[Bug] Claude lost automatic language detection for email recipients
Bug description
Claude no longer detects the appropriate language for email recipients. When composing an email to an English-speaking support team (support@anthropic.com), Claude wrote the entire email in Spanish despite the conversation thread being in English and the recipient being an American company.
Before ~September 16, Claude automatically detected the recipient's language context and composed emails accordingly. Now it defaults to the user's conversation language regardless of recipient.
Steps to reproduce
- Open a conversation with Claude in Spanish
- Ask Claude to compose/send an email to an English-speaking recipient (e.g., support@anthropic.com)
- The previous emails in the thread are in English
Expected behavior
Claude should detect that the recipient is English-speaking (American company, English thread) and write the email in English automatically.
Actual behavior
Claude writes the email entirely in Spanish to an American support team. The user has to manually point out the error and ask Claude to resend in English. Before September 16, this NEVER happened — Claude always matched the recipient's language automatically.
Environment
- Claude interface: claude.ai (web) + Claude Code
- Models affected: Opus 4.6, Opus 5, Sonnet
- OS: Debian 12 (Bookworm)
- Plan: Pro
Additional context
This is symptom #15 in an ongoing regression since ~September 16, 2026.
Related issues documenting other symptoms: #96308, #96329, #96745, #96892, #96327, #96350
The regression appears to have started with the Cowork-Chat merger update. All 15 documented symptoms affect multiple models.