Anthropic and OpenAI both cut prices on the same Tuesday, and Anthropic's cut arrives attached to three migration errors.
Claude Opus 5.5 (claude-opus-5-5) shipped on 22 September at $4 / $20 per MTok, against Opus 5's $5 / $25 — 20% off both sides, with a 1M token context window by default, 128k max output and always-on adaptive thinking.1 Hours earlier OpenAI shipped GPT-6 Sol at $2 / $10 and GPT-6 Luna at $0.10 / $0.50, each half its GPT-5.6 promotional price.2
The Anthropic cut is not a drop-in. Three request shapes that worked on Opus 5 now return 400 on Opus 5.5: thinking set to either disabled or enabled (omit the field and use effort instead), tool_choice of type any or tool (use auto with strict tool use), and, on the Claude API and Google Cloud, the computer_20251124 computer-use tool, which now requires computer_toolset_20260801. On Amazon Bedrock the old computer-use tool keeps working.1
So the discount is real and the migration is billable. A harness that pins tool_choice: any to force a call, or disables thinking on cheap turns, fails on the first request rather than degrading.
Worth moving to, but read the migration notes before you change the model string. The three 400s are the whole of the work, and each has a named replacement.
What else landed on the Claude API in the same note
Fast mode for Opus 5.5 is a research preview on the Claude API. No published limit or price for it, so there is nothing to evaluate yet.1
Tools can now be defined mid-conversation, in beta behind the inline-tools-2026-09-15 header. A tool_addition block carries a full tool definition, so you can add a tool, change its schema, or move a server tool to a newer version without editing tools and invalidating the prompt cache. With the MCP connector's mcp-client-2026-09-15 header the definition can be an MCP toolset, and the response records each server's fetched tool list in an mcp_tool_listing block that pins the list when you send it back.1
Opus 5.5 is also in the GitHub Copilot model picker as of the same day, and so are Sol and Luna.56