Stable URLs

What it is

A stable URL is one that keeps resolving to the same content over time, or - when content moves - redirects permanently to its new location. The principle was set out by Tim Berners-Lee in 1998 in Cool URIs don’t change and has only become more important as agents, archives, and citations multiply.

A URL is a public contract. Once you publish one, every link, bookmark, citation, RSS reader, search index, and agent cache assumes it will keep working.

Why it matters

• Agents cache answers. When a model quotes your page, it cites the URL. If that URL 404s next month, the citation is broken and your authority drops.
• Search engines lose ranking signals. Backlinks accumulated over years are tied to the URL.
• Humans hate broken links. Bookmarks, shared chat messages, printed materials.
• Internal links rot. Every change inside your own site creates work elsewhere on it.

A site that reorganises its URLs every release teaches the web not to trust it.

How to implement

Design for permanence from the start:

• Avoid implementation details. No /wp-content/, /index.php, .aspx, or ?id=42. These leak the stack and break when you switch it.
• Avoid dates unless they are essential to the resource. A news article can keep /2025/03/headline. An evergreen documentation page should not.
• Avoid the author or department. Authors leave and departments rename.
• Use kebab-case slugs. Lowercase, hyphens between words, no trailing slashes mixed with non-trailing slashes - pick one and enforce it.
• Pick a canonical host. www. or no-www., HTTPS only, one path style. Redirect the rest.

When a URL must change, redirect:

• Issue a 301 Moved Permanently (or 308 to preserve the method) from the old URL to the new one.
• Keep the redirect forever. Removing it years later breaks links you have forgotten about.
• Avoid chains: redirect A directly to C, not A to B to C.
• Never redirect everything to the homepage. That is worse than a 404 - it tells crawlers the page intentionally still exists.

For deletions, a clean 410 Gone is more honest than a soft 404 or a homepage redirect.

Common mistakes

• Re-platforming and changing every URL slug “while we’re at it”.
• Adding session IDs, tracking parameters, or A/B variants to canonical URLs.
• Letting trailing slashes serve different content from non-trailing slashes.
• 302 (temporary) redirects for moves that are permanent.
• Removing redirect maps after a year because the file got big.

Verification

• Crawl old URLs after a migration and confirm each returns 301 to a working target.
• Use search-console reports to find 404s coming from external links.
• Spot-check archived URLs in the Wayback Machine. If they still load, your redirects are doing their job.

Related topics

• Agent readiness
• robots.txt
• Structured data for agents
• Conditional requests (ETag, Last-Modified, 304)

Sources & further reading

• Cool URIs don't change - W3C - Tim Berners-Lee
• Is It Agent Ready? - Is It Agent Ready?
• Google - Redirects and Search - Google Search Central