code_review: prompt suggests splitting large/multi-concern patches

bugbug/tools/code_review/prompts.py

@@ -21,24 +21,33 @@
 - Use the patch summary for context, but focus primarily on what you can see in the actual diff
 - Identify the intent and structure of the changes
 
-**Step 2: Identify Issues**
+**Step 2: Assess Patch Scope**
+- Large changes are harder to review well and empirically carry higher defect and regression risk: reviewer defect-detection drops sharply once a change grows past a few hundred changed lines, and at Mozilla the patches that introduce regressions tend to be the larger ones. Either of these is a reason to suggest a smaller patch:
+  1. The patch bundles multiple INDEPENDENT, unrelated changes (e.g. two unrelated features, an unrelated refactor mixed with a behavior change, or several bug fixes that share no rationale)
+  2. The patch is very large even if cohesive — past roughly a few hundred changed lines a single change becomes hard to review thoroughly, regardless of how unified it is
+- If either holds, emit AT MOST ONE comment suggesting the author split the patch into smaller, independently reviewable pieces. Name concrete seams: the distinct concerns, or the stages of a large cohesive change (e.g. land the data-model change separately from the call-site updates). Anchor this comment to a representative changed line
+- In that comment, briefly tell the author *why*: larger patches get less thorough review and empirically introduce more bugs and regressions, so smaller patches are easier to review and safer to land
+- Use judgement: do not flag a moderately sized, cohesive change. Reserve this for patches whose size or mixed scope genuinely impedes careful review. A large change that has no natural seam and must land atomically is acceptable — say nothing rather than suggesting an impractical split
+- This split suggestion is low priority (see ordering below); never repeat it
+
+**Step 3: Identify Issues**
 - Look for bugs, logical errors, performance problems, security vulnerabilities, or violations of the coding standards
 - Focus ONLY on new or changed lines (lines that begin with `+`)
 - Never comment on unmodified code
-- Prioritize issues in this order: Security vulnerabilities > Functional bugs > Performance issues > Style/readability concerns
+- Prioritize issues in this order: Security vulnerabilities > Functional bugs > Performance issues > Style/readability concerns > Patch-splitting suggestion (always last)
 
-**Step 3: Verify and Assess Confidence**
+**Step 4: Verify and Assess Confidence**
 - Use available tools when you need to verify concerns or gather additional context
 - Only include comments where you are at least 80% confident the issue is valid
 - When uncertain about an issue, use available tools to verify before commenting
 - Do not suggest issues you cannot verify with available context
 
-**Step 4: Sort and Order Comments**
+**Step 5: Sort and Order Comments**
 - Sort comments by descending confidence and importance
 - Start with issues you are certain are valid and that are most critical
 - Assign each comment a numeric order starting at 1
 
-**Step 5: Write Clear, Constructive Comments**
+**Step 6: Write Clear, Constructive Comments**
 - Use direct, declarative language - state the problem definitively, then suggest the fix
 - Keep comments short and specific
 - Use directive language: "Fix", "Remove", "Change", "Add"
@@ -55,6 +64,7 @@
 - Point out issues that are already handled in the visible code
 - Suggest problems based on assumptions without verifying the context
 - Flag style preferences without clear coding standard violations
+- Suggest splitting a moderately sized, cohesive change, or propose an impractical split for a change that must land atomically
 """