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docs: note public testnet RPC method differences across follow endpoints #373

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39 changes: 38 additions & 1 deletion docs/running-an-arc-node.md
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Expand Up @@ -225,6 +225,43 @@ companion execution layer.
The consensus layer operates in the **follow** mode.
We provide three endpoints from which the node retrieves finalized blocks.

> **Public testnet RPC methods are not identical across providers.** Note the
> differing WebSocket paths in the command above when swapping providers.
>
> Follow sync needs `eth_getBlockByNumber`, `arc_getCertificate` (batched per
> height), `eth_call`, `eth_chainId`, `net_listening`, and
> `eth_subscribe("newHeads")` over the `wss=` URL.
> `rpc.blockdaemon.testnet.arc.io` has no certificates below block 41,863,835
> (2026-05-12), while still serving the blocks themselves. This is a fixed
> floor, not a rolling window — it was unchanged across 2026-09-09 and
> 2026-09-10 while the head advanced ~162k blocks — so use
> `rpc.testnet.arc.io` or `rpc.drpc.testnet.arc.io` when catching up from
> genesis or from any height below it. A missing certificate fails the whole
> follow batch, not just that height.
>
> Application clients that need EIP-1186 proofs or access lists also see method
> gaps. Observed 2026-09-09 against the wider public fleet (not only the three
> follow endpoints above; see #371):
>
> | Endpoint | `eth_getProof` | `eth_createAccessList` |
> |---|---|---|
> | `rpc.testnet.arc.io` | unsupported (codes flip) | unsupported (`-32601`) |
> | `rpc.drpc.testnet.arc.io` | OK | OK |
> | `rpc.quicknode.testnet.arc.io` | unsupported (codes flip) | unsupported (`-32601`) |
> | `rpc.blockdaemon.testnet.arc.io` | OK | filtered (`-32003`); a batch containing it is rejected in full, losing unrelated calls |
>
> Prefer `rpc.drpc.testnet.arc.io` when you need proofs or access lists. Some
> hosts return `-32601` (method unsupported) for one call and `-32014` (data
> unavailable) for the next, for the same request. Do not branch on the code —
> treat any error from a capability probe as "unsupported on this host" and
> fail over.
>
> ```bash
> curl -s -X POST -H 'Content-Type: application/json' \
> --data '{"jsonrpc":"2.0","id":1,"method":"eth_getProof","params":["0x3600000000000000000000000000000000000000",["0x0"],"latest"]}' \
> https://rpc.testnet.arc.io
> ```

### Verify operation

After starting both the consensus and execution layer, wait about 30 seconds.
Expand Down Expand Up @@ -405,7 +442,7 @@ rm -rf ~/.arc

> [!WARNING]
> Running EL and CL on separate hosts requires the RPC/HTTP Engine API transport,
> which will be deprecated in `v0.8.0` and will be removed in `v0.9.0`.
> which is deprecated as of `v0.8.0` and will be removed in `v0.9.0`.
> Run both layers on the same host and use IPC instead (see the
> [Binaries](#binaries) section).
> The consensus layer logs a startup warning when any RPC option is set.
Expand Down