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 skills add sandeepmvl/rails-skills --skill 05-safe-migrationsgit clone --depth 1 https://github.com/sandeepmvl/rails-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/sandeepmvl/rails-skills/05-safe-migrations)<a href="https://agentmods.dev/skills/sandeepmvl/rails-skills/05-safe-migrations"><img src="https://agentmods.dev/badge/skills/sandeepmvl/rails-skills/05-safe-migrations/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/sandeepmvl/rails-skills/05-safe-migrations"><img src="https://agentmods.dev/badge/skills/sandeepmvl/rails-skills/05-safe-migrations.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What 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 5.1 | $0.00133 | $0.04049 |
| Opus 5 | $0.00067 | $0.02024 |
| Sonnet 5 | $0.00027 | $0.00810 |
| Haiku 4.5 | $0.00013 | $0.00405 |
Grade A, and why
safe-migrations 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 12d ago.
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 — 396 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Safe Migrations
Every Rails developer ships a migration that locks the table for 90 seconds at some point. This skill prevents that. The rules are: never alter a column type, never change a column default on a large table in one step, never enforce NOT NULL on a backfill in the same migration, never index a 100M-row table without
CONCURRENTLY. AI agents botch every one of these by default.
Why this matters
A migration locks rows; an unsafe migration locks the whole table; a really unsafe migration locks the table for the duration of a pg_dump-sized table rewrite. Users see 502s. The fix isn't smarter Rails — it's deploy ordering and the strong_migrations gem.
The opinion
Install strong_migrations on day one. Treat every schema change as a multi-deploy split: add → backfill → enforce → cleanup. Use
CONCURRENTLYon Postgres for every index over ~10k rows. Backfill in batches of 1000 with throttling. Never trustchange_columnto be safe.
Counter-position: tiny apps (< 100k rows on all tables) can ignore this and just run migrations. We acknowledge that — but installing strong_migrations on day one means the first time you cross a threshold, you get a helpful error instead of an outage.
The mental model
A schema change is a contract between the schema and the deployed code. The code expects column X to exist (or not exist), to be NOT NULL (or nullable), to have a particular type. If the schema and the code disagree at any point during deploy, requests fail.
So every "big" schema change is at least two deploys:
- Deploy 1: schema change that's backwards-compatible with the old code.
- Deploy 2: code change that uses the new schema. Often followed by a third deploy to clean up the old column / constraint.
Strong_migrations enforces this by refusing the dangerous one-step migrations.
Core patterns
Pattern 1: Install strong_migrations
# Gemfile
gem "strong_migrations"
bin/rails generate strong_migrations:install
What ships with it
2 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
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.
- 12d ago First seen · 396 lines · 133 tokens per session scan A 2c98982dcaa5
safe-migrations is a skill published in the GitHub repository sandeepmvl/rails-skills (21 stars, last pushed 3mo ago), licensed MIT. It adds 133 tokens to every session and 4,049 once invoked, about $0.0007 per session on Opus 5. 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.
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