diff --git a/esphome/build_gen/arduino8266.py b/esphome/build_gen/arduino8266.py
index 9b3ecb007a..0a47cdf178 100644
--- a/esphome/build_gen/arduino8266.py
+++ b/esphome/build_gen/arduino8266.py
@@ -631,6 +631,10 @@ _PLAIN_LINKER_FLAGS = (
     "-nostdlib",
     "-rdynamic",
 )
+# The subset whose next token is an operand; unflagging the bare flag
+# would strand the operand. Operand-less members of the list above filter
+# whole-token from both the compile and link lines, as PlatformIO allows.
+_PLAIN_LINKER_OPERAND_FLAGS = ("-u", "-e")
 _PLAIN_LINKER_PREFIXES = ("-T", "-Xlinker")
 # Driver options, not ld options: -Wl, has no equivalent for these
 _PLAIN_DRIVER_LINK_PREFIXES = ("-fuse-ld=", "--specs=", "-specs=")
@@ -993,7 +997,7 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
     if plain := sorted(
         u
         for u in unflags
-        if u in _PLAIN_LINKER_FLAGS or u.startswith(_PLAIN_LINKER_PREFIXES)
+        if u in _PLAIN_LINKER_OPERAND_FLAGS or u.startswith(_PLAIN_LINKER_PREFIXES)
     ):
         raise EsphomeError(
             f"build_unflags cannot remove plain linker flag(s) "
@@ -1040,7 +1044,7 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
         f"buildtool = {_q(build_tool)}",
         f"ccache = {_q(ccache) if ccache else ''}",
         "",
-        # Rule names match SOURCE_KIND_FOR_SUFFIX values (c, cxx, asm)
+        # Rule names match SOURCE_KIND_FOR_SUFFIX values (c, cxx, asm, aspp)
         "rule c",
         "  command = $ccache $cc -MMD -MF $out.d $cflags $flags -c $in -o $out",
         "  depfile = $out.d",
@@ -1051,11 +1055,16 @@ def write_project(paths: InstalledPaths, ccache: str | None) -> bool:
         "  depfile = $out.d",
         "  deps = gcc",
         "  description = CXX $out",
-        "rule asm",
+        "rule aspp",
         "  command = $ccache $cc -MMD -MF $out.d -x assembler-with-cpp $asflags $flags -c $in -o $out",
         "  depfile = $out.d",
         "  deps = gcc",
         "  description = AS $out",
+        # Plain assembler, as SCons's ASCOM: no preprocessor, so no
+        # depfile and no $flags (defines/includes) either
+        "rule asm",
+        "  command = $ccache $cc -x assembler $asflags -c $in -o $out",
+        "  description = AS $out",
         "rule ar",
         f"  command = $python $buildtool ar {_q(toolchain_tool(paths.toolchain, 'ar'))} $out $out.rsp",
         "  rspfile = $out.rsp",
diff --git a/esphome/platformio/library.py b/esphome/platformio/library.py
index 89943c8aaa..2bbf48e77d 100644
--- a/esphome/platformio/library.py
+++ b/esphome/platformio/library.py
@@ -55,8 +55,9 @@ DEFAULT_BUILD_SRC_DIRS = "src"
 DEFAULT_BUILD_INCLUDE_DIR = "include"
 DEFAULT_BUILD_FLAGS = []
 # Suffix -> compiler kind (PlatformIO's CSUFFIXES/CXXSUFFIXES/ASSUFFIXES);
-# "asm" merges SCons's AS and ASPP sets. Per CXXSUFFIXES .C/.C++ are C++
-# here, even where SCons demotes .C on case-insensitive filesystems.
+# "aspp" is SCons's preprocessed-assembly set, "asm" its plain-assembler
+# set (no preprocessor, defines, or includes). Per CXXSUFFIXES .C/.C++ are
+# C++ here, even where SCons demotes .C on case-insensitive filesystems.
 SOURCE_KIND_FOR_SUFFIX: dict[str, str] = {
     ".c": "c",
     ".cpp": "cxx",
@@ -65,10 +66,10 @@ SOURCE_KIND_FOR_SUFFIX: dict[str, str] = {
     ".c++": "cxx",
     ".C": "cxx",
     ".C++": "cxx",
-    ".S": "asm",
-    ".spp": "asm",
-    ".SPP": "asm",
-    ".sx": "asm",
+    ".S": "aspp",
+    ".spp": "aspp",
+    ".SPP": "aspp",
+    ".sx": "aspp",
     ".s": "asm",
     ".asm": "asm",
     ".ASM": "asm",
diff --git a/tests/unit_tests/build_gen/test_arduino8266.py b/tests/unit_tests/build_gen/test_arduino8266.py
index 2ea5dca8aa..947778e400 100644
--- a/tests/unit_tests/build_gen/test_arduino8266.py
+++ b/tests/unit_tests/build_gen/test_arduino8266.py
@@ -346,7 +346,7 @@ def test_write_project_link_line_and_exclusions(tmp_path: Path) -> None:
     assert "core_esp8266_waveform_phase.cpp" not in content
     assert "core_esp8266_main.cpp.o" in content
     # Assembly and C sources compile through their own rules
-    assert "cont.S.o: asm" in content
+    assert "cont.S.o: aspp" in content
     assert "abi.c.o: c" in content
     # throw_stubs is force-included for ESPHome sources only, via one shared
     # srcflags variable rather than a copy of the flags line per edge
@@ -688,6 +688,30 @@ def test_build_config_nonosdk_precedence() -> None:
     assert _resolve_build_config(_defines()).nonosdk == "NONOSDK221"
 
 
+def test_write_project_plain_asm_rule_skips_preprocessor(tmp_path: Path) -> None:
+    """A lowercase .s source assembles plain (SCons AS), never through the
+    preprocessor rule that a .S source gets."""
+    paths = _make_framework(tmp_path)
+    core_dir = paths.framework / "cores" / "esp8266"
+    (core_dir / "lowlevel.s").write_text("nop\n")
+    _set_flags()
+    content = _write_ninja(paths)
+    assert "lowlevel.s.o: asm " in content
+    assert "rule asm\n  command = $ccache $cc -x assembler $asflags -c $in -o $out" in (
+        content
+    )
+
+
+def test_write_project_unflags_operandless_linker_flag(tmp_path: Path) -> None:
+    """build_unflags: -nostdlib filters whole-token from both lines, as
+    PlatformIO allows; only operand-taking flags hard-error."""
+    paths = _make_framework(tmp_path)
+    _set_flags()
+    CORE.build_unflags = {"-nostdlib"}
+    content = _write_ninja(paths)
+    assert "-nostdlib" not in content
+
+
 def test_write_project_unflagged_symbol_takes_its_dash_u(tmp_path: Path) -> None:
     """Unflagging a -u symbol drops the -u that carried it; a dangling -u
     would consume the next token and hand ld a symbol as an input file."""
diff --git a/tests/unit_tests/test_platformio_library.py b/tests/unit_tests/test_platformio_library.py
index 0d207d6951..653cf26c87 100644
--- a/tests/unit_tests/test_platformio_library.py
+++ b/tests/unit_tests/test_platformio_library.py
@@ -909,12 +909,12 @@ def test_split_flag_entry_non_string_is_clean() -> None:
 
 
 def test_source_kind_map_shape() -> None:
-    """The kind values the native compile rules key on, and the deliberate
-    AS/ASPP merge (.s and .S both map to asm)."""
+    """The kind values the native compile rules key on; the AS/ASPP split
+    matches SCons (.S preprocessed, .s plain assembler)."""
 
-    assert set(SOURCE_KIND_FOR_SUFFIX.values()) == {"c", "cxx", "asm"}
+    assert set(SOURCE_KIND_FOR_SUFFIX.values()) == {"c", "cxx", "asm", "aspp"}
     assert SOURCE_KIND_FOR_SUFFIX[".s"] == "asm"
-    assert SOURCE_KIND_FOR_SUFFIX[".S"] == "asm"
+    assert SOURCE_KIND_FOR_SUFFIX[".S"] == "aspp"
     assert SOURCE_KIND_FOR_SUFFIX[".c"] == "c"
     assert SOURCE_KIND_FOR_SUFFIX[".cpp"] == "cxx"
     # SCons's case-sensitive C++ suffixes: PIO compiles .C as C++