A 13 KB text file beat a smarter model: benchmarking AI codegen across 5 Angular state libraries

Disclosure up front: I maintain one of the five libraries tested (SignalTree), and it's the one that scored worst in the cold run — so this isn't a "look how good my thing is" post. The cross-library pattern and the fix were interesting enough that I wanted to put the numbers in front of people who use Copilot/Cursor/Claude Code every day. The whole harness is reproducible (one command, link at the bottom); I'd rather it get torn apart than taken on faith. Setup Libraries : NgRx (classic), NgRx SignalStore, Akita, Elf, SignalTree. Agents : Claude Sonnet 4.6, GPT-5.4, Gemini 3.1 Pro, Perplexity Sonar Pro, Claude Haiku 4.5, GPT-5.4-mini. 8 prompts : counter, paginated users, debounced search, derived totals, login form, undo/redo, deep nested state, multi-marker editor. 5 libs × 6 agents × 3 priming modes = 720 cells . Temperature 0. Identical prompt text per library (only the library name swapped). Scored on three orthogonal checks: idiomatic-pattern match, import resolution (does every import resolve to a real package), and method validity (do the called methods actually exist on the API). What this measures: one-shot generation. The agent gets the prompt, returns a file, we score it. Real interactive use — Cursor/Copilot with chat back-and-forth, where the model sees its own errors and gets a second try — is a different setting, and the lift could be larger or smaller there. This is the cold-shot case. Finding 1: cold accuracy basically tracks how much the library is in the training data No context provided, just "write this in library X": Library Cold score Akita 94% Elf 94% NgRx (classic) 91% NgRx SignalStore 86% SignalTree 49% The libraries that have been around for years, with thousands of blog posts and Stack Overflow answers, score in the 90s. The youngest/smallest library in the set scores ~49%. That gap isn't really a quality signal — it's a corpus signal. The models have simply seen orders of magnitude more Akita than SignalTree. Worth keeping in mind any