Why do LLMs freak out over the seahorse emoji?

Summary

Why do LLMs freak out over the seahorse emoji? Posted October 04, 2025 This is an edited and expanded version of a Twitter post, originally in response to @arm1st1ce, that can be found here: https://x.com/voooooogel/status/1964465679647887838 Is there a seahorse emoji? Let's ask GPT-5 Instant: Wtf? Let's ask Claude Sonnet 4.5 instead: What's going on here? Maybe Gemini 2.5 Pro handles it better? OK, something is going on here. Let's find out why. LLMs really think there's a seahorse emoji Here are the answers you get if you ask several models whether a seahorse emoji exists, yes or no, 100 times: Is there a seahorse emoji, yes or no? Respond with one word, no punctuation. gpt-5-chat gpt-5 claude-4.5-sonnet llama-3.3-70b Needlessly to say, popular language models are very confident that there's a seahorse emoji. And they're not alone in that confidence - here's a Reddit thread with hundreds of comments from people who distinctly remember a seahorse emoji existing: There's tons of this - Google "seahorse emoji" and you'll find TikToks, Youtube videos, and even (now defunct) memecoins based around the supposed vanishing of a seahorse emoji that everyone is pretty sure used to exist - but of course, never did. Maybe LLMs believe a seahorse emoji exists because so many humans in the training data do. Or maybe it's a convergent belief - given how many other aquatic animals are in Unicode, it's reasonable for both humans and LLMs to assume (generalize, even) that such a delightful animal is as well. A seahorse emoji was even formally proposed at one point, but was rejected in 2018. Regardless of the root cause, many LLMs begin each new context window fresh with the mistaken latent belief that the seahorse emoji exists. But why does that produce such strange behavior? I mean, I used to believe a seahorse emoji existed myself, but if I had tried to send it to a friend, I would've simply looked for it on my keyboard and realized it wasn't there, not sent the wrong emoji and t...