> ## Documentation Index
> Fetch the complete documentation index at: https://site.aspect.build/llms.txt
> Use this file to discover all available pages before exploring further.

# Optimizing your Bazel build

> Make CI builds faster and cheaper: download less with Build without the Bytes, and find the non-determinism that lowers your cache hit rate.

Try these when CI builds are slower or more expensive than they should be. They're independent: start with whichever matches what you see. For the CI runners themselves, see [Optimizing CI runners](/docs/aspect-workflows/enterprise/guides/optimizing-ci-runners).

## Enable Build without the Bytes (BWOB)

Bazel 7 defaults to `--remote_download_toplevel`. On CI, `--remote_download_minimal` downloads less; set it under `build:ci` on any version. [Build without the Bytes](/docs/aspect-workflows/platform/features/remote-cache#build-without-the-bytes) has the flag and where to put it.

Then turn `--config=ci` on for CI: pass `--config=ci` in the job's `bazel` calls, or add it automatically for Aspect CLI tasks in `.aspect/config.axl`:

```python title=".aspect/config.axl" theme={null}
load("@aspect//traits.axl", "BazelTrait")

def config(ctx: ConfigContext):
    is_ci = bool(ctx.std.env.var("CI"))
    if is_ci:
        ctx.traits[BazelTrait].extra_flags.extend([
            "--config=ci",
        ])
```

## Diagnose non-determinism and improve cache hit rate

A cache hit rate below 95% usually signals non-determinism: an action produces different outputs from the same inputs, so the actions that depend on it miss the cache.

1. Check the cache hit rate on the build's **Cache** tab in the Build Results UI. Target above 95%. The same tab lists the misses and their reasons. On a self-hosted deployment, the Grafana dashboards also chart the hit rate across builds.
2. Run [`aspect cache diff`](/docs/cli/tasks/cache_diff) on a commit CI has already built. It lists the actions that missed the cache and the tests they affect, without running any tests.
3. To find what makes a missed action non-deterministic, collect two execution logs on the same commit:
   * Pass `--execution_log_compact_file=/tmp/exec.log` to your `aspect build` or `aspect test` invocation.
   * For the second run, ensure a different runner handles the job: stop all runners and wait for the runner group to scale to zero (hosted by Aspect, ask Aspect to recycle the group), or retry from a previous day's commit to eliminate legitimate source-change misses.
   * Download the resulting log artifacts from each CI run.
4. Compare the two logs using Bazel's guide to [debugging remote cache hits](https://bazel.build/remote/cache-remote) to identify the non-deterministic actions.

<Tip>
  Execution log files are often large. Use a commit with no source changes for the second run to eliminate legitimate cache misses as a variable.
</Tip>