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Before reading every hunk in a change generated by a free or local coding model, run a short Git diff autopsy. The sequence below records which paths changed or disappeared, how many lines each path gained or lost, and where the work is concentrated in the repository:
git diff --summary BASE_REF TARGET_REF
git diff --name-status BASE_REF TARGET_REF
git diff --numstat BASE_REF TARGET_REF
git diff --shortstat BASE_REF TARGET_REF
git diff --dirstat=changes,0 BASE_REF TARGET_REFThese reports are triage signals, not a safety score. A deleted file can be an intentional migration, a replacement, or an error; a large line count can be generated code or a legitimate redesign. Use the output to choose what to inspect first, then read the relevant hunks and run the project’s normal validation.
What a diff autopsy can—and cannot—tell you
A diff autopsy is a structured pre-review pass over one comparison: two commits, two branches, or a revision and the current working tree. It answers three narrow questions:
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- Path status: Which files are new, modified, deleted, renamed, or copied?
- Churn: How many lines were added and removed per path, and what are the totals?
- Repository shape: Which directories contain most of the reported change?
Git’s official git-diff documentation defines these output formats. None of them establishes that a change is correct, that a defect exists, that tests pass, or that a particular model produced the code. Treat the reports as a map for review rather than a verdict on code quality or model quality.
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Set and record the comparison scope
Every number is meaningful only for the exact state being compared. Write the two revision names, or state that one side is the index or working tree, at the top of the worksheet. Use stable placeholders such as BASE_REF and TARGET_REF in notes so another reviewer can reproduce the report.
Comparing two revisions
git diff --summary BASE_REF TARGET_REFUse the same pair of references for every subsequent command. Do not mix a branch comparison with an unstaged working-tree comparison; their path lists and counts describe different snapshots.
Checking an in-progress change
git diff reports unstaged changes against the index. git diff --cached reports what is staged against the current commit. If the generated change is partly staged and partly unstaged, capture both reports and label them separately. A summary that covers only one side can hide files waiting in the other state.
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- Base and target commit IDs or branch names.
- Whether the comparison is staged, unstaged, or between commits.
- The date and reviewer name.
- Any pathspec or exclusion used to narrow the diff.
- Whether generated, vendor, or build directories were intentionally included.
This metadata prevents a later reviewer from treating a partial or stale report as the complete change set.
Capture path status before reading hunks
Use --summary for structural events
git diff --summary BASE_REF TARGET_REFGit describes --summary as a condensed report of newly added, deleted, renamed, and copied files. It is the quickest way to notice that a path vanished or that Git recognized a rename or copy. Mark each deletion in the worksheet for explicit review; do not infer intent from the status alone.
Use --name-status for a path-by-path list
git diff --name-status BASE_REF TARGET_REFThis form pairs each changed name with a status. It is convenient for a checklist because every path occupies a separate record. Keep the exact path, status, and any rename or copy information. When a path is reported as renamed, inspect both its old and new locations; a summary can tell you that the path moved, but not whether the resulting implementation still satisfies its callers.
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Deletion review checklist
- Is the deleted path explicitly removed by the requested task?
- Was a replacement file or API added, and do references point to it?
- Does the deletion remove tests, configuration, migrations, documentation, or generated assets that have different release implications?
- Is the old path expected to remain for compatibility with another package, platform, or deployment script?
- Does the commit message or issue explain the removal?
A deletion is a review prompt whose meaning depends on the repository and task. It is not evidence of a bug by itself.
Measure line churn at two levels
Per-path counts with --numstat
git diff --numstat BASE_REF TARGET_REF--numstat emits added and deleted line counts for each path in a machine-oriented form. Keep the two counts separately; additions and deletions answer different questions. A file with 500 additions and 500 deletions has very different review implications from one with 1,000 additions and no deletions, even though both involve 1,000 changed lines by a simple sum.
Binary files are represented with - counts rather than line numbers. Record that marker as “binary” instead of converting it to zero. A binary replacement needs an asset-specific review, because line arithmetic does not describe its size or semantic impact.
Aggregate totals with --shortstat
git diff --shortstat BASE_REF TARGET_REF--shortstat prints the final totals line from the stat output: the number of changed files and the aggregate insertions and deletions. Use it as the headline volume for the chosen comparison, while retaining --numstat to find the paths responsible for that volume.
Use ratios as prompts, not universal thresholds
You can calculate project-specific indicators such as deletion-to-addition ratio, the share of churn in one directory, or the number of files above an agreed review size. Git’s documentation supplies output formats, not a universal boundary for “suspicious” churn. If a team adopts a threshold, document the repository, file types, and comparison scope that make it meaningful. Do not present a locally chosen cutoff as a general rule for free-model changes.
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git diff --dirstat=changes,0 BASE_REF TARGET_REF--dirstat reports the distribution of relative change across subdirectories. The mode determines what “change” means:
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| Mode | What it counts | When it helps |
|---|---|---|
changes |
Removed and added source lines, while discounting pure code movement | Finding directories with substantive edits rather than simple rearrangement |
lines |
The regular line-based diff analysis | Measuring line churn when movement should remain visible in the distribution |
files |
Changed files, distributed by file count | Seeing whether a broad set of files was touched even when individual files are small |
The second value in changes,0 asks Git to include directories without applying a percentage cutoff. If you omit that value, the default cutoff can hide small contributors. Always record the selected mode because two dirstat reports with different modes answer different questions.
Read directory concentration carefully
A high percentage in one directory means that directory accounts for much of the selected statistic; it does not mean that directory is defective. A concentrated change may be expected when a task targets one subsystem. Conversely, a small percentage can still contain a critical configuration or security-sensitive file. Use dirstat to set review order, not to skip low-volume paths.
Build the worksheet
Keep one row per signal and link it to the raw command output. The following layout is compact enough for a pull request comment and detailed enough for a handoff:
| Step | Command | Record | Review question |
|---|---|---|---|
| Scope | git diff BASE_REF TARGET_REF |
Exact refs or tree states | Does this cover the change that was requested? |
| Path summary | git diff --summary BASE_REF TARGET_REF |
Added, deleted, renamed, copied paths | Which structural events need explanation? |
| Name/status | git diff --name-status BASE_REF TARGET_REF |
Status and path for every entry | Are any expected files absent or unexpected files present? |
| Per-file churn | git diff --numstat BASE_REF TARGET_REF |
Additions, deletions, binary markers | Which files dominate review effort? |
| Total churn | git diff --shortstat BASE_REF TARGET_REF |
Files changed and aggregate counts | Is the overall volume plausible for the task? |
| Directory shape | git diff --dirstat=changes,0 BASE_REF TARGET_REF |
Percent by directory and mode | Where should review begin, and what might be outside scope? |
Attach the raw output or save it with the review record. A copied number without its comparison scope, mode, and date cannot be reproduced reliably.
Triangulate the signals before opening individual hunks
The value comes from combinations, not isolated numbers. Use patterns such as these to decide where to look first:
| Observed pattern | Reasonable next action | What not to conclude |
|---|---|---|
| Deleted paths plus a large deletion count | Check whether the task calls for removal, then trace references and replacement files. | That the generated code is broken. |
| Many additions concentrated in a test or fixture directory | Confirm that fixtures are intentional and that the production code they exercise changed as expected. | That tests guarantee correctness. |
High files dirstat spread but modest line totals |
Review integration points and configuration touched across many directories. | That the change is low risk because each file is small. |
High changes concentration but a different lines distribution |
Investigate whether code movement or formatting accounts for the difference. | That one mode is “the true” repository shape. |
Binary markers in --numstat |
Review the asset diff, checksum, generation step, and packaging impact separately. | That the binary change has no line-level impact. |
Move from summaries to the relevant hunks
After marking the paths and directories that need attention, inspect the actual diff for those paths. Use a path-limited command so the review remains focused:
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git diff BASE_REF TARGET_REF -- path/to/fileFor a deletion, read the final version’s callers and configuration references as well as the removed file. For a rename, inspect both sides and search for imports, routes, build entries, and documentation that still use the old name. For a high-churn file, separate mechanical formatting or generated output from behavioral edits before deciding how deeply to review each hunk.
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Apply the method to free or local code-generation models
The workflow is useful when a change came from a free or locally hosted model because it limits the first pass to observable repository effects. You do not need to trust a model label or infer how the code was produced to ask whether the change matches the task.
- Freeze the scope. Save the commit or working-tree state that contains the generated output and record the intended task in one sentence.
- Run the path reports. Flag deletions, unexpected new directories, and renames before reading implementation details.
- Check volume against the request. A one-function request that changes dozens of unrelated directories deserves explanation, while a migration request may legitimately produce broad churn.
- Inspect concentration. Compare
changes,lines, andfilesdirstat views when formatting, moves, or generated assets make one view misleading. - Review behavior and validate. Read the hunks selected by the worksheet and run the project’s normal checks. Record failures separately from the diff statistics.
Do not claim that a summary proves a model is safe, unsafe, accurate, or responsible for a defect. The commands identify what changed in Git; they do not identify the authoring system or establish causation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common failure modes and recovery
The comparison range is wrong
Symptom: The report is unexpectedly tiny or includes months of unrelated work. Recovery: Reconfirm the base and target commits, rerun every command with the same pair, and replace the old worksheet rather than mixing totals.
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Untracked output is missing
Symptom: A generated file exists on disk but no path report mentions it. Recovery: Check the working-tree and index state, then include the file in a deliberate comparison after it is tracked or staged. Do not assume an absent path means the model did not create it.
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Renames obscure additions and deletions
Symptom: A file appears as renamed even though its contents changed substantially, or a move is shown as separate add/delete events. Recovery: Inspect the old and new paths and the hunk content. Treat status classification as a navigation aid, not a semantic judgment.
Binary counts look like zero
Symptom: A dash in --numstat is copied into a spreadsheet as zero. Recovery: Preserve the dash as a binary marker and route the asset to an appropriate review.
Generated or vendor files dominate churn
Symptom: The aggregate count is huge but hand-written code barely changed. Recovery: Keep the complete report, label generated paths, and create a second, clearly scoped view if the project’s policy permits excluding them. Never present the filtered view as the full diff.
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Symptom: Reviewers disagree about which directory received most of the change. Recovery: Check whether each person used changes, lines, or files, and compare like with like.
Set project-specific review rules
A team can turn the worksheet into a lightweight policy without pretending that Git supplies universal risk thresholds. Examples include:
- Require an explanation for every deleted production path.
- Ask for a second reviewer when aggregate churn exceeds the team’s documented norm for that task type.
- Require directory owners to review changes concentrated in their subtree.
- Separate generated artifacts from source changes in the review record.
- Store the exact commands, refs, and dirstat mode with the pull request.
Document the context for each rule: repository, language, generated-file policy, and task category. A threshold that makes sense for a small configuration patch may be useless for a generated client refresh. The preliminary repository-mining discussion in Linking Code and Documentation Churn: Preliminary Analysis likewise should not be turned into a universal numerical rule; its framing does not establish a cutoff for suspicious changes.
Printable pre-review checklist
- ☐ Base and target states are recorded.
- ☐ The comparison includes the intended staged, unstaged, or committed changes.
- ☐
--summaryoutput was captured. - ☐
--name-statuspaths and statuses were recorded. - ☐ Every deletion has a task-based explanation or an explicit review owner.
- ☐
--numstatadditions, deletions, and binary markers were preserved. - ☐
--shortstattotals match the same comparison scope. - ☐ Dirstat mode was recorded and directory concentration was noted.
- ☐ Generated, vendor, and build paths were identified rather than silently omitted.
- ☐ Relevant hunks were inspected after the summary pass.
- ☐ The project’s normal validation was run, and failures were recorded independently of churn.
Used consistently, this sheet makes the first minutes of review more deliberate: it exposes deleted paths, quantifies where the work sits, and gives reviewers a reproducible route into the hunks that matter. It does not replace engineering judgment, tests, or a careful reading of behavior.
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