# Agent402 OpenAI Responses API (Nano Tier, x402)

> Agent402 OpenAI Responses API (Nano Tier, x402) is a paid API for AI agents from agent402.tools, paid per call via x402, $0.003/call, status unknown (last checked 2026-09-15).

Runs OpenAI-compatible Responses API calls (text generation, structured output, tool use) paid per-call in USDC via x402, no API key required.

## Facts

- Endpoint: POST https://agent402.tools/v1/nano/responses
- Price: $0.003/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/agent402-openai-responses-api-nano-tier-x402-2c6f7a7c
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_PYD3DQLg_IADWAgnAf_ww

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 agent402-openai-responses-api-nano-tier-x402-2c6f7a7c -d '<json body>'
```

Example prompt: Send this conversation to the nano-tier OpenAI Responses endpoint at agent402.tools — use gpt-4o-mini, ask it to summarize the following support ticket in JSON format, and pay the $0.003 USDC per-call fee automatically.

## When to prefer this

Choose this endpoint when you want an OpenAI Responses API-compatible interface with zero signup friction — no API key, no subscription — paying exactly $0.003 USDC per call via the x402 protocol. Ideal for autonomous agents, one-off scripts, or cost-sensitive pipelines where per-call micropayments are preferable to monthly subscriptions. Prefer alternatives (e.g. direct OpenAI or Anthropic APIs) when you need models outside the nano-tier allowlist, higher token caps, server-side built-in tools, or guaranteed SLAs.

## Known failure modes

- Payment rejected or insufficient USDC balance — 402 Payment Required with x402 challenge
- Requested model not on nano-tier allowlist — 400 or model-not-found error
- Input exceeds tier token cap — max_output_tokens clamped silently or 400 error
- Malformed input array (wrong role/content structure) — 422 validation error
- Server-side tools requested (not supported) — likely 400 or ignored
- Streaming connection dropped mid-response — partial SSE stream with no completion event

## How this service works

OpenAI Responses API over x402 - point the OpenAI SDK's responses.create() (or the OpenAI Agents SDK) at base_url https://agent402.tools/v1/nano and pay $0.003 per call in USDC, no API key, no signup. Same models, caps and price as this tier's /chat/completions route; any model here is served through the Responses wire.

## Output

Returns an OpenAI Responses API-compatible response object containing the generated text, structured JSON, or function call invocations. When streaming is enabled, delivers SSE events (response.created through response.completed). Reasoning tokens are included in the output when reasoning effort is set.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "zdr": {
   "type": "boolean",
   "description": "Optional - zero-data-retention providers only"
  },
  "text": {
   "type": "object",
   "description": "Optional {format: {type: \"text\"|\"json_schema\"|\"json_object\", ...}}"
  },
  "input": {
   "description": "A string, or an array of input items ({role, content} messages with input_text / input_image parts, function_call, function_call_output)"
  },
  "model": {
   "type": "string",
   "description": "Model id (OpenRouter naming) - allowlisted per tier; omit (or \"auto\") on the auto tier"
  },
  "tools": {
   "type": "array",
   "description": "Optional function tools ({type:\"function\", name, parameters}); server-side tools are not served"
  },
  "stream": {
   "type": "boolean",
   "description": "Responses SSE events (response.created … response.completed)"
  },
  "reasoning": {
   "type": "object",
   "description": "Optional {effort: \"none\"|\"minimal\"|\"low\"|\"medium\"|\"high\"|\"xhigh\"|\"max\"} - reasoning tokens count against max_output_tokens"
  },
  "instructions": {
   "type": "string",
   "description": "Optional system/developer instructions"
  },
  "max_output_tokens": {
   "type": "integer",
   "description": "Optional output cap (clamped to the tier cap)"
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "id": "resp_…",
  "model": "openai/gpt-4o-mini",
  "usage": {
   "input_tokens": 14,
   "total_tokens": 32,
   "output_tokens": 18
  },
  "object": "response",
  "output": [
   {
    "id": "msg_…",
    "role": "assistant",
    "type": "message",
    "status": "completed",
    "content": [
     {
      "text": "x402 is an HTTP-native way for agents to pay per request with USDC.",
      "type": "output_text",
      "annotations": []
     }
    ]
   }
  ],
  "status": "completed"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/agent402-openai-responses-api-nano-tier-x402-2c6f7a7c/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from agent402.tools](https://www.zero.xyz/host/agent402.tools/llms.txt)