A Foundry service by InversoLabs · Live paid service · v0.1.54

A2A Callback Mailbox

Give asynchronous work a secure callback URL, suspend the expensive reasoning agent, and resume it only when useful data exists. Foundry provides durable ordered retrieval, replay protection, quotas, expiry, and structured output.

Purchase status: New paid x402 requests are currently accepted.

0.005 USDC per temporary mailbox · Base or Solana · x402

One provisioning payment includes all callback messages accepted during the selected mailbox lifetime; there is no per-message charge.

Save agent compute while work is pending

A 30-minute task checked every 15 seconds produces 120 polls. If an orchestrator invokes its reasoning model for every check, that means 120 unnecessary reasoning cycles before the result arrives. With a mailbox, the agent hands the remote service one callback URL, suspends, and resumes after a lightweight worker observes the stored event. Exact token savings depend on the buyer's model and orchestration design; plain HTTP checks consume no model tokens unless the buyer routes them through a model.

Why buy instead of build?

A mailbox is ready in about 500 ms. For 0.005 USDC, Foundry operates the HTTPS endpoint, persistence, authentication, deduplication, quotas, retention, and payment integration instead of requiring the agent to deploy and maintain that infrastructure.

Agent integration

POST https://callbacks.inversolabs.us/v1/services/agent-callback-mailbox/invoke
Idempotency-Key: your-stable-request-id
Content-Type: application/json

{"expectedTaskId":"task-123",
 "ingressCredential":"generate-a-random-secret-of-at-least-32-characters",
 "retrievalCredential":"generate-a-different-secret-of-at-least-32-characters"}

The first unsigned request returns an x402 402 Payment Required challenge. A compatible agent wallet signs the payment and retries automatically. Credentials remain caller-generated; Foundry never needs a customer wallet seed phrase.

Successful response

The following is an illustrative schema-valid response, not a recorded customer transaction:

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

Use the callback URL with the ingress credential, then retrieve ordered events with the separate retrieval credential. Exact schemas and error responses are available in OpenAPI.