Messages base (Anthropic-compatible)

Anthropic Messages API over x402 - point the Anthropic SDK (or Claude Code / the Agent SDK) at base_url https://agent402.tools/v1 and pay $0.02 per call in USDC, no API key, no signup. Send POST /v1/messages with the required fields max_tokens and 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, type, role, model, content and 2 more.

Same models, caps and price as this tier's /chat/completions route; any model here is served through the Messages wire (Claude natively, others translated). Depth: `effort` (low..max) on Claude 4.7+, thinking.budget_tokens on older Claude. Up to 32,000 input chars and 2048 output tokens; streaming supported. Omit "model" and the tier serves openai/gpt-4o-mini (named back in agent402_default_model); the price does not change.

Category: LLM gateway · Tags: llm ai inference anthropic-compatible messages-api claude gateway openrouter

TRY IN PLAYGROUND →

Parameters

Example request

curl -i -X POST https://agent402.tools/v1/messages \
  -H "Content-Type: application/json" \
  -d '{"model":"anthropic/claude-haiku-4.5","max_tokens":256,"messages":[{"role":"user","content":"Summarize x402 in one sentence."}]}'

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

Example response

{
  "id": "msg_…",
  "type": "message",
  "role": "assistant",
  "model": "anthropic/claude-sonnet-5",
  "content": [
    {
      "type": "text",
      "text": "x402 is an HTTP-native way for agents to pay per request with USDC."
    }
  ],
  "stop_reason": "end_turn",
  "usage": {
    "input_tokens": 14,
    "output_tokens": 18
  }
}

From an MCP client

catalog.call {
  "slug": "v1-chat-messages",
  "params": {
    "model": "anthropic/claude-haiku-4.5",
    "max_tokens": 256,
    "messages": [
      {
        "role": "user",
        "content": "Summarize x402 in one sentence."
      }
    ]
  }
}

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

Errors and behavior

• max_tokens and messages are 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 over x402, MPP or a prepaid credits key: settlement is cancelled when the tool fails. The exception is a Tempo push credential, a transfer the buyer sent before the call: it settles before the tool runs, so if the tool then fails the payment is recorded as a refund owed to the paying wallet.
• 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/messages 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 (an answer larger than 1 MB and a streamed response 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/messages", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "model": "anthropic/claude-haiku-4.5",
    "max_tokens": 256,
    "messages": [
      {
        "role": "user",
        "content": "Summarize x402 in one sentence."
      }
    ]
  }),
});

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