diff --git a/scripts/ci/pingora_edge_policy.py b/scripts/ci/pingora_edge_policy.py
index 0d3a2c0948..5c1e39d9e3 100644
--- a/scripts/ci/pingora_edge_policy.py
+++ b/scripts/ci/pingora_edge_policy.py
@@ -58,6 +58,10 @@
 MAX_FILE_BYTES = 1_048_576
 MAX_RESPONSE_BYTES = 16_777_216
 MAX_BLOB_BYTES = 100_000_000
+# Decoded-pixel bound for PNG validation, separate from the HTTP response cap:
+# a valid 2238x2052 RGBA screenshot decodes to 18.4 MB. 128 MiB covers 16-bit
+# RGBA up to 4K and 8-bit RGBA up to 6K while keeping zlib output bounded.
+MAX_PNG_DECODED_BYTES = 134_217_728
 REPOSITORY_RE = re.compile(r"^(?!.*(?:\.\.|\.$))[A-Za-z0-9_.-]+/(?!.*(?:\.\.|\.$))[A-Za-z0-9_.-]+$")
 SHA_RE = re.compile(r"^[0-9a-f]{40}$")
 # A base ref threaded into evaluate_pull_request may be either a branch name
@@ -833,7 +837,7 @@ def _is_complete_png(raw: bytes) -> bool:
                 pass_height = (height - y_start + y_step - 1) // y_step
                 row_bytes = (pass_width * channels * bit_depth + 7) // 8
                 expected_size += pass_height * (row_bytes + 1)
-                if expected_size > MAX_RESPONSE_BYTES:
+                if expected_size > MAX_PNG_DECODED_BYTES:
                     return False
                 scanlines.append((pass_height, row_bytes, pass_width))
             decoder = zlib.decompressobj()
diff --git a/tests/test_pingora_edge_policy.py b/tests/test_pingora_edge_policy.py
index 1a428556ad..aba34f6346 100644
--- a/tests/test_pingora_edge_policy.py
+++ b/tests/test_pingora_edge_policy.py
@@ -794,6 +794,33 @@ def chunk(kind: bytes, data: bytes) -> bytes:
     assert not policy._is_complete_png(signature + header + chunk(b"TEXT", b""))
 
 
+def test_png_decoded_size_is_not_bounded_by_the_http_response_cap() -> None:
+    """A valid 2238x2052 RGBA screenshot decodes past 16 MiB and is still a PNG.
+
+    Regression for late-life-anxiety-reanalysis#257: five macOS window
+    screenshots under a declared artifact prefix were rejected, then reported
+    as "not valid UTF-8", because the decoded-pixel bound reused the HTTP
+    response cap.
+    """
+
+    def chunk(kind: bytes, data: bytes) -> bytes:
+        payload = kind + data
+        return len(data).to_bytes(4, "big") + payload + zlib.crc32(payload).to_bytes(4, "big")
+
+    width, height = 2238, 2052
+    decoded = (b"\0" + b"\0" * (width * 4)) * height
+    assert len(decoded) > policy.MAX_RESPONSE_BYTES
+    header = width.to_bytes(4, "big") + height.to_bytes(4, "big") + bytes((8, 6, 0, 0, 0))
+    raw = (
+        policy.PNG_SIGNATURE
+        + chunk(b"IHDR", header)
+        + chunk(b"IDAT", zlib.compress(decoded))
+        + chunk(b"IEND", b"")
+    )
+    assert len(raw) < policy.MAX_FILE_BYTES
+    assert policy._is_complete_png(raw)
+
+
 def test_png_semantic_validation_fails_closed() -> None:
     """CRC-valid chunks still need a valid bounded PNG image stream."""
 
@@ -865,7 +892,7 @@ def indexed_png(
     assert not policy._is_complete_png(png(rgba, chunk(b"IDAT", zlib.compress(b"\0")), end))
     assert not policy._is_complete_png(png(rgba, chunk(b"IDAT", zlib.compress(b"\0\0\0\0\0") + b"x"), end))
     assert not policy._is_complete_png(png(rgba, chunk(b"IDAT", zlib.compress(b"\5\0\0\0\0")), end))
-    huge = (policy.MAX_RESPONSE_BYTES).to_bytes(4, "big") + (1).to_bytes(4, "big") + bytes((8, 6, 0, 0, 0))
+    huge = (policy.MAX_PNG_DECODED_BYTES // 4).to_bytes(4, "big") + (1).to_bytes(4, "big") + bytes((8, 6, 0, 0, 0))
     assert not policy._is_complete_png(png(huge, image, end))
 
     adam7 = (8).to_bytes(4, "big") * 2 + bytes((8, 6, 0, 0, 1))