diff --git a/esphome/build_helpers/idedata.py b/esphome/build_helpers/idedata.py
index be1f7fd25c..88c70aacab 100644
--- a/esphome/build_helpers/idedata.py
+++ b/esphome/build_helpers/idedata.py
@@ -258,6 +258,22 @@ def _cc_path_from_cxx(cxx_path: str) -> str:
     return f"{stem}{suffix}"
 
 
+def _cache_usable(cached: object) -> bool:
+    """Check a cached idedata dict against the guarantees of the write path.
+
+    Caches written by older versions predate the launcher rejection and the
+    include-union shape; serving one would bypass both. The dict check also
+    keeps "in" from substring-matching a bare JSON string.
+    """
+    if not isinstance(cached, dict) or "cc_path" not in cached:
+        return False
+    cxx_path = cached.get("cxx_path")
+    if not isinstance(cxx_path, str) or _is_launcher(cxx_path):
+        return False
+    includes = cached.get("includes")
+    return isinstance(includes, dict) and isinstance(includes.get("build"), list)
+
+
 def load_or_build_idedata(
     compile_commands: Path,
     elf_path: Path,
@@ -283,10 +299,11 @@ def load_or_build_idedata(
             # look like unexplained slow builds
             _LOGGER.warning("Discarding unreadable idedata cache %s: %s", cache, err)
         else:
-            # Rebuild pre-cc_path caches on the field, not the timestamp;
-            # the type check keeps "in" from substring-matching a string
-            if isinstance(cached, dict) and "cc_path" in cached:
+            if _cache_usable(cached):
+                # Re-stamp so a relocated build dir cannot serve a stale ELF path
+                cached["prog_path"] = str(elf_path)
                 return cached
+            _LOGGER.debug("Regenerating idedata: cache %s fails validation", cache)
 
     data = idedata_from_build(compile_commands, launcher)
     data["prog_path"] = str(elf_path)
@@ -320,6 +337,9 @@ def idedata_from_build(compile_commands: Path, launcher: str | None = None) -> d
     project-wide superset (as PlatformIO's idedata provides).
     """
     entries = json.loads(Path(compile_commands).read_text(encoding="utf-8"))
+    if not isinstance(entries, list):
+        # A TypeError here would escape IDEDATA_BEST_EFFORT_ERRORS
+        raise EsphomeError(f"{compile_commands} is not a compile-command list")
 
     representative = _pick_entry(entries)
     cxx_path, defines, rep_includes, cxx_flags = parse_entry(representative, launcher)
@@ -339,7 +359,7 @@ def idedata_from_build(compile_commands: Path, launcher: str | None = None) -> d
         # may hold different include sets.
         command = entry["command"]
         if "@" in command:
-            return f"unique:{entry['file']}"
+            return f"unique:{entry.get('output') or command}"
         return command.replace(entry.get("file", ""), "").replace(
             entry.get("output", ""), ""
         )
diff --git a/esphome/espidf/size_summary.py b/esphome/espidf/size_summary.py
index 5237495843..5338cd2135 100644
--- a/esphome/espidf/size_summary.py
+++ b/esphome/espidf/size_summary.py
@@ -37,7 +37,7 @@ _SIZE_SUFFIXES = {"K": 1024, "M": 1024 * 1024}
 def _parse_size(token: str) -> int:
     token = token.strip()
     if not token:
-        return 0
+        raise ValueError("blank partition size cell")
     if token.startswith(("0x", "0X")):
         return int(token, 16)
     suffix = token[-1].upper()
@@ -76,6 +76,14 @@ def print_summary(size_json: Path, partitions_csv: Path | None) -> None:
     summarize. Logs the cause at warning level, so a missing RAM/Flash line
     (which CI's memory-impact extraction greps for) is diagnosable.
     """
+    try:
+        _print_summary(size_json, partitions_csv)
+    except Exception as e:  # noqa: BLE001  # pylint: disable=broad-exception-caught
+        # Backstop for nested shapes the named guards below miss
+        _LOGGER.warning("Skipping size summary: %s", e)
+
+
+def _print_summary(size_json: Path, partitions_csv: Path | None) -> None:
     if not size_json.is_file():
         _LOGGER.warning("Skipping size summary: %s not found", size_json)
         return
diff --git a/tests/unit_tests/build_helpers/test_idedata.py b/tests/unit_tests/build_helpers/test_idedata.py
index dd2494a093..fec276a92c 100644
--- a/tests/unit_tests/build_helpers/test_idedata.py
+++ b/tests/unit_tests/build_helpers/test_idedata.py
@@ -574,11 +574,101 @@ def test_load_or_build_idedata_never_caches_a_launcher(tmp_path: Path) -> None:
     assert not cache.exists()
 
 
+@pytest.mark.parametrize(
+    "cached",
+    (
+        {"cc_path": "/x/gcc", "cxx_path": "/opt/homebrew/bin/ccache"},
+        {"cc_path": "/x/gcc", "cxx_path": "/tools/g++"},
+        {"cc_path": "/x/gcc", "cxx_path": "/tools/g++", "includes": {}},
+    ),
+    ids=("launcher-cxx", "no-includes", "no-build-list"),
+)
+def test_load_or_build_idedata_regenerates_invalid_cache(
+    tmp_path: Path, cached: dict
+) -> None:
+    """A cache written by an older version fails validation and regenerates."""
+    compile_commands = _write_compile_commands(tmp_path)
+    cache = tmp_path / "c.json"
+    cache.write_text(json.dumps(cached))
+    os.utime(cache, (compile_commands.stat().st_mtime + 5,) * 2)
+    with patch.object(idedata, "get_toolchain_includes", return_value=[]):
+        data = idedata.load_or_build_idedata(
+            compile_commands, tmp_path / "f.elf", cache
+        )
+    assert data["cxx_path"] == "/tools/g++"
+    assert "includes" in data
+
+
+def test_load_or_build_idedata_cache_hit_restamps_prog_path(tmp_path: Path) -> None:
+    """A served cache carries the current ELF path, not the one it was written with."""
+    compile_commands = _write_compile_commands(tmp_path)
+    cache = tmp_path / "c.json"
+    cache.write_text(
+        json.dumps(
+            {
+                "cc_path": "/tools/gcc",
+                "cxx_path": "/tools/g++",
+                "includes": {"build": [], "toolchain": []},
+                "prog_path": "/old/location/firmware.elf",
+            }
+        )
+    )
+    os.utime(cache, (compile_commands.stat().st_mtime + 5,) * 2)
+    data = idedata.load_or_build_idedata(
+        compile_commands, tmp_path / "firmware.elf", cache
+    )
+    assert data["prog_path"] == str(tmp_path / "firmware.elf")
+
+
+def test_idedata_from_build_non_list_compile_db_raises(tmp_path: Path) -> None:
+    """Valid JSON that is not a list raises by name, inside the best-effort tuple."""
+    compile_commands = tmp_path / "compile_commands.json"
+    for bad in ("{}", "null", '"text"'):
+        compile_commands.write_text(bad)
+        with pytest.raises(EsphomeError, match="not a compile-command list"):
+            idedata.idedata_from_build(compile_commands)
+
+
+def test_idedata_from_build_same_file_rsp_commands_never_dedupe(
+    tmp_path: Path,
+) -> None:
+    """Two objects built from one source with different .rsp files keep both
+    include sets; the rsp sentinel keys on the output, not the source."""
+    file = f"{ABS}build/src/esphome/core/shared.cpp"
+    entries = []
+    for name in ("a", "b"):
+        rsp = tmp_path / f"{name}.o.rsp"
+        rsp.write_text(f"-I{ABS}inc/{name}")
+        entries.append(
+            {
+                "directory": str(tmp_path),
+                "file": file,
+                "command": f"/tools/g++ @{rsp.name} -c {file} -o {name}.o",
+                "output": f"{name}.o",
+            }
+        )
+    compile_commands = tmp_path / "compile_commands.json"
+    compile_commands.write_text(json.dumps(entries))
+    with patch.object(idedata, "get_toolchain_includes", return_value=[]):
+        data = idedata.idedata_from_build(compile_commands)
+    joined = " ".join(data["includes"]["build"])
+    assert "inc/a" in joined and "inc/b" in joined
+
+
 def test_load_or_build_idedata_cache_hit_skips_rebuild(tmp_path: Path) -> None:
     """A valid cache newer than the compile DB is served without re-parsing."""
     compile_commands = _write_compile_commands(tmp_path)
     cache = tmp_path / "c.json"
-    cache.write_text(json.dumps({"cc_path": "/tools/gcc", "cached": True}))
+    cache.write_text(
+        json.dumps(
+            {
+                "cc_path": "/tools/gcc",
+                "cxx_path": "/tools/g++",
+                "includes": {"build": ["/inc"], "toolchain": []},
+                "cached": True,
+            }
+        )
+    )
     os.utime(cache, (compile_commands.stat().st_mtime + 5,) * 2)
     with patch.object(idedata, "idedata_from_build") as mock_build:
         data = idedata.load_or_build_idedata(
diff --git a/tests/unit_tests/test_size_summary.py b/tests/unit_tests/test_size_summary.py
index c5d117b953..8647eec67f 100644
--- a/tests/unit_tests/test_size_summary.py
+++ b/tests/unit_tests/test_size_summary.py
@@ -205,8 +205,19 @@ def test_print_summary_non_dict_json_warns(tmp_path, caplog) -> None:
 
 
 def test_print_summary_zero_app_partition_warns(tmp_path, caplog) -> None:
-    """A malformed partition row parsing to 0 must not render a 0% bar for
-    CI's memory-impact extraction to ingest."""
+    """A partition row with size 0 drops the Flash bar instead of rendering 0%."""
+    size_json = tmp_path / "size.json"
+    size_json.write_text(
+        '{"memory_types": {"DRAM": {"used": 1, "size": 2}}, "image_size": 100}'
+    )
+    partitions = tmp_path / "partitions.csv"
+    partitions.write_text("app0, app, ota_0, 0x10000, 0,\n")
+    print_summary(size_json, partitions)
+    assert "app partition size is" in caplog.text
+
+
+def test_print_summary_blank_partition_size_warns(tmp_path, caplog) -> None:
+    """A blank size cell raises ValueError by name instead of parsing to 0."""
     size_json = tmp_path / "size.json"
     size_json.write_text(
         '{"memory_types": {"DRAM": {"used": 1, "size": 2}}, "image_size": 100}'
@@ -214,4 +225,21 @@ def test_print_summary_zero_app_partition_warns(tmp_path, caplog) -> None:
     partitions = tmp_path / "partitions.csv"
     partitions.write_text("app0, app, ota_0, 0x10000, ,\n")
     print_summary(size_json, partitions)
-    assert "app partition size is" in caplog.text
+    assert "blank partition size cell" in caplog.text
+
+
+@pytest.mark.parametrize(
+    "payload",
+    (
+        '{"memory_types": []}',
+        '{"memory_types": {"DRAM": 5}}',
+        '{"memory_types": {"DRAM": {"used": "x", "size": "y"}}}',
+    ),
+    ids=("non-dict-memory-types", "scalar-region", "non-numeric-sizes"),
+)
+def test_print_summary_nested_shapes_never_raise(tmp_path, caplog, payload) -> None:
+    """The blanket guard keeps unexpected nested shapes from raising."""
+    size_json = tmp_path / "size.json"
+    size_json.write_text(payload)
+    print_summary(size_json, tmp_path / "partitions.csv")
+    assert "Skipping size summary" in caplog.text