> ## Documentation Index
> Fetch the complete documentation index at: https://docs.crust.moumen.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Build identity

> Why a git SHA is not enough to key a performance history on.

A SHA alone gives false continuity. Bundle sizes move with lockfiles, Next upgrades, Node versions
and config edits **at the same commit**.

So a build id hashes everything that can change the numbers:

```js theme={null}
buildId = sha256([
  gitSha,
  dirtyDiffHash,   // '' when the tree is clean
  lockfileHash,
  nextVersion,
  nodeMajor,
  bundler,         // 'turbopack' | 'webpack'
  configHash,      // the build's own resolved config
]).slice(0, 16)
```

<Note>
  The dirty-tree component matters most in practice. Most measuring happens on uncommitted work, and
  keying on the SHA alone makes ten experiments overwrite each other under one identity.
</Note>

Outside a git repository, a content fingerprint of the emitted chunk filenames stands in - those are
content-hashed by both bundlers, so it changes exactly when the output does.

## Run identity

A build can be measured many times. Each [synthetic run](/docs/guides/staging) gets a sortable id and an
environment fingerprint:

```text theme={null}
run.env = { machine, cpuCount, throttle, network, coldStart, browserVersion }
```

<Warning>
  Runs with different environment fingerprints must never be merged into one trend line. A laptop on
  battery versus CI looks exactly like a regression that is not one.
</Warning>

## Route identity

Trends are keyed on the **page file path**, not the URL pattern:

```json theme={null}
{ "id": "app/products/[slug]/page.tsx", "pattern": "/products/[slug]" }
```

Patterns change during refactors, and keying on them would silently restart the history. When a
file itself moves, [`.perf/aliases.json`](/docs/reference/files#aliases-json) stitches it back.

## Orphaned history

Squash merges destroy the pre-squash SHAs, so ancestry lookup finds nothing. Every snapshot also
stores a `sourceSignature` - a hash of the route table and module set - so an orphaned baseline can
be re-linked by content.