use-builtin-mechanism-not-parallel-invention

name	use-builtin-mechanism-not-parallel-invention
description	Use when adding, measuring, or shipping a feature in a project that already has a built-in facility (a benchmark, a chosen output format, an agreed convention) — reach for the existing mechanism instead of inventing a parallel one or silently regressing a decided choice, even if you don't explicitly remember the decision.

Use the built-in mechanism — don't invent a parallel one

When a project already provides a facility, USE IT. Do not roll your own, and do not silently regress an agreed decision. Two failure modes that trigger user fury:

1. Inventing parallel instrumentation. If the codebase has a built-in benchmark (e.g. dub-studio bench.rs: Bench::start/stage/finish → bench.json, gated by active.json "bench"), wire your new stage into IT (bench.stage("casting")). Do NOT sprinkle ad-hoc Instant::now() timers — you'll produce numbers that don't match the real per-stage/GPU/VRAM breakdown the built-in tool emits across ALL stages.

2. Silently regressing an agreed format/convention. If the team decided outputs are native JPG for speed, new artifacts must be JPG — not PNG because that's what a helper defaulted to. Check the surrounding convention before picking an extension/format. ffmpeg and the Rust image crate infer format from the path extension, so the fix is usually just the filename (char_0.jpg not .png) plus any hardcoded serving route.

Before claiming a feature is done

• Benchmark its real cost with the built-in tool, end-to-end on a WHOLE file, not a snippet. The user's fear is "this feature secretly eats 80% of total time." Report the honest ratio: feature_stage.sec / (analyze.total_sec + render.total_sec).
• The benchmark must run across all stages (analyze pipeline + render), because % of total is meaningless without the total.
• Then show the finished result от и до (start to finish), not just the intermediate.

Red flags (stop and use the built-in)

• "I'll just add a quick timer here" → there's already a bench; use it.
• "PNG is fine" when the project standardized on JPG → match the convention.
• "I'll measure just the casting stage" → measure the whole dub or the % is a lie.