Index Format

The following defines the format of the index. New features are occasionally added, which are only understood starting with the version of Cargo that introduced them. Older versions of Cargo may not be able to use packages that make use of new features. However, the format for older packages should not change, so older versions of Cargo should be able to use them.

Index Configuration

The root of the index contains a file named config.json which contains JSON information used by Cargo for accessing the registry. This is an example of what the config file looks like:

    "dl": "",
    "api": ""

The keys are:

  • dl: This is the URL for downloading crates listed in the index. The value may have the following markers which will be replaced with their corresponding value:

    • {crate}: The name of crate.
    • {version}: The crate version.
    • {prefix}: A directory prefix computed from the crate name. For example, a crate named cargo has a prefix of ca/rg. See below for details.
    • {lowerprefix}: Lowercase variant of {prefix}.
    • {sha256-checksum}: The crate’s sha256 checksum.

    If none of the markers are present, then the value /{crate}/{version}/download is appended to the end.

  • api: This is the base URL for the web API. This key is optional, but if it is not specified, commands such as cargo publish will not work. The web API is described below.

  • auth-required: indicates whether this is a private registry that requires all operations to be authenticated including API requests, crate downloads and sparse index updates.

Download Endpoint

The download endpoint should send the .crate file for the requested package. Cargo supports https, http, and file URLs, HTTP redirects, HTTP1 and HTTP2. The exact specifics of TLS support depend on the platform that Cargo is running on, the version of Cargo, and how it was compiled.

If auth-required: true is set in config.json, the Authorization header will be included with http(s) download requests.

Index files

The rest of the index repository contains one file for each package, where the filename is the name of the package in lowercase. Each version of the package has a separate line in the file. The files are organized in a tier of directories:

  • Packages with 1 character names are placed in a directory named 1.
  • Packages with 2 character names are placed in a directory named 2.
  • Packages with 3 character names are placed in the directory 3/{first-character} where {first-character} is the first character of the package name.
  • All other packages are stored in directories named {first-two}/{second-two} where the top directory is the first two characters of the package name, and the next subdirectory is the third and fourth characters of the package name. For example, cargo would be stored in a file named ca/rg/cargo.

Note: Although the index filenames are in lowercase, the fields that contain package names in Cargo.toml and the index JSON data are case-sensitive and may contain upper and lower case characters.

The directory name above is calculated based on the package name converted to lowercase; it is represented by the marker {lowerprefix}. When the original package name is used without case conversion, the resulting directory name is represented by the marker {prefix}. For example, the package MyCrate would have a {prefix} of My/Cr and a {lowerprefix} of my/cr. In general, using {prefix} is recommended over {lowerprefix}, but there are pros and cons to each choice. Using {prefix} on case-insensitive filesystems results in (harmless-but-inelegant) directory aliasing. For example, crate and CrateTwo have {prefix} values of cr/at and Cr/at; these are distinct on Unix machines but alias to the same directory on Windows. Using directories with normalized case avoids aliasing, but on case-sensitive filesystems it’s harder to support older versions of Cargo that lack {prefix}/{lowerprefix}. For example, nginx rewrite rules can easily construct {prefix} but can’t perform case-conversion to construct {lowerprefix}.

Name restrictions

Registries should consider enforcing limitations on package names added to their index. Cargo itself allows names with any alphanumeric, -, or _ characters. imposes its own limitations, including the following:

  • Only allows ASCII characters.
  • Only alphanumeric, -, and _ characters.
  • First character must be alphabetic.
  • Case-insensitive collision detection.
  • Prevent differences of - vs _.
  • Under a specific length (max 64).
  • Rejects reserved names, such as Windows special filenames like “nul”.

Registries should consider incorporating similar restrictions, and consider the security implications, such as IDN homograph attacks and other concerns in UTR36 and UTS39.

Version uniqueness

Indexes must ensure that each version only appears once for each package. This includes ignoring SemVer build metadata. For example, the index must not contain two entries with a version 1.0.7 and 1.0.7+extra.

JSON schema

Each line in a package file contains a JSON object that describes a published version of the package. The following is a pretty-printed example with comments explaining the format of the entry.

    // The name of the package.
    // This must only contain alphanumeric, `-`, or `_` characters.
    "name": "foo",
    // The version of the package this row is describing.
    // This must be a valid version number according to the Semantic
    // Versioning 2.0.0 spec at
    "vers": "0.1.0",
    // Array of direct dependencies of the package.
    "deps": [
            // Name of the dependency.
            // If the dependency is renamed from the original package name,
            // this is the new name. The original package name is stored in
            // the `package` field.
            "name": "rand",
            // The SemVer requirement for this dependency.
            // This must be a valid version requirement defined at
            "req": "^0.6",
            // Array of features (as strings) enabled for this dependency.
            "features": ["i128_support"],
            // Boolean of whether or not this is an optional dependency.
            "optional": false,
            // Boolean of whether or not default features are enabled.
            "default_features": true,
            // The target platform for the dependency.
            // null if not a target dependency.
            // Otherwise, a string such as "cfg(windows)".
            "target": null,
            // The dependency kind.
            // "dev", "build", or "normal".
            // Note: this is a required field, but a small number of entries
            // exist in the index with either a missing or null
            // `kind` field due to implementation bugs.
            "kind": "normal",
            // The URL of the index of the registry where this dependency is
            // from as a string. If not specified or null, it is assumed the
            // dependency is in the current registry.
            "registry": null,
            // If the dependency is renamed, this is a string of the actual
            // package name. If not specified or null, this dependency is not
            // renamed.
            "package": null,
    // A SHA256 checksum of the `.crate` file.
    "cksum": "d867001db0e2b6e0496f9fac96930e2d42233ecd3ca0413e0753d4c7695d289c",
    // Set of features defined for the package.
    // Each feature maps to an array of features or dependencies it enables.
    "features": {
        "extras": ["rand/simd_support"]
    // Boolean of whether or not this version has been yanked.
    "yanked": false,
    // The `links` string value from the package's manifest, or null if not
    // specified. This field is optional and defaults to null.
    "links": null,
    // An unsigned 32-bit integer value indicating the schema version of this
    // entry.
    // If this not specified, it should be interpreted as the default of 1.
    // Cargo (starting with version 1.51) will ignore versions it does not
    // recognize. This provides a method to safely introduce changes to index
    // entries and allow older versions of cargo to ignore newer entries it
    // doesn't understand. Versions older than 1.51 ignore this field, and
    // thus may misinterpret the meaning of the index entry.
    // The current values are:
    // * 1: The schema as documented here, not including newer additions.
    //      This is honored in Rust version 1.51 and newer.
    // * 2: The addition of the `features2` field.
    //      This is honored in Rust version 1.60 and newer.
    "v": 2,
    // This optional field contains features with new, extended syntax.
    // Specifically, namespaced features (`dep:`) and weak dependencies
    // (`pkg?/feat`).
    // This is separated from `features` because versions older than 1.19
    // will fail to load due to not being able to parse the new syntax, even
    // with a `Cargo.lock` file.
    // Cargo will merge any values listed here with the "features" field.
    // If this field is included, the "v" field should be set to at least 2.
    // Registries are not required to use this field for extended feature
    // syntax, they are allowed to include those in the "features" field.
    // Using this is only necessary if the registry wants to support cargo
    // versions older than 1.19, which in practice is only since
    // those older versions do not support other registries.
    "features2": {
        "serde": ["dep:serde", "chrono?/serde"]
    // The minimal supported Rust version (optional)
    // This must be a valid version requirement without an operator (e.g. no `=`)
    "rust_version": "1.60"

The JSON objects should not be modified after they are added except for the yanked field whose value may change at any time.

Note: The index JSON format has subtle differences from the JSON format of the Publish API and cargo metadata. If you are using one of those as a source to generate index entries, you are encouraged to carefully inspect the documentation differences between them.

For the Publish API, the differences are:

  • deps
    • name — When the dependency is renamed in Cargo.toml, the publish API puts the original package name in the name field and the aliased name in the explicit_name_in_toml field. The index places the aliased name in the name field, and the original package name in the package field.
    • req — The Publish API field is called version_req.
  • cksum — The publish API does not specify the checksum, it must be computed by the registry before adding to the index.
  • features — Some features may be placed in the features2 field. Note: This is only a legacy requirement for; other registries should not need to bother with modifying the features map. The v field indicates the presence of the features2 field.
  • The publish API includes several other fields, such as description and readme, which don’t appear in the index. These are intended to make it easier for a registry to obtain the metadata about the crate to display on a website without needing to extract and parse the .crate file. This additional information is typically added to a database on the registry server.
  • Although rust_version is included here, will ignore this field and instead read it from the Cargo.toml contained in the .crate file.

For cargo metadata, the differences are:

  • vers — The cargo metadata field is called version.
  • deps
    • name — When the dependency is renamed in Cargo.toml, cargo metadata puts the original package name in the name field and the aliased name in the rename field. The index places the aliased name in the name field, and the original package name in the package field.
    • default_features — The cargo metadata field is called uses_default_features.
    • registrycargo metadata uses a value of null to indicate that the dependency comes from The index uses a value of null to indicate that the dependency comes from the same registry as the index. When creating an index entry, a registry other than should translate a value of null to be and translate a URL that matches the current index to be null.
    • cargo metadata includes some extra fields, such as source and path.
  • The index includes additional fields such as yanked, cksum, and v.

Index Protocols

Cargo supports two remote registry protocols: git and sparse. The git protocol stores index files in a git repository and the sparse protocol fetches individual files over HTTP.

Git Protocol

The git protocol has no protocol prefix in the index url. For example the git index URL for is

Cargo caches the git repository on disk so that it can efficiently incrementally fetch updates.

Sparse Protocol

The sparse protocol uses the sparse+ protocol prefix in the registry URL. For example, the sparse index URL for is sparse+

The sparse protocol downloads each index file using an individual HTTP request. Since this results in a large number of small HTTP requests, performance is significantly improved with a server that supports pipelining and HTTP/2.

Sparse authentication

Cargo will attempt to fetch the config.json file before fetching any other files. If the server responds with an HTTP 401, then Cargo will assume that the registry requires authentication and re-attempt the request for config.json with the authentication token included.

On authentication failure (or a missing authentication token) the server may include a www-authenticate header with a Cargo login_url="<URL>" challenge to indicate where the user can go to get a token.

Registries that require authentication must set auth-required: true in config.json.


Cargo caches the crate metadata files, and captures the ETag or Last-Modified HTTP header from the server for each entry. When refreshing crate metadata, Cargo sends the If-None-Match or If-Modified-Since header to allow the server to respond with HTTP 304 “Not Modified” if the local cache is valid, saving time and bandwidth. If both ETag and Last-Modified headers are present, Cargo uses the ETag only.

Cache Invalidation

If a registry is using some kind of CDN or proxy which caches access to the index files, then it is recommended that registries implement some form of cache invalidation when the files are updated. If these caches are not updated, then users may not be able to access new crates until the cache is cleared.

Nonexistent Crates

For crates that do not exist, the registry should respond with a 404 “Not Found”, 410 “Gone” or 451 “Unavailable For Legal Reasons” code.

Sparse Limitations

Since the URL of the registry is stored in the lockfile, it’s not recommended to offer a registry with both protocols. Discussion about a transition plan is ongoing in issue #10964. The registry is an exception, since Cargo internally substitutes the equivalent git URL when the sparse protocol is used.

If a registry does offer both protocols, it’s currently recommended to choose one protocol as the canonical protocol and use source replacement for the other protocol.