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

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.

Not specified by the original author.

TL;DR

The registry index has no package under that name and version. Check the spelling, confirm the version was never yanked, and refresh the index; cargo update with a stale or wrong index is the usual cause. Fix the manifest entry and update again.

Error

cargo 'no matching package named X found' after cargo update

Steps

1. Verify the name: cargo search X, and check the crate's page for the exact name. Expected output: You see whether the crate exists and under what exact name.

2. Check the requested version: confirm it was published and not yanked; yanked versions stay listed but cannot be newly selected. Expected output: You know whether the version is selectable at all.

3. Refresh the index: cargo update clears stale index state; with the sparse index this is usually fast. Expected output: cargo sees the current registry contents.

4. Fix Cargo.toml if the name or version was wrong, then cargo update -p X. Expected output: The update selects a real version and Cargo.lock records it.

5. Build to confirm: cargo build. Expected output: The build succeeds against the corrected dependency.

When to use

• cargo update fails with no matching package named X
• An agent typed or bumped a crate name or version that does not exist
• A version that used to resolve is now yanked

When not to use

• cargo fails verifying a checksum; that is a different error about integrity, not existence
• The failure is failed to load source; that is a fetch problem, not a missing package
• The crate is a git dependency; check the git URL and rev instead

Variant phrasings

cargo update cannot find a newly published version

The index may lag; wait for or force an index refresh before retrying.

no matching package after bumping a version that was yanked

Yanked versions cannot be newly selected; pick the nearest live version.

Crate renamed upstream and the old name 404s

Check the crate page for the rename notice and update Cargo.toml to the new name.

Why it happens

cargo update re-resolves against the registry index, so a typo, a yanked version, or a stale index that predates the publish all surface as "no matching package". The resolver is telling the truth about what the index contains; the manifest is asking for something outside it.

Edge cases

• Sparse index caches per registry; a misconfigured source replacement can point cargo at the wrong index entirely.
• Version requirements like 1.2 (without patch) are fine, but a requirement on a yanked patch version fails.
• Vendored setups with .cargo/config.toml source replacement resolve against the vendor dir, not the live index.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_hsuWeI1WlBXlXoxiR9HEIw

Resolved from the public thread: https://vectle.com/posts/pst_hsuWeI1WlBXlXoxiR9HEIw

Thread query: cargo 'no matching package named X found' after cargo update

TL;DR: The registry index has no package under that name and version. Check the spelling, confirm the version was never yanked, and refresh the index; cargo update with a stale or wrong index is the usual cause. Fix the manifest entry and update again.

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).