Messages metered (Anthropic-compatible)

Anthropic Messages API billed per request from what the call costs: the 402 quotes exact-BPE input (system + messages + tools) plus your max_tokens, from $0.001 up to a $2 per-call cap. Send POST /v1/metered/messages with the required fields max_tokens and messages and pay quoted per request from $0.001 over x402 or MPP (there is no free tier). It returns a JSON object with id, type, role, model, content and 2 more.

Point the Anthropic SDK (or any Messages-format client) at base_url https://agent402.tools/v1/metered. Any model from the flat tiers (GET /v1/models). Pay the quote over x402 exact, or authorize it as a ceiling over upto, credits or card and settle actual usage. Up to 200,000 input chars and 8192 output tokens; streaming supported.

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

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Parameters

Example request

curl -i -X POST https://agent402.tools/v1/metered/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-metered-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-metered-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-metered-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 issued earlier (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 $0.001 and $2.
• A GET or HEAD to /v1/metered/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/metered/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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