Auto-generated session titles render in Russian for Ukrainian-language sessions

Environment

• Claude Code CLI 2.1.226, macOS 15 (Darwin 24.6.0)
• Model: claude-fable-5
• User-level ~/.claude/CLAUDE.md contains an explicit language directive: always respond in Ukrainian

Description

A session conducted entirely in Ukrainian (first prompt and all subsequent turns in Ukrainian, project files in Ukrainian) received an auto-generated session title in Russian.

Exact generated title: Простая система трекинга задач для РДУГ

Note the mixed output: the sentence is Russian, yet the acronym it contains («РДУГ», = ADHD) is the Ukrainian abbreviation (the Russian one would be «СДВГ»). The title generator conflated the two languages within a single line.

Steps to reproduce

1. Set a user-level CLAUDE.md instructing the assistant to always respond in Ukrainian.
2. Start a new session with a long first prompt written in Ukrainian.
3. Observe the auto-generated session title in the session list / resume picker.

Expected

The title is generated in the conversation's language (Ukrainian), or at minimum respects the CLAUDE.md language directive.

Actual

The title is generated in Russian. The main assistant respects the language directive; the background title-generation call does not.

Why this matters more than a cosmetic mislabel

Ukrainian and Russian are distinct languages, and conflating them is a known failure mode of smaller models. For Ukrainian users this is not a neutral localization nuance: Russian is the language of the state currently waging war against Ukraine, and a tool injecting it uninvited into a Ukrainian workspace reads as an error about user identity. Rough analogy: calling an Irish person English. Please treat severity accordingly.

Suggested fix

• Pass an explicit language hint (detected conversation language, or the CLAUDE.md language directive) into the title-generation prompt.
• Add uk/ru distinction cases to evals for auxiliary summarization calls; these two languages are conflated far more often than other pairs.