Foundry "cast: error sending request" RPC failure in CI

cast tried to send a JSON-RPC request and the HTTP client could not complete it. The RPC URL is missing, malformed, or the endpoint is unreachable from the runner, so no chain data is returned.

What this error means

A cast call, cast block, or cast send step fails with "Error: error sending request for url (...)" and the run exits non-zero.

Foundry

Error: error sending request for url (https://): builder error: relative URL without a base

Common causes

The RPC URL is unset or empty

cast reads --rpc-url or ETH_RPC_URL from an environment variable that is empty in CI, so it sends to an invalid base URL.

The endpoint is unreachable or rejecting requests

A wrong host, a rate-limited provider, or a network restriction stops the request from completing.

How to fix it

Provide a valid RPC URL from a secret

Set the endpoint explicitly and pass it to cast so requests reach a real node.

.github/workflows/ci.yml

env:
  ETH_RPC_URL: ${{ secrets.ETH_RPC_URL }}
run: cast block latest --rpc-url "$ETH_RPC_URL"

Use a local node for chain-independent steps

When the step does not need a live network, run anvil and point cast at it.

Terminal

anvil &
cast block latest --rpc-url http://127.0.0.1:8545

How to prevent it

Inject RPC URLs through secrets, not committed defaults.
Use a local anvil node for steps that do not require mainnet data.
Fail fast with a clear message when the RPC URL is empty.