Use Bazel Central Registry mirrors

Bazel builds using bzlmod (the default since Bazel 8.0) depend on the Bazel Central Registry (BCR) and on the hosts that serve module source archives. When either is unavailable, as in the December 2025 BCR outage, builds that need to fetch modules fail. Public mirrors let you keep building during such an outage.

A BCR fetch downloads two kinds of files, and each needs its own mirror:

• Registry files: MODULE.bazel, source.json, and metadata.json for each module version, served by https://bcr.bazel.build. Mirror these with --registry.
• Source archives: the module source code that source.json points to, usually on GitHub releases or mirror.bazel.build. Mirror these with --module_mirrors.

Bazel verifies every download against hashes stored in the registry and pinned in MODULE.bazel.lock, so a mirror can't change the contents of your build.

Mirror the registry

The BCR is built from the bazelbuild/bazel-central-registry GitHub repository, so you can read registry files straight from GitHub:

.bazelrc

common --registry=https://raw.githubusercontent.com/bazelbuild/bazel-central-registry/main/

Bazel searches registries in the order you list them and uses later registries only for modules that earlier ones don't provide. Setting --registry also stops Bazel from using the BCR by default. If you use your own registry too, list it before the BCR mirror:

.bazelrc

common --registry=https://registry.example.com/
common --registry=https://raw.githubusercontent.com/bazelbuild/bazel-central-registry/main/

Bazel records registry file URLs in MODULE.bazel.lock. After you change --registry, run bazel mod deps --lockfile_mode=update and commit the updated lockfile; otherwise builds with --lockfile_mode=error fail.

Mirror module source archives

In Bazel 8.4 and later, --module_mirrors lists URL prefixes that Bazel tries before a module's own source URLs. Cloudflare runs a public mirror:

.bazelrc

common --module_mirrors=https://bcr.cloudflaremirrors.com

Bazel adds the original URL, minus its scheme, to the mirror prefix. For example, https://github.com/foo/bar/archive/v1.tar.gz becomes https://bcr.cloudflaremirrors.com/github.com/foo/bar/archive/v1.tar.gz. Pass several comma-separated URLs to try more than one mirror. In Bazel 8.6 and later, you can set mirrors for a single registry with --module_mirrors=<registry>=<mirror>[,<mirror>...].

The Cloudflare mirror serves source archives only. It doesn't mirror the registry files, so use it with the registry mirror above.

Mirror other downloads

Some rule sets download tools and other files from mirror.bazel.build outside of Bzlmod, so --module_mirrors doesn't apply to them. To redirect those downloads, add a rewrite rule to a downloader config file:

.bazel_downloader.cfg

rewrite mirror.bazel.build/.* bcr.cloudflaremirrors.com/$0

.bazelrc

common --downloader_config=.bazel_downloader.cfg

Cloudflare mirrors only part of mirror.bazel.build. For files it lacks, host copies in your own bucket and rewrite to that bucket instead.

To route all downloads through your EngFlow cluster, see Remote Asset API.

Verify the configuration

Fetch all dependencies with an empty output base and no repository cache, so that Bazel downloads every file again:

Bash

bazel --output_base="$(mktemp -d)" fetch --repository_cache= //...