diff --git a/packages/angular/build/src/builders/library/builder.ts b/packages/angular/build/src/builders/library/builder.ts
index c3c2149adf5b..8fb7e047f09f 100644
--- a/packages/angular/build/src/builders/library/builder.ts
+++ b/packages/angular/build/src/builders/library/builder.ts
@@ -127,7 +127,10 @@ export async function* executeLibraryBuilder(
         poll,
         preserveSymlinks,
         signal,
-        watchFiles: watchedCompilationFiles,
+        watchFiles: [
+          ...watchedCompilationFiles,
+          ...normalizedOptions.assets.map((asset) => asset.input),
+        ],
       });
 
       context.addTeardown?.(() => void watcher?.close());
@@ -224,9 +227,9 @@ async function executeBuild(
     if (watcher) {
       watcher.add(Array.from(watchedCompilationFiles));
 
-      const { assetsToEmit } = actionContext;
-      if (assetsToEmit?.length) {
-        watcher.add(assetsToEmit.map((asset) => asset.source));
+      const { collectedAssets } = actionContext;
+      if (collectedAssets?.filesToEmit.length) {
+        watcher.add(collectedAssets.filesToEmit.map((asset) => asset.source));
       }
     }
   }
@@ -337,13 +340,19 @@ async function* runWatchLoop(
       buildState.hasEntryPointsChanges ||
       hasModifiedWatchedFile(changedFiles, watchedCompilationFiles, packageJsonPath);
 
-    const assetsToEmit = await collectAssetsToEmit(
+    const collectedAssets = await collectAssetsToEmit(
       assets,
       workspaceRoot,
       buildState.hasEmittedAssets ? changedFiles : undefined,
+      buildState.emittedAssets,
     );
 
-    if (!hasSourceChanges && !hasPackageJsonChanges && assetsToEmit.length === 0) {
+    if (
+      !hasSourceChanges &&
+      !hasPackageJsonChanges &&
+      collectedAssets.filesToEmit.length === 0 &&
+      collectedAssets.filesToDelete.length === 0
+    ) {
       continue;
     }
 
@@ -357,7 +366,7 @@ async function* runWatchLoop(
         context,
         buildState,
         modifiedFiles: changedFiles,
-        assetsToEmit,
+        collectedAssets,
       },
       withProgress,
       watcher,
diff --git a/packages/angular/build/src/builders/library/pipeline/assets.ts b/packages/angular/build/src/builders/library/pipeline/assets.ts
index 7b8329a7865a..33efe643eeda 100644
--- a/packages/angular/build/src/builders/library/pipeline/assets.ts
+++ b/packages/angular/build/src/builders/library/pipeline/assets.ts
@@ -14,27 +14,60 @@ import type { NormalizedLibraryOptions } from '../types';
 import { type DiskOutputFile, createDiskOutputFile } from './utils';
 
 /**
- * Resolves and collects configured library assets to be emitted to disk.
+ * Result of resolving and collecting library assets for a build iteration.
+ */
+export interface CollectedAssets {
+  /** Asset files to emit to disk, with POSIX source and destination paths. */
+  filesToEmit: DiskOutputFile[];
+
+  /** Previously emitted asset files whose source files were deleted and should be removed from output. */
+  filesToDelete: DiskOutputFile[];
+}
+
+interface AssetMatcher {
+  asset: NormalizedLibraryOptions['assets'][number];
+  prefix: string;
+  isMatch: picomatch.Matcher;
+}
+
+const assetMatcherCache = new WeakMap<NormalizedLibraryOptions['assets'], AssetMatcher[]>();
+
+/**
+ * Resolves and collects configured library assets to be emitted to or deleted from disk.
  *
- * @param assets The normalized asset patterns with POSIX paths.
- * @param workspaceRoot The normalized POSIX workspace root directory path.
+ * @param assets The normalized asset patterns with POSIX input and output paths.
+ * @param workspaceRoot The workspace root POSIX directory path.
  * @param modifiedFiles Optional set of modified POSIX file paths for incremental copying in watch mode.
- * @returns An array of disk file emission descriptors.
+ * @param emittedAssets Optional map of previously emitted POSIX source file paths to relative POSIX output paths.
+ * @returns The collected asset files to emit and delete.
  */
 export async function collectAssetsToEmit(
   assets: NormalizedLibraryOptions['assets'],
   workspaceRoot: string,
   modifiedFiles?: ReadonlySet<string>,
-): Promise<DiskOutputFile[]> {
+  emittedAssets?: ReadonlyMap<string, string>,
+): Promise<CollectedAssets> {
   if (assets.length === 0 || modifiedFiles?.size === 0) {
-    return [];
+    return { filesToEmit: [], filesToDelete: [] };
   }
 
   if (modifiedFiles) {
-    const matchers = createAssetMatchers(assets);
+    const matchers = getAssetMatchers(assets);
     const filesToEmit: DiskOutputFile[] = [];
+    const filesToDelete: DiskOutputFile[] = [];
 
     for (const file of modifiedFiles) {
+      let isFile: boolean | undefined;
+
+      const previousOutput = emittedAssets?.get(file);
+      if (previousOutput !== undefined) {
+        isFile = statSync(file, { throwIfNoEntry: false })?.isFile() ?? false;
+        if (!isFile) {
+          filesToDelete.push(createDiskOutputFile(file, previousOutput));
+          continue;
+        }
+      }
+
       for (const { asset, prefix, isMatch } of matchers) {
         if (!file.startsWith(prefix)) {
           continue;
@@ -45,28 +78,46 @@ export async function collectAssetsToEmit(
           continue;
         }
 
-        if (statSync(file, { throwIfNoEntry: false })?.isFile()) {
+        isFile ??= statSync(file, { throwIfNoEntry: false })?.isFile() ?? false;
+        if (isFile) {
           filesToEmit.push(createDiskOutputFile(file, path.posix.join(asset.output, relative)));
         }
       }
     }
 
-    return filesToEmit;
+    return { filesToEmit, filesToDelete };
   }
 
   const resolvedAssets = await resolveAssets(assets, workspaceRoot);
 
-  return resolvedAssets.map(({ source, destination }) => createDiskOutputFile(source, destination));
+  return {
+    filesToEmit: resolvedAssets.map(({ source, destination }) =>
+      createDiskOutputFile(source, destination),
+    ),
+    filesToDelete: [],
+  };
 }
 
-function createAssetMatchers(assets: NormalizedLibraryOptions['assets']) {
-  return assets.map((asset) => {
-    const isMatch = picomatch(asset.glob, {
-      dot: true,
-      ignore: [...DEFAULT_ASSET_IGNORE, ...(asset.ignore ?? [])],
+/**
+ * Returns cached or newly compiled picomatch matchers and POSIX input prefixes for each asset configuration.
+ *
+ * @param assets The normalized asset patterns with POSIX input and output paths.
+ * @returns An array of asset matcher descriptors.
+ */
+function getAssetMatchers(assets: NormalizedLibraryOptions['assets']): AssetMatcher[] {
+  let matchers = assetMatcherCache.get(assets);
+  if (!matchers) {
+    matchers = assets.map((asset) => {
+      const isMatch = picomatch(asset.glob, {
+        dot: true,
+        ignore: [...DEFAULT_ASSET_IGNORE, ...(asset.ignore ?? [])],
+      });
+      const prefix = asset.input.endsWith('/') ? asset.input : `${asset.input}/`;
+
+      return { asset, prefix, isMatch };
     });
-    const prefix = asset.input.endsWith('/') ? asset.input : `${asset.input}/`;
+    assetMatcherCache.set(assets, matchers);
+  }
 
-    return { asset, prefix, isMatch };
-  });
+  return matchers;
 }
diff --git a/packages/angular/build/src/builders/library/pipeline/build-action.ts b/packages/angular/build/src/builders/library/pipeline/build-action.ts
index f121048b1fc7..46efd576fc81 100644
--- a/packages/angular/build/src/builders/library/pipeline/build-action.ts
+++ b/packages/angular/build/src/builders/library/pipeline/build-action.ts
@@ -9,15 +9,15 @@
 import type { BuilderContext } from '@angular-devkit/architect';
 import path from 'node:path';
 import type { ComponentStylesheetBundler } from '../../../tools/esbuild/angular/component-stylesheets';
-import { emitFilesToDisk } from '../../../utils/disk-files';
+import { deleteFilesFromDisk, emitFilesToDisk } from '../../../utils/disk-files';
 import { assertIsError } from '../../../utils/error';
-import type { NormalizedLibraryOptions, SingleBuildState } from '../types';
-import { collectAssetsToEmit } from './assets';
+import type { BundleResult, NormalizedLibraryOptions, SingleBuildState } from '../types';
+import { type CollectedAssets, collectAssetsToEmit } from './assets';
 import { bundleEntryPoints } from './bundler';
 import { type LibraryCompilationDiagnostics, compileLibrary } from './compilation';
 import { collectEntryPointsToBundle } from './entry-points';
 import { generatePackageManifests } from './package-manifests';
-import type { DiskOutputFile, OutputFile } from './utils';
+import type { OutputFile } from './utils';
 
 /**
  * Result of executing a single library build iteration.
@@ -41,6 +41,9 @@ export function createSingleBuildState(): SingleBuildState {
     previousBundleResults: new Map(),
     pendingChangedEsmFiles: new Set(),
     pendingChangedDtsFiles: new Set(),
+    emittedBundleFiles: new Set(),
+    emittedManifestFiles: new Set(),
+    emittedAssets: new Map(),
   };
 }
 
@@ -76,12 +79,12 @@ export interface BuildActionContext {
   modifiedFiles?: Set<string>;
 
   /**
-   * Asset files to emit to disk for the current build iteration.
+   * Collected asset changes for the current build iteration.
    * Pre-collected in the watch loop to avoid redundant asset matching, or resolved and
    * populated by `buildAction` when omitted (such as during the initial build) so the
-   * caller can register their source paths with the file watcher.
+   * caller can register their POSIX source paths with the file watcher.
    */
-  assetsToEmit?: DiskOutputFile[];
+  collectedAssets?: CollectedAssets;
 }
 
 /**
@@ -133,8 +136,12 @@ export async function buildAction(actionContext: BuildActionContext): Promise<Bu
     }
 
     const filesToEmit: OutputFile[] = [];
+    const filesToDelete = new Set<string>();
+    let nextBundleFiles: Set<string> | undefined;
+    let nextManifestFiles: Set<string> | undefined;
 
     if (shouldCompileEntryPoints) {
+      const hadEntryPointsChanges = buildState.hasEntryPointsChanges;
       buildState.hasCompilationError = true;
       buildState.hasEntryPointsChanges = false;
 
@@ -212,6 +219,18 @@ export async function buildAction(actionContext: BuildActionContext): Promise<Bu
       for (const [name, bundleResult] of bundleOutput.bundleResults) {
         buildState.previousBundleResults.set(name, bundleResult);
       }
+
+      if (
+        bundleOutput.bundleResults.size > 0 ||
+        hadEntryPointsChanges ||
+        buildState.emittedBundleFiles.size === 0
+      ) {
+        nextBundleFiles = collectBundleOutputFiles(
+          buildState.previousBundleResults.values(),
+          buildState.emittedBundleFiles,
+          filesToDelete,
+        );
+      }
     }
 
     let hasTslibImport = false;
@@ -223,27 +242,45 @@ export async function buildAction(actionContext: BuildActionContext): Promise<Bu
     }
 
     if (!buildState.hasEmittedManifests || buildState.hasEmittedTslib !== hasTslibImport) {
-      filesToEmit.push(
-        ...(await generatePackageManifests(
-          {
-            ...options,
-            compilationMode: buildState.compilationMode ?? options.compilationMode ?? 'partial',
-          },
-          isWatchMode,
-          hasTslibImport,
-        )),
+      const manifestFiles = await generatePackageManifests(
+        {
+          ...options,
+          compilationMode: buildState.compilationMode ?? options.compilationMode ?? 'partial',
+        },
+        isWatchMode,
+        hasTslibImport,
       );
+
+      filesToEmit.push(...manifestFiles);
+
+      nextManifestFiles = new Set(manifestFiles.map((file) => file.path));
+      for (const prevFile of buildState.emittedManifestFiles) {
+        if (!nextManifestFiles.has(prevFile)) {
+          filesToDelete.add(prevFile);
+        }
+      }
     }
 
-    const resolvedAssetsToEmit = (actionContext.assetsToEmit ??= await collectAssetsToEmit(
+    const collectedAssets = (actionContext.collectedAssets ??= await collectAssetsToEmit(
       options.assets,
       options.workspaceRoot,
       buildState.hasEmittedAssets ? modifiedFiles : undefined,
+      buildState.emittedAssets,
     ));
-    filesToEmit.push(...resolvedAssetsToEmit);
+    filesToEmit.push(...collectedAssets.filesToEmit);
 
     await emitFilesToDisk(filesToEmit, (file) => path.posix.join(options.outputPath, file.path));
 
+    updateEmittedAssetState(buildState, collectedAssets, filesToDelete);
+
+    if (filesToDelete.size) {
+      await deleteFilesFromDisk(
+        Array.from(filesToDelete, (file) => path.join(options.outputPath, file)),
+      );
+    }
+
+    buildState.emittedBundleFiles = nextBundleFiles ?? buildState.emittedBundleFiles;
+    buildState.emittedManifestFiles = nextManifestFiles ?? buildState.emittedManifestFiles;
     buildState.hasEmittedManifests = true;
     buildState.hasEmittedTslib = hasTslibImport;
     buildState.hasEmittedAssets = true;
@@ -255,6 +292,79 @@ export async function buildAction(actionContext: BuildActionContext): Promise<Bu
   return { errors, warnings, logs };
 }
 
+/**
+ * Collects the active POSIX bundle output file paths across all entry points and queues any
+ * previously emitted bundle files that are no longer present for deletion.
+ *
+ * @param bundleResults Active bundle results across all entry points.
+ * @param previousBundleFiles Relative POSIX output paths emitted in the previous build iteration.
+ * @param filesToDelete Set of relative POSIX output paths queued for deletion.
+ * @returns The new set of active relative POSIX bundle output file paths.
+ */
+function collectBundleOutputFiles(
+  bundleResults: Iterable<BundleResult>,
+  previousBundleFiles: ReadonlySet<string>,
+  filesToDelete: Set<string>,
+): Set<string> {
+  const nextBundleFiles = new Set<string>();
+  for (const result of bundleResults) {
+    if (result.esmOutputFiles) {
+      for (const file of result.esmOutputFiles) {
+        nextBundleFiles.add(file);
+      }
+    }
+    if (result.dtsOutputFiles) {
+      for (const file of result.dtsOutputFiles) {
+        nextBundleFiles.add(file);
+      }
+    }
+  }
+
+  for (const prevFile of previousBundleFiles) {
+    if (!nextBundleFiles.has(prevFile)) {
+      filesToDelete.add(prevFile);
+    }
+  }
+
+  return nextBundleFiles;
+}
+
+/**
+ * Updates `buildState.emittedAssets` with newly emitted and deleted assets, queuing any
+ * deleted asset POSIX output paths that are not produced by another active asset for removal.
+ *
+ * @param buildState The state preserved across incremental builds.
+ * @param collectedAssets The collected asset files to emit and delete.
+ * @param filesToDelete Set of relative POSIX output paths queued for deletion.
+ */
+function updateEmittedAssetState(
+  buildState: SingleBuildState,
+  collectedAssets: CollectedAssets,
+  filesToDelete: Set<string>,
+): void {
+  const emittedAssets = buildState.emittedAssets;
+  const toDelete = collectedAssets.filesToDelete;
+  const toEmit = collectedAssets.filesToEmit;
+
+  for (const deletedAsset of toDelete) {
+    emittedAssets.delete(deletedAsset.source);
+  }
+
+  for (const emittedAsset of toEmit) {
+    emittedAssets.set(emittedAsset.source, emittedAsset.path);
+  }
+
+  if (toDelete.length) {
+    const activeAssetOutputs = new Set(emittedAssets.values());
+
+    for (const deletedAsset of toDelete) {
+      if (!activeAssetOutputs.has(deletedAsset.path)) {
+        filesToDelete.add(deletedAsset.path);
+      }
+    }
+  }
+}
+
 /**
  * Checks whether any modified file (excluding `package.json`) is part of the
  * tracked compilation file set.
diff --git a/packages/angular/build/src/builders/library/pipeline/bundler.ts b/packages/angular/build/src/builders/library/pipeline/bundler.ts
index c48942885083..a8057a126a7d 100644
--- a/packages/angular/build/src/builders/library/pipeline/bundler.ts
+++ b/packages/angular/build/src/builders/library/pipeline/bundler.ts
@@ -63,6 +63,9 @@ interface MultiBundleOutput {
   /** Map of bundle entry names to the set of virtual POSIX module IDs included in the bundle. */
   moduleIdsByBundle: Map<string, Set<string>>;
 
+  /** Map of bundle entry names to the set of relative POSIX output paths generated for the bundle. */
+  outputFilesByBundle: Map<string, Set<string>>;
+
   /** Set of bundle entry names that import 'tslib'. */
   tslibBundles: Set<string>;
 
@@ -117,6 +120,14 @@ export async function bundleEntryPoints(
         dtsOutput.moduleIdsByBundle.get(bundleName) ??
         previousBundleResult?.dtsModuleIds ??
         new Set(),
+      esmOutputFiles:
+        esmOutput.outputFilesByBundle.get(bundleName) ??
+        previousBundleResult?.esmOutputFiles ??
+        new Set(),
+      dtsOutputFiles:
+        dtsOutput.outputFilesByBundle.get(bundleName) ??
+        previousBundleResult?.dtsOutputFiles ??
+        new Set(),
       hasTslibImport: esmModuleIds
         ? esmOutput.tslibBundles.has(bundleName)
         : (previousBundleResult?.hasTslibImport ?? false),
@@ -232,11 +243,14 @@ function processRolldownOutput(
 ): Omit<MultiBundleOutput, 'warnings'> {
   const filesToEmit: MemoryOutputFile[] = [];
   const moduleIdsByBundle = new Map<string, Set<string>>();
+  const outputFilesByBundle = new Map<string, Set<string>>();
   const tslibBundles = new Set<string>();
   const chunksByFileName = new Map<string, OutputChunk>();
+  const outputFileNames = new Set<string>();
   const entryChunks: OutputChunk[] = [];
 
   for (const item of output) {
+    outputFileNames.add(item.fileName);
     filesToEmit.push(
       createMemoryOutputFile(
         path.posix.join(dir, item.fileName),
@@ -254,7 +268,9 @@ function processRolldownOutput(
 
   for (const entryChunk of entryChunks) {
     const modSet = new Set<string>();
+    const outSet = new Set<string>();
     moduleIdsByBundle.set(entryChunk.name, modSet);
+    outputFilesByBundle.set(entryChunk.name, outSet);
 
     const visited = new Set<OutputChunk>();
     const queue: OutputChunk[] = [entryChunk];
@@ -270,6 +286,11 @@ function processRolldownOutput(
       }
 
       visited.add(chunk);
+      outSet.add(path.posix.join(dir, chunk.fileName));
+      const mapFileName = `${chunk.fileName}.map`;
+      if (outputFileNames.has(mapFileName)) {
+        outSet.add(path.posix.join(dir, mapFileName));
+      }
 
       for (const modId of chunk.moduleIds) {
         if (modId[0] !== '\0') {
@@ -290,7 +311,7 @@ function processRolldownOutput(
     }
   }
 
-  return { filesToEmit, moduleIdsByBundle, tslibBundles };
+  return { filesToEmit, moduleIdsByBundle, outputFilesByBundle, tslibBundles };
 }
 
 /**
@@ -394,6 +415,7 @@ async function bundleAllEsm(
     return {
       filesToEmit: [],
       moduleIdsByBundle: new Map(),
+      outputFilesByBundle: new Map(),
       tslibBundles: new Set(),
       warnings: [],
     };
@@ -424,6 +446,7 @@ async function bundleAllDts(
     return {
       filesToEmit: [],
       moduleIdsByBundle: new Map(),
+      outputFilesByBundle: new Map(),
       tslibBundles: new Set(),
       warnings: [],
     };
diff --git a/packages/angular/build/src/builders/library/pipeline/entry-points.ts b/packages/angular/build/src/builders/library/pipeline/entry-points.ts
index 5cff4ca82136..955ff3fa1904 100644
--- a/packages/angular/build/src/builders/library/pipeline/entry-points.ts
+++ b/packages/angular/build/src/builders/library/pipeline/entry-points.ts
@@ -260,6 +260,10 @@ export function updateWatchedEntryPoints(
   );
 
   if (haveEntryPointsChanged(options.entryPoints, newEntryPoints)) {
+    for (const oldEntryPoint of options.entryPoints.values()) {
+      watchedCompilationFiles.delete(oldEntryPoint.entryFilePath);
+    }
+
     for (const entryPoint of newEntryPoints.values()) {
       watchedCompilationFiles.add(entryPoint.entryFilePath);
     }
diff --git a/packages/angular/build/src/builders/library/pipeline/entry-points_spec.ts b/packages/angular/build/src/builders/library/pipeline/entry-points_spec.ts
index 834b7aec6835..c79dc7c36c3e 100644
--- a/packages/angular/build/src/builders/library/pipeline/entry-points_spec.ts
+++ b/packages/angular/build/src/builders/library/pipeline/entry-points_spec.ts
@@ -7,7 +7,6 @@
  */
 
 import { join, resolve } from 'node:path';
-import { toPosixPath } from '../../../utils/path';
 import type {
   BundleResult,
   NormalizedEntryPoint,
@@ -530,7 +529,10 @@ describe('entry-points pipeline', () => {
         hasEntryPointsChanges: false,
       } as unknown as SingleBuildState;
 
-      const watchedCompilationFiles = new Set<string>();
+      const watchedCompilationFiles = new Set<string>([
+        join(projectRoot, 'src/public-api.ts'),
+        join(projectRoot, 'old-feature/src/public-api.ts'),
+      ]);
 
       const newPackageJson: PackageJsonData = {
         name: packageName,
@@ -553,13 +555,12 @@ describe('entry-points pipeline', () => {
       expect(options.entryPoints.has('old-feature')).toBeFalse();
       expect(buildState.previousBundleResults.has('old-feature')).toBeFalse();
       expect(buildState.previousBundleResults.has('.')).toBeTrue();
+      expect(watchedCompilationFiles.has(join(projectRoot, 'src/public-api.ts'))).toBeTrue();
       expect(
-        watchedCompilationFiles.has(toPosixPath(join(projectRoot, 'src/public-api.ts'))),
-      ).toBeTrue();
+        watchedCompilationFiles.has(join(projectRoot, 'old-feature/src/public-api.ts')),
+      ).toBeFalse();
       expect(
-        watchedCompilationFiles.has(
-          toPosixPath(join(projectRoot, 'new-feature/src/public-api.ts')),
-        ),
+        watchedCompilationFiles.has(join(projectRoot, 'new-feature/src/public-api.ts')),
       ).toBeTrue();
     });
 
@@ -657,9 +658,7 @@ describe('entry-points pipeline', () => {
       expect(options.entryPoints.get('.')?.entryFilePath).toBe(
         join(projectRoot, 'src/other-api.ts'),
       );
-      expect(
-        watchedCompilationFiles.has(toPosixPath(join(projectRoot, 'src/other-api.ts'))),
-      ).toBeTrue();
+      expect(watchedCompilationFiles.has(join(projectRoot, 'src/other-api.ts'))).toBeTrue();
     });
 
     it('should prune previousBundleResults when an entry point bundleName changes', () => {
diff --git a/packages/angular/build/src/builders/library/tests/behavior/watch-cleanup_spec.ts b/packages/angular/build/src/builders/library/tests/behavior/watch-cleanup_spec.ts
new file mode 100644
index 000000000000..b0096998496e
--- /dev/null
+++ b/packages/angular/build/src/builders/library/tests/behavior/watch-cleanup_spec.ts
@@ -0,0 +1,172 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import { executeLibraryBuilder } from '../../builder';
+import { BASE_OPTIONS, LIBRARY_BUILDER_INFO, describeLibraryBuilder } from '../setup';
+
+describeLibraryBuilder(executeLibraryBuilder, LIBRARY_BUILDER_INFO, (harness) => {
+  describe('Behavior: "Watch Mode Asset and Output Cleanup"', () => {
+    it('should emit newly created assets, respect ignore patterns, and clean up deleted assets in watch mode', async () => {
+      await harness.writeFiles({
+        'projects/lib/assets-dir/keep.txt': 'keep',
+      });
+
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        assets: [
+          {
+            glob: '**/*.txt',
+            input: 'projects/lib/assets-dir',
+            output: 'assets',
+            ignore: ['**/ignored.txt'],
+          },
+        ],
+        watch: true,
+      });
+
+      await harness.executeWithCases([
+        async ({ result }) => {
+          expect(result?.success).toBeTrue();
+          expect(harness.readFile('dist/lib/assets/keep.txt')).toBe('keep');
+
+          // Create an ignored file and a matching nested asset file
+          await harness.writeFiles({
+            'projects/lib/assets-dir/sub/ignored.txt': 'ignored',
+            'projects/lib/assets-dir/sub/added.txt': 'added',
+          });
+        },
+        async ({ result }) => {
+          expect(result?.success).toBeTrue();
+          expect(harness.readFile('dist/lib/assets/sub/added.txt')).toBe('added');
+          expect(harness.hasFile('dist/lib/assets/sub/ignored.txt')).toBeFalse();
+
+          // Remove the added asset file
+          await harness.removeFile('projects/lib/assets-dir/sub/added.txt');
+        },
+        ({ result }) => {
+          expect(result?.success).toBeTrue();
+          expect(harness.readFile('dist/lib/assets/keep.txt')).toBe('keep');
+          expect(harness.hasFile('dist/lib/assets/sub/added.txt')).toBeFalse();
+        },
+      ]);
+    });
+
+    it('should clean up output files when a secondary entry point is removed in watch mode', async () => {
+      await harness.writeFile(
+        'projects/lib/secondary/src/public-api.ts',
+        `export const secondary = 'secondary-value';`,
+      );
+
+      await harness.modifyFile('projects/lib/package.json', (content) => {
+        const pkg = JSON.parse(content);
+        pkg.exports = {
+          '.': './src/public-api.ts',
+          './secondary': './secondary/src/public-api.ts',
+        };
+
+        return JSON.stringify(pkg, null, 2);
+      });
+
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        declarationMap: true,
+        watch: true,
+      });
+
+      await harness.executeWithCases([
+        async ({ result }) => {
+          expect(result?.success).toBeTrue();
+          expect(harness.hasFile('dist/lib/fesm2022/lib.mjs')).toBeTrue();
+          expect(harness.hasFile('dist/lib/fesm2022/lib-secondary.mjs')).toBeTrue();
+          expect(harness.hasFile('dist/lib/fesm2022/lib-secondary.mjs.map')).toBeTrue();
+          expect(harness.hasFile('dist/lib/types/lib-secondary.d.ts')).toBeTrue();
+          expect(harness.hasFile('dist/lib/types/lib-secondary.d.ts.map')).toBeTrue();
+          expect(harness.hasFile('dist/lib/secondary/package.json')).toBeTrue();
+          expect(harness.hasFile('dist/lib/.npmignore')).toBeTrue();
+
+          // Remove the secondary entry point from package.json exports
+          const pkg = JSON.parse(harness.readFile('projects/lib/package.json'));
+          pkg.exports = {
+            '.': './src/public-api.ts',
+          };
+          await harness.writeFile('projects/lib/package.json', JSON.stringify(pkg, null, 2));
+        },
+        ({ result }) => {
+          expect(result?.success).toBeTrue();
+          expect(harness.hasFile('dist/lib/fesm2022/lib.mjs')).toBeTrue();
+          expect(harness.hasFile('dist/lib/fesm2022/lib-secondary.mjs')).toBeFalse();
+          expect(harness.hasFile('dist/lib/fesm2022/lib-secondary.mjs.map')).toBeFalse();
+          expect(harness.hasFile('dist/lib/types/lib-secondary.d.ts')).toBeFalse();
+          expect(harness.hasFile('dist/lib/types/lib-secondary.d.ts.map')).toBeFalse();
+          expect(harness.hasFile('dist/lib/secondary/package.json')).toBeFalse();
+          expect(harness.hasFile('dist/lib/.npmignore')).toBeFalse();
+
+          const pkg = JSON.parse(harness.readFile('dist/lib/package.json'));
+          expect(pkg.exports['./secondary']).toBeUndefined();
+        },
+      ]);
+    });
+
+    it('should prune removed nested secondary entry point files and update .npmignore in watch mode', async () => {
+      await harness.writeFiles({
+        'projects/lib/shared/src/public-api.ts': `export const SHARED = 'shared';\n`,
+        'projects/lib/testing/sub/src/public-api.ts': `export const SUB = 'sub';\n`,
+      });
+
+      await harness.modifyFile('projects/lib/package.json', (content) => {
+        const pkg = JSON.parse(content);
+        pkg.exports = {
+          '.': './src/public-api.ts',
+          './shared': './shared/src/public-api.ts',
+          './testing/sub': './testing/sub/src/public-api.ts',
+        };
+
+        return JSON.stringify(pkg, null, 2);
+      });
+
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        watch: true,
+      });
+
+      await harness.executeWithCases([
+        async ({ result }) => {
+          expect(result?.success).toBeTrue();
+          harness.expectFile('dist/lib/fesm2022/lib-shared.mjs').toExist();
+          harness.expectFile('dist/lib/fesm2022/lib-testing-sub.mjs').toExist();
+          harness.expectFile('dist/lib/types/lib-testing-sub.d.ts').toExist();
+          harness.expectFile('dist/lib/testing/sub/package.json').toExist();
+          expect(harness.readFile('dist/lib/.npmignore')).toContain('/testing/sub/package.json');
+
+          // Remove './testing/sub' while keeping './shared'
+          await harness.modifyFile('projects/lib/package.json', (content) => {
+            const pkg = JSON.parse(content);
+            pkg.exports = {
+              '.': './src/public-api.ts',
+              './shared': './shared/src/public-api.ts',
+            };
+
+            return JSON.stringify(pkg, null, 2);
+          });
+        },
+        ({ result }) => {
+          expect(result?.success).toBeTrue();
+          harness.expectFile('dist/lib/fesm2022/lib-shared.mjs').toExist();
+          harness.expectFile('dist/lib/fesm2022/lib-testing-sub.mjs').toNotExist();
+          harness.expectFile('dist/lib/fesm2022/lib-testing-sub.mjs.map').toNotExist();
+          harness.expectFile('dist/lib/types/lib-testing-sub.d.ts').toNotExist();
+          harness.expectFile('dist/lib/testing/sub/package.json').toNotExist();
+
+          const npmignore = harness.readFile('dist/lib/.npmignore');
+          expect(npmignore).toContain('/shared/package.json');
+          expect(npmignore).not.toContain('/testing/sub/package.json');
+        },
+      ]);
+    });
+  });
+});
diff --git a/packages/angular/build/src/builders/library/tests/options/assets_spec.ts b/packages/angular/build/src/builders/library/tests/options/assets_spec.ts
index ded7743675ff..a596cbfe4e62 100644
--- a/packages/angular/build/src/builders/library/tests/options/assets_spec.ts
+++ b/packages/angular/build/src/builders/library/tests/options/assets_spec.ts
@@ -15,6 +15,7 @@ describeLibraryBuilder(executeLibraryBuilder, LIBRARY_BUILDER_INFO, (harness) =>
       await harness.writeFiles({
         'projects/lib/assets-dir/file-a.png': 'PNG_A',
         'projects/lib/assets-dir/file-b.png': 'PNG_B',
+        'projects/lib/assets-dir/ignored.png': 'PNG_IGNORED',
         'projects/lib/assets-dir/file-c.svg': 'SVG_C',
       });
 
@@ -25,6 +26,7 @@ describeLibraryBuilder(executeLibraryBuilder, LIBRARY_BUILDER_INFO, (harness) =>
             glob: '**/*.png',
             input: 'projects/lib/assets-dir',
             output: 'assets',
+            ignore: ['ignored.png'],
           },
         ],
       });
@@ -34,6 +36,7 @@ describeLibraryBuilder(executeLibraryBuilder, LIBRARY_BUILDER_INFO, (harness) =>
 
       expect(harness.readFile('dist/lib/assets/file-a.png')).toBe('PNG_A');
       expect(harness.readFile('dist/lib/assets/file-b.png')).toBe('PNG_B');
+      expect(harness.hasFile('dist/lib/assets/ignored.png')).toBeFalse();
       expect(harness.hasFile('dist/lib/assets/file-c.svg')).toBeFalse();
     });
 
diff --git a/packages/angular/build/src/builders/library/types.ts b/packages/angular/build/src/builders/library/types.ts
index 2deeddf589e5..b6df21d12610 100644
--- a/packages/angular/build/src/builders/library/types.ts
+++ b/packages/angular/build/src/builders/library/types.ts
@@ -118,6 +118,12 @@ export interface BundleResult {
   /** Exact set of virtual POSIX DTS module IDs bundled into this entry point. */
   dtsModuleIds: ReadonlySet<string>;
 
+  /** Relative POSIX output paths of ESM bundle files and sourcemaps emitted for this entry point. */
+  esmOutputFiles?: ReadonlySet<string>;
+
+  /** Relative POSIX output paths of DTS bundle files and sourcemaps emitted for this entry point. */
+  dtsOutputFiles?: ReadonlySet<string>;
+
   /** Whether this entry point bundle imports 'tslib'. */
   hasTslibImport?: boolean;
 }
@@ -158,6 +164,15 @@ export interface SingleBuildState {
   /** Set of changed POSIX TypeScript declaration file paths pending bundling. */
   pendingChangedDtsFiles: Set<string>;
 
+  /** Set of relative POSIX output paths for bundle files emitted in the previous build iteration. */
+  emittedBundleFiles: Set<string>;
+
+  /** Set of relative POSIX output paths for package manifest files emitted in the previous build iteration. */
+  emittedManifestFiles: Set<string>;
+
+  /** Map of normalized POSIX asset source paths to their relative POSIX output paths. */
+  emittedAssets: Map<string, string>;
+
   /** Whether the previous build iteration encountered a compilation or diagnostic error. */
   hasCompilationError?: boolean;