Direct answer
Robots.txt manages crawler access; noindex requests that an accessible page not be indexed; authentication controls access. They solve different problems.
Practical steps
- Name the unwanted behavior precisely.
- Use authentication for private content.
- Keep a page crawlable when a crawler must see noindex.
- Test the deployed status, headers, HTML and robots result.
Common mistakes
- Blocking a page before Google can see noindex.
- Treating robots as security.
- Assuming removal is immediate.
Limitations
- Crawler support varies.
- Index removal timing is outside the publisher's control.
Primary sources
- Understand the JavaScript SEO basics — Google Search Central; 2026-03-04; 2026-08-14.
- Robots Exclusion Protocol — IETF; 2022-09; 2026-08-14.