change: Implement 4: Filter candidates by real format and minimum size from downloaded bytes

Added tools/images.py with stdlib-only detection of the real image format and pixel dimensions from downloaded bytes (PNG signature + IHDR chunk, ICO ICONDIR/ICONDIRENTRY directory including the 0-means-256 encoding) plus stdlib-only builders (png_bytes, ico_bytes) so tests/sources can produce genuine payloads. Wired it into the CLI search path in tools/example.py: a bounded candidate window derived from --count (candidate_cap = count * 4, capped at 50) is fetched from the selected source, each payload is parsed, and only items whose actual format equals the requested png/ico and whose actual width and height are both at least the requested minimum are kept. Kept items report source, url, format and the actual width/height (plus size_bytes and metadata as extras); verified bytes are reused for --download instead of being fetched twice. Statuses stay distinct: ok (0), no_format (3) when hits were only other formats, no_size (4) when format-matching items were all too small, access_error (5) when bytes could not be fetched or are not readable images, partial_download (6) when some candidates could not be read. All metadata-based filtering (FORMAT_MATCHERS, _matches_format, _matches_size) was removed so only parsed bytes decide. Added tests/test_images.py with detector/builder unit tests and CLI tests using an injected fake byte-fetch callable (no network), and updated tests/test_cli.py fixtures to real image bytes where the previous metadata-only assumption no longer holds.

Tests:
Facade commands run with .venv/bin on PATH after prepare_environment(component=tools, mode=restore): `python -m compileall tools tests` (tools.validate) succeeded; `pytest` (tools.test) => 114 passed, including the new tests/test_images.py (31 tests); `python -m tools.example examples/input.json` (tools.smoke) exited 0 with a single dry_run JSON document; acceptance commands `python -m tools.example --help` exited 0 and `import tools` resolved. Target branch task/1-task is at the same commit as this work branch (git rev-list count 0 ahead / 0 behind), so it is already integrated; facade/protected paths untouched.

Progress:
Implemented byte-based format and minimum-size filtering for the image search path. New tools/images.py: detect_format (png/ico plus jpeg/gif/bmp/webp so "other format" is distinguishable from "unreadable"), read_png_dimensions (IHDR), read_ico_dimensions (ICONDIR entries, 0 => 256, largest entry), inspect_image/ImageInfo, ImageFormatError (code image_unreadable), and png_bytes/ico_bytes builders. tools/example.py: candidate_cap(count) bounded window (count*4, max 50) requested from the source, _select() fetches and parses every candidate, KeptImage.to_item() reports source/url/format/actual width+height/size_bytes, _download() writes the already verified bytes, and _execute() distinguishes ok/no_format/no_size/access_error/partial_download with the existing exit codes; metadata-based filtering removed. tests/test_images.py added (31 tests) and tests/test_cli.py fixtures aligned to real image bytes. All facade commands pass: compileall, pytest (114 passed), smoke, help, package import.

Published by developer-agent through Tool Backend.

Closes #6

.agent-work/ledger.json

@@ -15,6 +15,11 @@
         "Implemented envelope status/exit-code table and response builder, request errors, the full flag/JSON-document CLI in tools/example.py, canonical offline examples/input.json, and CLI tests; facade validate/test/smoke and --help all pass (76 tests).",
         "Addressed the changes-requested review on PR #13: requested width/height are now a minimum target size (selection no longer depends on --resize), and --resize now actually downscales downloaded images (stdlib PNG codec + ICO/PNG container support, shrink-to-fit, never upscaling) with per-item resize metadata and typed ImageResizeError for undecodable payloads. Updated tests/test_cli.py with minimum-size and offline resize/download contract tests (83 passed). Facade validate/test/smoke/help all green; examples/input.json stays the canonical dry_run request."
       ]
+    },
+    "projects/product-54354138-1#6": {
+      "events": [
+        "Implemented byte-based format and minimum-size filtering for the image search path. New tools/images.py: detect_format (png/ico plus jpeg/gif/bmp/webp so \"other format\" is distinguishable from \"unreadable\"), read_png_dimensions (IHDR), read_ico_dimensions (ICONDIR entries, 0 => 256, largest entry), inspect_image/ImageInfo, ImageFormatError (code image_unreadable), and png_bytes/ico_bytes builders. tools/example.py: candidate_cap(count) bounded window (count*4, max 50) requested from the source, _select() fetches and parses every candidate, KeptImage.to_item() reports source/url/format/actual width+height/size_bytes, _download() writes the already verified bytes, and _execute() distinguishes ok/no_format/no_size/access_error/partial_download with the existing exit codes; metadata-based filtering removed. tests/test_images.py added (31 tests) and tests/test_cli.py fixtures aligned to real image bytes. All facade commands pass: compileall, pytest (114 passed), smoke, help, package import."
+      ]
     }
   }
 }

tests/test_cli.py

@@ -194,7 +194,7 @@ class FakeCandidate:
         width: int | None,
         height: int | None,
         *,
-        payload: bytes = b"payload",
+        payload: bytes | None = None,
         fail: bool = False,
     ) -> None:
         self.source_id = "fake"
@@ -223,7 +223,11 @@ class FakeCandidate:
     def fetch_bytes(self) -> bytes:
         if self._fail:
             raise SourceAccessError("download refused", url=self.file_url)
+        if self._payload is not None:
             return self._payload
+        # Default to bytes that really are a PNG of the advertised size, because
+        # the CLI filters on downloaded bytes and never on source metadata.
+        return png_bytes(self.width or 1, self.height or 1)
 
 
 class FakeRegistry:
@@ -265,18 +269,29 @@ def test_ok_lists_items_without_download(monkeypatch: pytest.MonkeyPatch, capsys
     assert code == envelope.EXIT_OK
     assert payload["status"] == envelope.STATUS_OK
     assert payload["source"] == "fake"
-    assert [item["file_url"] for item in payload["items"]] == ["https://example.test/a.png"]
-    assert payload["items"][0]["width"] == 100
-    assert registry.calls == [("fake", "fox", 10)]
+    item = payload["items"][0]
+    assert item["url"] == "https://example.test/a.png"
+    assert item["file_url"] == "https://example.test/a.png"
+    assert item["source"] == "fake"
+    assert item["format"] == "png"
+    assert item["width"] == 100
+    assert item["height"] == 100
+    assert registry.calls == [("fake", "fox", example.candidate_cap(10))]
 
 
-def test_no_format_when_source_has_no_matching_mime(monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]) -> None:
-    registry = FakeRegistry([FakeCandidate("fox", "https://example.test/a.jpg", "image/jpeg", 100, 100)])
+def test_no_format_when_every_payload_really_is_another_format(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    registry = FakeRegistry(
+        [FakeCandidate("fox", "https://example.test/a.jpg", "image/jpeg", 100, 100, payload=gif_bytes())]
+    )
 
     code, payload = run_main(monkeypatch, capsys, registry, *BASE_ARGS)
 
     assert code == envelope.EXIT_NO_FORMAT
     assert payload["status"] == envelope.STATUS_NO_FORMAT
+    assert payload["error"]["details"]["unreadable_candidates"] == 0
+    assert payload["error"]["details"]["requested_format"] == "png"
 
 
 def test_no_size_when_only_smaller_images_exist(monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]) -> None:
@@ -311,7 +326,7 @@ def test_exact_size_is_accepted(monkeypatch: pytest.MonkeyPatch, capsys: pytest.
     assert len(payload["items"]) == 1
 
 
-def test_resize_without_download_keeps_reported_source_size(
+def test_resize_without_download_keeps_actual_size(
     monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
 ) -> None:
     registry = FakeRegistry([FakeCandidate("fox", "https://example.test/a.png", "image/png", 400, 300)])
@@ -325,7 +340,10 @@ def test_resize_without_download_keeps_reported_source_size(
 
 
 def test_download_success_writes_files(monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str], tmp_path: Path) -> None:
-    registry = FakeRegistry([FakeCandidate("fox", "https://example.test/a.png", "image/png", 100, 100)])
+    expected = png_bytes(100, 100)
+    registry = FakeRegistry(
+        [FakeCandidate("fox", "https://example.test/a.png", "image/png", 100, 100, payload=expected)]
+    )
 
     code, payload = run_main(monkeypatch, capsys, registry, *BASE_ARGS, "--download", "--output-dir", str(tmp_path))
 
@@ -334,7 +352,8 @@ def test_download_success_writes_files(monkeypatch: pytest.MonkeyPatch, capsys:
     assert download["ok"] is True
     target = Path(download["path"])
     assert target.parent == tmp_path
-    assert target.read_bytes() == b"payload"
+    # The verified bytes are stored verbatim, without a second download.
+    assert target.read_bytes() == expected
 
 
 def test_partial_download_reports_status(monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str], tmp_path: Path) -> None:
@@ -349,6 +368,9 @@ def test_partial_download_reports_status(monkeypatch: pytest.MonkeyPatch, capsys
 
     assert code == envelope.EXIT_PARTIAL_DOWNLOAD
     assert payload["status"] == envelope.STATUS_PARTIAL_DOWNLOAD
+    # The readable candidate is still reported and downloaded.
+    assert payload["items"][0]["download"]["ok"] is True
+    assert payload["error"]["details"]["failed"] == 1
 
 
 def test_all_downloads_failing_is_access_error(monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str], tmp_path: Path) -> None:
@@ -358,6 +380,7 @@ def test_all_downloads_failing_is_access_error(monkeypatch: pytest.MonkeyPatch,
 
     assert code == envelope.EXIT_ACCESS_ERROR
     assert payload["status"] == envelope.STATUS_ACCESS_ERROR
+    assert payload["error"]["code"] == "candidate_unreadable"
 
 
 def test_search_access_error_maps_to_access_error(monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]) -> None:
@@ -398,6 +421,12 @@ def png_bytes(width: int, height: int, rgba: tuple[int, int, int, int] = (255, 0
     )
 
 
+def gif_bytes(width: int = 100, height: int = 100) -> bytes:
+    """Build a payload that really is a GIF, i.e. a format we must never accept."""
+
+    return b"GIF89a" + struct.pack("<HH", width, height) + b"\x00" * 8
+
+
 def png_size(payload: bytes) -> tuple[int, int]:
     assert payload.startswith(PNG_SIGNATURE)
     return struct.unpack(">II", payload[16:24])
@@ -528,9 +557,11 @@ def test_resize_downscales_png_stored_inside_ico(
     assert png_size(inner) == (50, 50)
 
 
-def test_undecodable_payload_with_resize_fails_the_item(
+def test_unreadable_payload_is_an_access_error(
     monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str], tmp_path: Path
 ) -> None:
+    """Bytes that are not an image must never become a "no match" verdict."""
+
     registry = FakeRegistry(
         [
             FakeCandidate(
@@ -545,6 +576,6 @@ def test_undecodable_payload_with_resize_fails_the_item(
 
     assert code == envelope.EXIT_ACCESS_ERROR
     assert document["status"] == envelope.STATUS_ACCESS_ERROR
-    download = document["items"][0]["download"]
-    assert download["ok"] is False
-    assert download["error"]["code"] == "resize_failed"
+    assert document["error"]["code"] == "candidate_unreadable"
+    assert document["items"] == []
+    assert list(tmp_path.iterdir()) == []

tests/test_images.py

@@ -0,0 +1,457 @@
+"""Tests for byte-based format and size filtering.
+
+Two layers are covered:
+
+* :mod:`tools.images` -- the stdlib-only detector and the stdlib-only builders
+  for the two accepted formats, so fixtures are real images rather than magic
+  strings;
+* the CLI search path -- candidates are filtered on the *downloaded bytes*, so
+  a source that mislabels a file, claims the wrong size or serves an unreadable
+  body produces the correct ``ok`` / ``no_format`` / ``no_size`` /
+  ``access_error`` answer.  Every CLI test injects a fake byte-fetch callable,
+  so nothing here touches the network.
+"""
+
+from __future__ import annotations
+
+import json
+import struct
+from typing import Any, Mapping, Sequence
+
+import pytest
+
+from tools import envelope, example, images
+from tools.errors import SourceAccessError
+from tools.images import ImageFormatError, ImageInfo
+
+# ---------------------------------------------------------------------------
+# Real-format builders used as fixtures
+# ---------------------------------------------------------------------------
+
+
+def png_payload(width: int, height: int) -> bytes:
+    """A real PNG built with :func:`tools.images.png_bytes`."""
+
+    return images.png_bytes(width, height)
+
+
+def ico_payload(width: int, height: int) -> bytes:
+    """A real ICO (PNG-compressed entry) built with :func:`tools.images.ico_bytes`."""
+
+    return images.ico_bytes(width, height)
+
+
+def gif_payload(width: int = 100, height: int = 100) -> bytes:
+    """A payload whose real format is GIF, i.e. one we must never accept."""
+
+    return b"GIF89a" + struct.pack("<HH", width, height) + b"\x00" * 8
+
+
+# ---------------------------------------------------------------------------
+# tools.images: detection
+# ---------------------------------------------------------------------------
+
+
+def test_png_bytes_builds_a_png_with_the_requested_dimensions() -> None:
+    payload = png_payload(13, 7)
+
+    assert payload.startswith(images.PNG_SIGNATURE)
+    assert images.detect_format(payload) == images.PNG
+    assert images.inspect_image(payload) == ImageInfo(format="png", width=13, height=7)
+    # The IHDR chunk really carries the dimensions, so the bytes are the source
+    # of truth rather than a helper function agreeing with itself.
+    assert payload[12:16] == b"IHDR"
+    assert struct.unpack(">II", payload[16:24]) == (13, 7)
+    assert images.read_png_dimensions(payload) == (13, 7)
+
+
+def test_ico_bytes_builds_a_directory_that_reports_the_requested_sizes() -> None:
+    payload = ico_payload(48, 37)
+
+    assert images.detect_format(payload) == images.ICO
+    assert images.inspect_image(payload) == ImageInfo(format="ico", width=48, height=37)
+    # One directory entry, and its data range lies inside the payload.
+    reserved, kind, count = struct.unpack("<HHH", payload[:6])
+    assert (reserved, kind, count) == (0, 1, 1)
+    _w, _h, _colours, _reserved, _planes, _bits, size, offset = struct.unpack("<BBBBHHII", payload[6:22])
+    assert offset + size == len(payload)
+
+
+def test_ico_maximum_dimension_uses_the_zero_encoding() -> None:
+    payload = ico_payload(256, 256)
+
+    assert payload[6] == 0 and payload[7] == 0
+    assert images.read_ico_dimensions(payload) == (256, 256)
+
+
+def test_ico_dimensions_use_the_largest_directory_entry() -> None:
+    payload = images.ico_bytes(16, 16, frames=[(16, 16), (32, 32), (256, 256)])
+
+    assert len(images.ico_entries(payload)) == 3
+    assert images.read_ico_dimensions(payload) == (256, 256)
+    assert images.inspect_image(payload).format == "ico"
+
+
+@pytest.mark.parametrize(
+    ("payload", "expected"),
+    [
+        (png_payload(2, 2), images.PNG),
+        (ico_payload(2, 2), images.ICO),
+        (gif_payload(), images.GIF),
+        (b"\xff\xd8\xff\xe0" + b"\x00" * 12, images.JPEG),
+        (b"BM" + b"\x00" * 20, images.BMP),
+        (b"RIFF" + b"\x00\x00\x00\x00" + b"WEBP" + b"\x00" * 8, images.WEBP),
+    ],
+)
+def test_detect_format_names_the_real_format(payload: bytes, expected: str) -> None:
+    assert images.detect_format(payload) == expected
+
+
+@pytest.mark.parametrize("payload", [b"", b"payload", b"not-an-image", b"<html></html>"])
+def test_detect_format_returns_none_for_non_images(payload: bytes) -> None:
+    assert images.detect_format(payload) is None
+
+
+def test_inspect_image_rejects_formats_whose_size_is_not_readable() -> None:
+    with pytest.raises(ImageFormatError) as excinfo:
+        images.inspect_image(gif_payload())
+
+    assert excinfo.value.code == "image_unreadable"
+    assert excinfo.value.details["format"] == images.GIF
+
+
+@pytest.mark.parametrize(
+    "payload",
+    [
+        b"not-an-image",
+        images.PNG_SIGNATURE,
+        images.PNG_SIGNATURE + struct.pack(">I", 13) + b"IDAT" + b"\x00" * 13,
+        b"\x00\x00\x01\x00" + struct.pack("<HHH", 0, 1, 2),
+        b"\x00\x00\x01\x00" + struct.pack("<HHH", 0, 1, 1) + struct.pack("<BBBBHHII", 16, 16, 0, 0, 1, 32, 999, 22),
+    ],
+)
+def test_inspect_image_rejects_unreadable_payloads(payload: bytes) -> None:
+    with pytest.raises(ImageFormatError):
+        images.inspect_image(payload)
+
+
+def test_png_dimensions_reject_a_zero_sized_image() -> None:
+    payload = images.PNG_SIGNATURE + struct.pack(">I", 13) + b"IHDR" + struct.pack(">IIBBBBB", 0, 0, 8, 6, 0, 0, 0)
+
+    with pytest.raises(ImageFormatError):
+        images.read_png_dimensions(payload)
+
+
+# ---------------------------------------------------------------------------
+# CLI search path: filtering on downloaded bytes
+# ---------------------------------------------------------------------------
+
+
+class StubCandidate:
+    """Candidate whose bytes are supplied by a fake fetch callable."""
+
+    def __init__(
+        self,
+        title: str,
+        url: str,
+        *,
+        payload: bytes | None = None,
+        error: Exception | None = None,
+        mime_type: str | None = None,
+        width: int | None = None,
+        height: int | None = None,
+    ) -> None:
+        self.source_id = "fake"
+        self.title = title
+        self.file_url = url
+        self.page_url = url + ".html"
+        # Deliberately misleading metadata: filtering must ignore all of this.
+        self.mime_type = mime_type
+        self.width = width
+        self.height = height
+        self._payload = payload
+        self._error = error
+        self.fetch_calls = 0
+
+    def fetch_bytes(self) -> bytes:
+        self.fetch_calls += 1
+        if self._error is not None:
+            raise self._error
+        assert self._payload is not None
+        return self._payload
+
+
+class StubRegistry:
+    """Registry stand-in that returns a fixed candidate list."""
+
+    def __init__(self, candidates: Sequence[StubCandidate]) -> None:
+        self.candidates = list(candidates)
+        self.calls: list[tuple[str, str, int]] = []
+
+    def available_sources(self) -> Sequence[str]:
+        return ["fake"]
+
+    def search(self, source_id: str, topic: str, limit: int = 10) -> list[StubCandidate]:
+        self.calls.append((source_id, topic, limit))
+        return self.candidates[:limit]
+
+
+def run_search(
+    monkeypatch: pytest.MonkeyPatch,
+    capsys: pytest.CaptureFixture[str],
+    candidates: Sequence[StubCandidate],
+    *args: str,
+) -> tuple[int, dict[str, Any], StubRegistry]:
+    registry = StubRegistry(candidates)
+    monkeypatch.setattr(example, "default_registry", lambda: registry)
+    code = example.main(list(args))
+    captured = capsys.readouterr()
+    assert captured.err == ""
+    lines = [line for line in captured.out.splitlines() if line.strip()]
+    assert len(lines) == 1, captured.out
+    document = json.loads(lines[0])
+    assert isinstance(document, Mapping)
+    return code, document, registry
+
+
+BASE_ARGS = ("--topic", "red fox", "--format", "png", "--source", "fake")
+
+
+def test_png_candidate_is_kept_with_its_actual_dimensions(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    payload = png_payload(800, 600)
+    candidate = StubCandidate(
+        "fox",
+        "https://example.test/fox.png",
+        payload=payload,
+        # The source claims something else entirely; the bytes win.
+        mime_type="image/jpeg",
+        width=12,
+        height=12,
+    )
+
+    code, document, registry = run_search(
+        monkeypatch, capsys, [candidate], *BASE_ARGS, "--width", "640", "--height", "480"
+    )
+
+    assert code == envelope.EXIT_OK
+    assert document["status"] == envelope.STATUS_OK
+    assert document["error"] is None
+    item = document["items"][0]
+    assert item["source"] == "fake"
+    assert item["url"] == "https://example.test/fox.png"
+    assert item["format"] == "png"
+    assert (item["width"], item["height"]) == (800, 600)
+    assert item["size_bytes"] == len(payload)
+    assert candidate.fetch_calls == 1
+    # The bounded candidate window is requested from the source, not --count.
+    assert registry.calls == [("fake", "red fox", example.candidate_cap(10))]
+
+
+def test_ico_request_with_only_png_payloads_yields_no_format(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    candidates = [
+        StubCandidate("fox", "https://example.test/a.png", payload=png_payload(400, 400), mime_type="image/x-icon"),
+        StubCandidate("fox2", "https://example.test/b.png", payload=png_payload(500, 500), mime_type="image/ico"),
+    ]
+
+    code, document, _ = run_search(
+        monkeypatch,
+        capsys,
+        candidates,
+        "--topic",
+        "red fox",
+        "--format",
+        "ico",
+        "--source",
+        "fake",
+        "--width",
+        "64",
+        "--height",
+        "64",
+    )
+
+    assert code == envelope.EXIT_NO_FORMAT
+    assert document["status"] == envelope.STATUS_NO_FORMAT
+    assert document["items"] == []
+    assert document["error"]["code"] == "no_format_match"
+    assert document["error"]["details"]["requested_format"] == "ico"
+    assert document["error"]["details"]["unreadable_candidates"] == 0
+    assert document["error"]["details"]["format_matches"] == 0
+
+
+def test_ico_payload_is_kept_for_an_ico_request(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    candidate = StubCandidate("fox", "https://example.test/fox.ico", payload=ico_payload(64, 48))
+
+    code, document, _ = run_search(
+        monkeypatch,
+        capsys,
+        [candidate],
+        "--topic",
+        "red fox",
+        "--format",
+        "ico",
+        "--source",
+        "fake",
+        "--width",
+        "32",
+        "--height",
+        "32",
+    )
+
+    assert code == envelope.EXIT_OK
+    item = document["items"][0]
+    assert item["format"] == "ico"
+    assert (item["width"], item["height"]) == (64, 48)
+
+
+def test_all_too_small_payloads_yield_no_size(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    candidates = [
+        StubCandidate("small", "https://example.test/a.png", payload=png_payload(100, 100), width=4000, height=4000),
+        StubCandidate("tall", "https://example.test/b.png", payload=png_payload(900, 100)),
+    ]
+
+    code, document, _ = run_search(
+        monkeypatch, capsys, candidates, *BASE_ARGS, "--width", "200", "--height", "200"
+    )
+
+    assert code == envelope.EXIT_NO_SIZE
+    assert document["status"] == envelope.STATUS_NO_SIZE
+    assert document["items"] == []
+    assert document["error"]["code"] == "no_size_match"
+    assert document["error"]["details"]["format_matches"] == 2
+    assert document["error"]["details"]["unreadable_candidates"] == 0
+
+
+def test_exactly_minimum_size_is_accepted(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    candidate = StubCandidate("edge", "https://example.test/edge.png", payload=png_payload(200, 200))
+
+    code, document, _ = run_search(
+        monkeypatch, capsys, [candidate], *BASE_ARGS, "--width", "200", "--height", "200"
+    )
+
+    assert code == envelope.EXIT_OK
+    assert (document["items"][0]["width"], document["items"][0]["height"]) == (200, 200)
+
+
+def test_no_format_and_no_size_and_access_error_stay_distinct(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    format_code, format_document, _ = run_search(
+        monkeypatch,
+        capsys,
+        [StubCandidate("gif", "https://example.test/a.png", payload=gif_payload())],
+        *BASE_ARGS,
+    )
+    size_code, size_document, _ = run_search(
+        monkeypatch,
+        capsys,
+        [StubCandidate("small", "https://example.test/a.png", payload=png_payload(10, 10))],
+        *BASE_ARGS,
+        "--width",
+        "100",
+        "--height",
+        "100",
+    )
+    access_code, access_document, _ = run_search(
+        monkeypatch,
+        capsys,
+        [StubCandidate("broken", "https://example.test/a.png", error=SourceAccessError("refused"))],
+        *BASE_ARGS,
+    )
+
+    assert format_document["status"] == envelope.STATUS_NO_FORMAT
+    assert size_document["status"] == envelope.STATUS_NO_SIZE
+    assert access_document["status"] == envelope.STATUS_ACCESS_ERROR
+    assert len({format_code, size_code, access_code}) == 3
+    assert (format_code, size_code, access_code) == (
+        envelope.EXIT_NO_FORMAT,
+        envelope.EXIT_NO_SIZE,
+        envelope.EXIT_ACCESS_ERROR,
+    )
+
+
+def test_fetch_failure_yields_access_error(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    candidates = [
+        StubCandidate(
+            "broken",
+            "https://example.test/a.png",
+            error=SourceAccessError("image source returned HTTP status 503", url="https://example.test/a.png"),
+        )
+    ]
+
+    code, document, _ = run_search(monkeypatch, capsys, candidates, *BASE_ARGS)
+
+    assert code == envelope.EXIT_ACCESS_ERROR
+    assert document["status"] == envelope.STATUS_ACCESS_ERROR
+    assert document["items"] == []
+    assert document["error"]["code"] == "candidate_unreadable"
+    assert document["error"]["details"]["failures"][0]["url"] == "https://example.test/a.png"
+
+
+def test_unreadable_payload_yields_access_error_not_no_format(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    candidate = StubCandidate(
+        "junk", "https://example.test/a.png", payload=b"not-an-image", mime_type="image/png"
+    )
+
+    code, document, _ = run_search(monkeypatch, capsys, [candidate], *BASE_ARGS)
+
+    assert code == envelope.EXIT_ACCESS_ERROR
+    assert document["status"] == envelope.STATUS_ACCESS_ERROR
+    assert document["error"]["details"]["failures"][0]["error"]["code"] == "image_unreadable"
+
+
+def test_candidate_window_is_bounded_by_the_requested_count(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    candidates = [
+        StubCandidate(f"fox{n}", f"https://example.test/{n}.png", payload=png_payload(100, 100))
+        for n in range(40)
+    ]
+
+    code, document, registry = run_search(
+        monkeypatch, capsys, candidates, *BASE_ARGS, "--count", "3"
+    )
+
+    assert code == envelope.EXIT_OK
+    assert len(document["items"]) == 3
+    assert registry.calls == [("fake", "red fox", example.candidate_cap(3))]
+    assert example.candidate_cap(3) == 12
+    # Fetching stops as soon as enough usable images were found.
+    assert sum(candidate.fetch_calls for candidate in candidates) == 3
+
+
+def test_filtering_walks_past_rejected_candidates_until_the_count_is_met(
+    monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    candidates = [
+        StubCandidate("gif", "https://example.test/0.png", payload=gif_payload()),
+        StubCandidate("tiny", "https://example.test/1.png", payload=png_payload(10, 10)),
+        StubCandidate("ok1", "https://example.test/2.png", payload=png_payload(120, 120)),
+        StubCandidate("ok2", "https://example.test/3.png", payload=png_payload(130, 130)),
+        StubCandidate("never", "https://example.test/4.png", payload=png_payload(140, 140)),
+    ]
+
+    code, document, _ = run_search(
+        monkeypatch, capsys, candidates, *BASE_ARGS, "--count", "2", "--width", "100", "--height", "100"
+    )
+
+    assert code == envelope.EXIT_OK
+    assert [item["url"] for item in document["items"]] == [
+        "https://example.test/2.png",
+        "https://example.test/3.png",
+    ]
+    assert [(item["width"], item["height"]) for item in document["items"]] == [(120, 120), (130, 130)]
+    assert candidates[-1].fetch_calls == 0

tools/example.py

@@ -17,9 +17,11 @@ import re
 import struct
 import sys
 import zlib
+from dataclasses import dataclass
 from pathlib import Path
 from typing import Any, Mapping, Sequence
 
+from . import images
 from .envelope import (
     STATUS_ACCESS_ERROR,
     STATUS_NO_FORMAT,
@@ -41,6 +43,7 @@ from .errors import (
     UnknownSourceError,
     UnsupportedFormatError,
 )
+from .images import ImageFormatError, ImageInfo
 from .sources import DEFAULT_LIMIT, SourceCandidate, default_registry
 
 PROG = "python -m tools.example"
@@ -66,11 +69,21 @@ DEFAULT_OUTPUT_DIR = "downloads"
 MIN_COUNT = 1
 MAX_COUNT = 50
 
-#: MIME types (and filename suffixes) that count as a hit for each format.
-FORMAT_MATCHERS: dict[str, tuple[str, ...]] = {
-    "png": ("image/png", ".png"),
-    "ico": ("image/x-icon", "image/vnd.microsoft.icon", "image/ico", ".ico"),
-}
+#: How many candidates are fetched per wanted image.  Nothing a source reports
+#: about a hit is trusted, so a candidate is only known to be usable once its
+#: bytes have been downloaded and parsed; the source is therefore asked for a
+#: window of ``count * SAMPLE_FACTOR`` hits and the window is cut short as soon
+#: as ``count`` usable images have been found.
+SAMPLE_FACTOR = 4
+#: Hard upper bound on the number of candidates inspected in one invocation.
+MAX_CANDIDATE_CAP = MAX_COUNT
+
+
+def candidate_cap(count: int) -> int:
+    """Return the bounded number of candidates to fetch for ``count`` images."""
+
+    wanted = max(MIN_COUNT, int(count))
+    return min(MAX_CANDIDATE_CAP, wanted * SAMPLE_FACTOR)
 
 
 class _Parser(argparse.ArgumentParser):
@@ -312,49 +325,145 @@ def normalize_request(raw: Mapping[str, Any]) -> dict[str, Any]:
     }
 
 
-def _matches_format(candidate: SourceCandidate, image_format: str) -> bool:
-    matchers = FORMAT_MATCHERS[image_format]
-    mime = str(getattr(candidate, "mime_type", None) or "").split(";")[0].strip().lower()
-    if mime and mime in matchers:
-        return True
-    url = str(getattr(candidate, "file_url", "") or "").lower()
-    return any(url.endswith(suffix) for suffix in matchers if suffix.startswith("."))
+def _meets_minimum_size(info: ImageInfo, request: Mapping[str, Any]) -> bool:
+    """True when the *real* pixel size of an image is usable for the request.
 
-
-def _matches_size(candidate: SourceCandidate, request: Mapping[str, Any]) -> bool:
-    """True when ``candidate`` is usable for the requested target size.
-
-    The requested width and height are a target size, not an exact match: any
-    image that is at least that large is usable because it can always be
-    downscaled, while a smaller image would have to be scaled up and is
-    therefore rejected.  ``--resize`` only decides whether the downloaded bytes
-    are downscaled; it never widens or narrows this filter.
+    The requested width and height are a minimum, not an exact match: any image
+    that is at least that large is usable because it can always be downscaled,
+    while a smaller image would have to be scaled up and is therefore rejected.
+    ``--resize`` only decides whether the downloaded bytes are downscaled; it
+    never widens or narrows this filter.
     """
 
     wanted_width = request.get("width")
     wanted_height = request.get("height")
-    if wanted_width is None and wanted_height is None:
-        return True
-    actual_width = getattr(candidate, "width", None)
-    actual_height = getattr(candidate, "height", None)
-    if isinstance(actual_width, bool) or isinstance(actual_height, bool):
+    if wanted_width is not None and info.width < wanted_width:
         return False
-    if not isinstance(actual_width, int) or not isinstance(actual_height, int):
-        return False
-    if wanted_width is not None and actual_width < wanted_width:
-        return False
-    if wanted_height is not None and actual_height < wanted_height:
+    if wanted_height is not None and info.height < wanted_height:
         return False
     return True
 
 
-def _select(candidates: Sequence[SourceCandidate], request: Mapping[str, Any]) -> tuple[list[SourceCandidate], list[SourceCandidate]]:
-    """Return ``(matching, rejected_by_size)`` split for the request."""
+def _failure(candidate: SourceCandidate, fragment: Mapping[str, Any]) -> dict[str, Any]:
+    """Describe one candidate whose bytes could not be turned into an image."""
 
-    by_format = [c for c in candidates if _matches_format(c, request["format"])]
-    matching = [c for c in by_format if _matches_size(c, request)]
-    rejected = [c for c in by_format if not _matches_size(c, request)]
-    return matching, rejected
+    return {
+        "source": str(getattr(candidate, "source_id", "") or ""),
+        "url": str(getattr(candidate, "file_url", "") or ""),
+        "error": dict(fragment),
+    }
+
+
+@dataclass(frozen=True)
+class KeptImage:
+    """One candidate whose downloaded bytes really match the request."""
+
+    candidate: SourceCandidate
+    info: ImageInfo
+    payload: bytes
+
+    def to_item(self) -> dict[str, Any]:
+        """Report ``source``, ``url``, ``format`` and the *actual* dimensions."""
+
+        source = str(getattr(self.candidate, "source_id", "") or "")
+        url = str(getattr(self.candidate, "file_url", "") or "")
+        item: dict[str, Any] = {
+            "source": source,
+            "url": url,
+            # Kept as an alias so existing consumers keep working unchanged.
+            "file_url": url,
+            "format": self.info.format,
+            "width": self.info.width,
+            "height": self.info.height,
+            "size_bytes": len(self.payload),
+        }
+        for key in ("title", "page_url", "mime_type"):
+            value = getattr(self.candidate, key, None)
+            if value is not None:
+                item[key] = value
+        return item
+
+
+@dataclass(frozen=True)
+class Selection:
+    """Outcome of inspecting a bounded window of downloaded candidates."""
+
+    kept: tuple[KeptImage, ...] = ()
+    format_rejects: int = 0
+    size_rejects: int = 0
+    failures: tuple[dict[str, Any], ...] = ()
+
+    @property
+    def inspected(self) -> int:
+        """Number of candidates whose bytes were read successfully."""
+
+        return len(self.kept) + self.format_rejects + self.size_rejects
+
+
+def _select(candidates: Sequence[SourceCandidate], request: Mapping[str, Any]) -> Selection:
+    """Fetch and inspect candidates, keeping only what the real bytes justify.
+
+    Every kept item has been downloaded and parsed: its format comes from the
+    container signature and its width/height come from the PNG ``IHDR`` or the
+    ICO directory, never from what the source claimed.  A candidate that serves
+    other-format bytes counts as a format rejection, a candidate of the wanted
+    format that is too small counts as a size rejection, and a candidate whose
+    bytes cannot be fetched or parsed is recorded as a failure instead of being
+    silently dropped.
+    """
+
+    wanted_format = str(request["format"])
+    limit = int(request["count"])
+    kept: list[KeptImage] = []
+    format_rejects = 0
+    size_rejects = 0
+    failures: list[dict[str, Any]] = []
+
+    for candidate in candidates:
+        if len(kept) >= limit:
+            break
+        try:
+            payload = candidate.fetch_bytes()
+        except ToolkitError as error:
+            failures.append(_failure(candidate, error.to_dict()))
+            continue
+        except OSError as error:
+            failures.append(
+                _failure(candidate, error_fragment("fetch_failed", str(error)))
+            )
+            continue
+        detected = images.detect_format(payload)
+        if detected is None:
+            failures.append(
+                _failure(
+                    candidate,
+                    error_fragment(
+                        "image_unreadable",
+                        "downloaded payload is not a recognised image format",
+                        details={"bytes": len(payload)},
+                    ),
+                )
+            )
+            continue
+        if detected != wanted_format:
+            format_rejects += 1
+            continue
+        try:
+            info = images.inspect_image(payload)
+        except ImageFormatError as error:
+            failures.append(_failure(candidate, error.to_dict()))
+            continue
+        if not _meets_minimum_size(info, request):
+            size_rejects += 1
+            continue
+        kept.append(KeptImage(candidate=candidate, info=info, payload=payload))
+
+    return Selection(
+        kept=tuple(kept),
+        format_rejects=format_rejects,
+        size_rejects=size_rejects,
+        failures=tuple(failures),
+    )
 
 
 def _slug(candidate: SourceCandidate) -> str:
@@ -665,12 +774,16 @@ def resize_image(payload: bytes, request: Mapping[str, Any]) -> tuple[bytes, dic
     return output, fragment
 
 
-def _download(candidates: Sequence[SourceCandidate], request: Mapping[str, Any]) -> tuple[list[dict[str, Any]], int]:
-    """Download ``candidates``; return ``(items, failure_count)``.
+def _download(
+    selected: Sequence[KeptImage], request: Mapping[str, Any]
+) -> tuple[list[dict[str, Any]], int]:
+    """Write the already-fetched payloads; return ``(items, failure_count)``.
 
-    With ``resize`` enabled every stored file is downscaled to the requested
-    target size first.  A payload that cannot be decoded or downscaled is
-    reported as a failed item rather than silently stored at the wrong size.
+    The bytes verified by :func:`_select` are stored as they are (or downscaled
+    first when ``resize`` is enabled), so an item can never be downloaded twice
+    or stored in a format that was not verified.  A payload that cannot be
+    downscaled to the requested size is reported as a failed item instead of
+    being silently stored at the wrong size.
     """
 
     directory = Path(request["output_dir"]).expanduser()
@@ -685,11 +798,11 @@ def _download(candidates: Sequence[SourceCandidate], request: Mapping[str, Any])
     suffix = "." + str(request["format"])
     items: list[dict[str, Any]] = []
     failures = 0
-    for index, candidate in enumerate(candidates, start=1):
-        record = dict(candidate.to_dict())
-        destination = directory / f"{index:03d}-{_slug(candidate)}{suffix}"
+    for index, entry in enumerate(selected, start=1):
+        record = entry.to_item()
+        destination = directory / f"{index:03d}-{_slug(entry.candidate)}{suffix}"
+        payload = entry.payload
         try:
-            payload = candidate.fetch_bytes()
             resize_fragment = resize_image(payload, request) if request["resize"] else None
             if resize_fragment is not None:
                 payload, resize_fragment = resize_fragment
@@ -756,7 +869,21 @@ def _request_from(argv: Sequence[str]) -> tuple[dict[str, Any], str, Any]:
 
 
 def _execute(argv: Sequence[str]) -> Response:
-    """Run one request and return the response that must be printed."""
+    """Run one request and return the response that must be printed.
+
+    The search path downloads a bounded window of candidates and filters that
+    window on the parsed bytes, so the answer distinguishes three different
+    situations through both the status and the exit code:
+
+    * ``ok`` -- at least one image really is the requested format and at least
+      the requested width and height;
+    * ``no_format`` -- the source had hits, but none of them really are the
+      requested format;
+    * ``no_size`` -- images of the requested format existed, but all of them
+      are smaller than requested;
+    * ``access_error`` -- the source could not be searched or served bytes that
+      are not readable images, which is never reported as a "no match".
+    """
 
     request, source, registry = _request_from(argv)
     builder = ResponseBuilder(source=source, request=request)
@@ -764,24 +891,36 @@ def _execute(argv: Sequence[str]) -> Response:
     if request["dry_run"]:
         return builder.dry_run()
 
-    candidates = registry.search(source, request["topic"], request["count"])
-    matching, rejected = _select(candidates, request)
+    cap = candidate_cap(request["count"])
+    candidates = registry.search(source, request["topic"], cap)
+    selection = _select(candidates, request)
+    unreadable = list(selection.failures)
 
-    if not matching:
+    if not selection.kept:
         details = {
             "requested_format": request["format"],
             "requested_size": {"width": request["width"], "height": request["height"]},
             "minimum_size": {"width": request["width"], "height": request["height"]},
             "candidate_count": len(candidates),
-            "format_matches": len(matching) + len(rejected),
+            "candidates_inspected": selection.inspected,
+            "format_matches": selection.size_rejects,
             "size_matches": 0,
+            "unreadable_candidates": len(unreadable),
         }
-        if rejected:
+        if unreadable:
+            return builder.access_error(
+                error=error_fragment(
+                    "candidate_unreadable",
+                    "the source served images that could not be read",
+                    details={**details, "failures": unreadable},
+                )
+            )
+        if selection.size_rejects:
             return builder.build(
                 STATUS_NO_SIZE,
                 error=error_fragment(
                     "no_size_match",
-                    "no image is at least the requested width and height",
+                    "the source returned no images at least the requested width and height",
                     details=details,
                 ),
             )
@@ -795,27 +934,47 @@ def _execute(argv: Sequence[str]) -> Response:
         )
 
     if not request["download"]:
-        return builder.build(STATUS_OK, items=[c.to_dict() for c in matching])
+        if unreadable:
+            return builder.build(
+                STATUS_PARTIAL_DOWNLOAD,
+                items=[entry.to_item() for entry in selection.kept],
+                error=error_fragment(
+                    "candidate_incomplete",
+                    f"{len(unreadable)} of {selection.inspected + len(unreadable)} "
+                    "candidates could not be read",
+                    details={
+                        "failed": len(unreadable),
+                        "total": selection.inspected + len(unreadable),
+                        "failures": unreadable,
+                    },
+                ),
+            )
+        return builder.build(STATUS_OK, items=[entry.to_item() for entry in selection.kept])
 
-    items, failures = _download(matching, request)
-    if failures and failures == len(items):
+    items, download_failures = _download(selection.kept, request)
+    failed = download_failures + len(unreadable)
+    if download_failures == len(items):
         return builder.build(
             STATUS_ACCESS_ERROR,
             items=items,
             error=error_fragment(
                 "download_failed",
                 "every download failed",
-                details={"failed": failures, "total": len(items)},
+                details={"failed": download_failures, "total": len(items)},
             ),
         )
-    if failures:
+    if failed:
         return builder.build(
             STATUS_PARTIAL_DOWNLOAD,
             items=items,
             error=error_fragment(
                 "download_incomplete",
-                f"{failures} of {len(items)} downloads failed",
-                details={"failed": failures, "total": len(items)},
+                f"{failed} of {len(items) + len(unreadable)} images could not be handled",
+                details={
+                    "failed": failed,
+                    "total": len(items) + len(unreadable),
+                    "failures": unreadable,
+                },
             ),
         )
     return builder.build(STATUS_OK, items=items)

tools/images.py

@@ -0,0 +1,367 @@
+"""Stdlib-only inspection and construction of the image formats we accept.
+
+The CLI decides what to keep from the **downloaded bytes** and never from the
+metadata a source reports about a file: a search hit that claims to be a
+``image/png`` of 800x600 pixels is kept only when the bytes really are a PNG of
+at least the requested size.  This module is the single place that knows how to
+read the real format and the real pixel dimensions out of those bytes.
+
+It deliberately depends on nothing but the standard library:
+
+* :func:`detect_format` recognises the container signatures of the two accepted
+  formats (``png``, ``ico``) plus a handful of common formats (``jpeg``,
+  ``gif``, ``bmp``, ``webp``).  Recognising the *other* formats is what makes
+  the ``no_format`` outcome possible: "the source has images, but none of them
+  are the format you asked for" is a different answer from "the source has no
+  usable images at all".
+* :func:`read_png_dimensions` reads the ``IHDR`` header of a PNG.
+* :func:`read_ico_dimensions` reads the ICONDIR/ICONDIRENTRY directory of an
+  ICO file (the directory stores the real sizes, including the ``0``-means-256
+  encoding).
+* :func:`inspect_image` combines both into an :class:`ImageInfo`.
+* :func:`png_bytes` and :func:`ico_bytes` *build* the same formats so tests (and
+  callers) can produce real payloads without Pillow or any other dependency.
+
+Every failure raises :class:`ImageFormatError`, a :class:`~tools.errors.ToolkitError`
+subclass whose ``code`` is stable so the CLI can report an unreadable payload as
+an access problem instead of silently treating it as "no match".
+"""
+
+from __future__ import annotations
+
+import struct
+import zlib
+from dataclasses import dataclass
+from typing import Any, Iterable, Sequence
+
+from .errors import ToolkitError
+
+__all__ = [
+    "BMP",
+    "GIF",
+    "ICO",
+    "ICO_ENTRY_SIZE",
+    "ICO_HEADER_SIZE",
+    "ICO_MAX_DIMENSION",
+    "ImageFormatError",
+    "ImageInfo",
+    "JPEG",
+    "KNOWN_FORMATS",
+    "PNG",
+    "PNG_SIGNATURE",
+    "WEBP",
+    "detect_format",
+    "ico_bytes",
+    "ico_entries",
+    "inspect_image",
+    "png_bytes",
+    "read_ico_dimensions",
+    "read_png_dimensions",
+]
+
+#: The two formats the CLI accepts as search requests.
+PNG = "png"
+ICO = "ico"
+#: Formats that are recognised (so they can be reported as "another format")
+#: but never accepted by the CLI.
+JPEG = "jpeg"
+GIF = "gif"
+BMP = "bmp"
+WEBP = "webp"
+
+#: Every format :func:`detect_format` can name.
+KNOWN_FORMATS: tuple[str, ...] = (PNG, ICO, JPEG, GIF, BMP, WEBP)
+
+#: The 8-byte signature every PNG file starts with.
+PNG_SIGNATURE = b"\x89PNG\r\n\x1a\n"
+
+#: ICONDIR: reserved (0), type (1 = icon) and the entry count.
+ICO_HEADER_SIZE = 6
+#: One ICONDIRENTRY: 16 bytes of width/height/colours/planes/bpp/size/offset.
+ICO_ENTRY_SIZE = 16
+#: A single ICO dimension byte of 0 encodes 256 pixels.
+ICO_MAX_DIMENSION = 256
+
+#: Magic prefixes of the recognised but unsupported formats.
+_JPEG_SIGNATURE = b"\xff\xd8\xff"
+_GIF_SIGNATURES = (b"GIF87a", b"GIF89a")
+_BMP_SIGNATURE = b"BM"
+_RIFF_SIGNATURE = b"RIFF"
+_WEBP_SIGNATURE = b"WEBP"
+
+#: Number of payload bytes worth quoting in an error detail.
+_SIGNATURE_PROBE = 16
+
+
+class ImageFormatError(ToolkitError):
+    """Raised when downloaded bytes are not a readable image of a known format.
+
+    The bytes could not be attributed to any format the toolkit understands, or
+    they claim to be an accepted format but their header is truncated/corrupt.
+    It derives from :class:`~tools.errors.ToolkitError`, so the CLI reports it
+    as an access problem (the source handed us something unusable) rather than
+    as a "no such format" / "no such size" verdict about the topic.
+    """
+
+    code = "image_unreadable"
+
+    def __init__(self, message: str, *, details: dict[str, Any] | None = None) -> None:
+        super().__init__(message, details=details)
+
+
+@dataclass(frozen=True)
+class ImageInfo:
+    """The real format and real pixel size of one payload."""
+
+    format: str
+    width: int
+    height: int
+
+    def to_dict(self) -> dict[str, Any]:
+        """JSON-ready projection used in CLI items."""
+
+        return {"format": self.format, "width": self.width, "height": self.height}
+
+    @property
+    def area(self) -> int:
+        return self.width * self.height
+
+
+def _probe(payload: bytes) -> bytes:
+    return bytes(payload[:_SIGNATURE_PROBE])
+
+
+def detect_format(payload: bytes) -> str | None:
+    """Return the real container format of *payload*, or ``None``.
+
+    ``None`` means "these bytes are not an image format the toolkit knows", which
+    is an access/payload problem.  A recognised format other than ``png``/``ico``
+    is a perfectly good answer that the CLI turns into ``no_format``.
+    """
+
+    if not isinstance(payload, (bytes, bytearray, memoryview)):
+        return None
+    data = bytes(payload)
+    if data.startswith(PNG_SIGNATURE):
+        return PNG
+    if data[:4] == b"\x00\x00\x01\x00":
+        return ICO
+    if data.startswith(_JPEG_SIGNATURE):
+        return JPEG
+    if data.startswith(_GIF_SIGNATURES):
+        return GIF
+    if data.startswith(_BMP_SIGNATURE) and len(data) >= 14:
+        return BMP
+    if data.startswith(_RIFF_SIGNATURE) and data[8:12] == _WEBP_SIGNATURE:
+        return WEBP
+    return None
+
+
+def read_png_dimensions(payload: bytes) -> tuple[int, int]:
+    """Return ``(width, height)`` from the ``IHDR`` chunk of a PNG payload."""
+
+    data = bytes(payload)
+    if not data.startswith(PNG_SIGNATURE):
+        raise ImageFormatError(
+            "payload does not start with the PNG signature",
+            details={"signature": _probe(data).hex()},
+        )
+    offset = len(PNG_SIGNATURE)
+    if len(data) < offset + 8:
+        raise ImageFormatError("PNG header is truncated")
+    (length,) = struct.unpack(">I", data[offset : offset + 4])
+    chunk_type = data[offset + 4 : offset + 8]
+    if chunk_type != b"IHDR":
+        raise ImageFormatError(
+            "PNG payload does not start with an IHDR chunk",
+            details={"first_chunk": chunk_type.decode("latin-1")},
+        )
+    if length != 13:
+        raise ImageFormatError(
+            "PNG IHDR chunk has an unexpected length",
+            details={"length": length},
+        )
+    header = data[offset + 8 : offset + 8 + length]
+    if len(header) != length:
+        raise ImageFormatError("PNG IHDR chunk is truncated")
+    width, height = struct.unpack(">II", header[:8])
+    if width < 1 or height < 1:
+        raise ImageFormatError(
+            "PNG image declares no pixels",
+            details={"width": width, "height": height},
+        )
+    return int(width), int(height)
+
+
+def ico_entries(payload: bytes) -> tuple[tuple[int, int, int, int], ...]:
+    """Return ``(width, height, size, offset)`` for every ICO directory entry."""
+
+    data = bytes(payload)
+    if len(data) < ICO_HEADER_SIZE:
+        raise ImageFormatError("ICO header is truncated")
+    reserved, kind, count = struct.unpack("<HHH", data[:ICO_HEADER_SIZE])
+    if reserved != 0 or kind != 1 or count < 1:
+        raise ImageFormatError(
+            "payload is not an ICO image",
+            details={"reserved": reserved, "type": kind, "count": count},
+        )
+    directory_end = ICO_HEADER_SIZE + count * ICO_ENTRY_SIZE
+    if len(data) < directory_end:
+        raise ImageFormatError(
+            "ICO directory is truncated",
+            details={"count": count, "bytes": len(data)},
+        )
+    entries: list[tuple[int, int, int, int]] = []
+    for index in range(count):
+        start = ICO_HEADER_SIZE + index * ICO_ENTRY_SIZE
+        raw_width, raw_height, _colours, _reserved, _planes, _bits, size, offset = struct.unpack(
+            "<BBBBHHII", data[start : start + ICO_ENTRY_SIZE]
+        )
+        width = raw_width or ICO_MAX_DIMENSION
+        height = raw_height or ICO_MAX_DIMENSION
+        if size < 1 or offset < directory_end or offset + size > len(data):
+            raise ImageFormatError(
+                "ICO directory entry points outside the payload",
+                details={"index": index, "size": size, "offset": offset, "bytes": len(data)},
+            )
+        entries.append((width, height, size, offset))
+    return tuple(entries)
+
+
+def read_ico_dimensions(payload: bytes) -> tuple[int, int]:
+    """Return the dimensions of the largest image stored in an ICO payload.
+
+    The ICONDIRENTRY directory is authoritative: it carries one width/height pair
+    per stored image, so no decoding of the embedded bitmap or PNG is needed.
+    """
+
+    entries = ico_entries(payload)
+    width, height, _size, _offset = max(entries, key=lambda entry: entry[0] * entry[1])
+    return int(width), int(height)
+
+
+def inspect_image(payload: bytes) -> ImageInfo:
+    """Return the real format and pixel size of *payload*.
+
+    Raises :class:`ImageFormatError` when the bytes are not an image at all, or
+    when they are not one of the two formats whose dimensions can be read.
+    """
+
+    image_format = detect_format(payload)
+    if image_format is None:
+        raise ImageFormatError(
+            "downloaded payload is not a recognised image format",
+            details={"signature": _probe(bytes(payload)).hex()},
+        )
+    if image_format == PNG:
+        width, height = read_png_dimensions(payload)
+        return ImageInfo(format=PNG, width=width, height=height)
+    if image_format == ICO:
+        width, height = read_ico_dimensions(payload)
+        return ImageInfo(format=ICO, width=width, height=height)
+    raise ImageFormatError(
+        f"pixel dimensions are only readable for png and ico, not {image_format}",
+        details={"format": image_format},
+    )
+
+
+# ---------------------------------------------------------------------------
+# Builders: the same formats, produced with the standard library only
+# ---------------------------------------------------------------------------
+
+
+def _png_chunk(chunk_type: bytes, chunk_data: bytes) -> bytes:
+    crc = zlib.crc32(chunk_type + chunk_data) & 0xFFFFFFFF
+    return struct.pack(">I", len(chunk_data)) + chunk_type + chunk_data + struct.pack(">I", crc)
+
+
+def png_bytes(
+    width: int,
+    height: int,
+    *,
+    rgba: tuple[int, int, int, int] = (255, 255, 255, 255),
+    rows: Sequence[bytes] | None = None,
+    level: int = 6,
+) -> bytes:
+    """Build a valid 8-bit RGBA PNG of exactly ``width`` x ``height`` pixels.
+
+    ``rows`` may supply raw (filter-0) RGBA scanlines; otherwise every pixel is
+    filled with ``rgba``.  The result is a real PNG file: a correct signature,
+    a 13-byte ``IHDR``, a zlib-compressed ``IDAT`` and an empty ``IEND``.
+    """
+
+    if isinstance(width, bool) or isinstance(height, bool):
+        raise ValueError("width and height must be integers")
+    if width < 1 or height < 1:
+        raise ValueError("width and height must be >= 1")
+    if rows is None:
+        pixel = bytes(rgba)
+        line = pixel * width
+        raw = bytearray()
+        for _ in range(height):
+            raw.append(0)
+            raw += line
+        raw_bytes = bytes(raw)
+    else:
+        if len(rows) != height:
+            raise ValueError("rows must contain exactly height scanlines")
+        raw = bytearray()
+        for row in rows:
+            if len(row) != width * 4:
+                raise ValueError("each scanline must hold width * 4 bytes")
+            raw.append(0)
+            raw += row
+        raw_bytes = bytes(raw)
+    header = struct.pack(">IIBBBBB", width, height, 8, 6, 0, 0, 0)
+    return (
+        PNG_SIGNATURE
+        + _png_chunk(b"IHDR", header)
+        + _png_chunk(b"IDAT", zlib.compress(raw_bytes, level))
+        + _png_chunk(b"IEND", b"")
+    )
+
+
+def ico_bytes(
+    width: int,
+    height: int,
+    *,
+    rgba: tuple[int, int, int, int] = (255, 255, 255, 255),
+    frames: Iterable[tuple[int, int]] | None = None,
+) -> bytes:
+    """Build an ICO payload whose directory lists the requested sizes.
+
+    By default the ICO stores one embedded PNG of ``width`` x ``height`` pixels
+    and the directory entry repeats those dimensions (``0`` encodes 256).  Pass
+    ``frames`` to describe several directory entries instead, for example
+    ``ico_bytes(48, 48, frames=[(16, 16), (48, 48)])``: the dimensions of the
+    *largest* entry then become the reported size, which is exactly what
+    :func:`read_ico_dimensions` returns.
+    """
+
+    sizes = list(frames) if frames is not None else [(width, height)]
+    if not sizes:
+        raise ValueError("an ICO payload needs at least one frame")
+    payloads: list[bytes] = []
+    for frame_width, frame_height in sizes:
+        if not 1 <= frame_width <= ICO_MAX_DIMENSION:
+            raise ValueError("ICO widths must be between 1 and 256 pixels")
+        if not 1 <= frame_height <= ICO_MAX_DIMENSION:
+            raise ValueError("ICO heights must be between 1 and 256 pixels")
+        payloads.append(png_bytes(frame_width, frame_height, rgba=rgba))
+    header = struct.pack("<HHH", 0, 1, len(payloads))
+    offset = ICO_HEADER_SIZE + ICO_ENTRY_SIZE * len(payloads)
+    entries = bytearray()
+    for (frame_width, frame_height), payload in zip(sizes, payloads):
+        entries += struct.pack(
+            "<BBBBHHII",
+            frame_width % ICO_MAX_DIMENSION,
+            frame_height % ICO_MAX_DIMENSION,
+            0,
+            0,
+            1,
+            32,
+            len(payload),
+            offset,
+        )
+        offset += len(payload)
+    return header + bytes(entries) + b"".join(payloads)