Compression test unmasks OpenRouter's stealth model as GLM

Someone on OpenRouter fed ox-alpha, an anonymous model sitting in a free preview slot, its own system prompt back as a user message. The model broke character and named itself GLM, built by Z.ai, per dejan.ai's compression-distance analysis. If that holds, OpenRouter's free tier is running a full production model behind an anonymous label, not a cheap stand-in.

A single broken-character response isn't proof on its own, so the analysis added Normalized Compression Distance, a technique that fingerprints a model by how well gzip compresses its output using another model's output as a reference. Similar models compress tightly against each other. Different ones don't. The method needs nothing beyond text a model already returns.

Run across 60 prompts against GPT-5.5, Claude Opus 5, two Gemini variants, and GLM-5.3, a k-NN classifier matched 7 of 14 ox-alpha queries to GLM-5.3. Opus 5 came in second with 3 matches, not close enough to call it a coin flip.

A separate coding benchmark points the same direction. The stealth model scored 52 of 63 on the Rails benchmark, with 28.6% Rails API recall, numbers that track GLM-5.3's known range rather than a smaller model wearing its outputs.

The identity is one preview slot's story. Compression distance itself carries over to the next one. It turns "which model is actually behind this endpoint" into something testable from outside, with no cooperation from the host and no API access beyond the responses it already returns.