> ## Documentation Index
> Fetch the complete documentation index at: https://docs.anyfast.ai/llms.txt
> Use this file to discover all available pages before exploring further.
# Claude Opus 5.5
> Use Anthropic Claude Opus 5.5 through AnyFast for long-running agentic coding, complex knowledge work, vision, and tool-based workflows.
Claude Opus 5.5 is Anthropic's Opus model for long-running agentic coding and knowledge work. Use the AnyFast model ID `claude-opus-5-5` with the native Anthropic Messages API at `POST /v1/messages`.
## Model specifications
| Specification | Value |
| ------------------------- | --------------------------------------- |
| Model ID | `claude-opus-5-5` |
| Context window | 1M tokens |
| Maximum output | 128K tokens |
| Input → output | Text and images → text |
| Thinking | Adaptive, always on |
| Default effort | `medium` |
| Effort levels | `low`, `medium`, `high`, `xhigh`, `max` |
| Reliable knowledge cutoff | June 2026 |
## What changed from Opus 5
* **Always-on thinking** — Omit `thinking` or set it to `{"type":"adaptive"}`. Both `disabled` and manual `enabled` modes return `400`.
* **New default effort** — Requests that omit `output_config.effort` use `medium`, rather than the `high` default used by Opus 5.
* **No forced tool use** — `tool_choice` supports `auto` and `none`. The `any` and named `tool` modes return `400`.
* **Preserved thinking** — Keep the conversation prefix unchanged and return prior thinking blocks exactly as received during tool-use and multi-turn workflows.
* **Refusal handling** — A safeguard refusal can return HTTP `200` with `stop_reason: "refusal"` and additional information in `stop_details`.
## Quick example
```bash cURL theme={null}
curl https://www.anyfast.ai/v1/messages \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"model": "claude-opus-5-5",
"max_tokens": 4096,
"output_config": { "effort": "medium" },
"messages": [
{
"role": "user",
"content": "Review this migration plan and identify the three highest-risk assumptions."
}
]
}'
```
```python Python theme={null}
import anthropic
client = anthropic.Anthropic(
api_key="YOUR_API_KEY",
base_url="https://www.anyfast.ai"
)
message = client.messages.create(
model="claude-opus-5-5",
max_tokens=4096,
output_config={"effort": "medium"},
messages=[
{
"role": "user",
"content": "Review this migration plan and identify the three highest-risk assumptions."
}
]
)
for block in message.content:
if block.type == "text":
print(block.text)
```
```python Streaming theme={null}
import anthropic
client = anthropic.Anthropic(
api_key="YOUR_API_KEY",
base_url="https://www.anyfast.ai"
)
with client.messages.stream(
model="claude-opus-5-5",
max_tokens=4096,
output_config={"effort": "medium"},
messages=[
{"role": "user", "content": "Create a phased rollout plan for this database migration."}
],
) as stream:
for text in stream.text_stream:
print(text, end="")
```
## Core parameters
| Parameter | Type | Required | Description |
| ---------------------- | ------- | -------- | --------------------------------------------------------------------------------------------------- |
| `model` | string | Yes | Must be `claude-opus-5-5`. |
| `messages` | array | Yes | Conversation turns with `user` or `assistant` roles. Text, images, and tool results are supported. |
| `max_tokens` | integer | Yes | Maximum output tokens across thinking and visible response text. Range: 1–128000. |
| `output_config.effort` | string | No | Controls thinking depth. Supports `low`, `medium`, `high`, `xhigh`, and `max`; default is `medium`. |
| `thinking.type` | string | No | Omit it or set it to `adaptive`. Thinking cannot be disabled. |
| `tools` | array | No | Tool definitions available to the model. |
| `tool_choice.type` | string | No | Supports `auto` and `none`. Forced `any` or a named `tool` is not supported. |
| `stream` | boolean | No | Enables SSE streaming. Default: `false`. |
| `stop_sequences` | array | No | Custom sequences that stop generation. |
Adaptive thinking is always on. Use `output_config.effort` to balance reasoning depth, latency, and token use. Leave enough room in `max_tokens` for both thinking and visible output, especially at `xhigh` or `max` effort.
Do not send `thinking: {"type":"disabled"}` or manual extended thinking with `budget_tokens`; both return `400`. Keep `tool_choice` at `auto` or `none` because forced `any` and named `tool` choices are also rejected.
## Tool use
Keep `tool_choice` at `auto` (the default) or use `none`. Describe each tool and its input schema precisely, then state in the prompt when the tool should be used. In a tool loop, inspect content blocks by `type` instead of assuming the first block contains text.
## Multi-turn conversations
When an assistant turn contains thinking blocks, send the complete turn back exactly as received. Treat the system prompt, tool definitions, and earlier messages as append-only. Changing content before an Opus 5.5 thinking block can invalidate the block and return `400`.
## Migrating from Opus 5
1. Change the model ID from `claude-opus-5` to `claude-opus-5-5`.
2. Remove `thinking: {"type":"disabled"}` and manual `budget_tokens` settings.
3. Set `output_config.effort` explicitly if you need behavior comparable across model versions.
4. Replace forced `tool_choice` modes with `auto` or `none`.
5. Update refusal handling to check `stop_reason` and `stop_details`, even when the HTTP status is `200`.
Anthropic released Claude Opus 5.5 on September 22, 2026. See the official [model overview](https://platform.claude.com/docs/en/models/opus-5-5/overview), [what's new](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5), and [announcement](https://www.anthropic.com/claude-opus-5-5).
View request fields, response blocks, and the interactive API playground.