[BUG] Repeated token generation ("until until until...") appearing in assistant output

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

During a long working session (extensive CSS debugging work), the assistant's text output occasionally got stuck repeating a single word dozens of times (e.g. "until until until...") before self-correcting mid-sentence. This happened at least twice in the same session, both times in the closing summary sentence after a long tool-use-heavy turn.

Model: Opus 5
Context: long session with many tool calls (Bash, Edit) before the failure
The text recovered on its own and finished the sentence coherently

VG of the claude code chat:
Fue más enrevesado de lo que parecía: la rejilla real es de 3 columnas × 2 filas de escritorio que en móvil se apilaba entera como una sola tarjeta. Para separar "Para casa" de "Para compartir" y mover "Explorar más" al final tuve que "desenvolver" las columnas con display:contents y reordenar con order — y por el camino encontré dos trampas reales (no cosas que adivinara): mi primera regla perdía la guerra de especificidad contra una regla ya existente (así que order no hacía nada), y había un nivel de anidación de más que no había visto (otro <ul> escondido dentro de cada columna). Las dos las until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until until untilundé midiendo con Playwright, no a ojo.

What Should Happen?

I asked to repeat the answer without the untils and described again what it wanted to writr

Error Messages/Logs

Steps to Reproduce

just a long session, i am working with claude code to update the menu of my wordpress webpage

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.221 Claude Code

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

im using the extension of claude code for visual studio code