forge-migration

forge-migration is a skill for Claude Code, Codex from ForgeyClap/claude-forge. It costs 42 tokens per session (1,508 once invoked), scanned A, original, MIT.

A playbook for updating old software through small, reversible changes instead of replacing everything at once. Characterization tests record what the existing system currently does, including undocumented behavior.

In plain words
What is it for?
Use it to modernize legacy systems, move features to a new platform, upgrade components, add feature flags, and plan a controlled cutover.
Why use it?
It reduces the risk of losing relied-on behavior during a rewrite or technology upgrade. Each change can be checked for matching results before the next part is moved.

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/forgeyclap/claude-forge/forge-migration
Any agent
npx skills add ForgeyClap/claude-forge --skill forge-migration
Clone the repo
git clone --depth 1 https://github.com/ForgeyClap/claude-forge

Made for: Claude Code, Codex.

Wrote 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.

agentmods badge for forge-migration

README.md
[![agentmods](https://agentmods.dev/badge/skills/forgeyclap/claude-forge/forge-migration.svg)](https://agentmods.dev/skills/forgeyclap/claude-forge/forge-migration)
Your own site
<a href="https://agentmods.dev/skills/forgeyclap/claude-forge/forge-migration"><img src="https://agentmods.dev/badge/skills/forgeyclap/claude-forge/forge-migration.svg" alt="Measured on agentmods" height="20"></a>
Per session 42 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 1,508 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 $0.00042 $0.01508
Opus 5 $0.00021 $0.00754
Sonnet 5 $0.00008 $0.00302
Haiku 4.5 $0.00004 $0.00151

Measured 4d ago against content hash 5daff7b3f3c0, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

forge-migration 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 4d 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.

.claude/skills/forge-migration/SKILL.md · 39 lines

How it starts

The opening of the file, as written. The whole thing — 39 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Forge playbook — Legacy modernization / migration

Do not duplicate ECC skills — defer to: learn-codebase / forge-deeplearn (prime the legacy system before touching it — mandatory here), test-driven-development + systematic-debugging (characterization + regression work), forge-worktrees (isolate each slice), /test-coverage. This file is orchestration only.

The governing principle is Michael Feathers' rule: you cannot safely change code you cannot characterize. Migration is not a rewrite — it is a sequence of small, reversible, parity-verified swaps behind a facade, each of which keeps the system working the whole time. The catastrophic failure mode is the "big-bang" rewrite that goes dark for months and cuts over into unverified behavior loss. Forge does not do that.

Hard rules

  • Characterization tests first. Before changing any legacy behavior, capture what it currently does with characterization / golden-master / approval tests — including the ugly, undocumented, "wrong-but-relied-on" behavior. These pin the existing contract so a migration can prove it preserved it. No behavior change lands before its characterization test exists and passes on the legacy code.
  • Strangler-fig, incremental cutover. Introduce a seam / facade / routing layer and move one slice at a time to the new implementation behind it. The old and new systems run side by side; the facade decides per-request which path serves. Never replace the whole thing at once.
  • Every step is reversible. Each slice cuts over behind a feature flag / route toggle with a proven route-back. Each step is independently deployable and independently revertible. If a slice misbehaves in production, you flip it back in seconds — you never have to "un-rewrite".
  • Parity verified before a slice is trusted. Old vs new must produce the same result: shadow/diff the new path against the old (GitHub-Scientist-style compare, or replayed inputs) and reconcile every discrepancy before the slice takes real traffic. Divergence is investigated, not waved through.
  • No big-bang rewrite. No months-long dark rewrite, no "we'll cut over everything on the weekend". If the plan can't be sliced, the first work package is to find the seams that make slicing possible.
  • Data migration is reversible and verified. Schema/data moves use dual-write or backfill-then-verify with row-count + checksum reconciliation and a rollback path. The old store stays authoritative until parity is proven; no destructive drop of the source until the new store is verified and a backup exists.

Read the full file on GitHub · 39 lines

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. 4d ago First seen · 39 lines · 42 tokens per session scan A 5daff7b3f3c0

Subscribe to this mod's changes

forge-migration is a skill published in the GitHub repository ForgeyClap/claude-forge (2 stars, last pushed 1mo ago), licensed MIT. It adds 42 tokens to every session and 1,508 once invoked, about $0.0002 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.