Opus 5.5 Upgrade: Fix 4 Breaking Changes Before They 400

Anthropic shipped Claude Opus 5.5 on September 22, the same afternoon OpenAI dropped GPT-6 Sol and Luna. The headline everyone is sharing is the price: $4 in and $20 out per million tokens, down from $5 and $25 on Opus 5, with cache reads at $0.20. The part fewer people are talking about is that swapping claude-opus-5 for claude-opus-5-5 is not a one-line change. Four request shapes that worked yesterday now return a 400, and a fifth change makes agent UIs go silent without any error at all.

This guide walks through each break, shows the exact error you will see, and builds three small pieces of Python you can drop into a real codebase: a request migrator that rewrites old kwargs, an agent loop that surfaces the new progress updates, and an append-only guard that catches the edits that now invalidate thinking blocks. Every snippet here was run against the current anthropic Python SDK (1.8.0) with a scripted fake client, and the API shapes come straight from Anthropic's Opus 5.5 migration guide.