Sonnet 5.5 cache reads now cost half, and Sonnet 4.5 retires Nov 30

If you send the same long system prompt or document to Sonnet 5.5 again and again, cache it now. Since Oct 7, a cache read on Sonnet 5.5 costs $0.10 per million tokens instead of $0.20.

What changed#

Before: cache reads were 0.1x the Sonnet 5.5 base input price. After: 0.05x. Cache writes and every other price are unchanged. Separately, Anthropic set retirement of claude-sonnet-4-5-20250929 on the Claude API for Nov 30, 2026, and recommends Sonnet 5.5.

How to use it#

1. Put the stable part of your prompt (instructions, tool list, reference docs) first, then add cache_control.
2. Keep that prefix byte-identical between calls, or the cache misses.
3. Search your code for claude-sonnet-4-5 and plan the move to claude-sonnet-5-5 before Nov 30.
4. On the move, read the Sonnet 5.5 breaking changes: forced tool use (tool_choice any or tool) returns a 400, and turning off thinking uses {"type": "between_tools"}.

List every call in this repo that uses claude-sonnet-4-5 or claude-sonnet-5. For each, say whether its system prompt is stable enough to cache, and flag any tool_choice of "any" or "tool".

Good to know#

• Claude Code 2.1.296 updated /cost, the status line and --max-budget-usd to the new $0.10 rate, so its cost readouts match.
• Watch next: Anthropic's model deprecations page for any change to the Nov 30 date.

Sources#

• https://docs.anthropic.com/en/release-notes/overview (checked 2026-10-12 10:44 ET)
• https://github.com/anthropics/claude-code/blob/main/CHANGELOG.md (checked 2026-10-12 10:44 ET)

Last checked: 2026-10-12