diff --git a/esphome/espidf/framework.py b/esphome/espidf/framework.py
index 13d99fd709..3418aa4597 100644
--- a/esphome/espidf/framework.py
+++ b/esphome/espidf/framework.py
@@ -748,8 +748,10 @@ def _prefetch_idf_tool_archives(
             ", ".join(entry["name"] for entry in entries),
         )
         # tools.json always carries sizes; should one be missing the combined
-        # bar could not be trusted, so show no bar at all (per-file bars from
-        # several threads would interleave) rather than a wrong one.
+        # bar could not be trusted, so show no bar at all rather than a wrong
+        # one. Unlike the library prefetch there is no sequential fallback:
+        # per-file bars from several threads would interleave, and skipping
+        # the prefetch would lose the resume workaround for #17703.
         sizes = [entry.get("size") or 0 for entry in entries]
         progress = BatchDownloadProgress(
             "Downloading ESP-IDF tools", sum(sizes) if all(sizes) else 0
diff --git a/esphome/platformio/library.py b/esphome/platformio/library.py
index 47c8d70fd6..d7e29d152e 100644
--- a/esphome/platformio/library.py
+++ b/esphome/platformio/library.py
@@ -900,8 +900,8 @@ def is_lib_ignored(name: str | None, lib_ignore: set[str]) -> bool:
 _DOWNLOAD_WORKERS = 4
 
 
-def _content_lengths(urls: list[str]) -> list[int]:
-    """Content-Length per URL via HEAD requests; 0 for any that fail."""
+def _content_lengths(urls: list[str]) -> list[int | None]:
+    """Content-Length per URL via HEAD requests; None when unknown."""
     import requests
 
     def head(url: str) -> int | None:
diff --git a/tests/unit_tests/test_platformio_library.py b/tests/unit_tests/test_platformio_library.py
index b6f87bca88..f832d444cf 100644
--- a/tests/unit_tests/test_platformio_library.py
+++ b/tests/unit_tests/test_platformio_library.py
@@ -629,6 +629,24 @@ def test_prefetch_wave_downloads_registry_archives_in_parallel(
     ]
 
 
+def test_prefetch_wave_unknown_size_falls_back_to_sequential(
+    setup_core, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """Any unknown HEAD size skips the parallel prefetch entirely so the
+    sequential downloads keep their per-file bars."""
+
+    def fail_download(self, force=False, salt="", namespace="", progress=None):
+        raise AssertionError("prefetched despite unknown size")
+
+    monkeypatch.setattr(ConvertedLibrary, "download", fail_download)
+    monkeypatch.setattr(lib, "_content_lengths", lambda urls: [1, None])
+    wave = [
+        ("a", ConvertedLibrary("a", "1.0", URLSource("https://x/a.tar.gz"))),
+        ("b", ConvertedLibrary("b", "1.0", URLSource("https://x/b.tar.gz"))),
+    ]
+    lib._prefetch_wave(wave, "", "idf")
+
+
 def test_content_lengths_head_requests(monkeypatch: pytest.MonkeyPatch) -> None:
     """Sizes come from HEAD Content-Length; a failing HEAD reads as 0 so
     the combined bar is skipped rather than wrong."""