Agent to Paid API: Build a Substream, Stream it to Your Own Database, Charge with x402

Agent to Paid API: Substreams → database → API → x402

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One prompt in, one paid endpoint out, with every command checked against official docs.

Build a Substream with agent skills, sink it into your own database, and charge per call with x402 on any supported chain. Worked example: Jupiter Lend (jupiter-lend-substreams).

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At a glance

Item	Detail
Steps	9, from installing skills to publishing
Payments	Any chain: whatever your facilitator supports
Database	Any host: Railway, your server, or managed
Sink	1 CLI: `substreams sink postgres`
Worked example	Jupiter Lend on Solana (jupiter-lend-substreams)

Who this is for

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AI agents

Grok Bot, Claude Code, Cursor: follow the steps in order and ask a human before any spend or publish.

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Data builders

Substreams developers who want their package to earn per call instead of sitting in a registry.

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API sellers

Anyone who wants fresh on-chain tables behind a 402 paywall without running an indexer from scratch.

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What you'll build

Plain-English prompt
   │  agent skills
   ▼
Substreams package (db_out)
   │  sink
   ▼
Postgres / ClickHouse (Railway or your own server)
   │  SQL
   ▼
Hono API (read-only)
   │  402 then pay
   ▼
x402 paywall (USDC on any supported chain)

No single public repo covers this whole path today. StreamingFast's skills cover build, test and sink; the x402 docs cover the paywall. This guide joins them, with lessons from running the Jupiter Lend API on Railway.

Resource	Build	Test	Sink	API	x402
substreams-skills	✅	✅	✅	—	—
Graph docs skills page	Setup	—	—	—	—
StreamingFast AI indexer blog	✅	Basic	—	—	—
SQL sink migration guide	—	—	✅	—	—
x402 seller quickstart	—	—	—	✅	✅
This guide	✅	✅	✅	✅	✅

Prerequisites

Item	Details
CLI	The `substreams` CLI, a recent version that includes `substreams sink postgres`
Data key	A Substreams API key (StreamingFast / The Graph Market, or Pinax) in `SUBSTREAMS_API_TOKEN` or `SUBSTREAMS_API_KEY`, or a `.substreams.env` file
Build tools	Rust with the `wasm32-unknown-unknown` target, and `buf` for protobuf
API	Node.js 20+ for Hono and the `@x402/*` packages
Hosting	Railway or your own server (Docker on a box or VPS); plan for disk, see step 4
Wallets	A receiving address on each chain you accept (EVM, Solana or another network your facilitator supports); public addresses only
Agent	Grok Bot, Claude Code, Cursor or VS Code 1.107+

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Secrets rule: Never paste API keys, database URLs, wallet keys or salts into chat, logs, READMEs or the Page. Keep them in Railway variables or a local env file that is git-ignored.

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1. Install the agent skills

Goal: give your agent the Substreams skills before it writes any code.

Claude Code: run the two commands below, then check with /skills.

claude plugin marketplace add streamingfast/substreams-skills
claude plugin install substreams-dev@streamingfast-substreams

Load without installing: clone the repo, then point Claude Code at it.

git clone https://github.com/streamingfast/substreams-skills.git
claude --plugin-dir ./substreams-skills