diff --git a/esphome/build_gen/arduino8266.py b/esphome/build_gen/arduino8266.py
index 731f52c2a5..86e4f70ad5 100644
--- a/esphome/build_gen/arduino8266.py
+++ b/esphome/build_gen/arduino8266.py
@@ -260,7 +260,7 @@ def _flag_defines(unflags: set[str], tokens: list[str]) -> dict[str, str]:
     """Map define name -> full ``NAME[=VALUE]`` for every -D build flag.
 
     ``tokens`` comes from one ``_lexed_build_flags()`` call shared with
-    ``_project_flags``, which already raised on any bare "-D".
+    ``_project_flags``, which already warned about and dropped any bare "-D".
     """
     defines: dict[str, str] = {}
     for tok in tokens:
@@ -486,7 +486,7 @@ def _project_flags(
     for tok in tokens:
         if tok in unflags:
             continue
-        # _lexed_build_flags raised on any bare -I/-D/-L/-l
+        # _lexed_build_flags warned about and dropped any bare -I/-D/-L/-l
         if tok.startswith("-Wl,"):
             link_flags.append(_shell_token(tok))
         elif tok.startswith("-L"):
@@ -505,6 +505,15 @@ def _project_flags(
                     f"Linker flag {tok} in build_flags is not routed to the "
                     "link line; use the -Wl, form"
                 )
+            if tok.startswith("-") and not tok.startswith(_COMPILE_FLAG_PREFIXES):
+                # The linker deny lists are not exhaustive; an unlisted
+                # link-only spelling would be inert on the -c compile line,
+                # so at least surface the odd shape
+                _LOGGER.warning(
+                    "Build flag %s is not a recognized compile-flag shape; "
+                    "it is passed to the compile line only",
+                    tok,
+                )
             compile_flags.append(_shell_token(tok))
     return compile_flags, link_flags, lib_dirs, libs
 
@@ -512,7 +521,19 @@ def _project_flags(
 # Plain-form linker flags rejected by _project_flags: inert on a -c compile
 # line, so the firmware would silently lack the requested link behavior
 # Best-effort, not exhaustive: an unlisted link-only spelling still falls
-# through to the compile line
+# through to the compile line, with a warning from the shape check
+_COMPILE_FLAG_PREFIXES = (
+    "-D",
+    "-I",
+    "-U",
+    "-W",
+    "-f",
+    "-m",
+    "-O",
+    "-g",
+    "-std=",
+    "-include",
+)
 _PLAIN_LINKER_FLAGS = (
     "-u",
     "-e",
diff --git a/tests/unit_tests/build_gen/test_arduino8266.py b/tests/unit_tests/build_gen/test_arduino8266.py
index c98360a2a2..361032dc4c 100644
--- a/tests/unit_tests/build_gen/test_arduino8266.py
+++ b/tests/unit_tests/build_gen/test_arduino8266.py
@@ -246,6 +246,29 @@ def test_lwip_low_memory_loses_to_listed_knobs() -> None:
     assert config.lwip_lib == "lwip2-1460"
 
 
+def test_lwip_ipv6_wins_over_default_knob() -> None:
+    """The shipping IPv6 config: network emits IPV6_LOW_MEMORY, esp8266
+    always emits HIGHER_BANDWIDTH_LOW_FLASH; IPv6 must win exactly as in
+    platformio-build.py's elif chain."""
+    config = _resolve(
+        "-DPIO_FRAMEWORK_ARDUINO_LWIP2_IPV6_LOW_MEMORY",
+        "-DPIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH",
+    )
+    assert config.lwip_lib == "lwip6-536-feat"
+    assert "LWIP_IPV6=1" in config.knob_defines
+
+
+def test_unrecognized_compile_flag_shape_warns(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """A link-only spelling missing from the deny lists still reaches the
+    compile line, but no longer silently."""
+    _set_flags("-shared")
+    compile_flags, _link_flags, _lib_dirs, _libs = _split_flags()
+    assert "-shared is not a recognized compile-flag shape" in caplog.text
+    assert "-shared" in compile_flags
+
+
 @pytest.mark.parametrize(
     ("knob", "expected"),
     [