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.
