side-effect

name	side-effect
description	Post-implementation side effect analysis. Checks consumer impact, shared state, UI components, function signatures, and layout cascade for unintended regressions.
argument-hint	[master|HEAD~N|commit-hash|file-path]

Side Effect Analysis

Post-implementation check for unintended side effects. Run after feature work, bug fixes, or refactoring to verify no existing functionality is broken.

Instructions

Step 1: Identify Changed Files

git diff --name-only HEAD

If no uncommitted changes, use $ARGUMENTS as the diff target (e.g., master, HEAD~3, a commit hash).

Step 2: For Each Changed File, Analyze Impact

Use an Explore agent to run the following checks in parallel across all changed files:

2a. Consumer Analysis

Find all files that import or reference the changed file:

• Direct imports (import ... from './changed-file')
• Re-exports through barrel files (index.ts)
• Dynamic references (string-based paths in tests, configs)

For each consumer, verify the change is compatible.

2b. Shared State Impact

Check if the change affects shared state that other components rely on:

Shared State	What to Check
Zustand stores	Store actions/selectors used by other components
TanStack Query caches	Query keys, cache shape, select transforms
WSS subscriptions	Channel names, payload schemas, merge logic
Context providers	Values consumed by children
localStorage/sessionStorage	Keys read by other features

2c. UI Component Side Effects

For shared UI components (components/ui/*), check ALL consumers:

• CSS selector changes: Do new selectors match unintended elements? (e.g., input:not([type="hidden"]) matching input[type="range"])
• Prop behavior changes: Does a default value change affect callers that rely on the old default?
• Event handler changes: Does timing or focus behavior affect other modals/dialogs?
• Animation/transition changes: Do new animations conflict with consumer expectations?

2d. Function Signature & Behavior

For changed utility functions (lib/**):

• Return type changes: Does null vs undefined vs "-" vs "0" differ from the replaced function?
• Edge case behavior: How does the new function handle null, NaN, empty string, 0?
• Formatting differences: Does the new formatter handle locale, trailing zeros, or precision differently?
• Import path changes: Are all consumers updated to the new import?

2e. Layout & Spacing Cascade

For CSS/Tailwind changes:

• Grid/flex changes: Do grid-cols-* or 1fr changes affect responsive breakpoints?
• Fixed-width → flexible: Does removing fixed widths cause content overflow or collapse?
• z-index changes: Do overlay/modal z-index changes conflict with other overlays?

Step 3: Risk Assessment

Classify each finding:

Risk	Criteria	Action
CRITICAL	Breaks existing functionality, data loss	Must fix before merge
HIGH	Visible regression in another feature	Should fix before merge
MEDIUM	Edge case behavior change, minor visual shift	Document and verify manually
LOW	Theoretical risk, unlikely to trigger	Note for awareness
NONE	Intentional change per requirements	Confirm alignment with spec

Step 4: Generate Report

SIDE EFFECT ANALYSIS: [CLEAN / ISSUES FOUND]

Changed files: N
Consumers analyzed: N

CRITICAL: [count]
HIGH:     [count]
MEDIUM:   [count]
LOW:      [count]

[For each finding above LOW:]
- File: path/to/file.tsx:line
  Risk: HIGH
  Impact: [what breaks and for whom]
  Fix: [specific remediation]

Step 5: Apply Fixes

• Fix all CRITICAL and HIGH issues immediately
• Run pnpm validate after fixes
• Re-run consumer checks for any files modified during fixes

Arguments

$ARGUMENTS can be:

• (empty) — Analyze uncommitted changes (git diff HEAD)
• master — Analyze all changes vs master branch
• HEAD~N — Analyze last N commits
• A commit hash — Analyze changes since that commit
• A file path — Analyze only that specific file's consumers