`tsc -b --watch` doesn't rebuild when an imported file changes, unless `incremental` is on

[Generalsimus]

🔎 Search Terms

tsc -b --watch, tsc --build --watch, imported file not rebuilt, incremental, tsbuildinfo

🕗 Version & Regression Information

• This is the behavior in every version I tried

I tried 6.0.3 and today's nightly (7.1.0-dev.20260924.1). In 7 it only happens without incremental or composite. In 6.0.3 it happens even with incremental.

⏯ Playground Link

No response

💻 Code

mkdir proj && cd proj
echo '{"compilerOptions":{"outDir":"out","module":"nodenext","target":"es2022"},"files":["index.ts"]}' > tsconfig.json
echo '{"type":"module"}' > package.json
echo 'import { s } from "./other.js"; export const x = s;' > index.ts
echo 'export const s = "AAA";' > other.ts
npx tsc -b --watch

# in another terminal, once the first build is done:
echo 'export const s = "BBB";' > other.ts

🙁 Actual behavior

Editing other.ts does nothing, and out/other.js still has "AAA". Editing index.ts does rebuild.

Here's what I got with the same project:

• tsc -b --watch: no rebuild
• tsc -b --watch with "incremental": true: rebuilds
• tsc -b --watch with "composite": true: rebuilds
• tsc --watch (no -b): rebuilds

Same thing if the imported file is outside include, like ../shared/s.ts.

🙂 Expected behavior

tsc -b --watch should rebuild when any file in the program changes, like it already does with incremental on, or like tsc --watch does.

Additional information about the issue

It looks like the build orchestrator only finds out about non-root files from the .tsbuildinfo, both when it checks events and when it picks directories to watch:

• TypeScript/tsc/internal/execute/build/orchestrator.go

  Lines 430 to 446 in df1a31e

  	if !rootChanged {
  	task.buildInfoEntryMu.Lock()
  	bi := task.buildInfoEntry
  	task.buildInfoEntryMu.Unlock()
  	if bi != nil && bi.buildInfo != nil {
  	buildInfoDir := tspath.GetDirectoryPath(string(bi.path))
  	for _, fileName := range bi.buildInfo.FileNames {
  	fp := o.toPath(o.resolveBuildInfoFileName(fileName, buildInfoDir))
  	if roots.Has(fp) {
  	continue
  	}
  	if _, changed := normalizedPaths[fp]; changed {
  	task.resetStatus()
  	needsUpdate.Store(true)
  	break
  	}
  	}

• TypeScript/tsc/internal/execute/build/orchestrator.go

  Lines 576 to 593 in df1a31e

  	// Non-root dependency directories from buildinfo (e.g. node_modules .d.ts files).
  	task.buildInfoEntryMu.Lock()
  	bi := task.buildInfoEntry
  	task.buildInfoEntryMu.Unlock()
  	if bi != nil && bi.buildInfo != nil {
  	buildInfoDir := tspath.GetDirectoryPath(string(bi.path))
  	roots := collections.NewSetFromItems(core.Map(task.resolved.FileNames(), o.toPath)...)
  	for _, fileName := range bi.buildInfo.FileNames {
  	absPath := o.host.FS().Realpath(o.resolveBuildInfoFileName(fileName, buildInfoDir))
  	fp := o.toPath(absPath)
  	if roots.Has(fp) {
  	continue
  	}
  	dir := tspath.GetDirectoryPath(absPath)
  	if !desiredDirs.Covered(dir) && watchmanager.CanWatchDirectory(dir) {
  	desiredDirs.Set(dir, false)
  	}
  	}

Without incremental or composite there's no buildinfo. So the change event for other.ts does come in (its folder is watched), but nothing picks it up and it gets dropped.

6.0 had the same gap, since its tsc -b only watched parsed.fileNames. So it's not a regression, but it's a bit surprising that in 7 the answer depends on incremental.

This came up in #64366, where I was asked to open a separate issue.

(I used an AI assistant to help write this up.)