Methodology
The compatibility engine is not a recommendation model. It is a set of explicit comparisons over typed specs, and this page describes every stage of how those specs get there.
A part is only useful to the engine if it can be described in numbers. That constraint drives the whole intake process.
#14 and 8/0 can be compared numerically; swivels get a breaking strain; beads and inline sinkers get a bore in millimetres; rods get line and lure ratings, power, and action; reels get drag and capacity.Every diagnostic in the studio is one of three shapes. Knowing which shape you are looking at tells you how much to trust it.
A bead bore of 0.9 mm against a line diameter of 0.86 mm is not a fit — sliding hardware needs roughly 0.15 mm of clearance or it binds and the bore edge saws the line under load. These rules are the least arguable ones on the site: both numbers are published, the comparison is arithmetic, and the required margin is stated in the diagnostic.
0.90 mm bore < 0.86 mm line + 0.15 mm clearance → error: bead will not run
A rig fails at its weakest link, and the weakest link is usually terminal hardware nobody looked at. If a swivel is rated 35 lb and the mainline is 40 lb, the swivel is the fuse and a hard hookset costs you the whole terminal end. Floors also cover reel drag against species pull, rod line rating against spooled test, and cast weight against rod lure rating.
swivel 35 lb < mainline 40 lb → error: swivel is the failure point
Each species carries a line test window, a hook size window, a leader window, a minimum sane drag, and scores for line shyness and abrasion risk. Falling outside a window raises a warning, never an error, because these are ranges within which most people catch most fish most of the time — not laws.
6 lb mainline < bass window 8–20 lb → warning: light for the target
Nothing is ever blocked. The studio will let you finish and export a build that is throwing errors, because sometimes you know something the engine does not.
When the engine says a species wants 8–20 lb line, that is the band in which the tackle behaves well for typical fish of that species in typical water. It is not a claim that 6 lb cannot land one — people land enormous fish on absurdly light line, and that is a skill result, not a spec result. Read every window as "you will have a normal day inside this range, and you are choosing a harder day outside it".
The same applies to rules of thumb. "Swivel at or above 1.5× line test" is a margin we think is worth keeping, not a manufacturer's instruction. It is written into the diagnostic text so you can argue with the number rather than with the tool.
Every price shown on this site is an approximate street-price estimate, rendered as~$X and rounded. It is not a live Amazon price, it is not scraped, and it is not updated when a retailer changes it. It exists so a build sheet can give you a rough total and so you can tell a $4 part from a $40 part while planning — nothing more. Confirm the actual price on the retailer's page before buying.
Outbound commercial links go through a cloaked /go/<id> path so there is one place that controls destinations. Where an item has a verified ASIN, that path resolves to the specific product page. Where it does not, it resolves to a tagged Amazon search for the product name instead. We do not guess ASINs — a plausible-looking but wrong product page is worse than a search result, because it silently sends you to the wrong thing. The mechanics are documented on the affiliate disclosure page.
This is the important section, and it is deliberately not buried at the bottom of a FAQ.
Rules change when they are shown to be wrong. If a diagnostic fires on a build you know is sound, or a catalog spec does not match the packaging in your hand, send it tocontact@fishhooklabs.com with the part and the number — see contact for what to include. Corrections are applied to the catalog or the rule directly, which fixes the problem for every build at once.