[fjakobs]

Problem

In development, the AppKit server builds the app with Vite (client) and tsx watch (server). Both only strip types. An app with type errors still renders in the dev preview, HMR applies the edits, and the server even runs the badly typed code. The dev log says nothing. The errors first appear when databricks apps validate or databricks apps deploy runs the app's typecheck script.

A developer, or a coding agent working in the dev loop, gets no signal until they ask to deploy. Then they have to fix, re-verify and deploy again.

A second gap made this worse: the template's typecheck script was tsc -b tsconfig.server.json && tsc -b tsconfig.client.json. When the server check failed, the && stopped the run, so client errors were never printed. Fixing the server errors and deploying again only revealed the client errors on the next attempt.

Fix

• Dev server reports type errors. A new Vite plugin, packages/appkit/src/plugins/server/typecheck-vite-plugin.ts, is registered in ViteDevServer and only runs in development. It runs the app's own typecheck script, the same check deployment validation runs, in the background:

  • once at startup (tsx watch restarts the process on server edits, so this covers server changes)
  • again after .ts/.tsx/.mts/.cts changes seen by Vite's watcher, debounced. The watcher also covers shared/, so regenerated appkit-types re-trigger the check.
  • at most one run at a time: a change during a run schedules one more run, and the stale result is not reported
  • errors are logged as [appkit:server:typecheck] Type errors found. The app still runs, but deployment validation fails until they are fixed: followed by the tsc output. No type errors. is logged on the first clean run and after errors are fixed.
  • the child process is stopped when the Vite server closes
  • it does nothing when package.json has no typecheck script

  It never blocks startup or requests, so the preview behaves exactly as before.

• Template: typecheck is now tsc -b. The root tsconfig.json already references both projects, so one run reports client and server errors together.

• Docs: local-development.mdx explains the background check. The LLM checklist in llm-guide.md, which ships in the package's CLAUDE.md/llms.txt, now says the typecheck script must pass and that the dev server logs type errors.

Tests

packages/appkit/src/plugins/server/tests/typecheck-vite-plugin.test.ts uses a real temp project with a small stand-in typecheck script and a fake watcher. It covers:

• the plugin is not applied without a typecheck script
• failures are logged with the script's output, then "No type errors." after a fix and a .tsx change
• non-TypeScript changes don't trigger a run
• closing the server stops an in-flight run without reporting it

Verification

The repro app was scaffolded from this template with databricks apps init --template ./template, plus one type error in client/src/App.tsx and one in server/server.ts.

Before (published AppKit):

• pnpm run dev: / returns 200 and the server runs the bad value. The dev log has no type errors, including after edits.
• databricks apps deploy against a Databricks workspace fails validation and reports only the server error.

After (this branch, packed and installed into the same app):

• pnpm run dev logs both errors about a second after startup, and / still returns 200.
• Fixing the client error leaves only the server error. Fixing the server error makes tsx restart, and the log says No type errors. Breaking the client again brings the error back.
• databricks apps validate, and databricks apps deploy against a Databricks workspace, report both client and server errors in one run.
• After shutdown, no tsc processes are left behind.
• With the errors fixed, the app (with this build of AppKit) deployed to the workspace: the deployment SUCCEEDED, the app is RUNNING, and the health check passed. The runtime log shows Mode: production (static) and no type-check activity, so the check stays dev-only.

Repo checks: pnpm build && pnpm docs:build, pnpm check:fix && pnpm -r typecheck, and pnpm test all pass.

This pull request and its description were written by Isaac.

[github-actions]

📦 Bundle size report

Compared against bundle-size-baseline.json (main).

@databricks/appkit

npm tarball (packed): 1.2 MB (+2.8 KB) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	1.3 MB (+2.8 KB)	461 KB (+1.3 KB)
Type declarations	465 KB	169 KB (-1 B)
Source maps	2.5 MB (+5.1 KB)	861 KB (+2.1 KB)
Other	11 KB	3.7 KB
Total	4.3 MB (+8.0 KB)	1.5 MB (+3.3 KB)

Per-entry composition (own code — deps external (as shipped))

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
.	107 KB (+495 B)	2.5 KB	109 KB (+495 B)	external	345 KB (+1.2 KB)
./beta	103 KB	482 B	104 KB	external	310 KB
./testing	45 KB (-1 B)	33 KB (+491 B)	78 KB (+490 B)	external	225 KB (+1.2 KB)
./tsdown	520 B	0 B	520 B	external	813 B
./type-generator	26 KB	0 B	26 KB	external	74 KB

Chunks:

Entry	Chunk	Load	Size (gz)
.	index.js	initial	102 KB
.	utils.js	initial	4.6 KB
.	remote-tunnel-manager.js	lazy	2.5 KB
./beta	beta.js	initial	83 KB
./beta	modular.js	initial	6.0 KB
./beta	stream-manager.js	initial	5.9 KB
./beta	service-context.js	initial	4.2 KB
./beta	databricks.js	initial	3.3 KB
./beta	client.js	initial	647 B
./beta	index.js	initial	20 B
./beta	supervisor-api.js	lazy	192 B
./beta	databricks.js	lazy	176 B
./beta	index.js	lazy	114 B
./testing	manifest.js	initial	32 KB
./testing	index.js	initial	10 KB
./testing	wide-event-emitter.js	initial	2.9 KB
./testing	index.js	lazy	28 KB
./testing	remote-tunnel-manager.js	lazy	2.5 KB
./testing	utils.js	lazy	1.8 KB
./tsdown	index.js	initial	520 B
./type-generator	index.js	initial	26 KB

@databricks/appkit-ui

npm tarball (packed): 397 KB (-4 B) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	429 KB	147 KB
Type declarations	267 KB	98 KB (-1 B)
Source maps	853 KB	286 KB
CSS	16 KB	3.2 KB
Total	1.5 MB	534 KB (-1 B)

Per-entry composition (consumer bundle — deps bundled, peerDeps external)

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
./js	5.3 KB	49 KB	55 KB	208 KB	14 KB
./js/beta	2.1 KB	0 B	2.1 KB	0 B	5.0 KB
./react	433 KB	49 KB	481 KB	1.3 MB	177 KB
./react/beta	4.9 KB	0 B	4.9 KB	0 B	12 KB

Chunks:

Entry	Chunk	Load	Size (gz)
./js	index.js	initial	5.2 KB
./js	chunk	initial	120 B
./js	apache-arrow	lazy	49 KB
./js/beta	beta.js	initial	2.1 KB
./react	index.js	initial	431 KB
./react	tslib	initial	2.1 KB
./react	apache-arrow	lazy	49 KB
./react/beta	beta.js	initial	4.9 KB

[github-actions]

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📦 Try this PR's app template

Scaffolds a new app from this PR's SDK build. Run it in any folder (requires the GitHub CLI — gh auth login — and the Databricks CLI):

gh run download 42243386058 -R databricks/appkit -n appkit-template-0.90.0-pr.d4601a2-fjakobs-la-2186-typecheck-in-build-loop-715 -D appkit-pr-715 \
  && unzip -o "appkit-pr-715/appkit-template-0.90.0-pr.d4601a2-fjakobs-la-2186-typecheck-in-build-loop-715.zip" -d "appkit-pr-715" \
  && databricks apps init --template "appkit-pr-715"

The template pins @databricks/appkit and @databricks/appkit-ui to tarballs built from this branch, so the scaffolded app runs against this PR's code.