Claude Sonnet 5 vs. Claude Sonnet 5.5: Upgrade Guide

Overview

Claude Sonnet 5 shipped on June 30, 2026 and became the default model on Claude's Free and Pro plans. Claude Sonnet 5.5 replaced it as the current Sonnet on September 28, 2026. Sonnet 5 stays available as a legacy model until at least June 30, 2027.

Same price, better caching

Both models cost $2 per 1M input tokens and $10 per 1M output tokens.

Cache reads drop from $0.20 to $0.10 per 1M tokens on Sonnet 5.5.

Both have a 1M-token context window and 128K max output, or up to 300K on the Message Batches API with a beta header.

The knowledge cutoff moves from January 2026 (Sonnet 5) to June 2026 (Sonnet 5.5).

Breaking changes in Sonnet 5.5

Up-front thinking can be turned off with the new between_tools setting.

Forced tool use returns an error.

Thinking blocks are tied to the model and the conversation.

On the Claude API and Google Cloud, the older computer_20251124 computer use tool is not accepted.

Setting temperature, top_p or top_k returns a 400 error.

Text between tool calls now comes back in thinking blocks, so apps that stream it need a display setting.

Recommendation

For new projects, start on Sonnet 5.5. For existing Sonnet 5 integrations, test tool forcing, sampling parameters and streaming between tool calls before switching.

No hands-on testing was conducted for this comparison. Every figure above comes from the official pages listed under Research sources, as published on or before October 22, 2026. Where a vendor has not published a number, we say so instead of estimating it.

Released	Sonnet 5: Jun 30, 2026 · Sonnet 5.5: Sep 28, 2026
Price per 1M tokens (input / output)	Both: $2 / $10
Cache read per 1M tokens	Sonnet 5: $0.20 · Sonnet 5.5: $0.10
Reliable knowledge cutoff	Sonnet 5: Jan 2026 · Sonnet 5.5: Jun 2026
Context / max output	Both: 1M / 128K (300K on Batch API, beta)