# A2A Callback Mailbox — Agent Capability Foundry

> A2A Callback Mailbox — Agent Capability Foundry is a paid API for AI agents from callbacks.inversolabs.us, paid per call via x402, $0.005/call, status unknown (last checked 2026-09-14).

Provisions a temporary, machine-payable callback mailbox that accepts A2A callback payloads from remote agents and holds them for retrieval.

## Facts

- Endpoint: POST https://callbacks.inversolabs.us/v1/services/agent-callback-mailbox/invoke
- Price: $0.005/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/a2a-callback-mailbox-agent-capability-foundry-43841455
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_gXQ6DOkDh4ASHYHpJUkSf

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 a2a-callback-mailbox-agent-capability-foundry-43841455 -d '<json body>'
```

Example prompt: Create a callback mailbox that can hold up to 100 events and 1 MB of data, expires in 60 minutes, and is locked to task ID 'task-abc-123' — I'll use the returned callback_url to hand off to a subagent.

## When to prefer this

Use this endpoint when you need a short-lived, machine-payable callback inbox without operating your own server — ideal for multi-agent pipelines where a coordinator must receive async results from subagents via a known URL. Prefer this over webhooks or polling when you need a disposable, authenticated ingress point with configurable TTL, event caps, and byte limits, especially in x402-native payment environments.

## Known failure modes

- Invalid parameter ranges (e.g. maxBytes below 65536 or above 4194304, ttlMinutes above 1440) return a 400 validation error
- Missing or malformed ingressCredential or retrievalCredential (must be 32–256 chars) returns 400
- Payment not included or insufficient USDC payment returns 402 Payment Required
- Service unavailability or provisioning failure returns 503
- Expired mailbox_id used for subsequent retrieval returns 404

## How this service works

Machine-payable capabilities for autonomous agents. This service accepts A2A callback payloads but is not itself an A2A protocol server and does not publish an A2A Agent Card.

## Output

Returns a JSON object containing the provisioned mailbox_id, a callback_url (the endpoint remote agents should POST to), an expires_at timestamp, the configured max_bytes and max_events limits, and the authentication_scheme (Bearer) used to secure ingress and retrieval.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "maxBytes": {
   "type": "integer",
   "maximum": 4194304,
   "minimum": 65536
  },
  "maxEvents": {
   "type": "integer",
   "maximum": 1000,
   "minimum": 1
  },
  "ttlMinutes": {
   "type": "integer",
   "maximum": 1440,
   "minimum": 1
  },
  "expectedTaskId": {
   "type": "string",
   "maxLength": 256,
   "minLength": 1
  },
  "ingressCredential": {
   "type": "string",
   "maxLength": 256,
   "minLength": 32
  },
  "retrievalCredential": {
   "type": "string",
   "maxLength": 256,
   "minLength": 32
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "max_bytes": 1048576,
  "expires_at": "2026-08-20T14:00:00+00:00",
  "mailbox_id": "example-mailbox-id",
  "max_events": 100,
  "callback_url": "https://callbacks.inversolabs.us/callbacks/example-mailbox-id",
  "authentication_scheme": "Bearer"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/a2a-callback-mailbox-agent-capability-foundry-43841455/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from callbacks.inversolabs.us](https://www.zero.xyz/host/callbacks.inversolabs.us/llms.txt)