Whyts

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 three 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@0.8.4 --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 0.8.5 · TypeScript 5.9.3 · repro-project/tsconfig.json

1. [measured] Recursive type instantiation: DeepKeys 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: 10082.6 ms, at most 98.8% of Check time (upper bound, not a saving) Docs: https://github.com/microsoft/TypeScript/wiki/Performance

Check 10.20s · Total 10.80s · 65 files · 1 compiler error · 2 findings in the full report. Use --verbose to see them.

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

A program-load action looks like this (vuetify 4.2.4, a project that only has import { createVuetify } from "vuetify"; the lines are shortened):

1. [measured] Program load: 421 dependency files, vuetify has 407; Check time is 0.01s of 4.22s

Where: ../node_modules/vuetify/lib/framework.d.ts:1:1 The program has 480 files: 421 in node_modules (13.8 MB of source), 58 compiler library files and 1 of your own. ... Memory used 577 MB in the traced run. Largest packages by files: vuetify 407 (12 MB), @vuetify/v0 3 (0 MB), vue 2 (0 MB), ... 409 of them (11.9 MB) come only through 218 typeof import() type references in ../node_modules/vuetify/lib/framework.d.ts, to ../node_modules/vuetify/lib/components/index.d.ts (192), ... Shortest import chain: index.ts -> ../node_modules/vuetify/lib/framework.d.ts -> ../node_modules/vuetify/lib/components/index.d.ts Fix: Each typeof import() reference in ... loads its target for every project that imports the package. Ask the maintainers to move them to an opt-in entry point. skipLibCheck skips checking these files, not loading them. ...

Rules:

• A diagnosis is measured when the compiler recorded it (the error or the trace events). The name-based walk over type aliases is syntax only: it can miss a cycle that runs through imports it cannot match by name, or through a type that is not a declaration of the program.
• The variance pattern needs 100 ms of union time in total, and only events of 50 ms or more count. It lists only types declared in the project. A type from a dependency gets no remedy, because the project cannot change it.
• The cost is the largest recorded interval for the place, shown as at most N% of Check time. It is an upper bound under tracing. It is not a predicted saving.
• In the compact report, a slow check without a known pattern shows only above 3% of Check time. A check that a diagnosis already explains by type name is not listed twice.
• The trace events exist in TypeScript 5.9, 6.0 and 7.0 (instantiateType_DepthLimit, getVariancesWorker). In TypeScript 7 they carry a checkerId, and whyts uses one checker, so the type ids stay unique.
• The program-load pattern needs all of these: 200 files or more from node_modules, a Total time of 1 s or more, a Check time of at most 1 s and at most 10% of the Total time, and one file that alone brings 100 files or more (and 25% of the dependency files). The compiler library files and your own files do not count as dependency files. A project with a normal amount of its own code has a larger Check time and does not match. The counters come from the traced run, where Memory used is higher than in a plain run.
• program-load is a measurement of the program, not of the check. It does not say that removing the files makes the check faster: the check time of such a program is already small. It reads typeof import('pkg') in declaration files as an import. A file that is also brought by a regular import is not counted as brought only by those references.
• callback-inference is a match of the call shape and of the time that the trace does not explain. The trace has no event for inference, so whyts does not prove that inference is the cost. It reads only the syntax of the callee declarations and of the argument. It does not know which overload the compiler chose. A type parameter that is wrapped in NoInfer deep inside the parameter type still matches: in telegraf (#2016), that form did not change the time, and NoInfer around the whole element type did.
• Remedies are short instructions. They name a direction. whyts did not measure any remedy on your project.

Real cases

whyts found the place in each case below. The tRPC fix is merged. The other fixes are open pull requests in those projects, so they are proposals until the maintainers merge them. The Vuetify row is a measurement in an issue, with no fix. Each number comes from my own measurement on the stated compiler version.

| Vue (#14323) | defineModel with a defineEmits call signature that ends in void makes OverloadUnionRecursive (@vue/shared) run to the depth limit. It is 100% of the Check time | A fix in [PR #15851](https://githu