migration-safety

A safety guide for changing the structure of a PostgreSQL or Supabase database that already contains data. It uses expand, backfill, and contract steps to reduce locking and data-loss risks.

In plain words
What is it for?
Writing paired up and down migrations, adding or changing columns, copying existing data in batches, and creating indexes on active tables.
Why use it?
It makes database changes reversible and avoids operations that can block a busy table or remove data unexpectedly.

Skill for Claude CodeCodex

Install

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.

agentmods
npx agentmods add skills/m-binimran/dev-pack/migration-safety
Any agent
npx skills add m-binimran/dev-pack --skill migration-safety
Clone the repo
git clone --depth 1 https://github.com/m-binimran/dev-pack

Made for: Claude Code, Codex.

Per session 50 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 379 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

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.

ModelPer sessionOnce invoked
Fable 5 $0.00050 $0.00379
Opus 5 $0.00025 $0.00189
Sonnet 5 $0.00010 $0.00076
Haiku 4.5 $0.00005 $0.00038

Measured 2d ago against content hash 3c6718230d44, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

migration-safety 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 2d 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.

skills/migration-safety/SKILL.md · 34 lines

What it actually says

migration-safety

Every migration must be reversible and must not lock a busy table or drop data by surprise. (The migration-validate hook checks your output — design to pass it.)

Always produce a pair

  • up: the change.
  • down: the exact rollback. If a change is truly irreversible (dropping data), say so loudly and require explicit confirmation.

Expand → backfill → contract (for column changes)

  1. Expand: add the new column/table, nullable, no constraint. Cheap, non-locking.
  2. Backfill: populate in batches (update ... where id between ...), not one giant statement.
  3. Contract: add the NOT NULL/constraint and drop the old column — in a later migration, after the app writes both.

Locking traps to avoid

  • ADD COLUMN ... NOT NULL with no DEFAULT on a non-empty table → fails / full rewrite. Add a default.
  • ALTER COLUMN TYPE → rewrites + locks. Prefer add-new-column + backfill.
  • Creating an index on a hot table → use CREATE INDEX CONCURRENTLY (outside a txn).

Output

  • up SQL, down SQL, and a one-line risk note (lock risk / data-loss risk / safe).
  • For destructive steps, an explicit "this cannot be undone" line.

Guardrails

  • No migration without a rollback path.
  • Never combine a destructive drop with the additive change in the same migration — separate them.
Changes

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.

  1. 2d ago First seen · 34 lines · 50 tokens per session scan A 3c6718230d44

Subscribe to this mod's changes

migration-safety is a skill published in the GitHub repository m-binimran/dev-pack (2 stars, last pushed 2mo ago), licensed MIT. It adds 50 tokens to every session and 379 once invoked, about $0.0003 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 2026-08-31.

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