Claude Sonnet 5.5
Save 72%Officialanthropic/claude-sonnet-5-5
Anthropic's latest Sonnet — successor to Sonnet 5 at the same price, the best balance of speed and intelligence.
- Context window
- 1M
- Max output
- 8K
- Input modalities
- text · image
- Output modalities
- text
- Capabilities
- Vision · Reasoning · Tool calling · Coding · Prompt cache · Paid cache writes · Strip Sampling Params
- Upstream release
- 2026-09-28
- RouteMux name
- anthropic/claude-sonnet-5-5
Try Claude Sonnet 5.5 here
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What is Claude Sonnet 5.5
Claude Sonnet 5.5 is Anthropic's newest Sonnet, released on September 28, 2026 as the successor to Claude Sonnet 5. Anthropic describes it as the best combination of speed and intelligence in its current lineup, and prices it exactly the same as Sonnet 5. It takes a 1M-token context window and vision input, with adaptive thinking on by default and a default reasoning effort of "high."
Through RouteMux it costs $0.56 per 1M input tokens against Anthropic's $2, and $2.80 per 1M output against $10 — the same model and the same Anthropic Messages API, routed through one key alongside every other model in the catalog. Failed and timed-out requests are not billed. Prompt caching is priced separately: reads are $0.056 per 1M tokens, and writes are $0.70 (5-minute) or $1.12 (1-hour) per 1M tokens.
Moving over from Sonnet 5 is mostly a model-name change, but Anthropic lists a few request shapes that now return an error. They are covered in the FAQ below.
Claude Sonnet 5.5 API pricing
Source: anthropic_x0.28 · Verified 2026-09-29
Cache read $0.056/M · Cache write $0.7/M
Estimate cost
Limited time20% extra Credits on your first top-up — up to 200 bonus Credits
Applies to your first top-up within 72 hours of signing up. Minimum $20, once per account.
Key capabilities
Same price as Sonnet 5
Anthropic kept Sonnet 5's pricing for the new model: $0.56 / 1M input and $2.80 / 1M output through RouteMux.
1M-token context
Enough to hold a mid-sized repository or a full document set in a single request.
Adaptive thinking, steered by effort
Thinking is on by default at high effort. Anthropic suggests medium for well-specified agentic tasks and medium or low for chat and latency-sensitive work.
Vision input and tool calling
Accepts images alongside text and supports native function calling, so screenshots, diagrams and tool results go in the same conversation.
Interface and protocols
Use any of these IDs to call this model via the API.
anthropic/claude-sonnet-5-5Replace the ROUTEMUX_KEY placeholder with your API key. Create one →
from anthropic import Anthropic
client = Anthropic(
base_url="https://api.routemux.com/anthropic",
api_key="<ROUTEMUX_KEY>",
)
message = client.messages.create(
model="anthropic/claude-sonnet-5-5",
max_tokens=1024,
messages=[{"role": "user", "content": "What is the meaning of life?"}],
)
print(message.content[0].text)Performance (last 30 days)
Claude Sonnet 5.5 FAQ
›What is Claude Sonnet 5.5?
Anthropic's latest Sonnet — successor to Sonnet 5 at the same price, the best balance of speed and intelligence.
›How large is the context window of Claude Sonnet 5.5?
Claude Sonnet 5.5 supports a context window of up to 1M tokens, with up to 8K output tokens per request.
›How much does the Claude Sonnet 5.5 API cost?
Through RouteMux, Claude Sonnet 5.5 costs $0.56 for input and $2.8 for output (USD per 1M tokens), below the official list price.
›How do I call Claude Sonnet 5.5 via API?
Claude Sonnet 5.5 is OpenAI-compatible: point your base URL at RouteMux and set the model field to anthropic/claude-sonnet-5-5 — no code changes needed.
›Which input and output modalities does Claude Sonnet 5.5 support?
Claude Sonnet 5.5 accepts text, image as input and produces text as output.
›When was Claude Sonnet 5.5 released?
Claude Sonnet 5.5 was released on September 28, 2026.
›Is this the same Claude Sonnet 5.5 as Anthropic's?
Yes — same model, same Anthropic Messages API surface. RouteMux routes your request through and bills your prepaid wallet instead of an Anthropic account.
›What breaks when I switch from Sonnet 5?
Per Anthropic: thinking type "disabled" now returns an error (send "between_tools" instead), and forced tool use (tool_choice "any" or "tool") is not supported — use "auto" with strict tool use. Non-default temperature, top_p and top_k are also rejected by Anthropic on this model; RouteMux drops those three parameters before forwarding, so SDKs that set them by default keep working.
›Should I use Sonnet 5.5 or Sonnet 5?
Sonnet 5.5 is Anthropic's current Sonnet at the same price. Sonnet 5 remains available if an existing integration relies on disabling thinking or forcing a tool call and you cannot change that yet.
Sources
Reviewed by Jerry Fan on 2026-09-29