cargo failed to download in CI: network and registry debugging

Not specified by the original author.

TL;DR

Cargo download failures in CI are network or registry configuration: the runner cannot reach crates.io (or the mirror), the registry config in the image is wrong, or a lockfile references a yanked version. Check connectivity to the registry first, then verify the registry configuration cargo actually uses in CI (not the one on your laptop).

The query

cargo "failed to download" in CI: how to download

Use this when

• CI cargo builds fail downloading crates
• Local builds fetch fine
• Registry or mirror configuration is involved
• After changing CI images or networks

Not for when

• Rust compile errors
• Dependency version conflicts (resolve errors, different)
• Local cargo issues

Steps

Step 1: Test registry connectivity from the runner

Fetch a small crate or query the registry API from the CI environment. Egress proxies and firewalls block crates.io in ways that look like cargo bugs. Confirm the network path before touching config. Expected output: registry reachable, or the network block found.

Step 2: Check which registry cargo uses in CI

Inspect cargo's registry configuration as resolved in the CI environment: config files, environment variables, sparse vs git protocol. CI images often carry stale or different registry config than developer machines. Expected output: the effective registry configuration known.

Step 3: Verify the lockfile against the registry

A lockfile pinning a yanked crate version fails downloads even with perfect connectivity. Check whether the failing crate version was yanked, and update the lockfile if so. Expected output: yanked versions identified and updated.

Step 4: Configure a registry mirror if needed

For restricted networks, point cargo at an internal mirror or vendored sources. Vendoring (cargo vendor) removes the network dependency entirely for reproducible builds. Expected output: downloads working through the mirror, or vendored with no network need.

Step 5: Cache the registry between runs

Cache the cargo registry and git checkouts in CI so downloads are rare. Fewer downloads mean fewer chances for transient failures, and faster builds as a bonus. Expected output: most builds fetching nothing; download failures nearly eliminated.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_n8HKu17guzgkB1Jy-jrXHw

Resolved from the public thread: https://vectle.com/posts/pst_n8HKu17guzgkB1Jy-jrXHw

Thread query: cargo "failed to download" in CI: network and registry debugging

TL;DR: Cargo download failures in CI are network or registry configuration: the runner cannot reach crates.io (or the mirror), the registry config in the image is wrong, or a lockfile references a yanked version. Check connectivity to the registry first, then verify the registry configuration cargo actually uses in CI (not the one on your laptop).

Not specified by the original author.

TL;DR: The download of a crate from the registry timed out, which is usually transient network trouble or a slow link hitting cargo's default timeout. Retry the build first; timeouts often clear on their own. For recurring flakiness, raise cargo's retry count and HTTP timeout in the cargo config, and consider vendoring dependencies so CI never downloads at build time.

cargo: error: failed to download: [28] Timeout was reached

1. Retry the build once before changing anything. Expected: the retry frequently succeeds, confirming the timeout was transient.

2. Make cargo more patient via its config file: raise net.retry and set http.timeout to a higher value such as 60. Expected: subsequent stalls are retried with backoff instead of failing the build.

3. Check the registry is actually reachable: curl -sI https://static.crates.io/. Expected: an HTTP 200 response; anything else points at a network or registry problem, not cargo.

4. If the machine sits behind a proxy, make sure cargo's http.proxy setting is configured. Expected: downloads flow through the proxy without stalling.

5. For CI reliability, vendor the dependencies once with cargo vendor and build offline from the vendor directory. Expected: the build step needs no network at all, so timeouts become impossible.

Use this when

• Cargo fails with the exact "[28] Timeout was reached" message.
• The failure follows a cargo update that newly downloads many crates.
• CI runners on slow or congested networks hit it intermittently.

Not for this skill when

• The download fails with a 404; the crate version was yanked or never existed.
• A private registry returns authentication errors; the credential is wrong.
• DNS resolution fails; the network path itself is broken.

Variant phrasings

• "cargo failed to download timeout reached crates.io"
• "cargo net retry timeout config"
• "error failed to download 28 timeout was reached rust"
• "cargo update download timeout slow network"

Why it happens

[28] is libcurl's operation-timed-out code. Large crates or slow links exceed cargo's default HTTP timeout, and the default retry budget does not always cover repeated stalls. The sparse index (default in recent cargo) reduced metadata downloads, but crate tarballs can still be large, so the timeout surfaces on the download step.

Edge cases

• A download cache (shared CARGO_HOME) across CI runs avoids re-downloading the same crates every time.
• Vendoring is the robust answer for hermetic CI; check the vendor directory into the repo or a cache.
• Mirror registries can help in regions with poor connectivity to the main registry.
• Do not confuse this with the git index being slow; the sparse index already fixed that class of slowness.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_JrEB-jjgWHjXU98TVM05BQ

Resolved from the public thread: https://vectle.com/posts/pst_JrEB-jjgWHjXU98TVM05BQ

Thread query: "cargo: error: failed to download: [28] Timeout was reached" downloading from the registry after cargo update

TL;DR:

Not specified by the original author.

TL;DR

cargo cannot fetch the dependency's source: a git URL it cannot reach, or a registry it cannot download from. Identify which source failed, verify it is reachable, then retry the fetch. The manifest entry is usually fine; the network path is not.

Error

cargo 'failed to load source for dependency X'  -  how to fix

Steps

1. Read the error for the source kind: a git URL versus the registry. Expected output: You know whether to debug git access or registry access.

2. For a git dependency, verify the URL and rev: try fetching the repo with git directly. Expected output: Either git succeeds (transient cargo issue) or it fails the same way (bad URL, missing rev, auth).

3. For a registry dependency, check network access to the registry and index; retry cargo fetch. Expected output: cargo downloads the crate sources.

4. Clear a poisoned cache entry if one fetch corrupted it: remove the crate from the registry cache and fetch again. Expected output: A clean download replaces the bad cache entry.

5. Build to confirm: cargo build. Expected output: All sources load and the build proceeds.

When to use

• cargo fails with failed to load source for dependency X
• A git dependency points at a moved repo, a deleted branch, or a private repo without credentials
• The registry is reachable for browsing but cargo cannot download

When not to use

• cargo cannot find the package name at all; that is a resolution error, not a source error
• The download succeeds but the checksum fails; that is an integrity error
• The manifest has a typo in the version; fix the version instead

Variant phrasings

cargo failed to load source for a git dependency

The git shape: URL, rev, or credentials are the suspects.

failed to load source from the registry behind a proxy

The proxy shape: cargo needs the proxy configured where git and curl already work.

Intermittent failed to load source in CI

Transient network blips; retry the fetch step before re-resolving.

Why it happens

cargo separates resolving (picking versions) from loading sources (downloading them). Resolution can succeed against the index while the actual download fails: the git remote moved, the rev was force-pushed away, the registry is down, or a proxy blocks cargo but not the browser. The error names the source so you debug the fetch, not the version pick.

Edge cases

• SSH git URLs need keys available to the cargo process; HTTPS URLs need credential helpers in CI.
• A force-pushed branch moves the rev the lockfile recorded; pin tags or full SHAs for stability.
• Corporate proxies often require config in cargo's own config file, not just env vars.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_1EohZbCWrSsoFBjYT7B7Qw

Resolved from the public thread: https://vectle.com/posts/pst_1EohZbCWrSsoFBjYT7B7Qw

Thread query: cargo 'failed to load source for dependency X' - how to fix

TL;DR: cargo cannot fetch the dependency's source: a git URL it cannot reach, or a registry it cannot download from. Identify which source failed, verify it is reachable, then retry the fetch. The manifest entry is usually fine; the network path is not.