refactor-legacy

A code-review command for planning safer changes to old Python code that has few or no tests.

In plain words
What is it for?
It reads Python files, identifies obstacles to testing and common code smells, then lays out a phased refactoring plan starting with tests.
Why use it?
It helps you understand existing behavior before changing it, reducing the risk of breaking undocumented behavior during refactoring.

Command

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 commands/mktoronto/python-clean-architecture/refactor-legacy
Clone the repo
git clone --depth 1 https://github.com/MKToronto/python-clean-architecture
Per session 16 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 506 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 $0.00016 $0.00506
Opus 5 $0.00008 $0.00253
Sonnet 5 $0.00003 $0.00101
Haiku 4.5 $0.00002 $0.00051

Measured 2d ago against content hash 399c97a369a4, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

refactor-legacy 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.

commands/refactor-legacy.md · 56 lines

What it actually says

Analyze the code at $ARGUMENTS (or the current working directory if no path given) and produce a step-by-step refactoring plan with testability as the first goal.

Process

  1. Read the code — Find and read ALL Python files in the target path recursively.

  2. Identify testability blockers — Look for:

    • Global state and module-level side effects
    • Hardcoded dependencies (self.x = ConcreteClass())
    • Mixed I/O and business logic (print/input in logic classes)
    • Missing abstractions (no Protocol interfaces)
    • Complex __init__ methods with setup logic
    • Functions with inconsistent return types
  3. Identify code smells — Scan for recurring refactoring opportunities:

    • Deep nesting (→ guard clauses)
    • Magic numbers (→ named constants)
    • String comparisons (→ Enums)
    • Scattered if/elif chains (→ data-driven rules)
    • Raw data structures (→ domain classes)
    • Catch-all exception handling (→ specific catches)
  4. Produce a phased plan:

    Phase 1: Add characterization tests

    • Capture current behavior with pinning tests
    • Don't fix anything yet — just lock down what exists

    Phase 2: Extract pure logic from side effects

    • Separate UI/IO from business logic
    • Move print/input into dedicated classes
    • Extract named helper functions from complex conditions

    Phase 3: Introduce Protocol interfaces

    • Define Protocol for external dependencies
    • Apply dependency injection at the composition root
    • Create stubs for testing

    Phase 4: Add proper unit tests

    • Test extracted pure logic with stubs
    • Verify business rules independently of I/O
  5. For each phase, show specific before/after code snippets from the actual codebase.

References:

  • ${CLAUDE_PLUGIN_ROOT}/skills/clean-architecture/references/refactoring-case-studies.md
  • ${CLAUDE_PLUGIN_ROOT}/skills/clean-architecture/references/testing-legacy-code.md
  • ${CLAUDE_PLUGIN_ROOT}/skills/clean-architecture/references/code-smells.md
  • ${CLAUDE_PLUGIN_ROOT}/skills/clean-architecture/references/dependency-injection.md
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 · 56 lines · 16 tokens per session scan A 399c97a369a4

Subscribe to this mod's changes

refactor-legacy is a command published in the GitHub repository MKToronto/python-clean-architecture (8 stars, last pushed 2mo ago), licensed MIT. It adds 16 tokens to every session and 506 once invoked, about $0.0001 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.