Link-Rot Detection

Concept of the skill

What it is: The scheduled audit mechanism that scans markdown content for external links that no longer resolve correctly. Mental model: Treat link checking as a batch health report, not a live page-rendering feature. Why it exists: Broken or redirected links erode trust quietly, so the project needs a repeatable scan with clear evidence. What it is NOT: It is not a one-off production incident investigation or UI design for broken-link badges. Adjacent concepts: Markdown link extraction, status-code classification, soft-404 detection, polite crawling. One-line analogy: It is a periodic inspection report for all outbound references. Common misconception: HTTP 200 means healthy; soft-404s, unrelated redirects, and rate-limit behavior also need interpretation.

Coverage

• Markdown link extraction — pulling every [text](url) and [text][ref] reference link out of every .md and .mdx file in the content tree
• External vs internal classification — what counts as external (different host) and what gets skipped (relative path, anchor link, mailto, tel)
• Status-code interpretation — 200 is healthy, 301/308 is a redirect (record the new URL but don't fail), 302/307 is transient (re-check next run), 404/410 is dead (flag), 5xx is transient (retry with backoff before flagging)
• Soft-404 detection — pages that return HTTP 200 but render an "unknown page" interstitial (compare body length / content against known soft-404 patterns)
• Rate-limiting and politeness — concurrent request budget per origin host, honoring robots.txt crawl-delay, exponential backoff on 429
• Reporting shape — the scanner's output is a structured report (JSON + markdown summary), not a live alert; the report is the audit artifact

Philosophy of the skill

External links rot. Every site that's older than two years has at least a few. The choice is between knowing about them on a schedule or finding out from a user. A periodic scan with a published report turns link health into a maintenance task instead of an emergency. The discipline is to be conservative — distinguish persistent failures (404 across 3 runs) from transient ones (one 503 on a Tuesday) — and to never let the scanner itself become a denial-of-service vector against the targets it's checking.

Verification

Before merging any change to the link-rot scanner or its config:

• [ ] The scanner extracts every link in [text](url) and [text][ref] form from .md and .mdx files; reference links resolve their targets before classification
• [ ] Internal links (relative paths, same-host absolute, anchors, mailto, tel) are explicitly excluded — confirmed by a fixture test with mixed link types
• [ ] Persistent failures are distinguished from transient ones — a link is only flagged after N consecutive failures across separate runs (default N=3)
• [ ] The scanner respects per-host concurrency limits and honors robots.txt crawl-delay; a single host getting many requests in tight sequence is a bug
• [ ] Soft-404 detection has a defined pattern set and an explicit "unknown" bucket for cases that don't match any known soft-404
• [ ] The scanner produces both a JSON report (machine-readable) and a markdown summary (human-readable) with link, status code, last-seen-OK timestamp, and recommendation

Do NOT Use When