---
title: "PUBSUB: subscribe a callbackUrl to a topic by its topicId..."
method: POST
path: "/api/pubsub/subscribe"
tags: ["pubsub"]
---

# PUBSUB: subscribe a callbackUrl to a topic by its topicId...

`POST /api/pubsub/subscribe`

## Request

- Base URL: `https://2s.io`
- URL: `https://2s.io/api/pubsub/subscribe`
- Auth: API key in header `PAYMENT-SIGNATURE`

PUBSUB: subscribe a callbackUrl to a topic by its topicId (the topic can belong to any wallet — you just need the id). CONFIRMATION REQUIRED: we immediately POST a one-time challenge to your callbackUrl with header X-2s-Confirmation-Token (and {type:"2s_subscription_confirmation", token} in the JSON body); to be CONFIRMED your endpoint must reply 2xx with the response body set to exactly that token, or JSON {"token":"<token>"}. Only confirmed subscribers receive published messages (this prevents subscribing a non-consenting URL). Response field "confirmed" tells you if it worked; re-call to retry. When the owner publishes, we POST the message to your callbackUrl, EIP-191-signed (verify with X-2s-Signature) and retried with backoff; a subscriber that repeatedly fails delivery is auto-disabled. Returns a subscriptionId for pubsub.unsubscribe. Up to 100 subscribers per topic, one per URL. Pay per call with x402.

## Request body

- object
  - `topicId` string, required: The topicId to subscribe to (from pubsub.create-topic).
  - `callbackUrl` string, uri, required: Where we POST published messages. Any http(s) URL; must echo the confirmation token to activate; body is signed.
  - `label` string: Optional free-text tag.

## Other responses

- `200` (unresolved $ref)
- `400`: Invalid request — not charged. error.code: BAD_REQUEST or INVALID_PARAM (parameter validation; error.details.issues lists each problem), MALFORMED_JSON, or a request-specific code. Other client errors: 405 METHOD_NOT_ALLOWED (wrong verb; see the Allow header), 413 PAYLOAD_TOO_LARGE.
- `402`: Payment required. Body is the x402 PaymentRequirements envelope with a multi-network accepts array; the per-call price is in accepts[].amount (and on the operation as x-2s-price). Sign for whichever rail you hold USDC on (EIP-3009 for Base, partial SPL transfer for Solana) and retry with the PAYMENT-SIGNATURE header (X-PAYMENT also accepted for v1 clients). Also PAYMENT_SETTLE_FAILED (error envelope) when a verified payment failed to settle: error.details.chargeStatus is "not_charged" (retry with a fresh payment) or "unknown" (the authorization may still settle before its deadline — check it before retrying).
- `404`: Not found — NOT_FOUND (or NO_SOURCES / NO_OPINIONS): the requested entity does not exist. Not charged.
- `429`: Too many requests — RATE_LIMITED: our per-IP / per-payer limiter. Not charged; retry after Retry-After.
- `5XX`: Server-side failure. 500 INTERNAL: unexpected error; quote error.errorId (also the X-2s-Error-Id header). 502 UPSTREAM_UNAVAILABLE (or another UPSTREAM_* code): the data provider failed — composite endpoints refuse unbilled when every source fails. 503 SERVICE_PAUSED: endpoint paused by the operator kill switch, retry after Retry-After; 503 SPEND_CAP: daily upstream budget exhausted. 504 UPSTREAM_TIMEOUT. Not charged.

---

[API](https://skmtc.dev/2s-io/apis/2s-the-most-everything-api.md) · [All operations](https://skmtc.dev/2s-io/apis/2s-the-most-everything-api/llms.txt) · [OpenAPI document](https://skmtc.dev/2s-io/apis/2s-the-most-everything-api/revisions/da4c5caf686e?raw)