Diff Computation Protocol

Delivery: this file is delivered via the explicit #file: import in code-review.agent.md, not via applyTo. Plugin and extension distributions strip the .github/ prefix, so an applyTo glob targeting .github/... would match nothing once distributed. Future coding-standards agents or prompts that need this guidance must import it with #file: rather than relying on applyTo.

Obtain the diff before reading any source files. Use the decision tree below to determine the appropriate method, then apply scope rules and large diff handling.

Decision Tree

Run git branch --show-current and git status --short to determine context. Match the first applicable case:

1. Branch review (user explicitly requests a specific branch or PR, or is on a feature branch that is not main and not detached HEAD): follow the Feature Branch Diff section. The pr-reference skill captures committed changes; a working-tree supplement captures staged, unstaged, and untracked changes. If the user says "review the PR" while on main or detached HEAD without naming a branch, PR, or commit, ask which branch or PR they want reviewed, then route to this case or to the Specific Commit section as appropriate.
2. Local uncommitted changes on main or detached HEAD (not on a feature branch, but edits exist that are not yet committed): follow the Uncommitted Changes section.
3. Selected code or #file references (user selects code in the editor or references #file:path/to/file.ext): follow the Selected Code section.
4. Specific commit review (user asks to review a particular commit): follow the Specific Commit section.
5. No reviewable content: inform the user that no diff could be determined and stop.

Feature Branch Diff

Invoke the pr-reference skill to compute the diff. The skill handles branch detection, merge-base resolution, file listing, non-source exclusions, and large diff chunking.

1. Generate the structured diff to an explicit output path so that path is a single source of truth the review agent reuses for diffPatchPath (overridable, not implicitly coupled to the skill default):

   generate.sh --base-branch auto --merge-base --exclude-ext min.js,min.css,map --output .copilot-tracking/pr/pr-reference.xml
   

   generate.ps1 -BaseBranch auto -MergeBase -ExcludeExt min.js,min.css,map -OutputPath .copilot-tracking/pr/pr-reference.xml
   

2. Get the changed file list:

   list-changed-files.sh --exclude-type deleted --format plain
   

   list-changed-files.ps1 -ExcludeType Deleted -Format Plain
   

3. For large diffs, use chunk planning and batched analysis:

   read-diff.sh --info       # chunk count and size summary
   read-diff.sh --chunk N    # read chunk N
   

   read-diff.ps1 -Info        # chunk count and size summary
   read-diff.ps1 -Chunk N     # read chunk N
   

If the changed-file list (list-changed-files.sh or list-changed-files.ps1) returns an empty list, stop and report "no reviewable content" per Decision Tree case 5.

Pass the diff output and file list as pre-computed input to the review agent so it skips its own scope detection.

Uncommitted Changes

• Unstaged: git diff HEAD
• Staged: git diff --cached
• Untracked (new files not yet staged): enumerate with git ls-files --others --exclude-standard, then read the full content of each file as the review input.

Selected Code

Use the provided code as the review input; no git diff is needed. Apply all loaded skills or review logic to the selected code. Skip artifact persistence since there is no branch context.

Specific Commit

git diff <commit>^..<commit>

Scope Rules

• Do not enumerate, list, or read source files before obtaining the diff or review input.
• Only lines present in the diff (added or modified lines) are in scope for findings.
• For selected code reviews (no diff context), all provided code lines are in scope.
• Read full file contents only for contextual understanding of diff lines, never as a source of findings.
• Pre-existing issues in unchanged code go in the Out-of-scope Observations table, clearly labelled and excluded from the verdict.

Large Diff Handling

Use the timeout parameter on terminal commands to prevent hanging on large repositories.

Changed Files	Strategy
Fewer than 20	Analyze all files with full diffs.
20 to 50	Group files by directory and analyze each group.
More than 50	Progressive batched analysis, processing 5-10 files at a time.

When a diff exceeds 2000 lines of combined changes or 500 lines in a single file, review the most recent commits individually using git log --oneline and git show --stat.

Non-Source Artifact Skip List

Skip these artifacts when computing and analyzing diffs:

• Lock files: package-lock.json, yarn.lock, pnpm-lock.yaml
• Minified bundles: .min.js, .min.css
• Source maps: .map
• Binaries
• Build output directories: /bin/, /obj/, /node_modules/, /dist/, /out/, /coverage/

Fallback (pr-reference skill unavailable)

If the pr-reference skill scripts are not found or fail, compute the diff manually:

1. Resolve the merge-base: git merge-base origin/<default-branch> HEAD
2. Generate the diff: git diff <merge-base>...HEAD
3. List changed files: git diff <merge-base>...HEAD --name-only
4. For uncommitted changes, supplement with git diff HEAD, git diff --cached, and git ls-files --others --exclude-standard

Apply the Non-Source Artifact Skip List and Large Diff Handling rules to the manual output.