# Fix: Discourse renders as unstyled HTML in Facebook's in-app browser on iPhone
**URL:** https://meta.discourse.org/t/fix-discourse-renders-as-unstyled-html-in-facebooks-in-app-browser-on-iphone/399440
**Category:** Bug
**Created:** [27 maart 2026 om 15:55 UTC](https://meta.discourse.org/t/fix-discourse-renders-as-unstyled-html-in-facebooks-in-app-browser-on-iphone/399440 "2026-03-27T15:55:21Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@shortmort37](https://meta.discourse.org/u/shortmort37)
#### Post date: [27 maart 2026 om 15:55 UTC](https://meta.discourse.org/t/fix-discourse-renders-as-unstyled-html-in-facebooks-in-app-browser-on-iphone/399440/1 "2026-03-27T15:55:21Z")
## Summary
If your Discourse site looks like plain, unstyled HTML when accessed via a link in the Facebook app on iPhone — no CSS, no JavaScript, no functionality — the cause is Discourse’s crawler detection incorrectly identifying Facebook’s in-app browser as a bot.
The fix is a one-line change via the Rails console.
* * *
## The symptom
Users clicking links to your Discourse site from the Facebook app on iPhone see a stripped-down, unstyled HTML page — essentially the crawler/noscript layout. No JavaScript runs, so features like image grids, lightboxes, and media players don’t work. The same links work correctly in Safari, Chrome, and Facebook’s in-app browser on Android and iPad.
* * *
## The cause
Facebook’s in-app browser on iPhone identifies itself with a user agent string containing the word `facebook`, for example:
```plaintext
Mozilla/5.0 (iPhone; CPU iPhone OS 18_7 like Mac OS X) AppleWebKit/605.1.15
(KHTML, like Gecko) Mobile/23D8133 Safari/604.1 MetaIAB Facebook
```
Discourse’s crawler detection (`CrawlerDetection`) checks user agents against the `crawler_user_agents` site setting, whose default value includes `facebook`:
```plaintext
rss|bot|spider|crawler|facebook|archive|wayback|ping|...
```
This causes Discourse to serve Facebook’s in-app browser the static crawler layout instead of the full Ember application — even though it’s a real browser being used by a real person.
You can confirm this is happening by checking your nginx access log for requests from this browser and noting the response payload size is dramatically smaller than normal (typically ~5KB vs ~35KB for a full topic page).
* * *
## The fix
Add `MetaIAB` to the `crawler_check_bypass_agents` site setting. This setting is specifically designed to exempt user agents from crawler treatment even when they match the crawler list.
**Note:** `crawler_check_bypass_agents` is a hidden site setting and does not appear in the standard admin UI. The Rails console is required.
### Via the Rails console
```ruby
SiteSetting.crawler_check_bypass_agents = "MetaIAB"
```
If the setting already has a value (e.g. `cubot`), append with a pipe separator:
```ruby
SiteSetting.crawler_check_bypass_agents = "cubot|MetaIAB"
```
The change takes effect immediately — no restart required.
* * *
## Why this works
The `CrawlerDetection.crawler?` method uses three settings in combination:
1. `non_crawler_user_agents` — if the UA matches this list, it _might_ be a real browser
2. `crawler_user_agents` — if it also matches this list, it’s treated as a crawler
3. `crawler_check_bypass_agents` — if it matches this list, it’s exempted from crawler treatment regardless
Facebook’s in-app browser UA contains `Safari`, which matches `non_crawler_user_agents`. It also contains `facebook`, which matches `crawler_user_agents`. Adding `MetaIAB` — a string unique to Facebook’s in-app browser UA — to `crawler_check_bypass_agents` causes Discourse to serve it the full application.
* * *
## Why Android and iPad are unaffected
- **Android** : Facebook’s in-app browser on Android sends a different user agent that does not contain `facebook`, so it passes crawler detection without issue.
- **iPad** : Facebook’s in-app browser on iPad also triggers the crawler layout, but because the iPad reports a wide `window.innerWidth` (~1180px), Discourse serves the desktop layout which happens to render adequately. The iPhone’s narrow viewport (~414px) triggers the mobile layout, which in crawler mode is completely non-functional.
* * *
## Additional note: meta-webindexer flooding
Separately, Facebook’s web indexer (`meta-webindexer/1.1`) may send a very high volume of requests to your site — potentially thousands per hour — all targeting the homepage. Unlike `meta-externalagent` (which handles OG link previews and should remain unblocked), `meta-webindexer` appears to serve no useful purpose for most Discourse installations.
If you observe this traffic in your logs, you can block it at the nginx level by adding it to your bot block configuration:
```nginx
"~*meta-webindexer" 1;
```
`meta-externalagent` should remain allowed, as it is responsible for scraping OG metadata when links are shared on Facebook.
---
### Author:  [@david](https://meta.discourse.org/u/david)
#### Post date: [27 maart 2026 om 15:59 UTC](https://meta.discourse.org/t/fix-discourse-renders-as-unstyled-html-in-facebooks-in-app-browser-on-iphone/399440/2 "2026-03-27T15:59:17Z")
Thanks @shortmort37. I made a PR to change the default in core to include MetaIAB:
[https://github.com/discourse/discourse/pull/38926](https://github.com/discourse/discourse/pull/38926)
---
### Author:  [@david](https://meta.discourse.org/u/david)
#### Post date: [28 maart 2026 om 08:00 UTC](https://meta.discourse.org/t/fix-discourse-renders-as-unstyled-html-in-facebooks-in-app-browser-on-iphone/399440/3 "2026-03-28T08:00:41Z")
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