Y!J-WSC
Y!J-WSC is a search-engine crawler operated by Yahoo Japan. Its job is to find, fetch, and index web pages so they can be returned in organic search results.
Traffic is regular and bounded by your robots.txt. Allowing it is generally how your site stays discoverable through the corresponding search engine, blocking it almost always reduces visibility there.
For most sites, search-engine crawlers are still the largest source of bot traffic and the largest source of human visitors that follow.
See Y!J-WSC on your own site
Match the User-Agent header on incoming requests against the pattern below.
regex
For higher confidence, also verify the source IP against the operator's published ranges. UA strings can be spoofed; IP ownership is harder to fake.
Renders JavaScript
No
IP verification
DNS reverse lookup
Crawl frequency
Continuous
Honors robots.txt
Yes
Honors Crawl-delay
Yes
Yahoo Japan runs 7 bots in total. Each one is a separate user-agent so you can allow or block them independently.
Should I let Y!J-WSC through?
In most cases, yes. Blocking traditional search crawlers reduces organic-search visibility. Allowing is the default for almost all sites. If volume gets noisy, rate-limit it before you block it outright.
Does blocking Y!J-WSC affect my Google rankings?
Only on Yahoo Japan Y!J-WSC feeds. Each search engine runs its own crawler, so blocking Y!J-WSC only removes you from that one index. Your visibility on Google, Bing, or anything else is untouched.
How do I confirm a request is really from Y!J-WSC?
Look at the User-Agent header in your access logs and match it against the strings listed above. Worth knowing that the User-Agent is easy to fake, so this check tells you "the traffic claims to be Y!J-WSC", not "the traffic is genuinely Y!J-WSC". If you need stronger guarantees, look for a reverse-DNS check or wait for Yahoo Japan to publish IP ranges.
What happens to my traffic if I block Y!J-WSC?
Your pages drop out of Yahoo Japan's index, which means losing the organic share you get from that engine. Not catastrophic if Yahoo Japan is a minor player, much more painful if it's a meaningful source of your traffic. Check your analytics for Yahoo Japan's actual referral share before deciding.
How is Y!J-WSC different from Yahoo Japan's other bots?
Yahoo Japan splits work across multiple user-agents so site owners can decide on each one independently. Training crawlers, live-fetch agents, search indexers, and agentic browsers each get their own name. Worth scanning the rest of the Yahoo Japan family above to see which ones actually matter for your site.
What's the cleanest way to control Y!J-WSC?
Two layers. Robots.txt for the polite crawlers that read it, and rules at your CDN or edge for the ones that don't. Rankly's Agent Experience handles both from a single config, so you can allow, block, rate-limit, or serve a stripped-down version per bot. Agent Analytics handles the observation half so you know which bots are actually worth a rule.