diff --git a/.github/actions/cache-arduino8266/action.yml b/.github/actions/cache-arduino8266/action.yml
index a9fff84d6c..15105004ec 100644
--- a/.github/actions/cache-arduino8266/action.yml
+++ b/.github/actions/cache-arduino8266/action.yml
@@ -16,6 +16,12 @@ runs:
       id: version
       shell: bash
       run: |
+        # The caller's install prefix must match the cached path below, or
+        # the cache silently stores/restores an empty directory
+        [ "$ESPHOME_ARDUINO8266_PREFIX" = "$HOME/.esphome-arduino8266" ] || {
+          echo "ESPHOME_ARDUINO8266_PREFIX is '$ESPHOME_ARDUINO8266_PREFIX', expected '$HOME/.esphome-arduino8266'" >&2
+          exit 1
+        }
         . venv/bin/activate
         key=$(python -c 'from esphome.components.esp8266 import RECOMMENDED_ARDUINO_FRAMEWORK_VERSION as f; from esphome.arduino8266.framework import TOOLCHAIN_VERSION as t; print(f"{f}-{t}")')
         [ -n "$key" ] || exit 1
diff --git a/esphome/__main__.py b/esphome/__main__.py
index f1a8934c00..2a6033d296 100644
--- a/esphome/__main__.py
+++ b/esphome/__main__.py
@@ -1999,6 +1999,15 @@ def command_analyze_memory(args: ArgsProtocol, config: ConfigType) -> int:
     from esphome.analyze_memory.cli import MemoryAnalyzerCLI
     from esphome.analyze_memory.ram_strings import RamStringsAnalyzer
 
+    # Refuse an unsupported toolchain before paying for a full compile
+    native_toolchain = _native_toolchain_module()
+    if native_toolchain is None and not CORE.using_toolchain_platformio:
+        _LOGGER.error(
+            "analyze-memory is not supported with the '%s' toolchain",
+            CORE.toolchain.value if CORE.toolchain else "unresolved",
+        )
+        return 1
+
     # Always compile to ensure fresh data (fast if no changes - just relinks)
     exit_code = write_cpp(config)
     if exit_code != 0:
@@ -2010,14 +2019,6 @@ def command_analyze_memory(args: ArgsProtocol, config: ConfigType) -> int:
 
     # Get idedata for analysis
     idedata = None
-    native_toolchain = _native_toolchain_module()
-
-    if native_toolchain is None and not CORE.using_toolchain_platformio:
-        _LOGGER.error(
-            "analyze-memory is not supported with the '%s' toolchain",
-            CORE.toolchain.value if CORE.toolchain else "unresolved",
-        )
-        return 1
     if native_toolchain is not None:
         objdump_path = str(native_toolchain.get_objdump_path())
         readelf_path = str(native_toolchain.get_readelf_path())
diff --git a/esphome/arduino8266/toolchain.py b/esphome/arduino8266/toolchain.py
index 6ae7b9c277..d1a5e6e461 100644
--- a/esphome/arduino8266/toolchain.py
+++ b/esphome/arduino8266/toolchain.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import json
 import logging
 from pathlib import Path
 import subprocess
@@ -131,6 +132,18 @@ def _write_compile_commands(
         # the memory analyzer) can't silently read outdated data.
         (build_dir / "compile_commands.json").unlink(missing_ok=True)
         raise EsphomeError(f"Could not generate compile_commands.json: {result.stderr}")
+    try:
+        entries = json.loads(result.stdout)
+    except ValueError:
+        entries = None
+    if not entries:
+        # compdb exits 0 with [] for unknown rule names; a renamed compile
+        # rule must fail the build, not silently strand every consumer
+        (build_dir / "compile_commands.json").unlink(missing_ok=True)
+        raise EsphomeError(
+            "ninja produced an empty compile database; the generator's rule "
+            "names no longer match"
+        )
     # write_file_if_changed keeps the mtime stable on no-op builds so the
     # idedata cache in get_idedata() stays valid.
     write_file_if_changed(build_dir / "compile_commands.json", result.stdout)
@@ -152,7 +165,8 @@ def _parse_app_size(build_dir: Path, paths: framework.InstalledPaths) -> int | N
     app_size = segment_length(ld_text, "irom0_0_seg")
     if app_size is None:
         _LOGGER.warning("irom0_0_seg not found in %s; skipping Flash summary", ld_path)
-    elif app_size == 0:
+        return None
+    if app_size == 0:
         _LOGGER.warning(
             "irom0_0_seg has zero length in %s; skipping Flash summary", ld_path
         )
@@ -169,13 +183,19 @@ def _print_size_summary(build_dir: Path, paths: framework.InstalledPaths) -> Non
     from esphome.build_helpers.size_summary import print_size_line
 
     size_tool = _toolchain_tool("size")
-    result = subprocess.run(
-        [str(size_tool), "-A", "-d", str(get_elf_path())],
-        capture_output=True,
-        text=True,
-        check=False,
-        close_fds=False,
-    )
+    try:
+        result = subprocess.run(
+            [str(size_tool), "-A", "-d", str(get_elf_path())],
+            capture_output=True,
+            text=True,
+            check=False,
+            close_fds=False,
+        )
+    except OSError as err:
+        # The summary is a bonus artifact like idedata; a truncated
+        # toolchain extraction must not discard an already-linked build
+        _LOGGER.warning("Could not summarize firmware size: %s", err)
+        return
     if result.returncode != 0:
         _LOGGER.warning("Could not summarize firmware size: %s", result.stderr)
         return
diff --git a/tests/unit_tests/components/esp8266/test_toolchain_validation.py b/tests/unit_tests/components/esp8266/test_toolchain_validation.py
index f7ef786ea1..6582180e19 100644
--- a/tests/unit_tests/components/esp8266/test_toolchain_validation.py
+++ b/tests/unit_tests/components/esp8266/test_toolchain_validation.py
@@ -24,7 +24,7 @@ from esphome.const import (
     CONF_VERSION,
     Toolchain,
 )
-from esphome.core import CORE
+from esphome.core import CORE, EsphomeError
 from esphome.types import ConfigType
 
 
@@ -163,3 +163,29 @@ def test_resolve_toolchain_rejects_unsupported() -> None:
     CORE.toolchain = Toolchain.SDK_NRF
     with pytest.raises(cv.Invalid, match="Unsupported toolchain 'sdk-nrf'"):
         _resolve_toolchain({})
+
+
+def test_run_compile_platformio_falls_through() -> None:
+    """Under toolchain: platformio the hook returns False without touching
+    the native backend; this is what keeps existing users on PlatformIO."""
+    CORE.toolchain = Toolchain.PLATFORMIO
+    with patch("esphome.arduino8266.toolchain.run_compile") as mock_native:
+        assert esp8266.run_compile(SimpleNamespace(), {}) is False
+    mock_native.assert_not_called()
+
+
+def test_run_compile_arduino_failure_raises() -> None:
+    """A non-zero native build fails by name instead of returning success."""
+    CORE.verbose = False
+    with (
+        patch("esphome.arduino8266.toolchain.run_compile", return_value=1),
+        pytest.raises(EsphomeError, match="native build failed"),
+    ):
+        esp8266.run_compile(SimpleNamespace(), {})
+
+
+def test_copy_files_native_skips_platformio_scripts(tmp_path: Path) -> None:
+    """The native build writes no PlatformIO extra scripts."""
+    CORE.build_path = tmp_path
+    esp8266.copy_files()
+    assert list(tmp_path.iterdir()) == []
diff --git a/tests/unit_tests/test_arduino8266_toolchain.py b/tests/unit_tests/test_arduino8266_toolchain.py
index bc267ca2a2..e7a15552cd 100644
--- a/tests/unit_tests/test_arduino8266_toolchain.py
+++ b/tests/unit_tests/test_arduino8266_toolchain.py
@@ -120,13 +120,33 @@ def test_run_compile_warns_when_idedata_fails(
 def test_write_compile_commands(tmp_path: Path) -> None:
     build_dir = tmp_path / "build"
     build_dir.mkdir()
+    entries = '[{"file": "a.cpp", "command": "cc"}]\n'
     with patch.object(
         toolchain.subprocess,
         "run",
-        return_value=MagicMock(returncode=0, stdout="[]\n"),
+        return_value=MagicMock(returncode=0, stdout=entries),
     ):
         toolchain._write_compile_commands(tmp_path / "ninja", build_dir, {})
-    assert (build_dir / "compile_commands.json").read_text() == "[]\n"
+    assert (build_dir / "compile_commands.json").read_text() == entries
+
+
+@pytest.mark.parametrize("stdout", ["[]\n", "not json"])
+def test_write_compile_commands_empty_db_raises(tmp_path: Path, stdout: str) -> None:
+    """An empty compile database (compdb exits 0 with [] for unknown rule
+    names) must fail the build and drop any stale database."""
+    build_dir = tmp_path / "build"
+    build_dir.mkdir()
+    (build_dir / "compile_commands.json").write_text("[stale]")
+    with (
+        patch.object(
+            toolchain.subprocess,
+            "run",
+            return_value=MagicMock(returncode=0, stdout=stdout),
+        ),
+        pytest.raises(EsphomeError, match="empty compile database"),
+    ):
+        toolchain._write_compile_commands(tmp_path / "ninja", build_dir, {})
+    assert not (build_dir / "compile_commands.json").exists()
 
 
 def test_write_compile_commands_failure_removes_stale_db(tmp_path: Path) -> None:
@@ -183,6 +203,18 @@ def test_print_size_summary(tmp_path: Path, capsys: pytest.CaptureFixture[str])
     assert "Flash: [====      ]  35.9% (used 375301 bytes from 1044464 bytes)" in out
 
 
+def test_print_size_summary_missing_size_tool_warns(
+    tmp_path: Path, caplog: pytest.LogCaptureFixture
+) -> None:
+    """A missing size binary degrades to a warning; the firmware already
+    linked and must not be discarded."""
+    with patch.object(
+        toolchain.subprocess, "run", side_effect=FileNotFoundError("no size")
+    ):
+        toolchain._print_size_summary(tmp_path, _paths(tmp_path))
+    assert "Could not summarize firmware size" in caplog.text
+
+
 def test_print_size_summary_no_app_size(
     tmp_path: Path, capsys: pytest.CaptureFixture[str]
 ) -> None: