diff --git a/esphome/platformio/library.py b/esphome/platformio/library.py
index 03ea7bee53..22da9cfc87 100644
--- a/esphome/platformio/library.py
+++ b/esphome/platformio/library.py
@@ -49,7 +49,10 @@ DEFAULT_BUILD_SRC_DIRS = "src"
 DEFAULT_BUILD_INCLUDE_DIR = "include"
 DEFAULT_BUILD_FLAGS = []
 # Source suffix -> compiler kind, PlatformIO's CSUFFIXES/CXXSUFFIXES/ASSUFFIXES
-# split. Native build generators map the kind to their compile rules.
+# split. Native build generators map the kind to their compile rules. "asm"
+# deliberately merges SCons's AS (.s/.asm) and ASPP (.S/.spp/.sx) sets: the
+# ninja rules compile all of them as assembler-with-cpp, whose asm-mode
+# preprocessor passes non-directive text through unchanged.
 SOURCE_KIND_FOR_SUFFIX: dict[str, str] = {
     ".c": "c",
     ".cpp": "cxx",
@@ -572,14 +575,14 @@ def lex_build_flags(entries: str | list[str], owner: str) -> list[str]:
     PlatformIO's ParseFlags does, and bare ``-I``/``-L``/``-l``/``-D``
     tokens re-glue to their argument across the whole stream.
     """
-    return join_flag_args(
-        (
-            token
-            for entry in ensure_list(entries)
-            for token in split_flag_entry(entry, owner)
-        ),
-        owner,
-    )
+    # Join per entry, as SCons's ParseFlags lexes each string independently:
+    # a dangling -I ending one entry must warn, not absorb the next entry's
+    # first token.
+    return [
+        token
+        for entry in ensure_list(entries)
+        for token in join_flag_args(split_flag_entry(entry, owner), owner)
+    ]
 
 
 # Flags whose argument may follow as a separate token; ParseFlags glues them
diff --git a/tests/unit_tests/build_helpers/test_idedata.py b/tests/unit_tests/build_helpers/test_idedata.py
index 4e403b04a6..26fc1d1ee7 100644
--- a/tests/unit_tests/build_helpers/test_idedata.py
+++ b/tests/unit_tests/build_helpers/test_idedata.py
@@ -3,6 +3,7 @@
 # pylint: disable=protected-access
 
 import json
+import logging
 import os
 from pathlib import Path
 from unittest.mock import MagicMock, patch
@@ -290,9 +291,10 @@ def test_parse_entry_recovers_from_unconfigured_launcher(
         f"{ABS}build/src/esphome/core/application.cpp",
         "/opt/homebrew/bin/ccache /tools/xtensa-lx106-elf-g++ -c a.cpp -o a.o",
     )
+    caplog.set_level(logging.DEBUG)
     cxx_path, _, _, _ = idedata.parse_entry(entry)
     assert cxx_path == "/tools/xtensa-lx106-elf-g++"
-    assert "WARNING" not in caplog.text
+    assert "Stripping unconfigured launcher" in caplog.text
 
 
 def test_parse_entry_keeps_launcher_without_program() -> None:
diff --git a/tests/unit_tests/test_espidf_component.py b/tests/unit_tests/test_espidf_component.py
index f873e35953..1baa22777c 100644
--- a/tests/unit_tests/test_espidf_component.py
+++ b/tests/unit_tests/test_espidf_component.py
@@ -1084,3 +1084,18 @@ def test_emit_idf_component_wires_esp32_target(tmp_path, monkeypatch):
     c.data = {"build": {"extraScript": "extra.py"}}
     _emit_idf_component(c)
     assert c.data["build"]["flags"] == ["-lesp32"]
+
+
+def test_build_flags_dangling_flag_does_not_cross_entries(
+    tmp_path, caplog: pytest.LogCaptureFixture
+) -> None:
+    """Each entry is lexed independently, as ParseFlags does: a dangling -I ending one
+    entry warns instead of absorbing the next entry's first token."""
+    (tmp_path / "src").mkdir()
+    c = IDFComponent("owner/name", "1.0", source=URLSource("http://dummy"))
+    c.path = tmp_path
+    c.data = {"build": {"flags": ["-Wall -I", "-DFOO=1"]}}
+    content = generate_cmakelists_txt(c)
+    assert "FOO=1" in content
+    assert "-I-DFOO" not in content
+    assert "Ignoring trailing '-I'" in caplog.text
diff --git a/tests/unit_tests/test_platformio_library.py b/tests/unit_tests/test_platformio_library.py
index f1a55dca60..c8f994bb04 100644
--- a/tests/unit_tests/test_platformio_library.py
+++ b/tests/unit_tests/test_platformio_library.py
@@ -553,3 +553,14 @@ def test_join_flag_args_trailing_bare_flag_warns(
 
     assert join_flag_args(["-Os", "-l"], "library x") == ["-Os"]
     assert "Ignoring trailing '-l'" in caplog.text
+
+
+def test_lex_build_flags_dangling_flag_does_not_cross_entries(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """Each entry is lexed independently, as ParseFlags does: a dangling -I
+    ending one entry warns instead of absorbing the next entry's first token."""
+    from esphome.platformio.library import lex_build_flags
+
+    assert lex_build_flags(["-Wall -I", "-DFOO=1"], "lib x") == ["-Wall", "-DFOO=1"]
+    assert "Ignoring trailing '-I'" in caplog.text