduino-esp32 uses a single library architecture id ("esp32") across
diff --git a/test/backend/unit/test_espidf_core_first.py b/test/backend/unit/test_espidf_core_first.py
index 10630528..d3622c8f 100644
--- a/test/backend/unit/test_espidf_core_first.py
+++ b/test/backend/unit/test_espidf_core_first.py
@@ -171,3 +171,43 @@ class TestManifestScope(unittest.TestCase):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+class TestGenericHeaderCollision(unittest.TestCase):
+    """Issue reported 2026-08-03 (nikas79): ESPAsyncWebServer includes
+    <Hash.h> (an ESP8266-core header) and the global scan resolved it to a
+    hexapod-robot library that happens to ship src/Hash.h. The whole library
+    was merged and its unrelated sources broke every ESP32 build with
+    'fatal error: SoftwareSerial.h' and friends."""
+
+    def setUp(self) -> None:
+        self.tmp = Path(tempfile.mkdtemp())
+
+    def tearDown(self) -> None:
+        shutil.rmtree(self.tmp, ignore_errors=True)
+
+    def test_generic_platform_header_never_resolves_to_a_user_lib(self):
+        ulibs = self.tmp / "Arduino" / "libraries"
+        # A library with a generic Hash.h, exactly like stemi-hexapod.
+        _mk(ulibs / "stemihexapod" / "library.properties",
+            "name=stemi-hexapod\narchitectures=esp32\n")
+        _mk(ulibs / "stemihexapod" / "src" / "Hash.h")
+        _mk(ulibs / "stemihexapod" / "src" / "Serial.cpp",
+            '#include <SoftwareSerial.h>\n')
+
+        c = ESPIDFCompiler()
+        c.arduino_path = ""
+        c._core_headers_cache = None
+        out = self.tmp / "project" / "user_libs"
+        out.mkdir(parents=True)
+
+        _names, hdr2comp = c._resolve_library_components(
+            ["Hash.h", "Server.h", "Serial.h"],
+            arduino_libs=ulibs, esp32_libs=None,
+            arduino_comp_name="arduino-esp32", user_libs_dir=out,
+        )
+        for header in ("Hash.h", "Server.h", "Serial.h"):
+            self.assertNotIn(header, hdr2comp, f"{header} must stay core-provided")
+        copied = [p.name for p in out.rglob("*") if p.is_file()]
+        self.assertNotIn("Serial.cpp", copied,
+                         "the unrelated library must NOT be merged")