rust-lang/cargo · error

invalid character `+` in dependency name: `+{toolchain}`

Error message

invalid character `+` in dependency name: `+{toolchain}`
    Use `cargo +{toolchain} add` if you meant to use the `{toolchain}` toolchain.

What it means

From parse_dependencies (src/bin/cargo/commands/add.rs:265-270). For each crate name passed to `cargo add`, if it begins with `+`, Cargo assumes the user confused a toolchain override with a dependency and bails, suggesting the correct `cargo +<toolchain> add` form. The `+` prefix is reserved by Cargo itself for toolchain selection.

Source

Thrown at src/bin/cargo/commands/add.rs:266

    };
    let default_features = default_features(matches);
    let optional = optional(matches);
    let public = public(matches);

    let mut crates = matches
        .get_many::<String>("crates")
        .into_iter()
        .flatten()
        .map(|c| (Some(c.clone()), None))
        .collect::<IndexMap<_, _>>();

    let mut infer_crate_name = false;

    for (crate_name, _) in crates.iter() {
        let crate_name = crate_name.as_ref().unwrap();

        if let Some(toolchain) = crate_name.strip_prefix("+") {
            anyhow::bail!(
                "invalid character `+` in dependency name: `+{toolchain}`
    Use `cargo +{toolchain} add` if you meant to use the `{toolchain}` toolchain."
            );
        }
    }

    if crates.is_empty() {
        if path.is_some() || git.is_some() {
            crates.insert(None, None);
            infer_crate_name = true;
        } else {
            unreachable!("clap should ensure we have some source selected");
        }
    }
    for feature in matches
        .get_many::<String>("features")
        .into_iter()
        .flatten()

View on GitHub (pinned to 0e07a15537)

Solutions

1. Move the toolchain before the subcommand: `cargo +<toolchain> add <crate>`.
2. If you did not mean a toolchain, remove the leading `+` from the dependency name.
3. Double-check shell quoting isn't prepending a stray `+`.

Example fix

// before
cargo add +nightly serde

// after
cargo +nightly add serde

// Normalize a toolchain+add invocation: ensure `+toolchain` precedes the subcommand
fn build_add_cmd(toolchain: Option<&str>, crate_name: &str) -> Vec<String> {
    let mut v: Vec<String> = vec!["cargo".into()];
    if let Some(tc) = toolchain.filter(|t| !t.is_empty()) {
        v.push(format!("+{tc}"));
    }
    v.push("add".into());
    assert!(!crate_name.starts_with('+'), "crate name must not start with +");
    v.push(crate_name.into());
    v
}

fn looks_like_toolchain_override(arg: &str) -> bool {
    arg.starts_with('+') && arg.len() > 1
}

When it happens

Trigger: Running `cargo add +nightly serde` instead of `cargo +nightly add serde`. Copying a toolchain-prefixed command and appending `add <crate>` in the wrong position.

Common situations: Misremembering argument order for toolchain overrides; shell scripts that concatenate `+toolchain` with the subcommand incorrectly; onboarding from a doc snippet with the wrong order.

Related errors

• invalid character `+` in package name: `+{toolchain}` Us
• argument for --color must be auto, always, or never, but fou
• feature `{feature}` must be qualified by the dependency it's
• feature `{feature}` is not allowed to use explicit `dep:` sy
• `{feature}` is unsupported when inferring the crate name, us

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/bbea9adecf0f7465.json. Report an issue: GitHub.