Match the report title.
Look for "dtb:uid mismatch EPUB" or the closest title listed above. If the platform wording is different, keep the original EPUBCheck, KDP, or Kindle Previewer message before choosing a fix.
dtb:uid mismatch and NCX identifier mismatch
Use this when the NCX dtb:uid value does not match the OPF package identifier. Compare content.opf and toc.ncx before you change metadata values.
The scan shows affected paths and repair decisions. It does not show manuscript text.
Not sure this kind of EPUB should be scanned here? Check unsupported or manual-only EPUB cases.
Scan report titles
- dtb:uid mismatch EPUB
- NCX identifier does not match OPF unique identifier
- NCX_001 identifier mismatch
- toc.ncx dtb:uid does not match the OPF identifier
Read the scan report
How to decide whether this report item matches dtb:uid mismatch and NCX identifier mismatch.
Use the report label, affected file, and repair decision together. A matching title alone is not enough if the file path or EPUB structure points to a different problem.
Check the affected file first.
Use the report's Affected area, Source file, Target file, or Problem type rows before editing content.opf, nav.xhtml, toc.ncx, XHTML, CSS, or image paths.
Follow the repair decision.
Repairable means EPUB Fixer found one clear structural change. Manual review or Not supported means use the source project, formatter, or platform workflow instead of guessing.
Handoff summary
What to send to a formatter if this is not safely repairable.
Copy the report summary instead of rewriting the EPUB from memory. A useful handoff includes the source file, target file or field, original error, repair decision, and next step.
Source file
Target path or field
Original error
Repair decision
Next step
1. Example report output
What the NCX_001 report should show.
The report should list the two identifier values and show which one the OPF package already treats as authoritative.
EPUBCheck message
ERROR(NCX-001): NCX identifier does not match OPF unique identifier.
OPF identifier
unique-identifier points to dc:identifier value urn:uuid:book-123.
NCX dtb:uid
toc.ncx head meta name="dtb:uid" content="urn:uuid:old-456".
Do this next
Align toc.ncx only if the OPF package already has one clear publication identifier.
Do not do this
Do not pick between ISBN, UUID, retailer, or draft identifiers automatically when the metadata itself is unclear.
A safe repair means keeping one existing publication identifier and making toc.ncx match it exactly.
2. Next step
Choose the OPF publication identifier first.
NCX_001 is small to edit, but easy to repair incorrectly if the OPF metadata already contains several competing identifiers.
Scan first
You have not checked both files yet.
List the OPF unique-identifier target and the toc.ncx dtb:uid value together before editing either one.
Safe repair
The NCX copied an old identifier.
Update the NCX dtb:uid only when the OPF package already has one clear publication identifier that should govern the file.
Manual review
The book has competing metadata identifiers.
Stop when the EPUB contains several ISBN, UUID, retailer, or draft values and the correct publication identifier is a publishing decision.
3. Concrete path example
A typical NCX_001 mismatch.
The OPF and NCX belong to the same book but point at different identifier values.
EPUBCheck: ERROR(NCX-001): NCX identifier does not match OPF unique identifier. Report: OPF publication identifier: urn:uuid:book-123 toc.ncx dtb:uid: urn:uuid:old-456 Fix decision: safe repair if the OPF identifier is the intended publication identifier
If the OPF publication identifier is itself ambiguous, choose manually before editing toc.ncx.
Quick decision
Decide whether the NCX copied an old identifier or the metadata is still unresolved.
For EPUB 2 navigation, toc.ncx should reflect the same publication identifier that content.opf already uses.
Scan first
You only see the NCX error.
Read the OPF unique-identifier target and the toc.ncx dtb:uid value together before editing metadata.
Safe fix
The OPF publication identifier is already clear.
Make toc.ncx match that identifier exactly, then validate again.
Stop
The metadata still needs a publishing choice.
Do not let an automatic repair choose between multiple ISBN, UUID, retailer, or draft identifiers.
Start here
Compare the OPF identifier and the NCX head value together.
NCX_001 is not about visible title or author text. It means the EPUB 2 navigation file names a different book identifier than the OPF package.
What it means
The book package and the NCX navigation file disagree about which identifier belongs to this publication.
Can it be fixed automatically?
EPUB Fixer can help when content.opf already has one clear publication identifier and toc.ncx only needs to match that value exactly.
What to check next
Open content.opf and find which dc:identifier the package unique-identifier points to.
What not to assume
It should stop when the metadata contains several plausible identifiers or when the correct publication identifier depends on a publishing workflow decision.
Common situations
Common NCX_001 cases.
Start with the OPF identifier the package already treats as the book identifier.
toc.ncx still has an old UUID.
The OPF package points to the current publication UUID, but toc.ncx still carries a value from an older export.
Update the NCX dtb:uid to the OPF publication identifier.
NCX uses an ISBN while OPF uses a UUID.
The EPUB contains more than one plausible identifier, so the two files no longer agree.
Choose the intended publication identifier manually before aligning the files.
The OPF identifier changed after metadata cleanup.
content.opf was edited, but toc.ncx was not regenerated or updated.
Repair only if the OPF unique identifier is already the correct one.
The EPUB contains inconsistent draft and publication metadata.
Converted files can carry test, retailer, and publication identifiers at the same time.
Treat this as manual metadata review instead of a blind NCX edit.
The NCX_001 identifier mismatch text
NCX_001: The NCX identifier does not match the OPF unique identifier.
Where NCX_001 usually appears
EPUBCheck, Kindle Previewer, or KDP reads an EPUB 2 style toc.ncx file and finds that its dtb:uid value does not match the publication identifier declared in content.opf.
What it means
NCX_001 means the NCX and OPF describe the same book with different identifiers.
The book package and the NCX navigation file disagree about which identifier belongs to this publication.
This often happens after metadata edits, EPUB 2 re-exports, or manual fixes where content.opf was updated but toc.ncx kept an older UUID or ISBN value.
Before you edit
Check content.opf and toc.ncx side by side.
Do not invent a new identifier. First confirm which OPF identifier already represents the publication, then see whether toc.ncx copied an older or different value.
- 1Open content.opf and find which dc:identifier the package unique-identifier points to.
- 2Copy the identifier value that package metadata already treats as the publication identifier.
- 3Open toc.ncx and find the head meta entry named dtb:uid.
- 4Compare the two identifier values exactly, including punctuation and case.
- 5Stop when the EPUB contains several ISBN, UUID, retailer, or draft identifiers and the publication identifier is not obvious.
Why KDP checks it
Why EPUB 2 validators care about NCX and OPF identifier alignment.
EPUB 3.3 package rules
The OPF file is the EPUB's map: metadata, file list, reading order, and navigation wiring. KDP and EPUBCheck use it to decide whether the book package is coherent.
EPUBCheck
EPUBCheck checks EPUB 2 and EPUB 3 files against the official rules and reports package, markup, link, and file-reference problems.
Can this be fixed safely?
When NCX_001 is safe to repair.
When automatic repair is safe
EPUB Fixer can help when content.opf already has one clear publication identifier and toc.ncx only needs to match that value exactly.
When you need manual review
It should stop when the metadata contains several plausible identifiers or when the correct publication identifier depends on a publishing workflow decision.
Before / after example
Before: content.opf identifies the book with one UUID, but toc.ncx still carries an older value. After: toc.ncx matches the OPF publication identifier exactly if that identifier is already the clear intended one.
Ready to retry?
Scan the EPUB metadata before editing toc.ncx.
The scan should show the OPF unique-identifier target, the dc:identifier value, and the NCX dtb:uid value so you can see whether one side is simply stale.
Upload EPUB to scanFAQ
Questions authors ask about NCX_001.
What does NCX_001 mean?
It means toc.ncx names a different book identifier than the OPF package. The two files should describe the same publication.
Is NCX_001 only for EPUB 2 style navigation?
Mostly, yes. It matters when the EPUB still uses toc.ncx as part of its navigation wiring.
Can EPUB Fixer repair NCX_001 automatically?
Only when content.opf already has one clear publication identifier and toc.ncx simply needs to match it.
Should I create a new identifier for toc.ncx?
No. Use the existing publication identifier only when it is already clear which one the OPF package should use.