Fusion vs JetBrains AI / Junie

Fusion vs JetBrains AI / JunieComparisonUpdated 2026-03-16

SUMMARY

Fusion (IDE) uses mdc configuration while JetBrains AI / Junie (IDE) uses markdown. They differ on 3 of 11 compared features. Both tools provide AI-assisted development but take different approaches to project configuration.

FEATURE COMPARISON

Feature	Fusion	JetBrains AI / Junie	Notes
Config format	mdc	markdown
Hierarchy support	✗	✗	Whether config files can be placed at multiple directory levels
Global config	✗	✗	User-wide configuration outside any project
Project config	✓	✓
Subdirectory scoping	✗	✗	Config files in subdirectories that apply only to that subtree
File inclusion / imports	✗	✗	Ability to include or reference other files from config
Ignore file	✗	.aiignore
IDE integration	✓	✓
Schema / structure	Hybrid (MDC frontmatter + Markdown)	Free-form (Markdown)	Whether config uses structured schema or free-form content
Git committed	✓	✓	Whether primary config files are intended to be version-controlled
Encoding	UTF-8	UTF-8

FILE MAPPING

Purpose	Fusion	JetBrains AI / Junie
Project rules	.builder/rules/*.mdc	.aiassistant/rules/*.md

SIDE-BY-SIDE CODE SAMPLES

Fusion

.builder/rules/*.mdc

mdc

---
description: React component conventions
globs: src/components/**/*.tsx
alwaysApply: false
---

# React Component Rules

## Structure
- Use functional components with TypeScript
- Define props interface above the component
- Export components as named exports

## Styling
- Use the project design system tokens for spacing and colors
- Prefer utility classes over custom CSS
- Keep component-specific styles colocated

## Patterns
- Wrap async data in Suspense boundaries
- Use forwardRef for reusable interactive components
- Keep components under 150 lines; extract hooks for logic

## Accessibility
- All interactive elements need aria-labels
- Use semantic HTML elements (nav, main, section)
- Ensure keyboard navigation works for custom widgets

JetBrains AI / Junie

.aiassistant/rules/*.md

markdown

# Kotlin Conventions

## Language Features
- Use Kotlin 2.x with coroutines for async work
- Prefer data classes for DTOs and value objects
- Use sealed classes/interfaces for domain errors
- Extension functions over utility classes

## Spring Boot
- Constructor injection (no field injection)
- Use @ConfigurationProperties over @Value
- Prefer WebFlux for reactive endpoints
- Use Spring Security with method-level security

## Testing
- JUnit 5 with Kotest assertions
- MockK for mocking (not Mockito)
- Testcontainers for integration tests
- Use @Nested for test organization

KEY DIFFERENCES

1. Config format

Fusion: mdc. JetBrains AI / Junie: markdown.

2. Ignore file

Fusion: Not supported. JetBrains AI / Junie: .aiignore.

3. Schema / structure

Fusion: Hybrid (MDC frontmatter + Markdown). JetBrains AI / Junie: Free-form (Markdown).

WHICH SHOULD I USE?

Recommendation

Choose based on your IDE/CLI preference. If you use Fusion, configure .builder/rules/*.mdc. If you use JetBrains AI / Junie, configure .aiassistant/rules/*.md. Many projects include config files for multiple tools so each team member can use their preferred editor.

PORTABILITY TIP

sync-instructions.sh

bash

# Maintain a single source of truth:
cp AI-INSTRUCTIONS.md CLAUDE.md
cp AI-INSTRUCTIONS.md .cursorrules
cp AI-INSTRUCTIONS.md AGENTS.md