Claude Sonnet 5.5

Use claude-sonnet-5-5 with the Anthropic Messages format. This document distinguishes the official request contract from the behavior observed in compatibility tests. Some advanced options do not yet behave as specified; review the limitations before depending on them.

See the model page for current input, output and cache prices.

Quick start

curl https://api.seedrouter.ai/v1/messages \
  -H "x-api-key: $SEEDROUTER_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-5-5",
    "max_tokens": 1024,
    "messages": [{"role": "user", "content": "Explain how a rainbow forms in three sentences."}]
  }'

POST /v1/messages accepts x-api-key or Bearer authentication and anthropic-version: 2023-06-01. Send an anthropic-beta header for features whose official documentation requires one. Keep credentials in server-side code.

The model also accepts basic OpenAI Chat Completions (POST /v1/chat/completions) and Responses (POST /v1/responses) requests. Use Messages for the native parameters described below; OpenAI format conversion does not provide every Anthropic feature.

Official parameter contract

Sonnet 5.5 has a 1M-token context window and a synchronous output limit of 128000 tokens. Batch-specific output limits do not apply to this endpoint. Optional properties have no application-supplied default unless the table says otherwise.

between_tools accepts only its type property and effort low, medium or high. Do not send display, budget_tokens or block_binding with it. Example:

{
  "model": "claude-sonnet-5-5",
  "max_tokens": 1024,
  "thinking": {"type": "between_tools"},
  "output_config": {"effort": "medium"},
  "messages": [{"role": "user", "content": "Explain this concept briefly."}]
}

Assistant prefilling is unsupported. To continue a pause_turn, replay the returned server-tool assistant content unchanged. Compaction summarizes existing history and is not an assistant prefill. Preserve thinking blocks and signatures exactly; do not move them between models or edit earlier history without following the official binding rules.

On the native Claude API, computer use requires computer_toolset_20260801; computer_20251124 is rejected. Advisor configurations using claude-opus-4-8, claude-opus-4-7 or claude-sonnet-5 are also rejected for this executor.

Fallback request fields

The official fallbacks beta retries eligible classifier refusals. It does not retry rate limits, overloads or server errors, and a refusal may remain unresolved. Send server-side-fallback-2026-07-01 for "default" or an explicit list; server-side-fallback-2026-06-01 supports the list only. Other dated versions are rejected.

An explicit list contains at most three entries with distinct models, none equal to the requested model. Allowed targets come from the beta Models API's allowed_fallback_models. Only model, max_tokens, thinking, output_config and speed are permitted per entry; overrides must be valid for that target. The July beta converts Sonnet 5.5 between_tools to Sonnet 5 disabled with omitted display when that fallback occurs. With the June beta, provide the Sonnet 5 thinking override yourself.

fallback_credit_token is for a separate retry after a refusal. A string selects strict redemption; an object adds mode. In strict mode, failed redemption rejects the retry. In best_effort mode, a token-layer failure can proceed at the normal price and is recorded in usage.fallback_credit; malformed tokens and combining credit with fallbacks still fail. Redemption also requires the eligible request, account, workspace, platform and five-minute window described in the official credit guide.

A benign request with fallbacks: "default", the July beta header and speed: "standard" returned the expected text. This establishes acceptance only: fallback execution and credit redemption have not been verified end to end here.

Media and tool inputs

Images use image blocks and PDFs use document blocks in a user message. The official source types include public URLs and base64 with the corresponding MIME type. The compatibility checks used a base64 PNG and a one-page base64 PDF, and verified the contents of the answers. They did not exercise every URL, file-size, image-resolution or PDF-page boundary.

Client-side tools use the standard tool_use → tool_result exchange. Keep tool-use IDs unchanged and return the result in a user message. A successful strict-tool example verifies that example's arguments, not every supported JSON Schema keyword.

Responses

A non-streaming response contains id, type: "message", role: "assistant", model, content, stop_reason, stop_sequence and usage, plus optional official fields such as container, diagnostics, context_management, stop_details and beta response fields. Content can include text, thinking, tool calls, tool results or other official block types; do not assume the first block is text.

With streaming, handle message_start, content_block_start, content_block_delta, content_block_stop, message_delta and message_stop. Errors can also occur inside a stream. Usage can include ordinary input/output tokens, thinking-token details, cache reads, and separate 5-minute / 1-hour cache creation counts.

An official classifier refusal is a normal response with stop_reason: "refusal" and stop_details, rather than an HTTP error. In a fallback response, model identifies the model that answered, fallback content blocks mark transitions, and usage.iterations describes the attempts. Inspect these fields instead of assuming the requested model served the response. These response behaviors remain unverified here.

Messages errors use {"type":"error","error":{"type":"...","message":"..."}}. Failed requests are not charged.

Compatibility verification: 2026-10-01

Beta tool execution, real MCP connections, Files API references, geographic residency, thinking-signature replay, full context/output ceilings, media boundary cases and refusal/fallback behavior have not been validated end to end. HTTP 200 and a returned model name do not authenticate which model ran or prove all supplied options took effect.

References

• Official Sonnet 5.5 overview
• Sonnet 5.5 changes and restrictions
• Messages parameter reference
• Per-message effort
• On-demand compaction
• Refusals and fallback
• Fallback credit
• OpenAPI schema
• Copyable Markdown