Mystery Model Union Alpha Suspected to Be Mistral, Leaks Full Chain-of-Thought

X user mariofilhoml made two key observations about the stealth-tested mystery model Union Alpha: it is suspected to originate from Mistral, and it leaked its full chain-of-thought (CoT) in a single request. Both remain unverified community observations with no official confirmation.

Confirmed

In one request, Union Alpha's chain-of-thought was directly exposed in the output: mariofilhoml reports the leaked reasoning text was about 15.8KB, roughly 21k tokens in full, and has been archived. The reasoning showed the model transcribing an image of an 18th-century Portuguese baptismal record manuscript.
During stealth testing, the model sends no reasoning tokens and bills only for the final answer; mariofilhoml speculates this is because no reasoning toggle was provided for the test period.

Unconfirmed

The Mistral attribution is solely mariofilhoml's inference, based on clues including the model answering China-related questions, an associated X account based in Spain, and Mistral infrastructure failing exactly when the model launched. None of this is officially confirmed.
As relayed by zainhas, some asked whether the CoT "leak" means reasoning is hidden by default or because it is a closed-source lab model; the characterization of CoT visibility remains under discussion.

Why it matters

A CoT leak usually means the model's reasoning chain was not properly hidden or truncated, exposing internal behavioral details. If the model is indeed a lab's stealth test product, such leaks and the missing reasoning toggle offer clues for identifying anonymous test models.

2026-09-17 ~ 2026-09-17 · 6 related posts

Primary sources

Mystery 'Union Alpha' model leaks its chain-of-thought during manuscript transcription task — mariofilhoml · 2026-09-17 04:15 (UTC+08:00)
Union Alpha Allegedly Leaks Its Chain-of-Thought in a User Request — mariofilhoml · 2026-09-17 04:11 (UTC+08:00)
Stealth Model That Hides Reasoning Tokens Likely From Mistral, Speculates User — mariofilhoml · 2026-09-17 05:41 (UTC+08:00)

[source] Union Alpha Allegedly Leaks Its Chain-of-Thought in a User Request — mariofilhoml · 2026-09-17
[source] Mystery 'Union Alpha' model leaks its chain-of-thought during manuscript transcription task — mariofilhoml · 2026-09-17
Union Alpha Allegedly Leaks Its Chain-of-Thought in a Single Request — zainhas · 2026-09-17
[source] Stealth Model That Hides Reasoning Tokens Likely From Mistral, Speculates User — mariofilhoml · 2026-09-17
Mystery model "Union" suspected to be Mistral: no reasoning tokens, only final answers — mariofilhoml · 2026-09-17

1 near-duplicate retellings: zainhas