Cargo and crates.io were developed in the rush leading up to the Rust 1.0 release to fill the needs for a tool to manage dependencies and a registry that people could use to share code. This rapid work resulted in these tools being connected with an API that initially didn't return the correct HTTP response status codes. After the Rust 1.0 release, Rust's stability guarantees around backward compatibility made this non-trivial to fix, as we wanted older versions of Cargo to continue working with the current crates.io API.

When an old version of Cargo receives a non-"200 OK" response, it displays the raw JSON body like this:

error: failed to get a 200 OK response, got 400
headers:
    HTTP/1.1 400 Bad Request
    Content-Type: application/json; charset=utf-8
    Content-Length: 171

body:
{"errors":[{"detail":"missing or empty metadata fields: description, license. Please see https://doc.rust-lang.org/cargo/reference/manifest.html for how to upload metadata"}]}

This was improved in pull request #6771, which was released in Cargo 1.34 (mid-2019). Since then, Cargo has supported receiving 4xx and 5xx status codes too and extracts the error message from the JSON response, if available.

On 2024-03-04 we will switch the API from returning "200 OK" status codes for errors to the new 4xx/5xx behavior. Cargo 1.33 and below will keep working after this change, but will show the raw JSON body instead of a nicely formatted error message. We feel confident that this degraded error message display will not affect very many users. According to the crates.io request logs only very few requests are made by Cargo 1.33 and older versions.

This is the list of API endpoints that will be affected by this change:

• GET /api/v1/crates
• PUT /api/v1/crates/new
• PUT /api/v1/crates/:crate/:version/yank
• DELETE /api/v1/crates/:crate/:version/unyank
• GET /api/v1/crates/:crate/owners
• PUT /api/v1/crates/:crate/owners
• DELETE /api/v1/crates/:crate/owners

All other endpoints have already been using regular HTTP status codes for some time.

If you are still using Cargo 1.33 or older, we recommend upgrading to a newer version to get the improved error messages and all the other nice things that the Cargo team has built since then.