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Claude Sonnet API (Pay-Per-Call)
x402
Claude Sonnet API (Pay-Per-Call) is a paid API for AI agents from x402.agentindex.world, paid per call via x402, $0.011264/call, status unknown (last checked 2026-10-02).
Provides pay-per-call access to Anthropic's claude-sonnet-4-5 model via OpenAI-compatible chat completions, billed per request via x402 micropayments with no API key required.
Price / call$0.011264
Success—
StatusUnknown
How this service works
Claude Sonnet API - pay per call, no API key. OpenAI-format chat completions on anthropic/claude-sonnet-5.5, pinned to Anthropic's own OpenRouter endpoint for reliability. You sign a fixed price computed from your max_tokens; unused tokens are not refunded. 20s server-side timeout - never charged if it fires; set your client timeout to 30s. Try GET /llm/claude-sonnet/sample.
Output
Returns an OpenAI-format chat completion object containing the assistant's generated message, finish reason, and token usage counts. The response mirrors the standard OpenAI /v1/chat/completions response shape so it can be parsed by any OpenAI-compatible client.
Example use cases
On-demand code explanation for agents
“I need Claude Sonnet to explain what this Python function does and suggest any improvements — limit the response to 500 tokens so I don't overpay.”
Customer support draft generation
“Use Claude Sonnet to write a polite, empathetic reply to this customer complaint about a delayed shipment — max 200 tokens.”
Ad-hoc reasoning in an agent pipeline
“Send this multi-turn conversation to Claude Sonnet and ask it to summarize the key decisions made so far — cap the output at 400 tokens.”
Technical details
Endpoint
GEThttps://x402.agentindex.world/llm/claude-sonnet?utm_source=zero.xyzWhen to prefer
Choose this endpoint when you need to call Claude Sonnet on a per-request basis without managing an Anthropic or OpenRouter API key, especially in agent pipelines that use x402 micropayment flows. It's ideal for low-to-medium frequency invocations where a fixed per-call price is preferable to subscription billing, and when OpenAI-format compatibility is required for drop-in use with existing tooling.
Side effects
- 20-second server-side timeout fires if the model takes too long — request is not charged but no output is returned; client should set a 30s timeout
- max_tokens exceeds 4096 cap — request may be rejected or capped silently
- Malformed messages array (missing role or content fields) causes a 400-level error
- x402 payment failure or insufficient balance prevents the request from being processed
- Model unavailability on OpenRouter may cause 5xx errors
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| messages | array | OpenAI-format chat messages: [{role, content}, ...]. |
| max_tokens | integer | Max completion tokens, capped at 4096 - also bounds the price ceiling. |
Tags
x402
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