Claude mobile web UI silently strips antThinking tag content
On November 15, user Kyrannio discovered a display-layer bug on Claude's mobile web client (model set to Fable 5.2, medium) related to an old artifacts control tag, progressively sharing details across multiple posts and ultimately reporting it to Anthropic.
Confirmed
- When a user message contains a literal antThinking angle-bracket tag, the message reaches the model fine and it responds accordingly; however, after sending, the UI strips the tag and everything between the tags, making the message appear silently removed or truncated.
- When the literal tag appears in a reply, the message display is likewise truncated from that point.
- The content is only hidden at the UI layer: copy-pasting that message reveals the full hidden text, which still exists at the data layer.
- The model can still read the stripped content internally—a hidden behavioral gap between UI display and what the model sees.
- The author has reported the bug to Anthropic.
Why it matters
- This means the message content users see in the interface may differ from what the model actually processes, opening room for hidden-injection or hidden-instruction abuse, and normal conversation content could also be silently swallowed.
- Since the data is not actually deleted, this "UI-only stripping" design choice is worth watching when auditing conversation content (e.g., verifying via copy-paste). Awaiting an official response or fix from Anthropic.
2026-11-15 ~ 2026-11-15 · 5 related posts
Primary sources
- Claude mobile web bug: replies truncated when model writes antThinking tag — Kyrannio · 2026-11-15
- [source] Claude mobile web bug: text inside antThinking tags silently stripped after sending — Kyrannio · 2026-11-15
- [source] Follow-up: copy-paste reveals the hidden antThinking text Claude's UI strips — Kyrannio · 2026-11-15
- Claude mobile web quirk: antThinking tag content silently stripped from sent messages — Kyrannio · 2026-11-15
- [source] Claude strips the <antThinking> tag from the UI but the model can still see it — Kyrannio · 2026-11-15