diff --git a/esphome/__main__.py b/esphome/__main__.py
index c641dff0c2..8e6d88f26e 100644
--- a/esphome/__main__.py
+++ b/esphome/__main__.py
@@ -863,11 +863,18 @@ def compile_program(args: ArgsProtocol, config: ConfigType) -> int:
         try:
             if toolchain.get_idedata() is None:
                 _LOGGER.warning("No idedata was generated for this build")
-        except (EsphomeError, OSError, RuntimeError, ValueError) as err:
+        except (
+            EsphomeError,
+            LookupError,
+            OSError,
+            RuntimeError,
+            ValueError,
+        ) as err:
             # The firmware already built; idedata is a bonus artifact here.
             # Broad on purpose: a vanished compiler (OSError), a failed
             # include probe (RuntimeError), or a truncated compile DB
-            # (ValueError) must not fail a successful build either.
+            # (ValueError/LookupError) must not fail a successful build
+            # either.
             _LOGGER.warning("Could not generate idedata: %s", err)
     else:
         from esphome.platformio import toolchain
diff --git a/esphome/platformio/extra_script.py b/esphome/platformio/extra_script.py
index 422cb84ee5..4a0a0dc7c5 100644
--- a/esphome/platformio/extra_script.py
+++ b/esphome/platformio/extra_script.py
@@ -131,6 +131,7 @@ class _FakeSConsEnv:
         }
         self.result = ExtraScriptResult()
         self._warned_methods: set[str] = set()
+        self._warned_keys: set[str] = set()
 
     # ----- SCons env API the common scripts use -----
 
@@ -141,11 +142,14 @@ class _FakeSConsEnv:
         for key, value in kwargs.items():
             if key not in _CAPTURED_KEYS:
                 # Diagnosable from the build log when a script configures
-                # something this shim does not translate
-                _LOGGER.warning(
-                    "PIO extra-script env.Append(%s=...) is not captured; ignoring",
-                    key,
-                )
+                # something this shim does not translate; once per key so a
+                # loop of Appends cannot spam
+                if key not in self._warned_keys:
+                    self._warned_keys.add(key)
+                    _LOGGER.warning(
+                        "PIO extra-script env.Append(%s=...) is not captured; ignoring",
+                        key,
+                    )
                 continue
             items = list(value) if isinstance(value, (list, tuple)) else [value]
             bucket = getattr(self.result, key.lower())
@@ -194,10 +198,19 @@ def run_extra_script(
     )
     try:
         source = script_path.read_text(encoding="utf-8")
-    except (OSError, UnicodeDecodeError) as err:
+    except OSError as err:
         # An unreadable declared script is a broken package, exactly like a
         # missing one; must not be quieter than that case
         raise EsphomeError(f"extraScript {script_path} is unreadable: {err}") from err
+    except UnicodeDecodeError as e:
+        # A content problem, best-effort like a SyntaxError below
+        _LOGGER.warning(
+            "PIO extra-script %s (in %s) is not UTF-8 (%r); ignoring its output",
+            script_path,
+            library_dir.name,
+            e,
+        )
+        return ExtraScriptResult()
     old_cwd = Path.cwd()
     try:
         # Inside the try: a SyntaxError in a vendored script is just as
diff --git a/tests/unit_tests/test_main.py b/tests/unit_tests/test_main.py
index 53a2d7faac..be8209b7c9 100644
--- a/tests/unit_tests/test_main.py
+++ b/tests/unit_tests/test_main.py
@@ -7265,6 +7265,7 @@ def test_command_idedata_incompatible_toolchain(tmp_path: Path) -> None:
         FileNotFoundError("no such compiler"),
         RuntimeError("Could not query builtin include dirs"),
         ValueError("no C++ translation unit found"),
+        KeyError("command"),
         None,  # replaced with EsphomeError inside (import is function-local)
     ],
 )
diff --git a/tests/unit_tests/test_platformio_extra_script.py b/tests/unit_tests/test_platformio_extra_script.py
index 753fda45a9..7e904183f1 100644
--- a/tests/unit_tests/test_platformio_extra_script.py
+++ b/tests/unit_tests/test_platformio_extra_script.py
@@ -330,12 +330,42 @@ def test_run_extra_script_sys_exit_zero_is_success(tmp_path, caplog) -> None:
 
 def test_run_extra_script_unreadable_raises(tmp_path) -> None:
     """An unreadable declared script is a broken package, like a missing one."""
+    from unittest.mock import patch
+
     from esphome.core import EsphomeError
     from esphome.platformio.extra_script import run_extra_script
 
     script = tmp_path / "extra.py"
-    script.write_bytes(b"\xff\xfe\x00bad")
-    with pytest.raises(EsphomeError, match="is unreadable"):
+    script.write_text("")
+    with (
+        patch("pathlib.Path.read_text", side_effect=OSError("denied")),
+        pytest.raises(EsphomeError, match="is unreadable"),
+    ):
         run_extra_script(
             script, library_dir=tmp_path, board_mcu="esp32", pio_platform="espressif32"
         )
+
+
+def test_run_extra_script_bad_encoding_is_best_effort(tmp_path, caplog) -> None:
+    """Undecodable content warns and skips, like a SyntaxError."""
+    from esphome.platformio.extra_script import run_extra_script
+
+    script = tmp_path / "extra.py"
+    script.write_bytes(b"\xff\xfe\x00bad")
+    result = run_extra_script(
+        script, library_dir=tmp_path, board_mcu="esp32", pio_platform="espressif32"
+    )
+    assert result.libs == []
+    assert "is not UTF-8" in caplog.text
+
+
+def test_uncaptured_append_key_warns_once(caplog) -> None:
+    """A loop of Appends to the same uncaptured key warns once."""
+    from esphome.platformio.extra_script import _FakeSConsEnv
+
+    env = _FakeSConsEnv(
+        board_mcu="esp8266", pio_env="esphome_esp8266", pio_platform="espressif8266"
+    )
+    env.Append(CPPPATH=["a"])
+    env.Append(CPPPATH=["b"])
+    assert caplog.text.count("env.Append(CPPPATH=...) is not captured") == 1