GPT-5.4 Loses 54% Retrieval Accuracy at 1M Tokens; Opus 4.6 Loses Only 15%

r/ChatGPT·high signal

A practitioner post (79↑, 22cmts) comparing long-context quality across models reveals GPT-5.4 loses 54% retrieval accuracy when scaling from 256K to 1M tokens, while Opus 4.6 only loses 15%. This makes Opus 4.6 significantly more reliable for long-context agentic tasks, large codebases, and document-heavy workflows. All major labs now claim 1M token windows but quality at scale varies dramatically.