Opus 4.6 regression: hallucinating dates and degraded accuracy after recent update

Description

Since a recent update to Claude Opus 4.6, there has been a noticeable regression in accuracy and reliability. Issues observed:

1. Date hallucination

When composing a formal email in September 2026, Claude wrote "20 de septiembre de 2025" instead of 2026. The current date was available in system context, yet Claude defaulted to its training cutoff year (May 2025).

2. Base64 reproduction corruption

When sending email attachments via Gmail MCP, Claude corrupts base64 strings in output tokens. Strings as short as ~1200 chars fail with "Base64 decoding failed". Only ~800 char strings reproduce correctly, and inconsistently.

Environment

• Model: claude-opus-4-6
• Session: Claude Code cloud container
• Tools: Gmail MCP, Google Drive MCP

Expected behavior

• Use current date from system context, not training cutoff year
• Reproduce base64 strings accurately in tool call parameters

Impact

Severe degradation for agentic workflows relying on accurate data and tool interoperability. Previously these tasks worked correctly