# Claude Opus 5.5 Claude Opus 5.5 is Anthropic's model for long-running agentic coding and knowledge work. Send the official Anthropic Messages request to SeedRouter: change the base URL and the API key, keep the body. The same model also answers the OpenAI Chat Completions and Responses formats. ## Model ID | Model ID | Context window | Max output | Thinking | Default effort | | ----------------- | -------------- | ----------- | ------------------- | -------------- | | `claude-opus-5-5` | 1M tokens | 128K tokens | Adaptive, always on | `medium` | See the [model page](https://seedrouter.ai/models/claude-opus-5-5#pricing) for current prices. ## Quick example ```bash 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-opus-5-5", "max_tokens": 1024, "messages": [{"role": "user", "content": "Summarize the trade-offs of event sourcing in three bullet points."}] }' ``` ```python import os import anthropic client = anthropic.Anthropic( api_key=os.environ["SEEDROUTER_API_KEY"], base_url="https://api.seedrouter.ai", ) message = client.messages.create( model="claude-opus-5-5", max_tokens=1024, messages=[{"role": "user", "content": "Summarize the trade-offs of event sourcing in three bullet points."}], ) print("".join(block.text for block in message.content if block.type == "text")) ``` ```javascript import Anthropic from "@anthropic-ai/sdk"; const client = new Anthropic({ apiKey: process.env.SEEDROUTER_API_KEY, baseURL: "https://api.seedrouter.ai", }); const message = await client.messages.create({ model: "claude-opus-5-5", max_tokens: 1024, messages: [{ role: "user", content: "Summarize the trade-offs of event sourcing in three bullet points." }], }); for (const block of message.content) if (block.type === "text") console.log(block.text); ``` ```go package main import ( "bytes" "fmt" "io" "net/http" "os" ) func main() { body := []byte(`{"model": "claude-opus-5-5", "max_tokens": 1024, "messages": [{"role": "user", "content": "Summarize the trade-offs of event sourcing in three bullet points."}]}`) req, _ := http.NewRequest("POST", "https://api.seedrouter.ai/v1/messages", bytes.NewReader(body)) req.Header.Set("x-api-key", os.Getenv("SEEDROUTER_API_KEY")) req.Header.Set("anthropic-version", "2023-06-01") req.Header.Set("Content-Type", "application/json") resp, err := http.DefaultClient.Do(req) if err != nil { panic(err) } defer resp.Body.Close() out, _ := io.ReadAll(resp.Body) fmt.Println(string(out)) } ``` ## Endpoints | Format | Method and path | Authentication | | ----------------------- | ---------------------------------------------------- | ----------------------------------------------------------------------------- | | Anthropic Messages | `POST https://api.seedrouter.ai/v1/messages` | `x-api-key: ` or `Authorization: Bearer `, plus `anthropic-version` | | OpenAI Chat Completions | `POST https://api.seedrouter.ai/v1/chat/completions` | `Authorization: Bearer ` | | OpenAI Responses | `POST https://api.seedrouter.ai/v1/responses` | `Authorization: Bearer ` | The Messages endpoint forwards your request body as sent, including optional fields, and returns the official response. An `anthropic-beta` header is passed on as well. Keep the API key in server-side code. ## Parameters | Name | Type | Required | Default | Notes | | --------------------------------------------------------------------------------------------------------- | ------------------- | -------- | ---------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `model` | string | Yes | — | `claude-opus-5-5`. | | `max_tokens` | integer | Yes | — | 0–128000. Includes the tokens spent on thinking. `0` only pre-warms the prompt cache. | | `messages` | object\[] | Yes | — | Alternating `user` and `assistant` turns; `content` is a string or an array of content blocks. The last turn must be `user`. Exception: to continue a `pause_turn` response, send its content back as the last `assistant` message. | | `system` | string or object\[] | No | — | System prompt. | | `thinking` | object | No | `{"type": "adaptive"}` | Thinking is adaptive and always on. `display`: `omitted` (default) or `summarized`. | | `output_config.effort` | enum | No | `medium` | `low`, `medium`, `high`, `xhigh`, `max`. Steers how much the model thinks. | | `output_config.format` | object | No | — | A JSON schema for structured output. | | `stop_sequences` | string\[] | No | — | Stop when one of these strings is generated. | | `stream` | boolean | No | `false` | Stream the response as server-sent events. | | `temperature` | number | No | — | Only `1` (the default) is accepted, for backwards compatibility; any other value returns a 400 error. Leave it out. | | `top_p` | number | No | — | Only values from `0.99` to `1` are accepted, for backwards compatibility; any other value returns a 400 error. Leave it out. | | `top_k` | integer | No | — | Not accepted: any value returns a 400 error. Leave it out. | | `tools` | object\[] | No | — | Tool definitions. | | `tool_choice` | object | No | — | `auto` or `none`; forcing a tool (`any` or `tool`) is not supported by this model. | | `metadata.user_id` | string | No | — | An opaque id for your end user, up to 512 characters. | | `cache_control` | object | No | — | Top-level prompt-cache breakpoint. | | `container`, `context_management`, `mcp_servers`, `diagnostics`, `service_tier`, `inference_geo`, `speed` | — | No | — | Forwarded as sent. | ## Thinking and effort Claude Opus 5.5 always uses adaptive thinking: the model decides how much to think, and `output_config.effort` steers it. Higher effort usually means more output tokens, a longer wait and a higher cost. With `thinking.display` set to `summarized`, the response includes `thinking` blocks you can show; with `omitted`, they are left out. Thinking tokens are billed as output tokens. ## Media inputs Images and PDFs go in a user turn's `content` as `image` and `document` blocks, with a `url` source or, as in the official API, a `base64` source: ```json {"role": "user", "content": [ {"type": "image", "source": {"type": "url", "url": "https://example.com/chart.png"}}, {"type": "text", "text": "What does this chart show?"} ]} ``` Replace the example URL with a publicly reachable file of your own. ## Billing dimensions See the [current rates on the model page](https://seedrouter.ai/models/claude-opus-5-5#pricing). A request is billed by the tokens it uses: * input tokens, * output tokens, including thinking, * prompt-cache reads, and * prompt-cache writes, with separate 5-minute and 1-hour rates. The charge is taken from the `usage` reported with the finished response. A request that fails is not charged. Your account's usage records show the exact charge for every request. ## Output A non-streaming request returns the official message object: ```json { "id": "msg_...", "type": "message", "role": "assistant", "model": "claude-opus-5-5", "content": [{"type": "text", "text": "..."}], "stop_reason": "end_turn", "usage": {"input_tokens": 18, "output_tokens": 4, "cache_read_input_tokens": 0, "cache_creation_input_tokens": 0} } ``` With `"stream": true` the response is a stream of the official events: `message_start`, `content_block_start`, `content_block_delta`, `content_block_stop`, `message_delta` and `message_stop`. The final `message_delta` carries the output token count. ## OpenAI-compatible formats The same model answers the OpenAI formats, so existing OpenAI code only needs a new base URL and model ID: ```python from openai import OpenAI client = OpenAI(api_key="YOUR_SEEDROUTER_KEY", base_url="https://api.seedrouter.ai/v1") chat = client.chat.completions.create( model="claude-opus-5-5", messages=[{"role": "user", "content": "Hello"}], ) response = client.responses.create(model="claude-opus-5-5", input="Hello") ``` These requests are converted to the Messages format, so a field with no Messages counterpart has no effect. Their responses carry the official OpenAI fields and may include a few extra usage fields; read `usage.total_tokens` and the official fields. ## Errors Errors on `/v1/messages` use the Anthropic shape, `{"type": "error", "error": {"type": "...", "message": "..."}}`; the other formats use `{"error": {"code": ..., "message": "..."}}`. The `code` is a code from the [common error catalog](https://seedrouter.ai/docs/api/errors). Failed requests are not charged. ## Tips * Start with the default effort and raise it only for tasks that need more thinking; effort changes both quality and cost. * Set `max_tokens` high enough for the thinking as well as the answer: it is one budget for both. * Put long, reused context first and mark it with `cache_control` so later requests read it from the cache at the lower rate. ## Related * [Claude Opus 5.5 playground and pricing](https://seedrouter.ai/models/claude-opus-5-5) * [Download OpenAPI](https://seedrouter.ai/docs/claude-opus-5-5.openapi.json) * [Copyable Markdown](https://seedrouter.ai/docs/claude-opus-5-5.md)