EPUB validation explainer
Why an EPUB opens locally but fails KDP.
A reader can open an EPUB while KDP, Kindle Previewer, or EPUBCheck still rejects the file. The difference is usually strict package validation, not whether the chapters are visible on your computer.
Why it happens
Local reading and publishing validation are different checks.
A reader can skip a broken reference.
Calibre or another local reader may ignore a missing image, stylesheet, or anchor. KDP and EPUBCheck still reject the package because the internal path does not resolve.
The book content can exist while the package map is wrong.
Chapters may be present, but container.xml, content.opf, manifest entries, spine order, nav.xhtml, or toc.ncx can point to the wrong file or id.
ZIP packaging rules are stricter than opening the file.
The EPUB wrapper still needs the right mimetype entry, container map, OPF path, and unambiguous package structure before a publishing system accepts it.
What to do next
Use the failed EPUB to find the strict blocker.
- 1
Save the exact KDP, Kindle Previewer, or EPUBCheck wording before re-exporting.
- 2
Check whether the message names a missing file, anchor, OPF field, manifest item, navigation file, container.xml, or mimetype rule.
- 3
Upload the same exported EPUB when you need the affected path and repairability decision.
- 4
Return to the source tool when the issue depends on missing content, chapter order, fixed-layout design, cover artwork, or visual formatting.
Route the error
Pick the guide that matches the wording you have.
Free EPUB validator for KDP upload errors
Use this when you want a broad scan before another publishing retry.
KDP EPUB upload failed and formatting errors
Use this when KDP rejected the exported EPUB or shows broad formatting wording.
Kindle Previewer conversion failed
Use this when Previewer fails, conversion stops, or enhanced typesetting cannot continue.
EPUBCheck error codes
Use this when you already have RSC, OPF, NCX, or PKG error output.
Common questions
Do not treat a local preview as publishing approval.
Does opening in Calibre mean the EPUB is valid?
No. A local reader can be tolerant. KDP, Kindle Previewer, and EPUBCheck are stricter about package paths, metadata, navigation, anchors, manifest entries, and ZIP packaging.
Should I re-export before scanning?
If you still have the EPUB that failed, scan that file first. The report can show whether the problem is a small package fix or a source export issue.
Can EPUB Fixer repair every file that opens locally but fails KDP?
No. It can help when there is one clear structural fix and the repaired EPUB passes EPUBCheck again. Missing content, ambiguous anchors, fixed-layout files, DRM, conversion, and visual formatting still need another workflow.