Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add rules/levifig/rails-instructions/performancegit clone --depth 1 https://github.com/levifig/rails-instructionsWhat it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5 | $0.00000 | $0.01108 |
| Opus 5 | $0.00000 | $0.00554 |
| Sonnet 5 | $0.00000 | $0.00222 |
| Haiku 4.5 | $0.00000 | $0.00111 |
Grade A, and why
performance scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 211 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Rails Performance Guide
Core Philosophy
- Measure before optimizing
- Optimize for the common case
- Keep solutions simple and maintainable
- Use Rails built-in performance features
- Monitor performance in production
Database Optimization Principles
- Design efficient schemas from the start
- Index foreign keys and queried columns
- Use database-specific features wisely
- Keep queries simple and focused
- Monitor slow query logs
Query Optimization
- Eliminate N+1 queries with includes
- Use select to limit returned columns
- Implement pagination for large datasets
- Use pluck for single column queries
- Batch process large operations
Indexing Strategy
- Index all foreign keys
- Create composite indexes for multi-column queries
- Use partial indexes for filtered queries
- Monitor index usage and effectiveness
- Remove unused indexes
Caching Principles
- Cache at multiple levels
- Use Russian doll caching for nested content
- Implement fragment caching for expensive views
- Cache database queries appropriately
- Monitor cache hit rates
Rails Cache Store
- Use Solid Cache for database-backed caching
- Configure Redis for high-traffic applications
- Set appropriate cache expiration
- Use cache keys that auto-expire
- Monitor cache memory usage
Fragment Caching
- Cache expensive view partials
- Use cache digests for automatic expiration
- Implement conditional caching
- Cache user-specific content carefully
- Profile view rendering times
Query Caching
- Leverage Rails query cache
- Use low-level caching for expensive calculations
- Cache aggregated data
- Implement counter caches
- Update caches strategically
Asset Optimization
- Use Propshaft for simple asset handling
- Implement CDN for asset delivery
- Compress assets appropriately
- Set proper cache headers
- Optimize image sizes
Background Job Performance
- Process heavy operations asynchronously
- Use appropriate queue priorities
- Batch similar operations
- Monitor job queue depth
- Implement job timeouts
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- yesterday First seen · 211 lines · 1,108 tokens per session scan A f186dc66df78
performance is a cursor rule published in the GitHub repository levifig/rails-instructions (54 stars, last pushed 1y ago), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 1,108 tokens. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
Other cursor rules, from other repositories
pytest-integration-tests
Below is an example test. Notice the following.
python-app
Cursor rule "python-app" from iloveitaly/llm-ide-rules, covering python app, factories and database & orm.
react
Cursor rule "react" from iloveitaly/llm-ide-rules, covering react, mock data, react hook form and styling.
creating-cursor-rules
Meta-rule for creating effective Cursor IDE rules with best practices, patterns, and examples.
react-router
Cursor rule "react-router" from iloveitaly/llm-ide-rules, covering react router, loading mock data, how to use clientloader and loading backend data.
pytest-tests
Below is an example test, you'll notice the following.