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A2A Callback Mailbox — Agent Capability Foundry
x402
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.
Price / call$0.005
Success—
StatusUnknown
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.
Example use cases
Async subagent result collection
“Spin up a callback mailbox that accepts up to 50 events, allows up to 512 KB of data, and expires in 30 minutes so my subagent fleet can POST their results back after finishing their tasks.”
Multi-step pipeline handoff inbox
“I'm kicking off a long-running agent task with ID 'pipeline-run-9871' — provision a mailbox for it that stays open for 90 minutes and can hold up to 200 events, so the downstream agent can report back when it's done.”
Short-lived agent webhook for testing
“Create a temporary callback mailbox that expires in 5 minutes and holds at most 10 events and 65 KB — I just need it to test whether my orchestrator correctly posts completion payloads to an A2A callback URL.”
Technical details
Endpoint
POSThttps://callbacks.inversolabs.us/v1/services/agent-callback-mailbox/invokeWhen to prefer
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.
Side effects
- 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
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| maxBytes | integer | |
| maxEvents | integer | |
| ttlMinutes | integer | |
| expectedTaskId | string | |
| ingressCredential | string | |
| retrievalCredential | string |
Output
Schema · Response
{
"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"
}
}Tags
x402
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