diff --git a/esphome/arduino8266/component.py b/esphome/arduino8266/component.py
index 89636ec6fa..2b3b5b5da3 100644
--- a/esphome/arduino8266/component.py
+++ b/esphome/arduino8266/component.py
@@ -93,9 +93,9 @@ def _library_info(name: str, read_path: Path, data: dict) -> ArduinoLibrary:
             lib.include_dirs.append(path.resolve())
 
     lib.sources = sorted(
-        Path(f).resolve()
+        path.resolve()
         for f in collect_filtered_files(read_path / src_dir, src_filter)
-        if Path(f).suffix in SRC_FILE_EXTENSIONS
+        if (path := Path(f)).suffix in SRC_FILE_EXTENSIONS
     )
     return lib
 
diff --git a/esphome/arduino8266/toolchain.py b/esphome/arduino8266/toolchain.py
index a434a53501..7e3aea3a13 100644
--- a/esphome/arduino8266/toolchain.py
+++ b/esphome/arduino8266/toolchain.py
@@ -4,7 +4,6 @@ from __future__ import annotations
 
 import logging
 from pathlib import Path
-import re
 import subprocess
 
 from esphome.arduino8266 import framework
@@ -43,7 +42,7 @@ def run_compile(config: ConfigType, verbose: bool) -> int:
     from esphome.build_gen import arduino8266 as build_gen
 
     paths = framework.check_and_install(CORE.data[KEY_CORE][KEY_FRAMEWORK_VERSION])
-    build_gen.write_project(paths)
+    ninja_changed = build_gen.write_project(paths)
 
     build_dir = get_build_dir()
     env = framework.get_build_env(paths["toolchain_path"])
@@ -58,7 +57,10 @@ def run_compile(config: ConfigType, verbose: bool) -> int:
     if rc != 0:
         return rc
 
-    _write_compile_commands(paths["ninja_path"], build_dir, env)
+    # The compile database is a pure function of build.ninja; skip its
+    # regeneration (a ninja spawn plus MBs of text) on unchanged builds.
+    if ninja_changed or not (build_dir / "compile_commands.json").is_file():
+        _write_compile_commands(paths["ninja_path"], build_dir, env)
     _print_size_summary(build_dir, paths["toolchain_path"])
     get_idedata()
     return 0
@@ -86,16 +88,13 @@ def _write_compile_commands(
 def _parse_app_size(build_dir: Path) -> int | None:
     """Read the app flash budget (irom0_0_seg length) from the linker script."""
     from esphome.build_gen.arduino8266 import get_flash_ld_path
+    from esphome.components.esp8266.build_surgery import segment_length
 
-    appsize_re = re.compile(r"irom0_0_seg\s*:.+len\s*=\s*(0x[\da-f]+)", re.IGNORECASE)
     try:
         ld_text = get_flash_ld_path(build_dir).read_text(encoding="utf-8")
     except OSError:
         return None
-    for line in ld_text.splitlines():
-        if match := appsize_re.search(line):
-            return int(match.group(1), 16)
-    return None
+    return segment_length(ld_text, "irom0_0_seg")
 
 
 def _print_size_summary(build_dir: Path, toolchain_path: Path) -> None:
diff --git a/esphome/build_gen/arduino8266.py b/esphome/build_gen/arduino8266.py
index 1429e51c40..1ebed13961 100644
--- a/esphome/build_gen/arduino8266.py
+++ b/esphome/build_gen/arduino8266.py
@@ -373,8 +373,12 @@ def _common_parent(paths: list[Path]) -> Path:
     return Path(os.path.commonpath([str(p.parent) for p in paths]))
 
 
-def write_project(paths: dict[str, Path]) -> None:
-    """Write the ninja build for the current configuration."""
+def write_project(paths: dict[str, Path]) -> bool:
+    """Write the ninja build for the current configuration.
+
+    Returns True when ``build.ninja`` changed, so the caller can skip work
+    derived purely from it (the compile database) on unchanged builds.
+    """
     from esphome.arduino8266.component import resolve_libraries
     from esphome.arduino8266.framework import ccache_path
 
@@ -551,7 +555,7 @@ def write_project(paths: dict[str, Path]) -> None:
     lines.append("default firmware.factory.bin firmware.ota.bin")
     lines.append("")
 
-    write_file_if_changed(build_dir / "build.ninja", "\n".join(lines))
+    return write_file_if_changed(build_dir / "build.ninja", "\n".join(lines))
 
 
 def get_flash_ld_path(build_dir: Path) -> Path:
diff --git a/esphome/components/esp8266/boards.py b/esphome/components/esp8266/boards.py
index 4571cff1c3..e5a9628e90 100644
--- a/esphome/components/esp8266/boards.py
+++ b/esphome/components/esp8266/boards.py
@@ -361,12 +361,30 @@ BOARDS = {
     },
 }
 
+"""
+ESP8266_BOARD_BUILD generate with:
+
+git clone https://github.com/platformio/platform-espressif8266
+python3 - <<'EOF'
+import json, glob, os
+for f in sorted(glob.glob("platform-espressif8266/boards/*.json")):
+    b = json.load(open(f))["build"]
+    extra = b["extra_flags"]
+    extra = extra.split() if isinstance(extra, str) else extra
+    defines = [
+        e[2:] for e in extra if e not in ("-DESP8266", "-DARDUINO_ARCH_ESP8266")
+    ]
+    entries = ", ".join(f'"{d}"' for d in defines) + ("," if len(defines) == 1 else "")
+    board = os.path.splitext(os.path.basename(f))[0]
+    print(f'    "{board}": {{"variant": "{b["variant"]}", "defines": ({entries})}},')
+EOF
+"""
+
 # Per-board Arduino core build metadata for the native (PlatformIO-free)
 # toolchain: the variant directory (supplies pins_arduino.h) and the
 # board-identity defines the PlatformIO builder passes via build.extra_flags.
 # -DESP8266 and -DARDUINO_ARCH_ESP8266 are shared by every board and added by
 # the generator; only the per-board defines are listed here.
-# Generated from platform-espressif8266 boards/*.json (see BOARDS note above).
 ESP8266_BOARD_BUILD = {
     "agruminolemon": {
         "variant": "agruminolemonv4",
diff --git a/esphome/components/esp8266/build_surgery.py b/esphome/components/esp8266/build_surgery.py
index 981204a4ff..8b09a3c80a 100644
--- a/esphome/components/esp8266/build_surgery.py
+++ b/esphome/components/esp8266/build_surgery.py
@@ -57,3 +57,12 @@ def apply_testing_memory_patches(content: str) -> str:
     content = _patch_segment_size(content, "iram1_0_seg", TESTING_IRAM_SIZE)
     content = _patch_segment_size(content, "dram0_0_seg", TESTING_DRAM_SIZE)
     return _patch_segment_size(content, "irom0_0_seg", TESTING_FLASH_SIZE)
+
+
+def segment_length(content: str, segment_name: str) -> int | None:
+    """Read a memory segment's length from linker script content."""
+    match = re.search(
+        rf"{segment_name}\s*:.+len\s*=\s*(0x[\da-fA-F]+)",
+        content,
+    )
+    return int(match.group(1), 16) if match else None
diff --git a/script/determine-jobs.py b/script/determine-jobs.py
index bac890d26e..c46529dfc6 100755
--- a/script/determine-jobs.py
+++ b/script/determine-jobs.py
@@ -513,16 +513,31 @@ ESP32_PLATFORMIO_TRIGGER_FILES = frozenset(
 )
 
 
+def _path_or_file_trigger(
+    files: list[str],
+    trigger_files: frozenset[str],
+    trigger_prefixes: tuple[str, ...],
+) -> bool:
+    """Whether any changed file matches the given infrastructure triggers."""
+    return any(
+        file in trigger_files or file.startswith(trigger_prefixes) for file in files
+    )
+
+
+@cache
+def _changed_components_closure(branch: str | None) -> frozenset[str]:
+    """Dependency closure of the changed components (shared by the
+    per-toolchain narrowing functions and cached per branch)."""
+    files = changed_files(branch)
+    component_files = [f for f in files if filter_component_and_test_files(f)]
+    return frozenset(get_components_with_dependencies(component_files, True))
+
+
 def _esp32_platformio_path_or_file_trigger(files: list[str]) -> bool:
     """Whether any changed file is a PlatformIO infrastructure / harness trigger."""
-    for file in files:
-        if file in ESP32_PLATFORMIO_TRIGGER_FILES:
-            return True
-        if any(
-            file.startswith(prefix) for prefix in ESP32_PLATFORMIO_TRIGGER_PATH_PREFIXES
-        ):
-            return True
-    return False
+    return _path_or_file_trigger(
+        files, ESP32_PLATFORMIO_TRIGGER_FILES, ESP32_PLATFORMIO_TRIGGER_PATH_PREFIXES
+    )
 
 
 # ESP-IDF infra: changes under esphome/espidf/ or to the IDF build generator
@@ -536,14 +551,9 @@ ESP_IDF_INFRA_TRIGGER_FILES = frozenset({"esphome/build_gen/espidf.py"})
 
 def _esp_idf_infra_changed(files: list[str]) -> bool:
     """Whether any changed file is ESP-IDF build/runner infrastructure."""
-    for file in files:
-        if file in ESP_IDF_INFRA_TRIGGER_FILES:
-            return True
-        if any(
-            file.startswith(prefix) for prefix in ESP_IDF_INFRA_TRIGGER_PATH_PREFIXES
-        ):
-            return True
-    return False
+    return _path_or_file_trigger(
+        files, ESP_IDF_INFRA_TRIGGER_FILES, ESP_IDF_INFRA_TRIGGER_PATH_PREFIXES
+    )
 
 
 def esp32_platformio_components_to_test(branch: str | None = None) -> list[str]:
@@ -586,10 +596,9 @@ def esp32_platformio_components_to_test(branch: str | None = None) -> list[str]:
     if core_changed(files) or _esp32_platformio_path_or_file_trigger(files):
         return sorted(ESP32_PLATFORMIO_TEST_COMPONENTS)
 
-    component_files = [f for f in files if filter_component_and_test_files(f)]
-    changed = get_components_with_dependencies(component_files, True)
-
-    return sorted(ESP32_PLATFORMIO_TEST_COMPONENTS & set(changed))
+    return sorted(
+        ESP32_PLATFORMIO_TEST_COMPONENTS & _changed_components_closure(branch)
+    )
 
 
 def should_run_esp32_platformio(branch: str | None = None) -> bool:
@@ -645,14 +654,9 @@ ESP8266_NATIVE_TRIGGER_FILES = frozenset(
 
 def _esp8266_native_path_or_file_trigger(files: list[str]) -> bool:
     """Whether any changed file is native-ESP8266 infrastructure / harness."""
-    for file in files:
-        if file in ESP8266_NATIVE_TRIGGER_FILES:
-            return True
-        if any(
-            file.startswith(prefix) for prefix in ESP8266_NATIVE_TRIGGER_PATH_PREFIXES
-        ):
-            return True
-    return False
+    return _path_or_file_trigger(
+        files, ESP8266_NATIVE_TRIGGER_FILES, ESP8266_NATIVE_TRIGGER_PATH_PREFIXES
+    )
 
 
 def esp8266_native_components_to_test(branch: str | None = None) -> list[str]:
@@ -667,10 +671,7 @@ def esp8266_native_components_to_test(branch: str | None = None) -> list[str]:
     if core_changed(files) or _esp8266_native_path_or_file_trigger(files):
         return sorted(ESP8266_NATIVE_TEST_COMPONENTS)
 
-    component_files = [f for f in files if filter_component_and_test_files(f)]
-    changed = get_components_with_dependencies(component_files, True)
-
-    return sorted(ESP8266_NATIVE_TEST_COMPONENTS & set(changed))
+    return sorted(ESP8266_NATIVE_TEST_COMPONENTS & _changed_components_closure(branch))
 
 
 def determine_cpp_unit_tests(
diff --git a/tests/unit_tests/build_gen/test_arduino8266.py b/tests/unit_tests/build_gen/test_arduino8266.py
index 8353d6c461..f7d2264e73 100644
--- a/tests/unit_tests/build_gen/test_arduino8266.py
+++ b/tests/unit_tests/build_gen/test_arduino8266.py
@@ -16,7 +16,16 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
+from esphome.build_gen import arduino8266
+from esphome.build_gen.arduino8266 import (
+    _defines_flags,
+    _flag_defines,
+    _flash_size_str,
+    _resolve_build_config,
+    get_flash_ld_path,
+)
 from esphome.components.esp8266.boards import BOARDS, ESP8266_BOARD_BUILD
+from esphome.components.esp8266.build_surgery import RATETABLE_RULE
 from esphome.components.esp8266.const import (
     KEY_BOARD,
     KEY_ESP8266,
@@ -55,7 +64,6 @@ def test_board_build_covers_every_board() -> None:
 
 
 def test_build_config_defaults() -> None:
-    from esphome.build_gen.arduino8266 import _flag_defines, _resolve_build_config
 
     _set_flags()
     config = _resolve_build_config(_flag_defines())
@@ -75,7 +83,6 @@ def test_build_config_defaults() -> None:
 def test_build_config_esphome_lwip_knob() -> None:
     """The lwIP variant ESPHome selects maps to the same defines and library
     as the PlatformIO builder."""
-    from esphome.build_gen.arduino8266 import _flag_defines, _resolve_build_config
 
     _set_flags("-DPIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH")
     config = _resolve_build_config(_flag_defines())
@@ -86,7 +93,6 @@ def test_build_config_esphome_lwip_knob() -> None:
 
 
 def test_build_config_knobs() -> None:
-    from esphome.build_gen.arduino8266 import _flag_defines, _resolve_build_config
 
     _set_flags(
         "-DPIO_FRAMEWORK_ARDUINO_ESPRESSIF_SDK305",
@@ -102,7 +108,6 @@ def test_build_config_knobs() -> None:
 
 
 def test_build_config_mmu_custom_requires_sizes() -> None:
-    from esphome.build_gen.arduino8266 import _flag_defines, _resolve_build_config
 
     _set_flags("-DPIO_FRAMEWORK_ARDUINO_MMU_CUSTOM")
     with pytest.raises(EsphomeError, match="MMU_IRAM_SIZE"):
@@ -122,11 +127,6 @@ def test_build_config_mmu_custom_requires_sizes() -> None:
 
 def test_defines_match_platformio_builder() -> None:
     """The exact define set the PlatformIO builder passes for nodemcuv2/dout."""
-    from esphome.build_gen.arduino8266 import (
-        _defines_flags,
-        _flag_defines,
-        _resolve_build_config,
-    )
 
     _set_flags("-DPIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH")
     assert _defines_flags(
@@ -183,9 +183,11 @@ def _make_framework(tmp_path: Path) -> dict[str, Path]:
     }
 
 
-def _write_ninja(paths: dict[str, Path]) -> str:
-    from esphome.build_gen import arduino8266
-
+def _write_ninja(
+    paths: dict[str, Path],
+    libraries: list | None = None,
+    ccache: str | None = None,
+) -> str:
     src = CORE.relative_src_path()
     (src / "esphome" / "components" / "esp8266").mkdir(parents=True, exist_ok=True)
     (src / "main.cpp").write_text("")
@@ -193,7 +195,11 @@ def _write_ninja(paths: dict[str, Path]) -> str:
 
     with (
         patch.object(arduino8266, "generate_ld_scripts"),
-        patch("esphome.arduino8266.framework.ccache_path", return_value=None),
+        patch(
+            "esphome.arduino8266.component.resolve_libraries",
+            return_value=libraries or [],
+        ),
+        patch("esphome.arduino8266.framework.ccache_path", return_value=ccache),
     ):
         arduino8266.write_project(paths)
     return (CORE.relative_pioenvs_path(CORE.name) / "build.ninja").read_text()
@@ -284,7 +290,6 @@ def test_build_config_lwip_variants(
     knob: str, lib: str, mss: int, features: int, ipv6: int
 ) -> None:
     """Every lwIP knob maps to the same defines and library as the PIO builder."""
-    from esphome.build_gen.arduino8266 import _flag_defines, _resolve_build_config
 
     _set_flags(f"-D{knob}")
     config = _resolve_build_config(_flag_defines())
@@ -321,14 +326,12 @@ def test_build_config_lwip_variants(
     ],
 )
 def test_build_config_mmu_variants(knob: str, expected: list[str]) -> None:
-    from esphome.build_gen.arduino8266 import _flag_defines, _resolve_build_config
 
     _set_flags(f"-D{knob}")
     assert _resolve_build_config(_flag_defines()).mmu_defines == expected
 
 
 def test_build_config_waveform_locked_phase() -> None:
-    from esphome.build_gen.arduino8266 import _flag_defines, _resolve_build_config
 
     _set_flags("-DPIO_FRAMEWORK_ARDUINO_WAVEFORM_LOCKED_PHASE", "-DFP_IN_IROM")
     config = _resolve_build_config(_flag_defines())
@@ -345,16 +348,13 @@ _COMMON_LD_H_OUTPUT = """\
 
 
 def _run_generate_ld_scripts(paths: dict[str, Path]) -> Path:
-    from esphome.build_gen import arduino8266
 
-    config = arduino8266._resolve_build_config(arduino8266._flag_defines())
+    config = _resolve_build_config(_flag_defines())
     arduino8266.generate_ld_scripts(paths, config, "eagle.flash.4m.ld")
     return CORE.relative_pioenvs_path(CORE.name, "ld")
 
 
 def test_generate_ld_scripts(tmp_path: Path) -> None:
-    from esphome.build_gen import arduino8266
-    from esphome.components.esp8266.build_surgery import RATETABLE_RULE
 
     paths = _make_framework(tmp_path)
     _set_flags("-DFP_IN_IROM")
@@ -375,7 +375,6 @@ def test_generate_ld_scripts(tmp_path: Path) -> None:
 
 
 def test_generate_ld_scripts_failure(tmp_path: Path) -> None:
-    from esphome.build_gen import arduino8266
 
     paths = _make_framework(tmp_path)
     result = MagicMock(returncode=1, stderr="nope")
@@ -387,7 +386,6 @@ def test_generate_ld_scripts_failure(tmp_path: Path) -> None:
 
 
 def test_generate_ld_scripts_testing_mode(tmp_path: Path) -> None:
-    from esphome.build_gen import arduino8266
 
     paths = _make_framework(tmp_path)
     (paths["framework_path"] / "tools" / "sdk" / "ld" / "eagle.flash.4m.ld").write_text(
@@ -403,7 +401,6 @@ def test_generate_ld_scripts_testing_mode(tmp_path: Path) -> None:
 
 def test_write_project_libraries_and_variant(tmp_path: Path) -> None:
     from esphome.arduino8266.component import ArduinoLibrary
-    from esphome.build_gen import arduino8266
 
     paths = _make_framework(tmp_path)
     variant_src = paths["framework_path"] / "variants" / "nodemcu" / "variant.cpp"
@@ -423,20 +420,9 @@ def test_write_project_libraries_and_variant(tmp_path: Path) -> None:
     )
     _set_flags("-DPIO_FRAMEWORK_ARDUINO_ENABLE_EXCEPTIONS")
 
-    src = CORE.relative_src_path()
-    (src / "esphome" / "components" / "esp8266").mkdir(parents=True, exist_ok=True)
-    (src / "main.cpp").write_text("")
-
-    with (
-        patch.object(arduino8266, "generate_ld_scripts"),
-        patch(
-            "esphome.arduino8266.component.resolve_libraries",
-            return_value=[library, headers_only],
-        ),
-        patch("esphome.arduino8266.framework.ccache_path", return_value="/cc/ccache"),
-    ):
-        arduino8266.write_project(paths)
-    content = (CORE.relative_pioenvs_path(CORE.name) / "build.ninja").read_text()
+    content = _write_ninja(
+        paths, libraries=[library, headers_only], ccache="/cc/ccache"
+    )
 
     assert "build libFrameworkArduinoVariant.a: ar" in content
     assert "build libMyLib.a: ar" in content
@@ -452,10 +438,9 @@ def test_write_project_libraries_and_variant(tmp_path: Path) -> None:
 
 
 def test_get_flash_ld_path(tmp_path: Path) -> None:
-    from esphome.build_gen import arduino8266
 
     CORE.testing_mode = True
-    assert arduino8266.get_flash_ld_path(tmp_path) == (
+    assert get_flash_ld_path(tmp_path) == (
         tmp_path / "ld" / "testing_eagle.flash.4m.ld"
     )
 
@@ -470,13 +455,12 @@ def test_get_flash_ld_path(tmp_path: Path) -> None:
             return_value="3.30102.0",
         ),
     ):
-        assert arduino8266.get_flash_ld_path(tmp_path) == (
+        assert get_flash_ld_path(tmp_path) == (
             tmp_path / "framework" / "tools" / "sdk" / "ld" / "eagle.flash.4m.ld"
         )
 
 
 def test_flash_size_str() -> None:
-    from esphome.build_gen.arduino8266 import _flash_size_str
 
     assert _flash_size_str("eagle.flash.4m.ld") == "4M"
     assert _flash_size_str("eagle.flash.512k.ld") == "512K"
diff --git a/tests/unit_tests/components/esp8266/test_build_surgery.py b/tests/unit_tests/components/esp8266/test_build_surgery.py
index a05c33f242..758aec22f7 100644
--- a/tests/unit_tests/components/esp8266/test_build_surgery.py
+++ b/tests/unit_tests/components/esp8266/test_build_surgery.py
@@ -62,3 +62,10 @@ def test_testing_memory_patches_enlarge_segments() -> None:
         "irom0_0_seg :                         org = 0x40201010, len = 0x2000000"
         in patched
     )
+
+
+def test_segment_length() -> None:
+    from esphome.components.esp8266.build_surgery import segment_length
+
+    assert segment_length(_FLASH_LD_SNIPPET, "irom0_0_seg") == 0xFEFF0
+    assert segment_length(_FLASH_LD_SNIPPET, "missing_seg") is None
diff --git a/tests/unit_tests/test_arduino8266_framework.py b/tests/unit_tests/test_arduino8266_framework.py
index 13eb5a4539..519d3c50d6 100644
--- a/tests/unit_tests/test_arduino8266_framework.py
+++ b/tests/unit_tests/test_arduino8266_framework.py
@@ -245,12 +245,9 @@ def test_ccache_path_disabled_by_env() -> None:
         assert framework.ccache_path() is None
 
 
-def test_ccache_path_no_binary() -> None:
-    with (
-        patch.dict(os.environ, {}, clear=False),
-        patch("shutil.which", return_value=None),
-    ):
-        os.environ.pop("ESPHOME_CCACHE_ENABLE", None)
+def test_ccache_path_no_binary(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("ESPHOME_CCACHE_ENABLE", raising=False)
+    with patch("shutil.which", return_value=None):
         assert framework.ccache_path() is None
 
 
@@ -263,12 +260,12 @@ def test_ccache_path_probe_failure() -> None:
         assert framework.ccache_path() is None
 
 
-def test_ccache_path_ok() -> None:
+def test_ccache_path_ok(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("ESPHOME_CCACHE_ENABLE", raising=False)
     with (
         patch("shutil.which", return_value="/usr/bin/ccache"),
         patch("subprocess.run"),
     ):
-        os.environ.pop("ESPHOME_CCACHE_ENABLE", None)
         assert framework.ccache_path() == "/usr/bin/ccache"
 
 
diff --git a/tests/unit_tests/test_arduino8266_toolchain.py b/tests/unit_tests/test_arduino8266_toolchain.py
index 6045f919bf..8223626807 100644
--- a/tests/unit_tests/test_arduino8266_toolchain.py
+++ b/tests/unit_tests/test_arduino8266_toolchain.py
@@ -184,3 +184,30 @@ def test_get_idedata_delegates(tmp_path: Path) -> None:
     assert compile_commands.name == "compile_commands.json"
     assert elf.name == "firmware.elf"
     assert cache.name == "test8266.json"
+
+
+def test_run_compile_skips_compdb_when_ninja_unchanged(tmp_path: Path) -> None:
+    """An unchanged build.ninja means the compile DB is already current."""
+    build_dir = toolchain.get_build_dir()
+    build_dir.mkdir(parents=True)
+
+    def run(regenerate_expected: bool) -> None:
+        with (
+            patch.object(framework, "check_and_install", return_value=_paths(tmp_path)),
+            patch.object(framework, "get_build_env", return_value={}),
+            patch("esphome.build_gen.arduino8266.write_project", return_value=False),
+            patch.object(
+                toolchain.subprocess, "run", return_value=MagicMock(returncode=0)
+            ),
+            patch.object(toolchain, "_write_compile_commands") as mock_compdb,
+            patch.object(toolchain, "_print_size_summary"),
+            patch.object(toolchain, "get_idedata"),
+        ):
+            assert toolchain.run_compile({CONF_ESPHOME: {}}, verbose=False) == 0
+        assert mock_compdb.called == regenerate_expected
+
+    # Missing compile DB: regenerated even though build.ninja is unchanged
+    run(regenerate_expected=True)
+    # Present compile DB + unchanged build.ninja: skipped
+    (build_dir / "compile_commands.json").write_text("[]")
+    run(regenerate_expected=False)
diff --git a/tests/unit_tests/test_espidf_component.py b/tests/unit_tests/test_espidf_component.py
index beebb7d4e4..c919953076 100644
--- a/tests/unit_tests/test_espidf_component.py
+++ b/tests/unit_tests/test_espidf_component.py
@@ -499,7 +499,7 @@ def test_parse_library_json(tmp_path):
     assert result["name"] == "test"
 
 
-def testparse_library_properties(tmp_path):
+def test_parse_library_properties(tmp_path):
     f = tmp_path / "library.properties"
     f.write_text(
         """
@@ -679,22 +679,22 @@ def test_node_key_registry_bare_name():
     assert (key, kind, locator) == ("bar", "registry", (None, "bar"))
 
 
-def testnormalize_dependencies_none():
+def test_normalize_dependencies_none():
     assert normalize_dependencies(None) == []
 
 
-def testnormalize_dependencies_list_form():
+def test_normalize_dependencies_list_form():
     deps = [{"name": "foo", "version": "1.0"}]
     assert normalize_dependencies(deps) == [{"name": "foo", "version": "1.0"}]
 
 
-def testnormalize_dependencies_dict_form():
+def test_normalize_dependencies_dict_form():
     out = normalize_dependencies({"nanopb/Nanopb": "^0.4.91", "BareName": "1.2.3"})
     assert {"name": "Nanopb", "owner": "nanopb", "version": "^0.4.91"} in out
     assert {"name": "BareName", "owner": None, "version": "1.2.3"} in out
 
 
-def testnormalize_dependencies_dict_form_nested_spec():
+def test_normalize_dependencies_dict_form_nested_spec():
     out = normalize_dependencies(
         {"nanopb/Nanopb": {"version": "^0.4.91", "platforms": "espidf"}}
     )
diff --git a/tests/unit_tests/test_main.py b/tests/unit_tests/test_main.py
index d5dc1e71d8..86d529c4a6 100644
--- a/tests/unit_tests/test_main.py
+++ b/tests/unit_tests/test_main.py
@@ -7168,3 +7168,21 @@ def test_write_cpp_file_arduino_toolchain_writes_no_project(tmp_path: Path) -> N
 
     mock_write_cpp.assert_called_once()
     mock_pio_project.assert_not_called()
+
+
+def test_write_cpp_file_platformio_toolchain_writes_project(tmp_path: Path) -> None:
+    """The default toolchain writes the PlatformIO project files."""
+    setup_core(platform=PLATFORM_ESP8266, tmp_path=tmp_path, name="test")
+
+    with (
+        patch("esphome.writer.write_cpp") as mock_write_cpp,
+        patch("esphome.build_gen.platformio.write_project") as mock_pio_project,
+        patch.object(
+            type(CORE), "cpp_main_section", new_callable=PropertyMock
+        ) as mock_section,
+    ):
+        mock_section.return_value = ""
+        assert main.write_cpp_file() == 0
+
+    mock_write_cpp.assert_called_once()
+    mock_pio_project.assert_called_once()