Sample EPUB file (valid)

Handle it in code

Each snippet below was run against this exact file with Python 3.14.4 and Node 24.15.0 when this page was built, and the output shown is what it printed.

Python

Follow container.xml to the package document and confirm it exists, which is exactly what a reader does first.

import re, zipfile

with zipfile.ZipFile("valid.epub") as z:
    first = z.infolist()[0]
    if first.filename != "mimetype" or first.compress_type != zipfile.ZIP_STORED:
        raise SystemExit("rejected: mimetype must be the first entry, stored uncompressed")
    container = z.read("META-INF/container.xml").decode("utf-8")
    opf = re.search(r'full-path="([^"]+)"', container).group(1)
    print("ok" if opf in z.namelist() else f"rejected: container.xml points at {opf}, which is missing")

Run against this file, it prints: ok