Claude Sonnet 5.5 Launch: Opus-Level Coding, Same Price

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Key Highlights

• Opus-level capability at Sonnet pricing: Anthropic reports 70.6% on Terminal-Bench 4.0, above Opus 5.5’s 66.4%; GDPval-AA knowledge work scores 1844, essentially tied with Opus 5.5’s 1846
• Unchanged pricing: $2 input / $10 output per million tokens and $0.20 cache reads, identical to Sonnet 5 and half the price of Opus 5.5
• Faster and more token-efficient: Anthropic says output is more than 30% faster than Sonnet 5 and the same work takes fewer tokens, cutting per-task cost by up to about 30%
• Live on APIYI: claude-sonnet-5-5 and claude-sonnet-5-5-thinking, over both OpenAI-compatible and Anthropic-native endpoints, with all four billing items matching the official price
• Five breaking changes: thinking: disabled returns 400 (use between_tools), forced tool use returns 400, thinking blocks are bound to the model and conversation, computer use only accepts the new toolset, and advisor pairings are narrower. top_p / top_k are no longer supported and return an error in APIYI testing

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Background

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Detailed Analysis

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Core Features

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Benchmarks (Vendor-Reported)

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Independent Evaluation (Artificial Analysis)

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For Developers: Breaking Changes

• Progress text between tool calls now arrives in thinking blocks: with the default display: "omitted" their text is empty, so an interface that streams that text to users goes quiet between tool calls. Use between_tools or set thinking.display to get it back
• Safety classifiers: a declined request returns HTTP 200 with stop_reason: "refusal", and stop_details names one of five categories (cyber / bio / frontier_llm / reasoning_extraction / general_harms). This is the first Sonnet with cyber and anti-distillation classifiers; higher-risk cyber requests fall back to Sonnet 5 on the provider side

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Technical Specifications

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APIYI Test Results

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Practical Applications

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Recommended Use Cases

1. Everyday coding: bug fixes, small repository changes, and terminal agent tasks; Terminal-Bench 4.0 shows the biggest gain
2. Documents, slides, and spreadsheets: an area Anthropic highlights, with AA-Briefcase and GDPval-AA close to Opus 5.5
3. High-frequency agents and support workflows: faster output and fewer tool-call steps suit latency-sensitive multi-turn tool use
4. Replacing Sonnet 5 and some Opus 5.5 workloads: a same-price upgrade from Sonnet 5; well-scoped tasks on Opus 5.5 are worth evaluating on Sonnet 5.5 at about half the cost

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Code Examples

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Anthropic-Native Format

import anthropic

client = anthropic.Anthropic(
    api_key="sk-your-apiyi-key",
    base_url="https://api.apiyi.com"
)

message = client.messages.create(
    model="claude-sonnet-5-5",
    max_tokens=32000,
    output_config={"effort": "medium"},  # default is high; Anthropic suggests medium for well-specified agentic coding
    messages=[
        {"role": "user", "content": "Find the root cause of this failing test and propose a minimal patch."}
    ]
)

for block in message.content:
    if block.type == "text":
        print(block.text)

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Turning Off Up-Front Thinking (Replaces disabled)

message = client.messages.create(
    model="claude-sonnet-5-5",
    max_tokens=8000,
    thinking={"type": "between_tools"},  # disabled returns 400
    output_config={"effort": "low"},      # between_tools only works with low / medium / high
    messages=[{"role": "user", "content": "Rewrite this paragraph to be more concise."}]
)

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OpenAI-Compatible Format

from openai import OpenAI

client = OpenAI(
    api_key="sk-your-apiyi-key",
    base_url="https://api.apiyi.com/v1"
)

response = client.chat.completions.create(
    model="claude-sonnet-5-5",
    # do not send top_p / top_k (they error); temperature is accepted but has no effect
    messages=[
        {"role": "user", "content": "Review this code and point out potential bugs and improvements."}
    ]
)

print(response.choices[0].message.content)

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Six-Step Migration Checklist from Sonnet 5

1. Change the model name: claude-sonnet-5 → claude-sonnet-5-5
2. Replace the thinking-off setting: change thinking: {"type": "disabled"} to between_tools, with effort no higher than high
3. Remove forced tool use: set tool_choice to auto and use strict: true to keep tool input valid
4. Drop sampling parameters: top_p / top_k return an error and temperature has no effect, so send none of them; stop using manual budget_tokens too and control thinking depth with effort
5. Recalibrate effort: the same level does not produce the same amount of thinking as on Sonnet 5, so re-run your evaluation instead of carrying the old setting over
6. Keep history append-only: pass thinking blocks back unchanged and do not edit system / tools / earlier messages between requests

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Pricing and Availability

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Pricing

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Groups and Endpoints

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Stack with Recharge Promotions

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Summary and Recommendations

1. Sonnet 5 users: upgrade directly, but go through the six-step checklist first, especially disabled → between_tools, forced tool use, and sampling parameters
2. Opus 5.5 users: evaluate well-scoped coding and document tasks on Sonnet 5.5 first at about half the cost; keep complex architecture decisions and long research chains on Opus 5.5
3. Cost control: do not default to max effort; start at medium / high and raise it only if results call for it