.bazelrc flags you should enable

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Alex Eagle
·Apr 28, 2022·

2 min read

I suggest you add these lines to your .bazelrc file as early in your project as possible. Add one at a time and let the dust settle, as they can be breaking.

See a complete .bazelrc file here: https://github.com/aspect-build/bazel-examples/blob/main/bazelrc/.bazelrc

  • build --sandbox_default_allow_network=false: ensure that you don't accidentally make non-hermetic actions/tests which depend on remote services. Tag an individual target with tags=["requires-network"] to opt-out of the enforcement.

  • test --incompatible_exclusive_test_sandboxed: fix a bug where Bazel didn't enable sandboxing for tests with tags=["exclusive"]

  • build --incompatible_strict_action_env: don't let environment variables like $PATH sneak into the build, which can cause massive cache misses when they change.

  • build --nolegacy_external_runfiles: improve performance of sandbox by skipping the older my_repo/external/other_repo symlinks. Note, some rules may fail under this flag, please file issues with the rule author.

  • startup --host_jvm_args=-DBAZEL_TRACK_SOURCE_DIRECTORIES=1: ensure that the Bazel server notices when a directory changes, if you have a directory listed in the srcs of some target.

  • build --experimental_remote_merkle_tree_cache --experimental_remote_merkle_tree_cache_size=[XX]: Improve remote cache checking speed by memorizing merkle tree calculations, and tweak the amount of memory allocated to it

  • build --remote_local_fallback: If the grpc remote cache connection fails, it will fail the build, add this so it falls back to the local cache.

  • build --heap_dump_on_oom: helps you debug when Bazel runs out of memory

  • build --incompatible_remote_results_ignore_disk: If you have both --noremote_upload_local_results and --disk_cache, then this fixes a bug where Bazel doesn't write to the local disk cache as it treats as a remote cache.

  • build --incompatible_default_to_explicit_init_py: fix the wrong default that comes from Google's internal monorepo by using __init__.py to delimit a Python package

  • build --noexperimental_check_output_files --noexperimental_check_external_repository_files: Speed up all builds by not checking if output files have been modified. Note, the second one is only in Bazel 6.0 nightlies at present.

  • test --test_verbose_timeout_warnings: Bazel's default for test_timeout is medium (5 min), but most tests should instead be short (1 min).

  • build --incompatible_remote_build_event_upload_respect_no_cache: Don't upload artifacts referenced in the BEP if the action can't be cached remotely.

As the very last line in .bazelrc:

  • try-import %workspace%/.bazelrc.user: allow developers to add repo-specific overrides in their own personal .bazelrc.user file. Make sure this is git-ignored.


  • build --experimental_inprocess_symlink_creation: allows spaces in filenames which are inputs to actions.

  • test --build_tests_only: change the behavior of bazel test to not bother building targets which aren't dependencies of the tests. Matches some developer expectations.


These flags are not a good idea:

  • --workspace_status_command: this should only be enabled for release builds where stamping is desirable, otherwise you spend time running git commands on every build.
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