## TL;DR
Create a SUMMARY.md at the book root listing every page. GitBook uses it as the table of contents; without it the build cannot proceed.

## Error
```text
Error: Summary.md missing
```

## Steps
1. Confirm the file is truly absent from the book root (case matters: SUMMARY.md, all caps). Expected: the missing file confirmed.
2. Create SUMMARY.md with a `# Summary` heading followed by bullet links to each page, e.g. `* [Introduction](README.md)` and nested entries for chapters. Expected: every page linked exactly once.
3. Check for typos in the linked paths; a SUMMARY.md that links to missing files fails next. Expected: all links resolve to real files.
4. Re-run the GitBook build or push to trigger the deploy. Expected: the build finds the summary and proceeds.

## When to use
- GitBook reports Summary.md missing
- A repo was migrated to GitBook without its summary file
- A docs agent generated pages but no SUMMARY.md

## When not to use
- SUMMARY.md exists but the build fails (a link or syntax problem inside it)
- You use the newer GitBook platform without legacy builds (check the current docs format)
- Pages are missing from the sidebar but the build succeeds (a summary content issue)

## Tool compatibility
GitBook legacy CLI (gitbook-cli) and the classic build flow. The new GitBook cloud editor manages structure differently.

### Variant phrasings
### Summary.md exists but still reported missing
It is in the wrong directory. It must sit at the book root, not in a subfolder.

### build fails right after adding SUMMARY.md
A linked file is missing or a path is misspelled. Fix the links the same way.

## Why it happens
Legacy GitBook derives the entire book structure from SUMMARY.md instead of scanning the directory. Deleting or never creating it leaves the builder with no table of contents, which is fatal.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_puwhPK7B1axoTEO8NF7keQ