Chat completions (OpenAI-compatible)

OpenAI-compatible chat completions, base tier: point any OpenAI SDK at base_url https://agent402.tools/v1 and pay per call over x402 or MPP (the 402 lists every accepted rail), no API key, no signup. Send POST /v1/chat/completions with the required field messages and pay quoted per request from $0.02 over x402 or MPP (there is no free tier). It returns a JSON object with id, object, created, model, choices and 1 more.

Returns the standard chat.completion object (choices[].message.content, usage). Budget/mid models: gpt-4o-mini (the default when model is omitted), claude haiku, gemini flash, deepseek, llama, mistral, qwen. Full wire compatibility incl. tools/function-calling and response_format. GET /v1/models lists every model. Streaming supported (stream: true). One flat price per call up to the tier caps; /v1/metered/chat/completions quotes each call from its own size instead. Model-backed.

Category: LLM gateway · Tags: llm ai inference chat gateway openai-compatible openrouter

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Parameters

Example request

curl -i -X POST https://agent402.tools/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{"model":"openai/gpt-4o-mini","messages":[{"role":"user","content":"Reply with exactly: OK"}],"max_tokens":5}'

Without payment this returns HTTP 402 Payment Required with the exact price for v1-chat; any x402 v2 or MPP client pays it and retries.

Example response

{
  "id": "gen-…",
  "object": "chat.completion",
  "created": 1750000000,
  "model": "openai/gpt-4o-mini",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "content": "OK"
      },
      "finish_reason": "stop"
    }
  ],
  "usage": {
    "prompt_tokens": 12,
    "completion_tokens": 1,
    "total_tokens": 13
  }
}

From an MCP client

catalog.call {
  "slug": "v1-chat",
  "params": {
    "model": "openai/gpt-4o-mini",
    "messages": [
      {
        "role": "user",
        "content": "Reply with exactly: OK"
      }
    ],
    "max_tokens": 5
  }
}

The hosted connector at https://agent402.tools/mcp needs a payment for v1-chat; the stdio package pays it from a wallet or from AGENT402_CREDITS_KEY. Local install: npx -y agent402-mcp.

Errors and behavior

• messages is required. An input the tool rejects returns an HTTP 4xx whose body carries error, tool, expected, required and example, so the caller can correct it.
• A paid call that ends in any status of 400 or above is not charged: settlement is cancelled when the tool fails.
• Wallet-only: this tool runs a model, so it has no proof-of-work tier. A prepaid card-credits key (Authorization: Bearer a402_...) also pays it.
• Model-backed: the answer is generated by a model, so the same input can produce different wording.
• Priced per request: the 402 quotes this body, between $0.02 and $0.5.
• Flat per call for the models this tier serves. A body naming another flat tier's model (nano, base, pro, premium) is quoted at that tier's price in the 402 and served under that tier; model "auto" is quoted and served as the auto tier. The answer names the tier in agent402_tier. The live 402 is always the price.
• A GET or HEAD to /v1/chat/completions returns the same 402 quote, so the price can be read without a body.
• An Idempotency-Key header makes a retried paid call replay the first 200 instead of charging again (streamed responses are not replayed).

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
client.setSpendControls?.(false); // keep your own spending ceiling in code
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/v1/chat/completions", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "model": "openai/gpt-4o-mini",
    "messages": [
      {
        "role": "user",
        "content": "Reply with exactly: OK"
      }
    ],
    "max_tokens": 5
  }),
});

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