---
title: "PUBSUB: publish a message to a topic YOU own, fanning out a..."
method: POST
path: "/api/pubsub/publish"
tags: ["pubsub"]
---

# PUBSUB: publish a message to a topic YOU own, fanning out a...

`POST /api/pubsub/publish`

## Request

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

PUBSUB: publish a message to a topic YOU own, fanning out a signed callback to every CONFIRMED subscriber. Returns the number of subscribers it was delivered to. Only the topic owner (the x402 payer who created it) can publish. Message is arbitrary JSON (≤64 KB). Priced by confirmed-subscriber count (a small base + per subscriber), so you pay for the fan-out you request. Delivery is at-least-once with retry + signature.

## Request body

- object
  - `topicId` string, required: The topic you own (from pubsub.create-topic).
  - `message` unknown

## 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)