diff --git a/packages/angular/build/src/builders/library/builder.ts b/packages/angular/build/src/builders/library/builder.ts
index 84b933e7ceb6..c3c2149adf5b 100644
--- a/packages/angular/build/src/builders/library/builder.ts
+++ b/packages/angular/build/src/builders/library/builder.ts
@@ -111,13 +111,10 @@ export async function* executeLibraryBuilder(
     );
 
     // Track all referenced compilation files for watch mode
-    const watchedCompilationFiles = new Set<string>([
-      toPosixPath(tsConfigPath),
-      toPosixPath(packageJsonPath),
-    ]);
+    const watchedCompilationFiles = new Set<string>([tsConfigPath, packageJsonPath]);
 
     for (const entryPoint of normalizedOptions.entryPoints.values()) {
-      watchedCompilationFiles.add(toPosixPath(entryPoint.entryFilePath));
+      watchedCompilationFiles.add(entryPoint.entryFilePath);
     }
 
     if (isWatchMode) {
@@ -237,6 +234,17 @@ async function executeBuild(
 
 /**
  * Runs the watch loop, rebuilding the library as watched files are modified.
+ *
+ * @param watcher The file watcher instance.
+ * @param options The normalized library builder options with POSIX paths.
+ * @param stylesheetBundler The component stylesheet bundler instance.
+ * @param watchedCompilationFiles Set of POSIX file paths tracked for compilation.
+ * @param context The Architect builder context.
+ * @param withProgress Progress wrapper function (`withSpinner` or `withNoProgress`).
+ * @param buildState The state preserved across incremental builds.
+ * @param buildAction The library build action function to execute.
+ * @param hasModifiedWatchedFile Helper to check whether watched compilation files changed.
+ * @param signal Optional abort signal to terminate the watch loop.
  */
 async function* runWatchLoop(
   watcher: BuildWatcher,
@@ -253,7 +261,6 @@ async function* runWatchLoop(
   const { collectAssetsToEmit } = await import('./pipeline/assets');
 
   const { workspaceRoot, packageJsonPath, assets, clearScreen } = options;
-  const posixPackageJsonPath = toPosixPath(packageJsonPath);
 
   for await (const changes of watcher) {
     if (signal?.aborted) {
@@ -293,13 +300,13 @@ async function* runWatchLoop(
 
     // Check if package.json was modified
     let hasPackageJsonChanges = false;
-    if (changedFiles.has(posixPackageJsonPath)) {
+    if (changedFiles.has(packageJsonPath)) {
       try {
         const packageJson = await loadPackageJson(packageJsonPath);
-        options.packageJson = packageJson;
         if (!packageJson.name) {
           throw new Error(`The package.json at '${packageJsonPath}' must contain a 'name'.`);
         }
+        options.packageJson = packageJson;
         options.packageName = packageJson.name;
 
         updateWatchedEntryPoints(
@@ -328,7 +335,7 @@ async function* runWatchLoop(
       !buildState.singleProgramCache ||
       buildState.hasCompilationError ||
       buildState.hasEntryPointsChanges ||
-      hasModifiedWatchedFile(changedFiles, watchedCompilationFiles, posixPackageJsonPath);
+      hasModifiedWatchedFile(changedFiles, watchedCompilationFiles, packageJsonPath);
 
     const assetsToEmit = await collectAssetsToEmit(
       assets,
@@ -388,7 +395,7 @@ function logMessages(
  * @param logger The Architect logger API.
  * @param startTime High-resolution timestamp recorded when the build iteration started.
  * @param success Whether the build iteration succeeded without errors.
- * @param outputPath Optional output directory path to log on successful builds.
+ * @param outputPath Optional POSIX output directory path to log on successful builds.
  */
 function logBuildResult(
   logger: logging.LoggerApi,
diff --git a/packages/angular/build/src/builders/library/options.ts b/packages/angular/build/src/builders/library/options.ts
index b7747fb6db27..14bc4514027d 100644
--- a/packages/angular/build/src/builders/library/options.ts
+++ b/packages/angular/build/src/builders/library/options.ts
@@ -13,6 +13,7 @@ import { normalizeAssetPatterns } from '../../utils';
 import { supportColor } from '../../utils/color';
 import { assertIsError } from '../../utils/error';
 import { normalizeCacheOptions } from '../../utils/normalize-cache';
+import { toPosixPath } from '../../utils/path';
 import {
   generateSearchDirectories,
   getTailwindConfig,
@@ -24,14 +25,25 @@ import { loadPackageJson } from './pipeline/utils';
 import type { Schema as LibraryBuilderOptions } from './schema';
 import type { NormalizedLibraryOptions, PackageJsonData } from './types';
 
+/**
+ * Normalizes raw library builder options into {@link NormalizedLibraryOptions} with all
+ * file and directory paths normalized to POSIX format.
+ *
+ * @param context The Architect builder context.
+ * @param projectName The name of the library project.
+ * @param options The raw library builder schema options.
+ * @returns A promise resolving to the normalized library options.
+ */
 export async function normalizeLibraryOptions(
   context: BuilderContext,
   projectName: string,
   options: LibraryBuilderOptions,
 ): Promise<NormalizedLibraryOptions> {
-  const { workspaceRoot } = context;
+  const workspaceRoot = toPosixPath(context.workspaceRoot);
   const projectMetadata = await context.getProjectMetadata(projectName);
-  const { projectRoot, projectSourceRoot } = getProjectRootPaths(workspaceRoot, projectMetadata);
+  const projectRootPaths = getProjectRootPaths(workspaceRoot, projectMetadata);
+  const projectRoot = toPosixPath(projectRootPaths.projectRoot);
+  const projectSourceRoot = toPosixPath(projectRootPaths.projectSourceRoot);
 
   const {
     tsConfig,
@@ -50,8 +62,8 @@ export async function normalizeLibraryOptions(
     clearScreen,
   } = options;
 
-  const resolvedTsConfigPath = path.resolve(workspaceRoot, tsConfig);
-  const packageJsonPath = path.join(projectRoot, 'package.json');
+  const resolvedTsConfigPath = toPosixPath(path.resolve(workspaceRoot, tsConfig));
+  const packageJsonPath = path.posix.join(projectRoot, 'package.json');
 
   let packageJson: PackageJsonData;
   try {
@@ -99,7 +111,7 @@ export async function normalizeLibraryOptions(
     }
     defaultAssets.push({
       glob: 'README.md',
-      input: path.dirname(entryPoint.entryFilePath),
+      input: path.posix.dirname(entryPoint.entryFilePath),
       output: entryPoint.name,
     });
   }
@@ -109,7 +121,11 @@ export async function normalizeLibraryOptions(
     workspaceRoot,
     projectRoot,
     projectSourceRoot,
-  );
+  ).map((asset) => ({
+    ...asset,
+    input: toPosixPath(path.resolve(workspaceRoot, asset.input)),
+    output: toPosixPath(asset.output),
+  }));
 
   const cacheOptions = normalizeCacheOptions(
     projectMetadata,
@@ -119,7 +135,7 @@ export async function normalizeLibraryOptions(
   );
 
   const styleIncludePaths = (stylePreprocessorOptions?.includePaths ?? []).map((p: string) =>
-    path.resolve(workspaceRoot, p),
+    toPosixPath(path.resolve(workspaceRoot, p)),
   );
 
   const searchDirectories = await generateSearchDirectories([projectRoot, workspaceRoot]);
@@ -133,7 +149,9 @@ export async function normalizeLibraryOptions(
     projectRoot,
     packageName,
     packageJson,
-    outputPath: path.resolve(workspaceRoot, options.outputPath ?? path.join('dist', projectName)),
+    outputPath: toPosixPath(
+      path.resolve(workspaceRoot, options.outputPath ?? path.posix.join('dist', projectName)),
+    ),
     deleteOutputPath,
     packageJsonPath,
     tsConfigPath: resolvedTsConfigPath,
diff --git a/packages/angular/build/src/builders/library/pipeline/assets.ts b/packages/angular/build/src/builders/library/pipeline/assets.ts
index e1e46f4b9f08..7b8329a7865a 100644
--- a/packages/angular/build/src/builders/library/pipeline/assets.ts
+++ b/packages/angular/build/src/builders/library/pipeline/assets.ts
@@ -9,7 +9,6 @@
 import { statSync } from 'node:fs';
 import path from 'node:path';
 import picomatch from 'picomatch';
-import { toPosixPath } from '../../../utils/path';
 import { DEFAULT_ASSET_IGNORE, resolveAssets } from '../../../utils/resolve-assets';
 import type { NormalizedLibraryOptions } from '../types';
 import { type DiskOutputFile, createDiskOutputFile } from './utils';
@@ -17,9 +16,9 @@ import { type DiskOutputFile, createDiskOutputFile } from './utils';
 /**
  * Resolves and collects configured library assets to be emitted to disk.
  *
- * @param assets The normalized asset patterns.
- * @param workspaceRoot The workspace root directory path.
- * @param modifiedFiles Optional set of modified file paths for incremental copying in watch mode.
+ * @param assets The normalized asset patterns with POSIX paths.
+ * @param workspaceRoot The normalized POSIX workspace root 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.
  */
 export async function collectAssetsToEmit(
@@ -32,25 +31,22 @@ export async function collectAssetsToEmit(
   }
 
   if (modifiedFiles) {
-    const matchers = createAssetMatchers(assets, workspaceRoot);
+    const matchers = createAssetMatchers(assets);
     const filesToEmit: DiskOutputFile[] = [];
 
     for (const file of modifiedFiles) {
-      const resolvedFile = path.isAbsolute(file) ? file : path.resolve(workspaceRoot, file);
-      const posixFile = toPosixPath(resolvedFile);
-
-      for (const { asset, posixInputPrefix, isMatch } of matchers) {
-        if (!posixFile.startsWith(posixInputPrefix)) {
+      for (const { asset, prefix, isMatch } of matchers) {
+        if (!file.startsWith(prefix)) {
           continue;
         }
 
-        const relative = posixFile.slice(posixInputPrefix.length);
+        const relative = file.slice(prefix.length);
         if (!isMatch(relative)) {
           continue;
         }
 
-        if (statSync(resolvedFile, { throwIfNoEntry: false })?.isFile()) {
-          filesToEmit.push(createDiskOutputFile(resolvedFile, path.join(asset.output, relative)));
+        if (statSync(file, { throwIfNoEntry: false })?.isFile()) {
+          filesToEmit.push(createDiskOutputFile(file, path.posix.join(asset.output, relative)));
         }
       }
     }
@@ -63,15 +59,14 @@ export async function collectAssetsToEmit(
   return resolvedAssets.map(({ source, destination }) => createDiskOutputFile(source, destination));
 }
 
-function createAssetMatchers(assets: NormalizedLibraryOptions['assets'], workspaceRoot: string) {
+function createAssetMatchers(assets: NormalizedLibraryOptions['assets']) {
   return assets.map((asset) => {
-    const absInput = path.resolve(workspaceRoot, asset.input);
-    const posixInput = toPosixPath(absInput).replace(/\/+$/, '');
     const isMatch = picomatch(asset.glob, {
       dot: true,
       ignore: [...DEFAULT_ASSET_IGNORE, ...(asset.ignore ?? [])],
     });
+    const prefix = asset.input.endsWith('/') ? asset.input : `${asset.input}/`;
 
-    return { asset, posixInputPrefix: `${posixInput}/`, isMatch };
+    return { asset, prefix, isMatch };
   });
 }
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 6d40f5fe4383..887ede84a74a 100644
--- a/packages/angular/build/src/builders/library/pipeline/build-action.ts
+++ b/packages/angular/build/src/builders/library/pipeline/build-action.ts
@@ -11,7 +11,6 @@ import path from 'node:path';
 import type { ComponentStylesheetBundler } from '../../../tools/esbuild/angular/component-stylesheets';
 import { emitFilesToDisk } from '../../../utils/emit-files-to-disk';
 import { assertIsError } from '../../../utils/error';
-import { toPosixPath } from '../../../utils/path';
 import type { NormalizedLibraryOptions, SingleBuildState } from '../types';
 import { collectAssetsToEmit } from './assets';
 import { bundleEntryPoints } from './bundler';
@@ -49,7 +48,7 @@ export function createSingleBuildState(): SingleBuildState {
  * Context required to execute a single library build iteration.
  */
 export interface BuildActionContext {
-  /** Normalized options for the library build. */
+  /** Normalized options for the library build with POSIX paths. */
   options: NormalizedLibraryOptions;
 
   /** The Architect builder context. */
@@ -62,7 +61,7 @@ export interface BuildActionContext {
   isWatchMode: boolean;
 
   /**
-   * Set of file paths tracked for compilation and configuration
+   * Set of POSIX file paths tracked for compilation and configuration
    * (including `tsconfig.json`, `package.json`, entry points, and referenced source,
    * template, and stylesheet files). Updated during compilation and used to determine
    * whether file changes require recompiling entry points, excluding asset files so
@@ -73,7 +72,7 @@ export interface BuildActionContext {
   /** State preserved across incremental builds in watch mode. */
   buildState: SingleBuildState;
 
-  /** Set of file paths modified since the last build iteration. */
+  /** Set of POSIX file paths modified since the last build iteration. */
   modifiedFiles?: Set<string>;
 
   /**
@@ -91,6 +90,7 @@ export interface BuildActionContext {
  * manifest generation, and asset copying.
  *
  * @param actionContext The build action state and configuration.
+ * @returns A promise resolving to the build action result containing errors, warnings, and logs.
  */
 export async function buildAction(actionContext: BuildActionContext): Promise<BuildActionResult> {
   const {
@@ -106,10 +106,10 @@ export async function buildAction(actionContext: BuildActionContext): Promise<Bu
   const warnings: string[] = [];
   const logs: string[] = [];
 
-  const posixPackageJsonPath = toPosixPath(options.packageJsonPath);
+  const { packageJsonPath } = options;
   const { pendingChangedEsmFiles, pendingChangedDtsFiles } = buildState;
 
-  if (!modifiedFiles || modifiedFiles.has(posixPackageJsonPath)) {
+  if (!modifiedFiles || modifiedFiles.has(packageJsonPath)) {
     buildState.hasEmittedManifests = false;
   }
 
@@ -120,7 +120,7 @@ export async function buildAction(actionContext: BuildActionContext): Promise<Bu
     buildState.hasEntryPointsChanges ||
     pendingChangedEsmFiles.size > 0 ||
     pendingChangedDtsFiles.size > 0 ||
-    hasModifiedWatchedFile(modifiedFiles, watchedCompilationFiles, posixPackageJsonPath);
+    hasModifiedWatchedFile(modifiedFiles, watchedCompilationFiles, packageJsonPath);
 
   try {
     if (!buildState.hasEmittedManifests) {
@@ -172,13 +172,12 @@ export async function buildAction(actionContext: BuildActionContext): Promise<Bu
         pendingChangedDtsFiles.add(file);
       }
 
-      const itemsToBundle = collectEntryPointsToBundle(options, buildState, logs);
-
       let bundleOutput: Awaited<ReturnType<typeof bundleEntryPoints>> | undefined;
       let diagnostics: LibraryCompilationDiagnostics;
       if (hasStylesheetErrors) {
         diagnostics = await diagnosePromise;
       } else {
+        const itemsToBundle = collectEntryPointsToBundle(options, buildState, logs);
         try {
           [bundleOutput, diagnostics] = await Promise.all([
             bundleEntryPoints(itemsToBundle, esmFiles, dtsFiles, {
@@ -243,7 +242,7 @@ export async function buildAction(actionContext: BuildActionContext): Promise<Bu
     ));
     filesToEmit.push(...resolvedAssetsToEmit);
 
-    await emitFilesToDisk(filesToEmit, (file) => path.join(options.outputPath, file.path));
+    await emitFilesToDisk(filesToEmit, (file) => path.posix.join(options.outputPath, file.path));
 
     buildState.hasEmittedManifests = true;
     buildState.hasEmittedTslib = hasTslibImport;
@@ -262,16 +261,16 @@ export async function buildAction(actionContext: BuildActionContext): Promise<Bu
  *
  * @param modifiedFiles Set of POSIX file paths modified since the last build.
  * @param watchedCompilationFiles Set of POSIX file paths tracked for compilation.
- * @param posixPackageJsonPath The normalized POSIX path to the library's `package.json`.
+ * @param packageJsonPath The normalized POSIX path to the library's `package.json`.
  * @returns `true` if a watched compilation file was modified; otherwise `false`.
  */
 export function hasModifiedWatchedFile(
   modifiedFiles: ReadonlySet<string>,
   watchedCompilationFiles: ReadonlySet<string>,
-  posixPackageJsonPath: string,
+  packageJsonPath: string,
 ): boolean {
   for (const file of modifiedFiles) {
-    if (file !== posixPackageJsonPath && watchedCompilationFiles.has(file)) {
+    if (file !== packageJsonPath && watchedCompilationFiles.has(file)) {
       return true;
     }
   }
diff --git a/packages/angular/build/src/builders/library/pipeline/bundler.ts b/packages/angular/build/src/builders/library/pipeline/bundler.ts
index 5cd89a4b110d..c48942885083 100644
--- a/packages/angular/build/src/builders/library/pipeline/bundler.ts
+++ b/packages/angular/build/src/builders/library/pipeline/bundler.ts
@@ -60,7 +60,7 @@ interface MultiBundleOutput {
   /** In-memory files generated by the bundle invocation. */
   filesToEmit: MemoryOutputFile[];
 
-  /** Map of bundle entry names to the set of virtual module IDs included in the bundle. */
+  /** Map of bundle entry names to the set of virtual POSIX module IDs included in the bundle. */
   moduleIdsByBundle: Map<string, Set<string>>;
 
   /** Set of bundle entry names that import 'tslib'. */
@@ -75,9 +75,9 @@ interface MultiBundleOutput {
  * using at most 2 Rolldown instances total (1 for all .mjs bundles, 1 for all .d.ts bundles).
  *
  * @param items The collection of entry point inputs with change statuses.
- * @param esmFiles Map of virtual ESM output file paths to their content.
- * @param dtsFiles Map of virtual TypeScript declaration file paths to their content.
- * @param options The normalized library builder options.
+ * @param esmFiles Map of virtual POSIX ESM output file paths to their content.
+ * @param dtsFiles Map of virtual POSIX TypeScript declaration file paths to their content.
+ * @param options The normalized library builder options with POSIX paths.
  * @returns A promise resolving to the bundled output files and bundle cache results.
  */
 export async function bundleEntryPoints(
@@ -135,7 +135,7 @@ export async function bundleEntryPoints(
  *
  * @param entryPoints The entry points to include in the bundle input map.
  * @param dtsMode Whether the input map is for declaration files (.d.ts) rather than ESM files (.js).
- * @returns A record mapping bundle names to virtual entry file paths.
+ * @returns A record mapping bundle names to virtual POSIX entry file paths.
  */
 function resolveEntryInputMap(
   entryPoints: readonly NormalizedEntryPoint[],
@@ -143,20 +143,18 @@ function resolveEntryInputMap(
 ): Record<string, string> {
   const input: Record<string, string> = {};
   for (const { bundleName, entryFilePath } of entryPoints) {
-    const posixPath = toPosixPath(entryFilePath);
     input[bundleName] = dtsMode
-      ? posixPath.replace(/\.([cm]?ts)$/, '.d.$1')
-      : posixPath.replace(/\.([cm]?)ts$/, '.$1js');
+      ? entryFilePath.replace(/\.([cm]?ts)$/, '.d.$1')
+      : entryFilePath.replace(/\.([cm]?)ts$/, '.$1js');
   }
 
   return input;
 }
 
 /**
- * Creates a Rolldown plugin that loads modules from an in-memory file map
- * and prevents illegal cross-entry-point sibling imports.
+ * Creates a Rolldown plugin that resolves and loads modules from an in-memory file map.
  *
- * @param files Map of virtual file paths to their content.
+ * @param files Map of virtual POSIX file paths to their content.
  * @param extensions Candidate extensions to probe when resolving extensionless module specifiers.
  * @param includeMap Whether source map files should also be loaded from the memory map.
  * @returns A Rolldown plugin instance.
@@ -183,12 +181,7 @@ function createMemoryFileLoaderPlugin(
           return { id, external: true };
         }
 
-        const posixId = toPosixPath(id);
-        const importerPosix = toPosixPath(importer);
-        const resolved =
-          posixId[0] === '.'
-            ? path.posix.join(path.posix.dirname(importerPosix), posixId)
-            : posixId;
+        const resolved = id[0] === '.' ? path.posix.join(path.posix.dirname(importer), id) : id;
 
         let resolvedCandidate: string | undefined;
         if (files.has(resolved)) {
@@ -230,7 +223,7 @@ function createMemoryFileLoaderPlugin(
  * the transitive set of module IDs belonging to each entry point bundle.
  *
  * @param output The array of output chunks and assets from Rolldown.
- * @param dir The destination output directory prefix.
+ * @param dir The destination POSIX output directory prefix.
  * @returns The processed multi-bundle output.
  */
 function processRolldownOutput(
@@ -312,10 +305,10 @@ const IGNORED_ROLLDOWN_WARNING_CODES: ReadonlySet<string> = new Set([
 /**
  * Executes a Rolldown bundle build across all provided entry inputs.
  *
- * @param input Map of bundle names to virtual entry file paths.
+ * @param input Map of bundle names to virtual POSIX entry file paths.
  * @param plugins Rolldown plugins to use during bundling.
  * @param extension Output file extension ('mjs' or 'd.ts').
- * @param options The normalized library options.
+ * @param options The normalized library options with POSIX paths.
  * @returns A promise resolving to the multi-bundle output.
  */
 async function executeMultiBundle(
@@ -367,7 +360,7 @@ async function executeMultiBundle(
   try {
     const { output } = await bundle.generate({
       format: 'es',
-      dir: path.join(outputPath, dir),
+      dir: path.posix.join(outputPath, dir),
       entryFileNames: `[name].${extension}`,
       chunkFileNames: `[name]-[hash].${extension}`,
       sourcemap,
@@ -388,8 +381,8 @@ async function executeMultiBundle(
  * Bundles JavaScript ESM output files for all provided entry points.
  *
  * @param entryPoints The entry points to bundle.
- * @param esmFiles Map of virtual ESM output file paths to contents.
- * @param options The normalized library options.
+ * @param esmFiles Map of virtual POSIX ESM output file paths to contents.
+ * @param options The normalized library options with POSIX paths.
  * @returns A promise resolving to the multi-bundle output.
  */
 async function bundleAllEsm(
@@ -418,8 +411,8 @@ async function bundleAllEsm(
  * Bundles TypeScript declaration (.d.ts) output files for all provided entry points.
  *
  * @param entryPoints The entry points to bundle.
- * @param dtsFiles Map of virtual declaration output file paths to contents.
- * @param options The normalized library options.
+ * @param dtsFiles Map of virtual POSIX declaration output file paths to contents.
+ * @param options The normalized library options with POSIX paths.
  * @returns A promise resolving to the multi-bundle output.
  */
 async function bundleAllDts(
diff --git a/packages/angular/build/src/builders/library/pipeline/compilation.ts b/packages/angular/build/src/builders/library/pipeline/compilation.ts
index afc5c319854e..5a795b463bae 100644
--- a/packages/angular/build/src/builders/library/pipeline/compilation.ts
+++ b/packages/angular/build/src/builders/library/pipeline/compilation.ts
@@ -35,19 +35,19 @@ export interface LibraryCompilationDiagnostics {
  * Output of the unified library compilation step.
  */
 export interface LibraryCompilationOutput {
-  /** Read-only map of emitted JavaScript files. */
+  /** Read-only map of emitted JavaScript files keyed by POSIX path. */
   readonly esmFiles: ReadonlyMap<string, string>;
 
-  /** Read-only map of emitted TypeScript declaration files. */
+  /** Read-only map of emitted TypeScript declaration files keyed by POSIX path. */
   readonly dtsFiles: ReadonlyMap<string, string>;
 
-  /** Set of JavaScript file paths that were modified or newly emitted in this iteration. */
+  /** Set of POSIX JavaScript file paths that were modified or newly emitted in this iteration. */
   readonly changedEsmFiles: ReadonlySet<string>;
 
-  /** Set of declaration file paths that were modified or newly emitted in this iteration. */
+  /** Set of POSIX declaration file paths that were modified or newly emitted in this iteration. */
   readonly changedDtsFiles: ReadonlySet<string>;
 
-  /** Set of all source and dependency file paths referenced during compilation. */
+  /** Set of all POSIX source and dependency file paths referenced during compilation. */
   readonly referencedFiles: ReadonlySet<string>;
 
   /** Cached compilation state for subsequent incremental builds. */
@@ -70,10 +70,10 @@ export interface LibraryCompilationOutput {
  * Compiles all library entry points in a single TypeScript and Angular compilation pass.
  *
  * @param entryPoints The collection of normalized library entry points to compile.
- * @param options The normalized library builder options.
+ * @param options The normalized library builder options with POSIX paths.
  * @param stylesheetBundler The component stylesheet bundler instance.
  * @param cached Optional cached compilation state from a previous build iteration.
- * @param modifiedFiles Optional set of modified file paths for incremental compilation.
+ * @param modifiedFiles Optional set of modified POSIX file paths for incremental compilation.
  * @returns A promise resolving to the compilation output containing emitted files and diagnostics.
  */
 export async function compileLibrary(
@@ -179,13 +179,12 @@ export async function compileLibrary(
 
     if (effectiveModifiedFiles) {
       for (const modifiedFile of effectiveModifiedFiles) {
-        const posixModified = toPosixPath(modifiedFile);
-        if (allReferencedFiles.has(posixModified)) {
+        if (allReferencedFiles.has(modifiedFile)) {
           continue;
         }
 
-        const basePathWithoutExt = posixModified.replace(/(?:\.d\.[cm]?ts|\.[cm]?[jt]sx?)$/i, '');
-        if (basePathWithoutExt === posixModified || existsSync(posixModified)) {
+        const basePathWithoutExt = modifiedFile.replace(/(?:\.d\.[cm]?ts|\.[cm]?[jt]sx?)$/i, '');
+        if (basePathWithoutExt === modifiedFile || existsSync(modifiedFile)) {
           continue;
         }
 
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 7797f069fade..5cff4ca82136 100644
--- a/packages/angular/build/src/builders/library/pipeline/entry-points.ts
+++ b/packages/angular/build/src/builders/library/pipeline/entry-points.ts
@@ -38,9 +38,9 @@ export function getEntryPointBundleName(packageName: string, entryPointName = '.
  * @param posixKey Normalized POSIX key without trailing slashes.
  * @param isPrimary Whether this is the primary entry point.
  * @param targetPath The relative file path string from exports.
- * @param projectRoot The library project root directory.
+ * @param projectRoot The library project root POSIX directory path.
  * @param packageName The root package name (e.g. `@my/lib`).
- * @returns The normalized entry point descriptor.
+ * @returns The normalized entry point descriptor with POSIX paths.
  */
 function normalizeEntryPoint(
   key: string,
@@ -65,7 +65,7 @@ function normalizeEntryPoint(
   const subpath = isPrimary ? '.' : `./${name}`;
   const displayName = (isPrimary ? packageName : `${packageName}/${name}`).toLowerCase();
   const bundleName = getEntryPointBundleName(packageName, name);
-  const entryFilePath = path.resolve(projectRoot, targetPath);
+  const entryFilePath = toPosixPath(path.resolve(projectRoot, targetPath));
 
   if (!/(?<!\.d)\.(?:ts|mts)$/.test(entryFilePath)) {
     throw new Error(
@@ -117,10 +117,10 @@ function resolveTarget(value: unknown): string | undefined {
  * Normalizes all entry points from the library's `package.json` `exports` field.
  *
  * @param rawExports The `exports` field from `package.json`.
- * @param projectRoot The library project root directory.
- * @param packageJsonPath Path to `package.json` for error reporting.
+ * @param projectRoot The library project root POSIX directory path.
+ * @param packageJsonPath POSIX path to `package.json` for error reporting.
  * @param packageName The root package name (e.g. `@my/lib`).
- * @returns A Map of normalized entry points keyed by name.
+ * @returns A Map of normalized entry points keyed by name with POSIX entry file paths.
  */
 export function normalizeEntryPoints(
   rawExports: unknown,
@@ -206,7 +206,11 @@ export function normalizeEntryPoints(
 }
 
 /**
- * Determines whether two sets of normalized entry points differ in keys, paths, or subpaths.
+ * Determines whether two sets of normalized entry points differ in keys, POSIX entry paths, or subpaths.
+ *
+ * @param oldEntryPoints The previous map of normalized entry points.
+ * @param newEntryPoints The new map of normalized entry points.
+ * @returns `true` if any entry point was added, removed, or modified; otherwise `false`.
  */
 export function haveEntryPointsChanged(
   oldEntryPoints: ReadonlyMap<string, NormalizedEntryPoint>,
@@ -234,6 +238,12 @@ export function haveEntryPointsChanged(
 
 /**
  * Updates watched files, entry points in options, and cached bundle results when package.json entry points change.
+ *
+ * @param packageJson The reloaded `package.json` data.
+ * @param options The normalized library builder options.
+ * @param buildState The state preserved across incremental builds.
+ * @param watchedCompilationFiles Set of POSIX file paths tracked for compilation.
+ * @param packageJsonPath Normalized POSIX path to `package.json`.
  */
 export function updateWatchedEntryPoints(
   packageJson: PackageJsonData,
@@ -251,7 +261,7 @@ export function updateWatchedEntryPoints(
 
   if (haveEntryPointsChanged(options.entryPoints, newEntryPoints)) {
     for (const entryPoint of newEntryPoints.values()) {
-      watchedCompilationFiles.add(toPosixPath(entryPoint.entryFilePath));
+      watchedCompilationFiles.add(entryPoint.entryFilePath);
     }
 
     for (const name of buildState.previousBundleResults.keys()) {
@@ -315,8 +325,8 @@ export function collectEntryPointsToBundle(
 /**
  * Checks whether any file in `changedFiles` belongs to the given entry point's `moduleIds`.
  *
- * @param moduleIds Set of module IDs referenced by the entry point's previous bundle.
- * @param changedFiles Set of file paths that changed during compilation.
+ * @param moduleIds Set of virtual POSIX module IDs referenced by the entry point's previous bundle.
+ * @param changedFiles Set of POSIX file paths that changed during compilation.
  * @returns `true` if at least one changed file is part of `moduleIds`; otherwise `false`.
  */
 function hasEntryPointChanges(
diff --git a/packages/angular/build/src/builders/library/pipeline/package-manifests.ts b/packages/angular/build/src/builders/library/pipeline/package-manifests.ts
index 90e5d5bbd94a..98ed88a506e6 100644
--- a/packages/angular/build/src/builders/library/pipeline/package-manifests.ts
+++ b/packages/angular/build/src/builders/library/pipeline/package-manifests.ts
@@ -188,7 +188,7 @@ let cachedTslibRange: string | undefined;
 
 /**
  * Get the tslib range used by the Angular compiler.
- * @param workspaceRoot path to the workspace root
+ * @param workspaceRoot path to the workspace root.
  * @returns tslib range
  */
 async function getAngularTslibRange(workspaceRoot: string): Promise<string> {
diff --git a/packages/angular/build/src/builders/library/pipeline/stylesheet-bundler.ts b/packages/angular/build/src/builders/library/pipeline/stylesheet-bundler.ts
index 74c44f3a1491..971741d7a2e2 100644
--- a/packages/angular/build/src/builders/library/pipeline/stylesheet-bundler.ts
+++ b/packages/angular/build/src/builders/library/pipeline/stylesheet-bundler.ts
@@ -10,6 +10,9 @@ import { ComponentStylesheetBundler } from '../../../tools/esbuild/angular/compo
 import type { BundleStylesheetOptions } from '../../../tools/esbuild/stylesheets/bundle-options';
 import type { NormalizedLibraryOptions } from '../types';
 
+/**
+ * Subset of normalized library options required to configure the component stylesheet bundler.
+ */
 export type LibraryStylesheetBundlerOptions = Pick<
   NormalizedLibraryOptions,
   | 'workspaceRoot'
@@ -25,7 +28,7 @@ export type LibraryStylesheetBundlerOptions = Pick<
 /**
  * Creates a stylesheet bundler instance configured for library compilation.
  *
- * @param options The normalized library builder options.
+ * @param options The normalized library builder options with POSIX paths.
  * @param incremental Whether incremental watch mode is enabled.
  * @param target The esbuild target environments derived from browserslist.
  * @returns A new ComponentStylesheetBundler instance.
diff --git a/packages/angular/build/src/builders/library/pipeline/utils.ts b/packages/angular/build/src/builders/library/pipeline/utils.ts
index 9092a73005ef..8ef8cf7fe9f3 100644
--- a/packages/angular/build/src/builders/library/pipeline/utils.ts
+++ b/packages/angular/build/src/builders/library/pipeline/utils.ts
@@ -7,6 +7,7 @@
  */
 
 import fs from 'node:fs/promises';
+import { toPosixPath } from '../../../utils/path';
 import type { PackageJsonData } from '../types';
 
 const IS_DTS_FILE_REGEXP = /\.d\.[cm]?ts$/i;
@@ -28,7 +29,7 @@ export const TYPES_OUTPUT_DIR = 'types';
 export interface MemoryOutputFile {
   origin: 'memory';
 
-  /** The destination path where the file should be written. */
+  /** The relative POSIX destination path where the file should be written. */
   path: string;
 
   /** The contents of the file as either a string or byte array. */
@@ -41,10 +42,10 @@ export interface MemoryOutputFile {
 export interface DiskOutputFile {
   origin: 'disk';
 
-  /** The path to the source file on disk. */
+  /** The POSIX path to the source file on disk. */
   source: string;
 
-  /** The destination path where the file should be copied. */
+  /** The relative POSIX destination path where the file should be copied. */
   path: string;
 }
 
@@ -54,26 +55,26 @@ export interface DiskOutputFile {
 export type OutputFile = MemoryOutputFile | DiskOutputFile;
 
 /**
- * Creates an output file descriptor for an existing file on disk.
+ * Creates an output file descriptor for an existing file on disk with POSIX paths.
  *
  * @param source The path to the source file on disk.
- * @param path The destination path where the file should be copied.
- * @returns A {@link DiskOutputFile} descriptor.
+ * @param path The relative destination path where the file should be copied.
+ * @returns A {@link DiskOutputFile} descriptor with POSIX `source` and `path`.
  */
 export function createDiskOutputFile(source: string, path: string): DiskOutputFile {
   return {
     origin: 'disk',
-    source,
-    path,
+    source: toPosixPath(source),
+    path: toPosixPath(path),
   };
 }
 
 /**
- * Creates an output file descriptor for an in-memory file.
+ * Creates an output file descriptor for an in-memory file with a POSIX destination path.
  *
- * @param path The destination path where the file should be written.
+ * @param path The relative destination path where the file should be written.
  * @param contents The contents of the file as either a string, byte array, or JSON object.
- * @returns A {@link MemoryOutputFile} descriptor.
+ * @returns A {@link MemoryOutputFile} descriptor with a POSIX `path`.
  */
 export function createMemoryOutputFile(
   path: string,
@@ -81,7 +82,7 @@ export function createMemoryOutputFile(
 ): MemoryOutputFile {
   return {
     origin: 'memory',
-    path,
+    path: toPosixPath(path),
     contents:
       typeof contents === 'string' || contents instanceof Uint8Array
         ? contents
diff --git a/packages/angular/build/src/builders/library/types.ts b/packages/angular/build/src/builders/library/types.ts
index d360cfe2c868..2deeddf589e5 100644
--- a/packages/angular/build/src/builders/library/types.ts
+++ b/packages/angular/build/src/builders/library/types.ts
@@ -28,7 +28,7 @@ export interface NormalizedEntryPoint {
   /** Base name of the output bundle (e.g. 'my-lib' or 'my-lib-testing'). */
   bundleName: string;
 
-  /** Absolute path to entry file. */
+  /** Absolute POSIX path to the entry file. */
   entryFilePath: string;
 
   /** Is this the primary entry point ('.')? */
@@ -59,21 +59,40 @@ export interface PackageJsonData {
 
 /**
  * Normalized options for the library builder.
+ * All file and directory paths are normalized to POSIX format.
  */
 export interface NormalizedLibraryOptions {
+  /** Absolute POSIX path to the workspace root directory. */
   workspaceRoot: string;
+
+  /** Absolute POSIX path to the library project root directory. */
   projectRoot: string;
+
   packageName: string;
   packageJson: PackageJsonData;
+
+  /** Absolute POSIX path to the build output directory. */
   outputPath: string;
+
   deleteOutputPath: boolean;
+
+  /** Absolute POSIX path to the library's `package.json`. */
   packageJsonPath: string;
+
+  /** Absolute POSIX path to the TypeScript configuration file. */
   tsConfigPath: string;
+
   entryPoints: Map<string, NormalizedEntryPoint>;
   inlineStyleLanguage: 'css' | 'less' | 'sass' | 'scss';
+
+  /** Absolute POSIX paths to style preprocessor include directories. */
   styleIncludePaths: string[];
+
   sass?: StylesheetPluginsass;
+
+  /** Normalized asset patterns with POSIX `input` and `output` paths. */
   assets: ReturnType<typeof normalizeAssetPatterns>;
+
   compilationMode?: 'partial' | 'full';
   declarationMap?: boolean;
   allowedNonPeerDependencies: RegExp[];
@@ -93,10 +112,10 @@ export interface NormalizedLibraryOptions {
  * Cached module ID sets for a bundled entry point.
  */
 export interface BundleResult {
-  /** Exact set of virtual ESM module IDs bundled into this entry point. */
+  /** Exact set of virtual POSIX ESM module IDs bundled into this entry point. */
   esmModuleIds: ReadonlySet<string>;
 
-  /** Exact set of virtual DTS module IDs bundled into this entry point. */
+  /** Exact set of virtual POSIX DTS module IDs bundled into this entry point. */
   dtsModuleIds: ReadonlySet<string>;
 
   /** Whether this entry point bundle imports 'tslib'. */
@@ -110,13 +129,13 @@ export interface SingleProgramCache {
   /** The active Angular compilation instance. */
   readonly compilationInstance: AngularCompilation;
 
-  /** In-memory map of emitted JavaScript files keyed by relative output path. */
+  /** In-memory map of emitted JavaScript files keyed by POSIX path. */
   readonly esmFiles: Map<string, string>;
 
-  /** In-memory map of emitted TypeScript declaration files keyed by relative output path. */
+  /** In-memory map of emitted TypeScript declaration files keyed by POSIX path. */
   readonly dtsFiles: Map<string, string>;
 
-  /** Set of file paths that failed during stylesheet bundling or compilation. */
+  /** Set of POSIX file paths that failed during stylesheet bundling or compilation. */
   readonly failedFiles?: ReadonlySet<string>;
 }
 
@@ -133,10 +152,10 @@ export interface SingleBuildState {
   /** Map of entry point names to their previous bundle results. */
   previousBundleResults: Map<string, BundleResult>;
 
-  /** Set of changed ESM file paths pending bundling. */
+  /** Set of changed POSIX ESM file paths pending bundling. */
   pendingChangedEsmFiles: Set<string>;
 
-  /** Set of changed TypeScript declaration file paths pending bundling. */
+  /** Set of changed POSIX TypeScript declaration file paths pending bundling. */
   pendingChangedDtsFiles: Set<string>;
 
   /** Whether the previous build iteration encountered a compilation or diagnostic error. */
diff --git a/packages/angular/build/src/utils/delete-output-dir.ts b/packages/angular/build/src/utils/delete-output-dir.ts
index 45084760793d..1f676e77b759 100644
--- a/packages/angular/build/src/utils/delete-output-dir.ts
+++ b/packages/angular/build/src/utils/delete-output-dir.ts
@@ -17,8 +17,9 @@ export async function deleteOutputDir(
   outputPath: string,
   emptyOnlyDirectories?: string[],
 ): Promise<void> {
-  const resolvedOutputPath = resolve(root, outputPath);
-  if (resolvedOutputPath === root) {
+  const resolvedRoot = resolve(root);
+  const resolvedOutputPath = resolve(resolvedRoot, outputPath);
+  if (resolvedOutputPath === resolvedRoot) {
     throw new Error('Output path MUST not be project root directory!');
   }
 
diff --git a/packages/angular/build/src/utils/normalize-asset-patterns.ts b/packages/angular/build/src/utils/normalize-asset-patterns.ts
index d97b6e605fa4..66365e10df22 100644
--- a/packages/angular/build/src/utils/normalize-asset-patterns.ts
+++ b/packages/angular/build/src/utils/normalize-asset-patterns.ts
@@ -24,14 +24,15 @@ export function normalizeAssetPatterns(
 
   // When sourceRoot is not available, we default to ${projectRoot}/src.
   const sourceRoot = projectSourceRoot || path.join(projectRoot, 'src');
-  const resolvedSourceRoot = path.resolve(workspaceRoot, sourceRoot);
-  const resolvedProjectRoot = path.resolve(workspaceRoot, projectRoot);
+  const resolvedWorkspaceRoot = path.resolve(workspaceRoot);
+  const resolvedSourceRoot = path.resolve(resolvedWorkspaceRoot, sourceRoot);
+  const resolvedProjectRoot = path.resolve(resolvedWorkspaceRoot, projectRoot);
 
   return assetPatterns.map((assetPattern) => {
     // Normalize string asset patterns to objects.
     if (typeof assetPattern === 'string') {
       const assetPath = path.normalize(assetPattern);
-      const resolvedAssetPath = path.resolve(workspaceRoot, assetPath);
+      const resolvedAssetPath = path.resolve(resolvedWorkspaceRoot, assetPath);
       let root: string;
 
       // Check if the string asset is within sourceRoot.
@@ -39,8 +40,8 @@ export function normalizeAssetPatterns(
         root = resolvedSourceRoot;
       } else if (resolvedAssetPath.startsWith(resolvedProjectRoot)) {
         root = resolvedProjectRoot;
-      } else if (resolvedAssetPath.startsWith(workspaceRoot)) {
-        root = workspaceRoot;
+      } else if (resolvedAssetPath.startsWith(resolvedWorkspaceRoot)) {
+        root = resolvedWorkspaceRoot;
       } else {
         throw new Error(`The ${assetPattern} asset path must be within the workspace root.`);
       }
@@ -67,11 +68,11 @@ export function normalizeAssetPatterns(
       }
 
       // Output directory for both is the relative path from the root to input.
-      const output = path.relative(root, path.resolve(workspaceRoot, input));
+      const output = path.relative(root, path.resolve(resolvedWorkspaceRoot, input));
 
       assetPattern = { glob, input, output };
     } else {
-      if (!isSubDirectory(workspaceRoot, assetPattern.input)) {
+      if (!isSubDirectory(resolvedWorkspaceRoot, assetPattern.input)) {
         throw new Error(`The ${assetPattern.input} asset path must be within the workspace root.`);
       }