the `registry.index` config value is no longer…

Error message

the `registry.index` config value is no longer supported
Use `[source]` replacement to alter the default index for crates.io.

What it means

Cargo no longer supports the `registry.index` configuration value; check_registry_index_not_set fails fast whenever it is present. Users must instead use a `[source]` replacement table to redirect the default crates.io index. This is an intentional deprecation error to migrate users to the source-replacement mechanism.

Solutions

Delete the `registry.index` key from the config file named in the error.
If you need a different index, add a source replacement: [source.crates-io] replace-with = "my-mirror" plus a [source.my-mirror] registry = "..." section.
Re-run the cargo command to confirm the deprecation error is gone.

Example fix

// before: ~/.cargo/config.toml
[registry]
index = "https://my-mirror.example.com/registry"

// after: ~/.cargo/config.toml
[source.crates-io]
replace-with = "my-mirror"

[source.my-mirror]
registry = "https://my-mirror.example.com/registry"

Defensive patterns

Strategy: validation

Validate before calling

// grep all config scopes before running cargo
for f in /etc/cargo/config ~/.cargo/config.toml .cargo/config.toml; do
  grep -n 'index' "$f" 2>/dev/null | grep -i registry && echo "remove registry.index in $f"
done

Try / catch

match config.check_registry_index_not_set() {
    Err(e) if e.to_string().contains("registry.index") => {
        // migrate to [source] replacement, then retry
    }
    other => other?,
}

Prevention

Audit old config files when upgrading Cargo; `registry.index` was removed long ago.
Use [source.crates-io] replace-with for mirrors, never registry.index.
Pin CI images to a config-audited state so stale global config never leaks in.

When it happens

Trigger: Calling check_registry_index_not_set (invoked during registry-dependent operations like cargo publish/login) when any config source defines `registry.index` — e.g. a `[registry]` section containing `index = "..."`, or the corresponding `CARGO_REGISTRY_INDEX`-style environment config.

Common situations: Older pre-2018 .cargo/config files written for legacy Cargo carrying over to modern Cargo; corporate mirror setups copied from outdated docs; CI images with stale global cargo config.

Understand the failure class

Background: "is deprecated and will be removed" — deprecation warnings for old API names, keywords, and options, and how to migrate before the removal release — this error's family across 29 libraries.

Related errors

AI-assisted analysis of rust-lang/cargo@98a09e7e7d (2026-09-22). Data as JSON: /api/errors/2d535d5f6a412a52. Report an issue: GitHub.

Appendix: source

Thrown at src/context/mod.rs:1719

        if let Some(index) = self.get_string(["registries", registry, "index"])? {
            self.resolve_registry_index(&index).with_context(|| {
                format!(
                    "invalid index URL for registry `{}` defined in {}",
                    registry, index.definition
                )
            })
        } else {
            bail!(
                "registry index was not found in any configuration: `{}`",
                registry
            );
        }
    }

    /// Returns an error if `registry.index` is set.
    pub fn check_registry_index_not_set(&self) -> CargoResult<()> {
        if self.get_string(["registry", "index"])?.is_some() {
            bail!(
                "the `registry.index` config value is no longer supported\n\
                Use `[source]` replacement to alter the default index for crates.io."
            );
        }
        Ok(())
    }

    fn resolve_registry_index(&self, index: &Value<String>) -> CargoResult<Url> {
        // This handles relative file: URLs, relative to the config definition.
        let base = index
            .definition
            .root(self.cwd())
            .join("truncated-by-url_with_base");
        // Parse val to check it is a URL, not a relative path without a protocol.
        let _parsed = index.val.into_url()?;
        let url = index.val.into_url_with_base(Some(&*base))?;
        if url.password().is_some() {
            bail!("registry URLs may not contain passwords");

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