Rumble Fish has been awarded a grant from the Stellar Community Fund (SCF #45) to build x402 on Stellar: an open-source payment facilitator and Bazaar discovery layer that lets AI agents find, price, and pay for API endpoints and MCP tools on a per-request basis, in USDC or any SEP-41 asset.

This is our third SCF award this year. It follows the SCF #41 grant for the Soroban-first Block Explorer, now live as Soroban Scan, and the SCF #42 grant for the Stellar Prices API. This time we were selected in the RFP Track (Developer Tooling), as one of three teams funded under the x402 Facilitator with Bazaar RFP. You can read the full round results in the SCF #45 Round Recap.

Why AI Agents Need a Different Payment Model

The way we pay for APIs today was designed for humans. You sign up, enter a card, pick a monthly plan, and copy an API key into your config. An AI agent that needs one price quote or one contract lookup can't fill in a signup form, and a $49 monthly subscription for a single call makes no sense for either side. x402 fixes this by reviving the long-dormant HTTP 402 "Payment Required" status code. When an agent calls a paid endpoint, the server responds with payment terms. The agent signs a payment, a facilitator verifies and settles it on-chain, and the request goes through. Payment becomes part of the HTTP request itself, with no accounts or keys involved.

Stellar is a natural home for this pattern. Settlement costs around 0.0023 XLM per transaction, a fraction of a cent, which makes sub-cent pricing per request economically viable in a way it isn't on most other networks.

What We're Building for x402 on Stellar

The project has four main parts. The facilitator implements the x402 v2 verify, settle, and supported endpoints, accepts any SEP-41 token, and sponsors network fees so payers don't need to hold XLM. The Bazaar is a discovery catalog with ranked search based on relevance, quality score, and curation, so agents can find paid services by describing what they need. An MCP-native discovery server exposes search and paid calls as tools, which means agent runtimes can discover and pay for services without custom integration code. And SDKs in TypeScript and Rust cover both buyers and sellers. The Rust seller crate will be the first seller-side Rust integration in the x402 ecosystem.

We're also adding a usage-based "upto" settlement scheme. The standard exact-price model breaks down for metered services, where the final cost depends on how many rows a query returns or how many tokens a model generates. With upto, the client authorizes a ceiling, and the server settles the actual amount used, never more. A Soroban proxy contract enforces the ceiling, expiry, and single-use nonces on-chain. We're submitting the scheme specification to the x402 Technical Steering Committee so it can become part of the protocol for every network, not only Stellar. Delivery happens in three tranches: a testnet foundation, a feature-complete release with the facilitator on pubnet and an external security audit underway, and a mainnet launch with audit findings resolved, a public conformance report listing settled mainnet transaction hashes, and a developer guide published to the Stellar Developer Docs.

Real Services Instead of Demo Endpoints

The RFP requires example integrations. We could have built a few synthetic endpoints that return hardcoded JSON, and they would pass every test. They would also tell us very little about how the infrastructure behaves under real use. So we're gating our own production services behind x402 instead. Soroban Scan and the Prices API will both accept x402 payments through the new Rust crate, alongside a canonical Express/TypeScript reference example. Real queries from real users hit edge cases that demo endpoints never will, and the facilitator gets production traffic from day one. It also gives both projects a sustainable monetization path that doesn't depend on grant funding.

What the Team Says

Marek Kowalski, CEO/CTO of Rumble Fish, sees this grant as the point where the team's earlier Stellar work comes together:

"Agents are becoming real consumers of APIs, and they need to pay for what they use without a human filling in forms. Stellar has the fees and the settlement speed to make per-request payments work. We already run production services on Stellar, so we're building this as a team that will use it every day, not as a demo."

What This Means for the Stellar Ecosystem

Agentic payments are one of the fastest-moving areas in the industry right now, and the networks that offer the best developer experience for them will attract the services agents end up paying for. A facilitator, a discovery layer, MCP support, and SDKs in two languages give Stellar a complete stack for AI agent payments, available as open source for any team to run or build on.

For us, this completes a line of infrastructure work: an explorer that makes Soroban readable, a price API that makes asset data consistent, and now a payment layer that makes both of them, and any other Stellar service, accessible to autonomous agents. You can track the project's progress on the Stellar Community Fund. If you're building on Stellar and need a technical partner who knows the ecosystem inside out, our team is ready to help, so get in touch.