Whyts MCP server

whyts

Find out why your TypeScript project is slow.

A small CLI that turns compiler diagnostics, traces, and import relationships into evidence you can act on. It says where the check is slow, and for two known patterns it also says why and what to do. No account, API key, or LLM required.

npx whyts

Run it in the folder of your tsconfig.json. It runs the real compiler once and prints at most three actions. To prove that a type change helped, use npx whyts compare (see below). To give the same evidence to an AI coding agent, use claude mcp add --transport stdio whyts -- npx -y whyts-mcp.

Found with whyts: a 7.4 s type check that went to 0.06 s (TanStack Form) and vue-tsc going from 80 s to 0.56 s (Vue). See Real cases.

Try it

Requires Node.js 20+ and a TypeScript project with dependencies installed.

npx whyts --project .

For repeatable runs, pin a version: npx whyts@0.8.3 --project .

To run from a checkout:

git clone https://github.com/musatoktas/whyts.git
cd whyts
npm ci --ignore-scripts
npm run demo
node src/cli.js --project /path/to/your/tsconfig.json

Diagnoses and remedies

By default, whyts prints at most three actions and one summary line. Each action has a pattern, a location, a short remedy and a link. The image above is a recorded run. Use --verbose for the full report.

whyts 0.8.2 · TypeScript 5.9.3 · repro-project/tsconfig.json

1. [measured] Recursive type instantiation: DeepKeys<Values>
   Where: k.ts:4:17
   TS2589 at this place: Type instantiation is excessively deep and possibly infinite.
   It hit the instantiation limit inside DeepKeysAndValuesImpl (../src/util-types.ts:151).
   DeepKeysAndValuesImpl refers to itself: DeepKeysAndValuesImpl -> DeepKeyAndValueArray -> DeepKeysAndValuesImpl.
   Type argument JsonData (k.ts:2): JsonData refers to itself.
   Fix: Stop the expansion of DeepKeysAndValuesImpl when it meets a type it already visited. Or add a depth limit to it. Check that the fix keeps the same results.
   Cost: 13118 ms, at most 99.5% of Check time (upper bound, not a saving)
   Docs: https://github.com/microsoft/TypeScript/wiki/Performance

Check 13.18s · Total 13.55s · 65 files · 1 compiler error · 1 finding in the full report. Use --verbose to see them.

Two patterns have a remedy. Each one was seen in a real project before it was added.

Rules:

Shortened. The full README is on GitHub.