diff --git a/esphome/arduino8266/component.py b/esphome/arduino8266/component.py
index a39f3f9a8b..2e0de9a57d 100644
--- a/esphome/arduino8266/component.py
+++ b/esphome/arduino8266/component.py
@@ -26,6 +26,7 @@ from esphome.platformio.library import (
     ConvertedLibrary,
     InvalidLibrary,
     LibraryBackend,
+    _parse_library_json,
     check_library_data,
     collect_filtered_files,
     convert_libraries,
@@ -128,10 +129,19 @@ def _library_info(name: str, read_path: Path, data: dict) -> ArduinoLibrary:
 
 
 def _bundled_library(framework_path: Path, name: str) -> ArduinoLibrary:
-    """A library bundled with the Arduino core, read from the framework tree."""
+    """A library bundled with the Arduino core, read from the framework tree.
+
+    ``library.json`` wins over ``library.properties`` when both exist, as in
+    PlatformIO's LibBuilderFactory; only the JSON manifest can carry a
+    ``build`` section (srcDir, srcFilter, flags).
+    """
     lib_dir = framework_path / "libraries" / name
-    manifest = lib_dir / "library.properties"
-    data = parse_library_properties(manifest) if manifest.is_file() else {}
+    manifest_json = lib_dir / "library.json"
+    if manifest_json.is_file():
+        data = _parse_library_json(manifest_json)
+    else:
+        manifest = lib_dir / "library.properties"
+        data = parse_library_properties(manifest) if manifest.is_file() else {}
     return _library_info(name, lib_dir, {"name": name, **data})
 
 
diff --git a/esphome/build_gen/build_tool.py b/esphome/build_gen/build_tool.py
index bfcf4128eb..19275a8eaf 100644
--- a/esphome/build_gen/build_tool.py
+++ b/esphome/build_gen/build_tool.py
@@ -21,9 +21,11 @@ def main() -> int:
         # Remove first: ``ar rc`` replaces members but never drops ones whose
         # source was removed from the build, which would leak stale objects.
         Path(archive).unlink(missing_ok=True)
-        return subprocess.run(
-            [ar, "rc", archive, f"@{rspfile}"], check=False
-        ).returncode
+        # Expand the response file here instead of passing @rspfile: GNU ar
+        # treats backslashes in response files as escapes, corrupting Windows
+        # paths ("sub\a.o" -> "suba.o").
+        objects = Path(rspfile).read_text(encoding="utf-8").split()
+        return subprocess.run([ar, "rc", archive, *objects], check=False).returncode
     if mode == "copy":
         src, dst = sys.argv[2:4]
         shutil.copyfile(src, dst)
diff --git a/esphome/platformio/library.py b/esphome/platformio/library.py
index 6283a566e4..719df2eeec 100644
--- a/esphome/platformio/library.py
+++ b/esphome/platformio/library.py
@@ -562,7 +562,7 @@ def split_flag_entry(entry: str, owner: str) -> list[str]:
         raise EsphomeError(f"Malformed build flag {entry!r} in {owner}: {err}") from err
 
 
-def lex_build_flags(entries, owner: str) -> list[str]:
+def lex_build_flags(entries: str | list[str], owner: str) -> list[str]:
     """Shell-lex a manifest ``build.flags`` list into joined tokens.
 
     The composition every backend needs: each entry is lexed the way
diff --git a/tests/unit_tests/build_gen/test_build_tool.py b/tests/unit_tests/build_gen/test_build_tool.py
index 82ca90a8c7..0c8eb00f9d 100644
--- a/tests/unit_tests/build_gen/test_build_tool.py
+++ b/tests/unit_tests/build_gen/test_build_tool.py
@@ -27,7 +27,8 @@ def test_ar_removes_stale_archive(tmp_path: Path) -> None:
     ):
         assert build_tool.main() == 0
     assert not archive.exists()
-    assert mock_run.call_args[0][0] == ["ar-bin", "rc", str(archive), f"@{rsp}"]
+    # The rspfile is expanded by the shim (GNU ar would escape backslashes)
+    assert mock_run.call_args[0][0] == ["ar-bin", "rc", str(archive), "a.o"]
 
 
 def test_copy(tmp_path: Path) -> None:
@@ -61,3 +62,28 @@ def test_runs_as_script(tmp_path: Path) -> None:
     )
     assert result.returncode == 0
     assert dst.read_text() == "x"
+
+
+def test_ar_expands_rspfile_without_escaping(tmp_path) -> None:
+    """Backslash paths survive: the shim expands the rspfile itself instead
+    of letting GNU ar treat backslashes as escapes."""
+    rsp = tmp_path / "objs.rsp"
+    rsp.write_text("obj/a.o\nsub\\b.o\n")
+    with (
+        patch.object(
+            build_tool.sys,
+            "argv",
+            ["build_tool", "ar", "ar-bin", str(tmp_path / "lib.a"), str(rsp)],
+        ),
+        patch.object(
+            build_tool.subprocess, "run", return_value=MagicMock(returncode=0)
+        ) as mock_run,
+    ):
+        assert build_tool.main() == 0
+    assert mock_run.call_args[0][0] == [
+        "ar-bin",
+        "rc",
+        str(tmp_path / "lib.a"),
+        "obj/a.o",
+        "sub\\b.o",
+    ]
diff --git a/tests/unit_tests/test_arduino8266_component.py b/tests/unit_tests/test_arduino8266_component.py
index 222a881f85..edc81be6cb 100644
--- a/tests/unit_tests/test_arduino8266_component.py
+++ b/tests/unit_tests/test_arduino8266_component.py
@@ -304,3 +304,18 @@ def test_resolve_libraries_lib_ignore_covers_bundled_dependencies(
         libs = component.resolve_libraries(framework)
 
     assert [lib.name for lib in libs] == ["some__External"]
+
+
+def test_bundled_library_prefers_library_json(tmp_path: Path) -> None:
+    """A bundled library.json wins over library.properties (PIO semantics);
+    its build section is honored."""
+    framework = _make_framework(tmp_path)
+    lib_dir = framework / "libraries" / "GDBStub"
+    (lib_dir / "custom").mkdir(parents=True)
+    (lib_dir / "custom" / "gdb.cpp").write_text("")
+    (lib_dir / "library.properties").write_text("name=GDBStub\n")
+    (lib_dir / "library.json").write_text(
+        '{"name": "GDBStub", "build": {"srcDir": "custom"}}'
+    )
+    lib = component._bundled_library(framework, "GDBStub")
+    assert [s.name for s in lib.sources] == ["gdb.cpp"]