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Fix Mastodon URL previews not showing anything useful #19231

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reivilibre merged 2 commits into from
Dec 16, 2025
+11 −4
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Fix Mastodon URL previews not showing anything useful #19231

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cfa64f5
Fix Mastodon URL previews not showing anything useful
redstrate Nov 26, 2025
0b4eb34
Try to fix up comment describing our choice of preference
reivilibre Dec 16, 2025
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1 change: 1 addition & 0 deletions changelog.d/19231.bugfix
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix a bug where Mastodon posts (and possibly other embeds) have the wrong description for URL previews.
14 changes: 10 additions & 4 deletions synapse/media/url_previewer.py
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Expand Up @@ -328,10 +328,16 @@ async def _do_preview(self, url: str, user: UserID, ts: int) -> bytes:
# response failed or is incomplete.
og_from_html = parse_html_to_open_graph(tree)

# Compile the Open Graph response by using the scraped
# information from the HTML and overlaying any information
# from the oEmbed response.
og = {**og_from_html, **og_from_oembed}
# Compile an Open Graph response by combining the oEmbed response
# and the information from the HTML, with information in the HTML
# preferred.
#
# The ordering here is intentional: certain websites (especially
# SPA JavaScript-based ones) including Mastodon and YouTube provide
# almost complete OpenGraph descriptions but only stubs for oEmbed,
# with further oEmbed information being populated with JavaScript,
# that Synapse won't execute.
og = og_from_oembed | og_from_html
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@reivilibre reivilibre Nov 28, 2025

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maybe at this point it would be good to add a comment explaining why this order matters (otherwise I'm half worried someone will come and flip-flop it the other way in a couple of months when another site is better the other way around).

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@redstrate redstrate Nov 28, 2025

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Done, it now says why the order matters - not only the type of site but also the two examples shown off in the PR.

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@mistydemeo mistydemeo Dec 11, 2025

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Just a note on the comment - you have your comment the wrong way around. This is actually preferring OpenGraph info from the HTML over from oEmbed, and that's why it's helping. SPA sites like Mastodon are providing only stub information in their oEmbed, and it's the OpenGraph data in the HTML that has useful information.


await self._precache_image_url(user, media_info, og)
else:
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