rust-lang/cargo · error · anyhow::Error

`resolver` setting `{}` is not valid, valid options are "1",

Error message

`resolver` setting `{}` is not valid, valid options are "1", "2" or "3"

What it means

`ResolveBehavior::from_manifest` parses the `resolver` key under `[package]` (or workspace) in `Cargo.toml`. Only the literal strings `"1"`, `"2"`, `"3"` are accepted; anything else (e.g. `"true"`, `"v2"`, `"4"`, or a number) bails with this message listing the valid options.

Source

Thrown at src/resolver/types.rs:124

/// Resolver behavior, used to opt-in to new behavior that is
/// backwards-incompatible via the `resolver` field in the manifest.
#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
pub enum ResolveBehavior {
    /// V1 is the original resolver behavior.
    V1,
    /// V2 adds the new feature resolver.
    V2,
    /// V3 changes version preferences
    V3,
}

impl ResolveBehavior {
    pub fn from_manifest(resolver: &str) -> CargoResult<ResolveBehavior> {
        match resolver {
            "1" => Ok(ResolveBehavior::V1),
            "2" => Ok(ResolveBehavior::V2),
            "3" => Ok(ResolveBehavior::V3),
            s => anyhow::bail!(
                "`resolver` setting `{}` is not valid, valid options are \"1\", \"2\" or \"3\"",
                s
            ),
        }
    }

    pub fn to_manifest(&self) -> String {
        match self {
            ResolveBehavior::V1 => "1",
            ResolveBehavior::V2 => "2",
            ResolveBehavior::V3 => "3",
        }
        .to_owned()
    }
}

/// Options for how the resolve should work.
#[derive(Clone, Debug, Eq, PartialEq, Hash)]

View on GitHub (pinned to 0e07a15537)

Solutions

Set `resolver = "2"` (or `"1"`/`"3"`) as a quoted string in `[package]` or `[workspace]`.
Upgrade Cargo if you intended a newer resolver value not yet supported by your toolchain.
Remove the `resolver` key entirely to use the default for your edition (edition 2021+ defaults to V2).

Example fix

# before
[package]
resolver = "v2"
# after
[package]
resolver = "2"

Defensive patterns

Strategy: validation

Validate before calling

# Validate the resolver key is one of the allowed strings:
tomlq -r '.package.resolver // .workspace.resolver' Cargo.toml \
  | grep -qE '^[123]$' || echo 'resolver must be "1", "2", or "3"'

Prevention

Only ever write `resolver = "2"` (or omit it on edition 2021+).
Keep Cargo.toml linters in CI to catch typos.
Upgrade Cargo if you need a newer resolver value.

When it happens

Trigger: Setting `resolver = "v2"`, `resolver = 2` (unquoted integer is a TOML error separately), `resolver = "4"`, or any non-`"1"/"2"/"3"` string in `[package]`/`[workspace]`. The `match resolver { s => bail!(...) }` catch-all arm fires.

Common situations: Typos in `Cargo.toml`; copying a config snippet from outdated docs; using a future resolver value on an older Cargo; mixing up `resolver` (behavior selector) with edition.

Related errors

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/9f69a66f82ea2e2d.json. Report an issue: GitHub.