diff --git a/esphome/arduino/library.py b/esphome/arduino/library.py
index f1871c7302..a5d4a6f515 100644
--- a/esphome/arduino/library.py
+++ b/esphome/arduino/library.py
@@ -99,14 +99,18 @@ def _resolve_src_dir(name: str, read_path: Path, build: dict) -> str:
 
 
 def _reject_unsupported_link_fields(name: str, data: dict) -> None:
-    for dropped_key in ("precompiled", "ldflags"):
-        if data.get(dropped_key):
-            # PIO's Arduino lib builder honors these; building without them
-            # would fail at link with no stated cause
-            raise EsphomeError(
-                f"Library {name} declares {dropped_key}, which this backend "
-                "does not support"
-            )
+    # PIO's Arduino lib builder honors these; building without them would
+    # fail at link with no stated cause. library.properties values are
+    # strings, so "false" (the spec's explicit opt-out) is not a declaration.
+    precompiled = data.get("precompiled")
+    if precompiled and str(precompiled).strip().lower() != "false":
+        raise EsphomeError(
+            f"Library {name} declares precompiled, which this backend does not support"
+        )
+    if data.get("ldflags"):
+        raise EsphomeError(
+            f"Library {name} declares ldflags, which this backend does not support"
+        )
 
 
 def _resolve_lib_archive(name: str, data: dict, build: dict) -> bool:
@@ -218,18 +222,14 @@ def _collect_lib_sources(
             len(dropped),
             ", ".join(sorted(dropped)),
         )
-    if (
-        not lib.sources
-        and ("srcFilter" in build or "srcDir" in build)
-        and not any(Path(f).suffix.lower() in LIBRARY_HEADER_SUFFIXES for f in matched)
+    if not lib.sources and not any(
+        Path(f).suffix.lower() in LIBRARY_HEADER_SUFFIXES for f in matched
     ):
-        # Matched headers mean a header-only library; a declared filter
-        # matching nothing (or only inert files) is a manifest/tree problem.
-        # The truly empty tree raises via _assert_tree_has_code.
-        _LOGGER.warning(
-            "Library %s declares srcFilter/srcDir but no source files matched",
-            name,
-        )
+        # Matched headers mean a header-only library; a filter matching
+        # nothing (or only inert files) is a manifest/tree problem whether
+        # or not it was declared. The truly empty tree raises via
+        # _assert_tree_has_code.
+        _LOGGER.warning("Library %s: no source files matched", name)
 
 
 def _library_info(name: str, read_path: Path, data: dict) -> ArduinoLibrary:
diff --git a/esphome/build_gen/build_tool.py b/esphome/build_gen/build_tool.py
index dbf988c1c7..d2497acce6 100644
--- a/esphome/build_gen/build_tool.py
+++ b/esphome/build_gen/build_tool.py
@@ -67,9 +67,11 @@ def _run_ar(ar: str, archive: str, rspfile: str) -> int:
 def _run_copy(src: str, dst: str) -> int:
     try:
         shutil.copyfile(src, dst)
-    except OSError:
-        # Never leave a partially written output (e.g. a firmware image)
-        Path(dst).unlink(missing_ok=True)
+    except OSError as err:
+        # Never leave a partially written output (e.g. a firmware image);
+        # SameFileError means dst IS src, where unlinking destroys the input
+        if not isinstance(err, shutil.SameFileError):
+            Path(dst).unlink(missing_ok=True)
         raise
     return 0
 
diff --git a/tests/unit_tests/build_gen/test_build_tool.py b/tests/unit_tests/build_gen/test_build_tool.py
index 5698e8521e..05bc2483de 100644
--- a/tests/unit_tests/build_gen/test_build_tool.py
+++ b/tests/unit_tests/build_gen/test_build_tool.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 from pathlib import Path
+import shutil
 import subprocess
 import sys
 from unittest.mock import MagicMock, patch
@@ -188,6 +189,20 @@ def test_surplus_arguments_error(capsys: pytest.CaptureFixture[str]) -> None:
     assert "expected 2 arguments, got 3" in capsys.readouterr().err
 
 
+def test_copy_same_file_keeps_the_input(tmp_path: Path) -> None:
+    """A same-file copy (dst IS src) must not unlink the input."""
+    src = tmp_path / "firmware.bin"
+    src.write_bytes(b"image")
+    with (
+        patch.object(
+            build_tool.sys, "argv", ["build_tool", "copy", str(src), str(src)]
+        ),
+        pytest.raises(shutil.SameFileError),
+    ):
+        build_tool.main()
+    assert src.read_bytes() == b"image"
+
+
 def test_copy_failure_leaves_no_partial_output(tmp_path: Path) -> None:
     """A failed copy unlinks the destination; a partial firmware image must
     never be left on disk."""
diff --git a/tests/unit_tests/test_arduino_library.py b/tests/unit_tests/test_arduino_library.py
index c99a62816f..9a349c2ea6 100644
--- a/tests/unit_tests/test_arduino_library.py
+++ b/tests/unit_tests/test_arduino_library.py
@@ -189,7 +189,7 @@ def test_library_info_declared_filter_matches_nothing_warns(
     data = {"build": {"srcFilter": ["+<nothing/*>"]}}
     lib = component._library_info("x", read_path, data)
     assert not lib.sources
-    assert "declares srcFilter/srcDir but no source files matched" in caplog.text
+    assert "no source files matched" in caplog.text
 
 
 def test_empty_converted_tree_raises_at_emit(tmp_path: Path) -> None:
@@ -606,6 +606,44 @@ def test_library_info_unsupported_link_fields_raise(tmp_path: Path) -> None:
         component._library_info("x", read_path, {"ldflags": "-lfoo", "build": {}})
 
 
+@pytest.mark.parametrize("value", ["false", "False", " false ", "", False, None])
+def test_library_info_precompiled_opt_out_accepted(
+    tmp_path: Path, value: object
+) -> None:
+    """Manifest values are strings; precompiled=false is the spec's
+    explicit opt-out, not a declaration."""
+    read_path = tmp_path / "lib"
+    (read_path / "src").mkdir(parents=True)
+    (read_path / "src" / "stub.cpp").write_text("")
+    data = {"build": {}}
+    if value is not None:
+        data["precompiled"] = value
+    component._library_info("x", read_path, data)
+
+
+@pytest.mark.parametrize("value", ["full", True, "weird"])
+def test_library_info_precompiled_set_raises(tmp_path: Path, value: object) -> None:
+    """Both full (Arduino's other legal value) and unknown spellings fail safe."""
+    read_path = tmp_path / "lib"
+    (read_path / "src").mkdir(parents=True)
+    (read_path / "src" / "stub.cpp").write_text("")
+    with pytest.raises(EsphomeError, match="declares precompiled"):
+        component._library_info("x", read_path, {"precompiled": value, "build": {}})
+
+
+def test_library_info_default_filter_matching_nothing_warns(
+    tmp_path: Path, caplog: pytest.LogCaptureFixture
+) -> None:
+    """The empty-match warning is not gated on a declared srcFilter/srcDir;
+    a default-filter src/ holding only inert files warns too."""
+    read_path = tmp_path / "lib"
+    (read_path / "src").mkdir(parents=True)
+    (read_path / "src" / "keywords.txt").write_text("")
+    lib = component._library_info("x", read_path, {"build": {}})
+    assert not lib.sources
+    assert "no source files matched" in caplog.text
+
+
 def test_library_info_unmapped_sources_warn(
     tmp_path: Path, caplog: pytest.LogCaptureFixture
 ) -> None: