diff --git a/esphome/build_gen/arduino8266.py b/esphome/build_gen/arduino8266.py
index 1d72971434..ff946f321a 100644
--- a/esphome/build_gen/arduino8266.py
+++ b/esphome/build_gen/arduino8266.py
@@ -495,6 +495,9 @@ def write_project(paths: InstalledPaths) -> bool:
         cache_key="arduino8266",
     )
 
+    if not src_dir.is_dir():
+        # Generated project state, not install state: clean-all would not help
+        raise EsphomeError(f"Generated source directory {src_dir} is missing")
     # A missing install directory would otherwise surface as a wall of
     # include errors; failing here names the path instead.
     include_dirs = [
@@ -505,7 +508,7 @@ def write_project(paths: InstalledPaths) -> bool:
         sdk / "lwip2" / "include",
         variant_dir,
     ]
-    for required in include_dirs:
+    for required in include_dirs[1:]:
         if not required.is_dir():
             raise EsphomeError(
                 f"{_INCOMPLETE_INSTALL}: missing {required}; {_CLEAN_HINT}"
@@ -534,7 +537,14 @@ def write_project(paths: InstalledPaths) -> bool:
         + common
         + get_project_cxx_compile_flags()
     )
-    asflags = _ASFLAGS + defines + includes + project_compile_flags
+    # PlatformIO's ASPPCOM carries defines and includes but not CCFLAGS,
+    # so only -D/-I user flags reach assembly there; match it.
+    asflags = (
+        _ASFLAGS
+        + defines
+        + includes
+        + [f for f in project_compile_flags if f.startswith(("-D", "-I"))]
+    )
 
     # build_unflags applies to the framework flag sets too (compile and link),
     # as under PlatformIO (a silently ignored ``build_unflags: -Os`` would
@@ -546,7 +556,7 @@ def write_project(paths: InstalledPaths) -> bool:
     if esp8266_data[KEY_SCANF_FLOAT]:
         link_flags += ["-u", "_scanf_float"]
     link_flags += project_link_flags
-    link_flags += [flag for lib in libraries for flag in lib.link_flags]
+    link_flags += [_shell_token(flag) for lib in libraries for flag in lib.link_flags]
     flash_ld = _active_flash_ld_name(flash_ld_name)
     link_flags += ["-T", flash_ld]
 
@@ -616,7 +626,7 @@ def write_project(paths: InstalledPaths) -> bool:
         f"asflags = {' '.join(asflags)}",
         f"linkflags = {' '.join(link_flags)}",
         f"libdirflags = {' '.join(f'-L{_q(d)}' for d in lib_dirs)}",
-        f"libflags = {' '.join(f'-l{lib}' for lib in system_libs)}",
+        f"libflags = {' '.join(_shell_token(f'-l{lib}') for lib in system_libs)}",
         "",
     ]
 
@@ -625,7 +635,8 @@ def write_project(paths: InstalledPaths) -> bool:
         core_exclude |= _CORE_EXCLUDE_WAVEFORM
 
     archives = []
-    variant_sources = _collect_sources(variant_dir) if variant_dir.is_dir() else []
+    # variant_dir existence was already enforced with the include dirs
+    variant_sources = _collect_sources(variant_dir)
     if variant_sources:
         objs = _ninja_compile_edges(lines, variant_sources, variant_dir, "variant")
         lines.append(f"build libFrameworkArduinoVariant.a: ar {' '.join(objs)}")
diff --git a/tests/unit_tests/build_gen/test_arduino8266.py b/tests/unit_tests/build_gen/test_arduino8266.py
index 8c658bf842..27e682d124 100644
--- a/tests/unit_tests/build_gen/test_arduino8266.py
+++ b/tests/unit_tests/build_gen/test_arduino8266.py
@@ -676,3 +676,50 @@ def test_ninja_path_escaping() -> None:
     """Build-statement paths and command-line paths escape differently."""
     assert arduino8266._e("a b:$c") == "a$ b$:$$c"
     assert arduino8266._q("/a b/$x") == '"/a b/$$x"'
+
+
+def test_write_project_asm_excludes_non_define_user_flags(tmp_path: Path) -> None:
+    """The ASPPCOM command under PlatformIO never sees CCFLAGS, so only -D/-I user flags
+    reach assembly compiles."""
+    paths = _make_framework(tmp_path)
+    _set_flags("-DUSER_KNOB=1", "-Wno-volatile")
+    content = _write_ninja(paths)
+    asflags = next(line for line in content.splitlines() if line.startswith("asflags"))
+    assert "-DUSER_KNOB=1" in asflags
+    assert "-Wno-volatile" not in asflags
+    cxxflags = next(
+        line for line in content.splitlines() if line.startswith("cxxflags")
+    )
+    assert "-Wno-volatile" in cxxflags
+
+
+def test_write_project_returns_changed(tmp_path: Path) -> None:
+    """The documented contract: True when build.ninja changed, False on an
+    identical regeneration (pins byte-stable output too)."""
+    paths = _make_framework(tmp_path)
+    _set_flags()
+    src = CORE.relative_src_path()
+    (src / "esphome" / "components" / "esp8266").mkdir(parents=True, exist_ok=True)
+    (src / "main.cpp").write_text("")
+    with (
+        patch.object(arduino8266, "generate_ld_scripts"),
+        patch("esphome.arduino.library.resolve_libraries", return_value=[]),
+        patch("esphome.arduino8266.framework.ccache_path", return_value=None),
+    ):
+        assert arduino8266.write_project(paths) is True
+        assert arduino8266.write_project(paths) is False
+
+
+def test_write_project_missing_src_dir_raises(tmp_path: Path) -> None:
+    """A missing generated source tree is its own error, not an install one."""
+    paths = _make_framework(tmp_path)
+    _set_flags()
+    with (
+        patch.object(arduino8266, "generate_ld_scripts"),
+        patch("esphome.arduino.library.resolve_libraries", return_value=[]),
+        patch.object(
+            arduino8266.CORE, "relative_src_path", return_value=tmp_path / "nope"
+        ),
+        pytest.raises(EsphomeError, match="source directory"),
+    ):
+        arduino8266.write_project(paths)