Design: DIY vs. a Heavy Dependency

Before reaching for an existing single-purpose tool — especially one with a GUI, a bundled runtime, an unaudited binary, or that only runs on one platform — evaluate whether the underlying operation is simple and well-documented enough to reimplement directly, in code that can be read start to finish.

This is not a mandate to always DIY. Most of the time the existing tool is still the right call. But the evaluation should always happen before defaulting to "just install the tool that does this."

When to Use This Skill

The Trigger

Run the evaluation whenever the tool about to be depended on is:

• Single-purpose — does exactly one narrow thing for this project, not a general-purpose runtime/library reused elsewhere too.
• A GUI wrapping something mechanical — the "app" is a UI on top of a well-defined operation ("point this at a file and click a button").
• Not cross-platform, or otherwise poorly maintained, one-off, or distributed as an opaque bundled binary that can't easily be audited.
• Heavier than the job warrants — bundles a runtime (Electron, Qt, PyInstaller) or drags in a chain of transient dependencies just to perform one well-scoped operation.

The Decision

DIY is likely right when

• The underlying protocol/algorithm/format is documented, or has an authoritative open-source reference implementation to verify against — not something to be guessed at from scratch.
• The reimplementation is bounded — tens to a few hundred lines, not a multi-week undertaking re-deriving something genuinely hard.
• What results is auditable: one file, or a small readable set, that a future session (or the same person in six months) can read top to bottom and trust, instead of a black-box binary.
• It removes a disproportionate dependency chain relative to the value (a GUI app + its bundled runtime + a mount step, vs. one script + one well-known library).

DIY is wrong when

• The tool encapsulates real, hard-to-verify complexity — cryptographic correctness, hardware timing, safety-critical logic — where a subtle bug in a reimplementation is a worse outcome than keeping the dependency.
• No authoritative reference exists to verify against; the result would be unverified guesswork shipped with confidence it hasn't earned.
• The dependency is already cheap and well-audited — a single well-known CLI installable per the Tool Installation Priority (mise → uv → bun → cargo/go → brew).
• The existing tool is actively maintained and the switching cost (in time, or in correctness risk) clearly exceeds the dependency it removes.

If DIYing: Verify Against an Authoritative Source

A reimplementation is only as good as its verification. Never trust a from-scratch port of a protocol or algorithm on memory alone.

1. Find an authoritative reference — the original implementation, a spec, an RFC, or (for reverse-engineered protocols) two or more independent, converging open-source implementations.
2. Cross-check constants and structure directly from source, not from memory or a summarized web result.
3. Write a diff test. Run the new implementation and the reference implementation's core logic side by side on the same inputs — including edge cases and boundary conditions — and assert byte-for-byte equality. Don't eyeball two implementations and declare them equivalent; make a script prove it.
4. Attribute and license correctly. Vendoring or closely porting someone else's documented algorithm still carries their attribution and license terms — a reimplementation that quietly drops credit isn't actually more transparent than the binary it replaced.

Step 3 is the one most often skipped and the one that pays. In the worked example below it caught a real behavioral divergence before it shipped.

Worked Example (2026-07, nintendo-switch-cfw)

A Switch CFW project depended on CrystalRCM.app — a mounted, unaudited macOS GUI binary — to perform one operation: inject a fusée-gelée payload over USB into a console in RCM mode. The exploit's memory layout and USB protocol are fixed and well-documented (CVE-2018-6242), with byte-identical reference implementations across multiple independent open-source forks going back to 2018. A textbook DIY candidate.

Replaced with a ~215-line, single-file, uv run-executable script (PEP 723 inline deps: just pyusb). Before trusting it:

• Cross-checked every constant and the full algorithm against two independent forks' actual source (not a summary), plus the assembly source for the one binary blob that had to stay vendored (a 124-byte relocator stub, verified byte-identical across forks).
• Wrote a diff test against a verbatim port of the reference algorithm across six cases (empty, tiny, a real downloaded payload, and the exact overflow boundary) — caught one subtle behavioral divergence at the boundary case.
• Smoke-tested the live CLI path (arg parsing, error paths, USB device enumeration) as far as possible without physical hardware.

Net: a mounted GUI app of unknown provenance replaced by one auditable script plus pyusb/libusb — both well-known, independently useful libraries, not another single-purpose black box.

Rationale

Single-purpose GUI tools accumulate silently. Each is individually "just one dependency," but across a portfolio they add up to a pile of differently-built, non-cross-platform, unaudited binaries with no shared conventions — the opposite of the consistency such a portfolio otherwise optimizes for.

The fix isn't "never use existing tools." It's making the evaluation a habit: before installing the app, ask whether the operation is simple and documented enough to own directly, verify it properly if so, and fall back to the heavy dependency when the evaluation says it's genuinely warranted.

Related

• code-quality-plugin:code-dep-audit — auditing dependencies you already have for vulnerabilities and staleness; this skill governs the prior question of whether to take the dependency at all
• agent-patterns-plugin:verify-before-plan — the same "check reality before committing to an approach" instinct
• YAGNI (~/.claude/rules/code-quality.md) — DIY only when the win is real; don't reimplement speculatively, or for a tool that isn't actually being removed
• ~/.claude/rules/verify-upstream-before-patching.md — go to the authoritative source rather than working from memory
• ~/.claude/rules/offload-to-deterministic-substrate.md — the diff-test step is that principle applied to correctness verification itself