diff --git a/esphome/espidf/framework.py b/esphome/espidf/framework.py
index e012117b4f..39c036e6d7 100644
--- a/esphome/espidf/framework.py
+++ b/esphome/espidf/framework.py
@@ -785,6 +785,7 @@ def _prefetch_idf_tool_archives(
         # The installer downloads anything missing itself; never let the
         # prefetch become a new way for the install to fail.
         _LOGGER.warning("ESP-IDF tool prefetch failed: %s", e)
+        _LOGGER.debug("Prefetch failure detail", exc_info=True)
 
 
 def _check_esphome_idf_framework_install(
diff --git a/esphome/platformio/extra_script.py b/esphome/platformio/extra_script.py
index 921fe2fd5d..363c40561f 100644
--- a/esphome/platformio/extra_script.py
+++ b/esphome/platformio/extra_script.py
@@ -114,10 +114,19 @@ class _FakeSConsEnv:
         self.result = ExtraScriptResult()
         self._warned_methods: set[str] = set()
         self._warned_keys: set[str] = set()
+        self._warned_gets: set[str] = set()
 
     # ----- SCons env API the common scripts use -----
 
     def get(self, key: str, default: str | None = None) -> str | None:
+        if key not in self._vars and key not in self._warned_gets:
+            # A script branching on an unmodelled var silently takes the
+            # default branch; make that diagnosable from a normal build log
+            self._warned_gets.add(key)
+            _LOGGER.warning(
+                "PIO extra-script env.get(%r) is not modelled; returning the default",
+                key,
+            )
         return self._vars.get(key, default)
 
     def __getitem__(self, key: str) -> str:
@@ -140,6 +149,12 @@ class _FakeSConsEnv:
             bucket = getattr(self.result, key.lower())
             bucket.extend(items)
 
+    # Same keys, same flattened capture; ordering/dedup don't matter since
+    # the consumer re-orders anyway
+    Prepend = Append
+    AppendUnique = Append
+    PrependUnique = Append
+
     # ----- Everything else is a no-op so unsupported scripts don't crash -----
 
     def __getattr__(self, name: str):
@@ -267,7 +282,7 @@ def captured_as_build_flags(
     flags.extend(
         f"-L{shlex.quote(_anchored(path))}" for path in _strs(result.libpath, "LIBPATH")
     )
-    flags.extend(f"-l{lib}" for lib in _strs(result.libs, "LIBS"))
+    flags.extend(f"-l{shlex.quote(lib)}" for lib in _strs(result.libs, "LIBS"))
     for define in result.cppdefines:
         # SCons also accepts dict/list CPPDEFINES; formatting those blind
         # would hand the compiler garbage like -D{'FOO': '1'}
@@ -277,6 +292,8 @@ def captured_as_build_flags(
             flags.append(f"-D{define}")
         else:
             _LOGGER.warning("Ignoring unsupported CPPDEFINES entry %r", define)
-    flags.extend(_strs(result.linkflags, "LINKFLAGS"))
-    flags.extend(_strs(result.cppflags, "CPPFLAGS"))
+    # Each captured entry is one argv token in SCons; quote so the
+    # lex_build_flags round-trip cannot split a spaced value into two
+    flags.extend(shlex.quote(f) for f in _strs(result.linkflags, "LINKFLAGS"))
+    flags.extend(shlex.quote(f) for f in _strs(result.cppflags, "CPPFLAGS"))
     return flags
diff --git a/esphome/platformio/library.py b/esphome/platformio/library.py
index ce74d828da..e7ce437570 100644
--- a/esphome/platformio/library.py
+++ b/esphome/platformio/library.py
@@ -925,6 +925,12 @@ def _content_lengths(urls: list[str]) -> list[int | None]:
     """Content-Length per URL via HEAD requests; None when unknown."""
     import requests
 
+    from esphome.happy_eyeballs import ensure_happy_eyeballs
+
+    # Same convention as every other network call: without it a broken-IPv6
+    # network burns the full timeout per HEAD before falling back
+    ensure_happy_eyeballs()
+
     def head(url: str) -> int | None:
         try:
             resp = requests.head(url, timeout=10, allow_redirects=True)
diff --git a/tests/unit_tests/test_platformio_extra_script.py b/tests/unit_tests/test_platformio_extra_script.py
index 81de70cdf7..a4eeb5bf41 100644
--- a/tests/unit_tests/test_platformio_extra_script.py
+++ b/tests/unit_tests/test_platformio_extra_script.py
@@ -432,6 +432,41 @@ def test_run_extra_script_bad_encoding_is_best_effort(tmp_path, caplog) -> None:
     assert "is not UTF-8" in caplog.text
 
 
+@pytest.mark.parametrize("method", ("Prepend", "AppendUnique", "PrependUnique"))
+def test_append_variants_capture_like_append(method: str) -> None:
+    """Prepend/AppendUnique/PrependUnique write the captured keys too."""
+    env = _FakeSConsEnv(
+        board_mcu="esp8266", pio_env="esphome_esp8266", pio_platform="espressif8266"
+    )
+    getattr(env, method)(LIBS=["algobsec"], LIBPATH=["lib"])
+    assert env.result.libs == ["algobsec"]
+    assert env.result.libpath == ["lib"]
+
+
+def test_env_get_unknown_key_warns_once(caplog) -> None:
+    """A script branching on an unmodelled env var is diagnosable."""
+    env = _FakeSConsEnv(
+        board_mcu="esp8266", pio_env="esphome_esp8266", pio_platform="espressif8266"
+    )
+    assert env.get("BOARD") is None
+    assert env.get("BOARD", "d1") == "d1"
+    assert env.get("BOARD_MCU") == "esp8266"
+    assert caplog.text.count("env.get('BOARD') is not modelled") == 1
+    assert "BOARD_MCU" not in caplog.text
+
+
+def test_spaced_linkflag_survives_relexing(tmp_path) -> None:
+    """A captured argv token with a space stays one token after lexing."""
+    result = ExtraScriptResult(
+        linkflags=["-Wl,-T my linker.ld"], cppflags=["-include my hdr.h"]
+    )
+    flags = captured_as_build_flags(result, library_dir=tmp_path)
+    assert lex_build_flags(flags, "test") == [
+        "-Wl,-T my linker.ld",
+        "-include my hdr.h",
+    ]
+
+
 def test_uncaptured_append_key_warns_once(caplog) -> None:
     """A loop of Appends to the same uncaptured key warns once."""