FuelLabs/sway · error · anyhow::Error

Fetching gas costs from testnet is currently not implemented

Error message

Fetching gas costs from testnet is currently not implemented.

What it means

Identical stub to the Mainnet arm: GasCostsSource::provide_gas_costs() returns a hard-coded anyhow error for Self::Testnet because fetching live gas costs is not implemented (sway#7472). Only the BuiltIn arm returns Ok, by deserializing the packaged gas_costs_values.json.

Source

Thrown at forc-test/src/lib.rs:208

    Testnet,
    File(String),
}

impl GasCostsSource {
    pub fn provide_gas_costs(&self) -> Result<GasCostsValues, anyhow::Error> {
        match self {
            // Values in the `gas_costs_values.json` are taken from the `chain-configuration` repository:
            //      chain-configuration/upgradelog/ignition/consensus_parameters/<version>.json
            // Update these values when there are changes to the on-chain gas costs.
            Self::BuiltIn => Ok(serde_json::from_str(include_str!(
                "../gas_costs_values.json"
            ))?),
            // TODO: (GAS-COSTS) Fetch actual gas costs from mainnet/testnet and JSON file.
            //       See: https://github.com/FuelLabs/sway/issues/7472
            Self::Mainnet => Err(anyhow::anyhow!(
                "Fetching gas costs from mainnet is currently not implemented."
            )),
            Self::Testnet => Err(anyhow::anyhow!(
                "Fetching gas costs from testnet is currently not implemented."
            )),
            Self::File(_file_path) => Err(anyhow::anyhow!(
                "Loading gas costs from a JSON file is currently not implemented."
            )),
        }
    }
}

impl FromStr for GasCostsSource {
    type Err = anyhow::Error;

    fn from_str(value: &str) -> Result<Self, Self::Err> {
        match value {
            "built-in" => Ok(Self::BuiltIn),
            "mainnet" => Ok(Self::Mainnet),
            "testnet" => Ok(Self::Testnet),
            file_path => Ok(Self::File(file_path.to_string())),

View on GitHub (pinned to 47e5e902fa)

Solutions

Run `forc test` with the built-in source (default, or explicitly `--gas-costs built-in`)
Track/upgrade to a forc version implementing testnet fetching (sway#7472)
If you maintain the tool, implement the arm by fetching consensus parameters and deserializing into GasCostsValues

Example fix

# before
forc test --gas-costs testnet

# after
forc test --gas-costs built-in

Defensive patterns

Strategy: validation

Validate before calling

let source = GasCostsSource::from_str(value)?;
if matches!(source, GasCostsSource::Testnet | GasCostsSource::Mainnet | GasCostsSource::File(_)) {
    anyhow::bail!("only 'built-in' gas costs are supported by this forc version");
}

Type guard

fn supported(src: &GasCostsSource) -> bool {
    matches!(src, GasCostsSource::BuiltIn)
}

Try / catch

if let Err(e) = source.provide_gas_costs() {
    if e.to_string().contains("testnet") {
        // fall back to built-in values for now
        return GasCostsSource::BuiltIn.provide_gas_costs();
    }
    return Err(e);
}

Prevention

Default to omitting --gas-costs (BuiltIn is the default variant)
Wrap forc invocations in scripts and validate the supported source list once per forc version

When it happens

Trigger: `forc test --gas-costs testnet`, or GasCostsSource::from_str("testnet") followed by provide_gas_costs(). Parsing "testnet" succeeds; the failure happens at gas-cost provisioning time.

Common situations: Developers targeting testnet who expect cost profiles to mirror the live network, or CI configs pinned to `--gas-costs testnet` copied from templates for a forc version that never supported it.

Related errors

AI-assisted analysis of FuelLabs/sway@47e5e902fa (2026-08-16). Data as JSON: /api/errors/394891a3c54f94d4. Report an issue: GitHub.