Documentation

One key. One bill. Every chain.

Everything below is served by the same gateway you'll call — chains, limits, and tools are rendered live, never stale. Machine-readable version: /llms.txt.

Try it live — no signup

Pick a chain and a method, hit send — the request goes through the real gateway: same relay, same automatic failover, same upstreams. You'll see the response time and which provider actually served you.

// the JSON-RPC response appears here

Demo methods only — sign up free for the full JSON-RPC surface with your own key. Full reference in the docs.

Quickstart

Sign up (free tier, e-mail only).
Create an API key in the dashboard — the fy_… token is shown exactly once.
Relay a request:

curl -X POST https://flexyield.io/rpc/eth_mainnet \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"eth_blockNumber"}'

Authentication

Every endpoint takes the same bearer key: Authorization: Bearer fy_…. Keys are account-scoped; issue one per app or agent, set an optional monthly request cap per key, and revoke instantly from the dashboard.

Supported chains

Slug	Network
`arbitrum_one`	Arbitrum One
`base_mainnet`	Base Mainnet
`eth_mainnet`	Ethereum Mainnet
`optimism_mainnet`	OP Mainnet
`polygon_pos`	Polygon PoS
`solana_mainnet`	Solana Mainnet

Every chain is served by multiple upstream providers with automatic failover and circuit breakers — measured uptime and latency are public on /status.

Endpoints

Endpoint	What it does
`POST /rpc/{chain}`	JSON-RPC 2.0 relay to the best healthy upstream. 2xx bodies pass through verbatim. Omitting `{chain}` defaults to `eth_mainnet`.
`GET /data/{chain}/balances/{address}`	Native balance on every chain; ERC-20 token balances on chains with a data-capable upstream.
`GET /data/{chain}/history/{address}`	Recent asset transfers (sent + received, newest first). EVM chains.

Rate limits & response headers

Free: 1,000,000 requests/month included · burst up to 100 requests per 15 minutes — free.

Builder: Unlimited requests · fair-use burst up to 1,800 requests/min.

Pro: 15,000,000 requests/month included · burst up to 3,000 requests/min.

Pro (yearly): 15,000,000 requests/month included · burst up to 3,000 requests/min.

Every authenticated response carries X-RateLimit-Limit and X-RateLimit-Remaining; plans with a monthly included quota add X-Quota-Limit and X-Quota-Remaining. A 429 adds Retry-After (seconds). An exhausted monthly quota answers with error code -32007; optional per-key monthly caps answer with -32006.

Supply is tiered by design: free-tier traffic is served by the public RPC endpoints in our pool (with the same automatic failover). Premium credentialed providers are onboarding and will serve paid plans first once live; the data API activates with premium supply (paid plans only).

Errors

Code	Meaning
`-32001`	Invalid or missing API key (HTTP 401)
`-32005`	Rate limit exceeded (HTTP 429, see `Retry-After`)
`-32006`	Monthly request cap reached for this key (HTTP 429)
`-32007`	Plan's monthly included requests used up (HTTP 429)
`-32009`	Account suspended (HTTP 403)
`-32010`	Data API requires a paid plan (HTTP 403)
`-32600` / `-32602` / `-32700`	Invalid request / params / JSON
`-32603`	All upstream endpoints unavailable (HTTP 502)

Every error includes error.data.request_id — quote it when contacting hello@flexyield.io and we can trace the exact call.

For AI agents (MCP)

The gateway is an MCP server at https://flexyield.io/mcp (HTTP+SSE). Account tools authenticate with the same bearer API key. Current tools:

create_api_key — Create a new FlexYield API key for the authenticated account. Returns the plaintext token exactly once — store it securely, it cannot be retrieved again.
revoke_api_key — Revoke one of the authenticated account's API keys. Requests using the key stop working immediately. Idempotent.
usage_report — Report the authenticated account's RPC gateway usage: total requests, success rate, latency (avg + p95), per-key volume/latency/errors/last-used, and per-method and per-chain counts.
list_plans — List FlexYield's public subscription plans: prices in integer USD cents, billing interval, monthly included request quota, and burst rate limits. No authentication required.
join_waitlist — Join the FlexYield early-access waitlist. Qualifier must be one of: developer, investor, curious. No authentication required.
get_balances — Native balance (all supported chains) and ERC-20 token balances (chains with a data-capable upstream) for an address. Chains: e.g. eth_mainnet, base_mainnet, arbitrum_one, optimism_mainnet, polygon_pos, solana_mainnet.
get_history — Recent asset transfers (sent + received, newest first) for an address on chains with a data-capable upstream. EVM chains only in v1.