Before you upload an EPUB, validate it: Apple Books says every book must pass the latest version of EPUBCheck, Kobo runs every upload through EPUB validation, and KDP recommends checking the file in Kindle Previewer. Then check what validators do not tell you in plain words, such as missing alt text, a broken link or a missing cover. ProofSlip checks an EPUB on Windows for package and navigation, broken links, missing alt text, and cover and accessibility details, and compares the title and ISBN in your book details with the EPUB. It does not replace EPUBCheck, which is free and is the check Apple names.
Check your EPUB’s links, alt text, cover and book details on your PC before you upload.
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What do the retailers actually require?
Each store says something different, so it helps to read their own words.
| Retailer | What it says about EPUB checking | Source |
|---|---|---|
| Apple Books | ”All books submitted to Apple Books must pass the latest version of EPUBCheck.” Its portal also validates files on upload. All files must be listed in the manifest, URIs must be encoded, and only UTF-8 and UTF-16 are allowed | Apple Books for Authors, Apple Books Asset Guide, Book Validation |
| Kobo Writing Life | Every book, whatever format you upload, is run through EPUB validation. Critical errors must be fixed before publishing; non-critical errors can be published, but OverDrive will not accept files with any validation errors | Kobo Writing Life, Validating and Testing Your eBooks |
| Amazon KDP | Supports EPUB files that meet the Kindle Publishing Guidelines and recommends validating the file with Kindle Previewer before upload | KDP supported eBook formats |
Retailer pages checked on 17 October 2026. Kobo’s own EPUB spec on GitHub adds that it runs incoming files through EPUBCheck 4.2.6, and that warnings may pass ingestion but still fail its content QA.
Apple’s Prepare your book page also asks for accessibility descriptions for images, widgets and diagrams, and says images inside the book should not exceed 4 million pixels.
What is EPUBCheck, and do I still need it?
EPUBCheck is the W3C’s free, open source conformance checker for EPUB, released under the BSD license. The latest release at the time of writing is v5.4.0, from September 2026. It runs from the command line or as a Java library, so you need Java installed. Kobo’s help suggests the free Pagina EPUB validator as a graphical option.
Yes, you still need it. Apple names EPUBCheck specifically, and its error list is what Apple and Kobo will see. Run it on the final file every time.
What EPUBCheck gives you is a list of conformance messages with file paths and line numbers. It is precise but terse. It is not designed to tell you which images lack descriptions or whether your cover is in place in plain language, and it does not know your book’s ISBN.
What about accessibility checks?
The W3C’s EPUB Accessibility 1.1 is the standard for accessible EPUBs. It requires accessibility metadata in every conforming publication: accessMode, accessibilityFeature and accessibilityHazard, with accessibilitySummary and accessModeSufficient recommended. It notes that laws such as the European Union’s Directive 2019/882 set accessibility requirements for digital publications.
Ace by DAISY is a free, open source tool for checking EPUB accessibility against that specification. It runs as a command line tool or as the Ace App, and it produces an HTML report focused on violations, metadata, document outlines and images. If you need a formal accessibility conformance claim, Ace is the tool built for it.
How do the tools compare?
| Need | EPUBCheck | Ace by DAISY | ProofSlip |
|---|---|---|---|
| Price | Free, open source | Free, open source | One-time purchase, 15-day free trial |
| EPUB conformance, as Apple requires | Yes, the reference tool | Runs accessibility tests | Package and navigation checks; not a replacement for EPUBCheck |
| Broken links | Reports missing internal references | Not its focus | Yes |
| Missing alt text | Not its focus | Yes | Yes |
| Accessibility metadata | Not its focus | Yes | Accessibility details |
| Cover | Not its focus | Not its focus | Yes |
| Title and ISBN compared with your book details | No | No | Yes, with ISBN check digits |
| Print PDF and Word manuscript in the same run | No | No | Yes |
| Interface | Command line, Java | Command line or Ace App | Windows app, plain-language findings |
EPUBCheck and Ace facts from their official pages above, checked 17 October 2026. ProofSlip facts from its Microsoft Store listing.
The fair summary: EPUBCheck is required and free, Ace is the specialist for accessibility, and ProofSlip is the plain-language check for the problems authors most often miss, alongside the print files for the same book.
How do I check an EPUB with ProofSlip?
ProofSlip checks print PDFs, EPUBs and Word manuscripts on your PC.
- Export the EPUB from Vellum, Atticus, Sigil, InDesign, Calibre or whatever you use.
- Have your book details ready. ProofSlip’s book details check covers ISBN check digits, categories and keywords, and compares the title and ISBN with what is inside the EPUB.
- Drop the EPUB on the window. Add the print interior PDF and cover too if you have them; the whole job is checked at once.
- Read the package and navigation findings. A missing table of contents or a navigation problem shows here.
- Fix broken links. Each broken link is listed so you can find it in your source file.
- Add missing alt text. Every image that conveys information needs a description, added in your authoring tool.
- Check the cover and accessibility details. Make sure the cover is in place and the accessibility metadata says what your book offers.
- Export the reports. Export every report at once. Then run EPUBCheck on the same final file before you upload.
ProofSlip never changes the EPUB. Fix problems in the tool that made it, export again and check the new file.
Check the EPUB, the print PDF and the book details together during the 15-day free trial.
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What are the most common EPUB problems?
| Problem | Why it matters | Usual fix |
|---|---|---|
| Image without alt text | Screen reader users hear nothing, and Apple asks for descriptions | Add a description in your authoring tool |
| Broken internal link | A table of contents entry or footnote goes nowhere | Re-link the target heading or note, then export again |
| File not in the manifest | Apple says books with unmanifested files fail import | Export again from the authoring tool rather than adding files by hand |
| Spaces in file names | Apple notes unencoded spaces in URIs as a common failure | Rename internal files without spaces |
| Title or ISBN mismatch | The EPUB says one thing and the store listing another | Update the EPUB metadata or your book details so they agree |
| Missing accessibility metadata | EPUB Accessibility 1.1 requires it | Fill in the accessibility fields in your tool |
When is a different tool better?
- You need the official conformance result. That is EPUBCheck, every time, for free.
- You need a formal accessibility conformance claim. Ace by DAISY is built for evaluating against EPUB Accessibility 1.1.
- You publish only to Kindle. Kindle Previewer is free and is the tool KDP recommends.
- You are on a Mac. ProofSlip runs on Windows only.
Frequently asked questions
Does Apple Books require EPUBCheck?
Yes. Apple says all books submitted to Apple Books must pass the latest version of EPUBCheck.
Can I publish on Kobo with validation errors?
Kobo says non-critical errors can be published, but critical errors must be fixed, and OverDrive will not accept files with any validation errors.
Does ProofSlip replace EPUBCheck?
No. Run EPUBCheck on the final file. ProofSlip adds plain-language checks for links, alt text, cover, accessibility details and book details.
Does ProofSlip fix my EPUB?
No. It never changes your files. You fix problems in your authoring tool and export again.
Is my book uploaded anywhere?
No. There is no account and no upload. Files never leave your PC.
Try ProofSlip
ProofSlip is a one-time purchase in the Microsoft Store, with a 15-day free trial. Drop your EPUB, interior PDF and cover on the window and read the findings before you upload. For the print side, see our print-ready PDF checklist, check PDF fonts embedded, or the ProofSlip product page.
Upload an EPUB you have already checked.
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Amazon, Kindle and Kindle Direct Publishing are trademarks of Amazon.com, Inc. or its affiliates. Apple and Apple Books are trademarks of Apple Inc. Kobo, EPUB and DAISY are trademarks of their owners. Microsoft and Windows are trademarks of the Microsoft group of companies. Other names are trademarks of their owners. ProofSlip is not affiliated with or endorsed by any of them.
Last updated 17 October 2026. Published by Automata Labs, a software company in Canada.