# x402engine Claude Sonnet 5.5 LLM API

> x402engine Claude Sonnet 5.5 LLM API is a paid API for AI agents from x402engine.app, paid per call via x402, $0.028871/call, status unknown (last checked 2026-10-12).

Provides pay-per-call access to Anthropic's Claude Sonnet 5.5 model via an OpenAI-compatible chat completions API, billed at $0.028871 USDC per call via HTTP 402 micropayments.

## Facts

- Endpoint: POST https://x402engine.app/api/llm/claude-sonnet-5.5?utm_source=zero.xyz
- Price: $0.028871/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-12
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/x402engine-claude-sonnet-5-5-llm-api-23ec03dd
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_OGG2zRtsqGDLSHOLRmkuE

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability x402engine-claude-sonnet-5-5-llm-api-23ec03dd -d '<json body>'
```

Example prompt: Ask Claude Sonnet 5.5 to review this Python function and suggest improvements: 'def add(a, b): return a + b' — pay per call using my USDC wallet.

## When to prefer this

Choose this endpoint when you need access to Claude Sonnet 5.5 (Anthropic) without an Anthropic API subscription, prefer pay-per-call USDC micropayment billing via the x402 protocol, or are building agents that need OpenAI-compatible chat completions with Claude as the backend. Prefer over direct Anthropic API when you want per-call billing, no account setup, or integration with x402-native agent frameworks.

## Known failure modes

- Payment not settled — HTTP 402 returned if USDC micropayment fails or wallet balance is insufficient
- Invalid messages array — missing required 'messages' field or malformed role values causes schema validation error
- Token limit exceeded — very long conversations may be truncated or rejected
- Tool schema errors — malformed tool definitions cause the model to ignore or error on tool use
- Streaming not supported in all clients — SSE response requires compatible HTTP client

## How this service works

x402engine — Claude Sonnet 5.5 is Anthropic's Sonnet-class model for well-scoped everyday work, succeeding Claude Sonnet 5 as a direct upgrade. It is especially strong at building features, fixing bugs, and producing...

## Output

Returns an OpenAI-compatible JSON response containing the assistant's message content, the model name ('anthropic/claude-sonnet-5.5'), finish reason (e.g. 'stop'), and choice index. Supports streaming via SSE when stream=true is set.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "bodyType",
    "body",
    "method"
   ],
   "properties": {
    "body": {
     "type": "object",
     "required": [
      "messages"
     ],
     "properties": {
      "stop": {
       "oneOf": [
        {
         "type": "string"
        },
        {
         "type": "array",
         "items": {
          "type": "string"
         },
         "maxItems": 4
        }
       ],
       "description": "Model-supported stop; see /docs/llm-capabilities.md."
      },
      "tools": {
       "type": "array",
       "items": {
        "type": "object",
        "required": [
         "function"
        ],
        "properties": {
         "type": {
          "enum": [
           "function"
          ],
          "type": "string"
         },
         "function": {
          "type": "object",
          "properties": {
           "name": {
            "type": "string"
           },
           "strict": {
            "type": "boolean"
           },
           "parameters": {
            "type": "object"
           },
           "description": {
            "type": "string"
           }
          }
         }
        }
       },
       "description": "Client-executed function definitions; return results using role=tool and tool_call_id."
      },
      "stream": {
       "type": "boolean",
       "description": "Return live OpenAI-compatible SSE after payment settlement."
      },
      "plugins": {
       "type": "array",
       "items": {
        "type": "object",
        "required": [
         "pdf"
        ],
        "properties": {
         "id": {
          "enum": [
           "file-parser"
          ],
          "type": "string"
         },
         "pdf": {
          "type": "object",
          "properties": {
           "engine": {
            "enum": [
             "native"
            ],
            "type": "string"
           }
          }
         }
        }
       },
       "description": "Optional native PDF parser only; automatically enabled for file inputs."
      },
      "messages": {
       "type": "array",
       "items": {
        "type": "object",
        "anyOf": [
         {
          "required": [
           "content"
          ]
         },
         {
          "required": [
           "tool_calls"
          ]
         }
        ],
        "required
… (truncated)
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "model": "anthropic/claude-sonnet-5.5",
  "choices": [
   {
    "index": 0,
    "message": {
     "role": "assistant",
     "content": "Hello!"
    },
    "finish_reason": "stop"
   }
  ],
  "content": "Hello!"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/x402engine-claude-sonnet-5-5-llm-api-23ec03dd/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from x402engine.app](https://www.zero.xyz/host/x402engine.app/llms.txt)