Claude Opus 5.5 and Sonnet 5.5: API Pricing, Migration Changes, and Model Choice
Two Generation Changes in September: The Claude Lineup Now
In September 2026, three positions in Anthropic's current lineup changed: on September 1, Fable 5.1 succeeded Claude Fable 5; on September 22, Opus 5.5 launched; on September 28, Sonnet 5.5 launched.
The change buyers should notice most is that the official default recommendation has changed. Anthropic's models overview now says: if you're unsure which model to use, start with Claude Opus 5.5 for most workloads; use Fable 5.1 for demanding reasoning and long-horizon agentic work, or when Opus 5.5 at higher effort still falls short.
Current Lineup and Pricing
| Model | Model ID | Input/Output (per million tokens) | Cache read | Context | Max output | Official positioning |
|---|---|---|---|---|---|---|
| Claude Fable 5.1 | claude-fable-5-1 | $10/$50 | $0.25 | 1M | 128K | Demanding reasoning and long-horizon agents |
| Claude Opus 5.5 | claude-opus-5-5 | $4/$20 | $0.20 | 1M | 128K | Long-running agentic coding and knowledge work |
| Claude Sonnet 5.5 | claude-sonnet-5-5 | $2/$10 | $0.20 | 1M | 128K | Best combination of speed and intelligence |
| Claude Haiku 4.5 | claude-haiku-4-5-20251001 | $1/$5 | $0.10 | 200K | 64K | Fastest |
Sources: Anthropic's official models overview and pricing pages (checked 2026-09-30), USD. Opus 5, Sonnet 5, Opus 4.8/4.7/4.6/4.5, and Sonnet 4.6/4.5 are legacy (still available).
Claude Opus 5.5: A Cheaper New Default
Pricing: Lower Than Opus 5 on Every Line
| Item | Opus 5.5 | Opus 5 |
|---|---|---|
| Input | $4 | $5 |
| Output | $20 | $25 |
| 5-minute cache write | $5 | $6.25 |
| 1-hour cache write | $8 | $10 |
| Cache read | $0.20 | $0.50 |
| Batch API (input/output) | $2/$10 | $2.50/$12.50 |
The biggest gap is the cache read: on Opus 5.5 it is 5% of the input price (10% on most models), so the more often you resend the same system prompt, the more you save.
Key Specs
- 1M-token context and up to 128K output tokens per request (up to 300K on the Batch API with a beta header)
- Adaptive thinking is always on and cannot be turned off; control thinking depth with the effort parameter instead. The default effort on the Claude API is
medium - Fast mode (research preview) is available on the Claude API at $8/$40
Claude Sonnet 5.5: An Upgrade at the Same Price
Sonnet 5.5 is priced the same as Sonnet 5 ($2/$10, cache reads $0.20), positioned as the best combination of speed and intelligence and a step up from Sonnet 5. It also has a 1M context and 128K output; adaptive thinking is on by default, but the lowest setting, between_tools, turns off up-front thinking (at high effort or below). The default effort on the Claude API is high.
Sonnet 5 was originally launched with introductory pricing through the end of August; Anthropic has confirmed that $2/$10 is now the standard price and that the previously scheduled increase to $3/$15 on September 1 will not occur, so Sonnet 5 and Sonnet 5.5 currently cost the same.
Cost Math (Arithmetic, Not Benchmarks)
Assume 50 million input tokens and 10 million output tokens per month, no caching:
| Model | Input cost | Output cost | Monthly total |
|---|---|---|---|
| Fable 5.1 | $500 | $500 | $1,000 |
| Opus 5.5 | $200 | $200 | $400 |
| (Reference) Opus 5 | $250 | $250 | $500 |
| Sonnet 5.5 | $100 | $100 | $200 |
| Haiku 4.5 | $50 | $50 | $100 |
If 40 million of the 50 million input tokens hit the cache, Opus 5.5 input costs $8 (cached) + $40 (uncached) = $48, versus $20 + $50 = $70 on Opus 5. This simplified math ignores cache-write costs, which are billed separately at 1.25x (5-minute) or 2x (1-hour) the input price.
⚠️ Watch token counts when comparing across generations: Anthropic notes that Claude 4.7 and later models use a newer tokenizer that produces roughly 30% more tokens for the same text (depending on content). If you're moving from Sonnet 4.6 or earlier, don't compare unit prices alone.
Migrating from Opus 5 or Sonnet 5: Changes That Return 400
Of the changes listed in the official release notes and migration guides, these will make existing code fail outright:
- Forced tool use is no longer supported:
tool_choiceofanyortoolreturns 400 (on both Opus 5.5 and Sonnet 5.5). Useautowith strict tool use instead - Thinking can't be disabled on Opus 5.5: sending
thinking: {"type": "disabled"}or{"type": "enabled", ...}returns 400 — omit thethinkingfield and use effort - To turn off up-front thinking on Sonnet 5.5: send
thinking: {"type": "between_tools"}instead of"disabled" - Sonnet 5.5 rejects custom sampling parameters: non-default
temperature,top_p, ortop_kreturns 400 - Computer use tool version change: on the Claude API and Google Cloud, the earlier
computer_20251124is no longer accepted; usecomputer_toolset_20260801. On Amazon Bedrock the older version still works
Anthropic also lists "thinking blocks are tied to the model and the conversation" as a change; in addition, thinking blocks produced by Sonnet 5.5 work only in the account that produced them (or a linked account), and when another account sends one, the API drops the block and the request still succeeds. Applications that replay thinking content should be retested.
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How to Choose
| Workload | Recommendation | Why |
|---|---|---|
| Unsure — want to ship first | Opus 5.5 | Anthropic's stated default |
| High-volume support, content rewriting, small code fixes | Sonnet 5.5 | Half the price of Opus 5.5 and faster |
| Classification, routing, real-time responses | Haiku 4.5 | Cheapest and fastest, but only a 200K context |
| Hardest reasoning, research-style agent tasks | Fable 5.1 | Upgrade when Opus 5.5 at higher effort still falls short |
Haiku 4.5 remains in the current lineup, but its official retirement commitment is "not sooner than October 15, 2026," so long-running projects should keep the flexibility to switch models.
FAQ: Claude Opus 5.5 and Sonnet 5.5
How much does Claude Opus 5.5 cost?
$4 per million input tokens and $20 per million output tokens, $0.20 for cache reads, and half price on the Batch API ($2/$10).
Is Opus 5.5 cheaper than Opus 5?
Yes. Opus 5 is $5/$25; Opus 5.5 is lower on input, output, cache writes, and cache reads, with input and output each 20% cheaper.
When did Claude Sonnet 5.5 launch, and did the price go up?
It launched on 2026-09-28 at the same price as Sonnet 5: $2/$10.
Can we still use Opus 5 and Sonnet 5?
Yes. Anthropic lists them as legacy (still available), with retirement commitments of "not sooner than 2027-07-24" for Opus 5 and "not sooner than 2027-06-30" for Sonnet 5. New projects should still start on the 5.5 generation.
What code changes does Opus 5.5 need?
The three most common: remove thinking disabled/enabled settings, change tool_choice from any/tool to auto, and move computer use to the new toolset.
Conclusion: Default to Opus 5.5, Push Volume Work Down to Sonnet 5.5
Three Core Recommendations
- Default new projects to Opus 5.5: 20% cheaper than Opus 5 and Anthropic's recommended starting point
- Always cache long prompts you resend: Opus 5.5 cache reads cost just 5% of the input price
- Run a test pass before migrating:
tool_choice,thinking, and sampling parameters are the settings most likely to return 400
Ready to Adopt Claude 5.5?
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Further Reading
- Claude Fable 5 Complete Guide
- Claude API Purchase Guide
- Claude API Pricing Explained
- GPT-6 Astra, 6.1 Sol, Luna: Which to Pick
References
- Models overview — Claude Platform Docs (checked 2026-09-30)
- Pricing — Claude Platform Docs (checked 2026-09-30)
- Claude Opus 5.5 — Claude Platform Docs
- Claude Sonnet 5.5 — Claude Platform Docs
- Release notes — Claude Platform Docs
- Model deprecations — Claude Platform Docs
- Claude Opus 5.5 is now available on AWS — AWS What's New (2026-09-22)
- Claude Sonnet 5.5 now available on AWS — AWS What's New (2026-09-28)
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