feat(@angular/build): add prerenderFormat option to prerender routes as <route>.html

[JohannesHoppe]

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Prerendering always writes a route to <route>/index.html. On static hosts, this forces a choice between clean URLs and no redirects. You can't have both:

• Clean URLs, but redirects: links use /<route>. Every direct request (search engines, bookmarks, shared links) is first redirected (301/308) to /<route>/, and the Angular router then removes the trailing slash again.
• No redirects, but trailing slashes everywhere: links have to use /<route>/, and the app needs TrailingSlashPathLocationStrategy to keep the slash in the address bar.

Issue Number: #29173

What is the new behavior?

A new application builder option prerenderFormat makes both possible: clean URLs without a trailing slash, served directly with a status code 200.

• "directory" (default): /foo/bar is written to foo/bar/index.html, unchanged.
• "file": /foo/bar is written to foo/bar.html, so hosts that serve <route>.html for /<route> respond to /foo/bar without a redirect.

Mirrors Astro's build.format ('directory' | 'file'); Next.js, SvelteKit and Hugo offer the same choice via trailingSlash/uglyURLs.

• The root route (after removing the baseHref option) stays index.html, so / and /<locale>/ keep being served by the host's directory index.
• A route that cannot be written safely to <route>.html keeps <route>/index.html, and the build reports a warning that names the route. This applies to routes whose last segment is index in any casing (for example /index, /Index or /docs/index, whose file would be the index.html of the parent path), to routes whose file would be the index output of the application (such as index.csr.html), and to routes whose file name is already used by another route (compared case-insensitively).
• "file" is only considered when the build does not produce a server. The @angular/ssr runtime looks up prerendered pages as <route>/index.html (AngularServerApp.buildServerAssetPathFromRequest, CommonEngine.retrieveSSGPage), and with a server there is no redirect to avoid, since the generated server.ts serves static files with redirect: false. With outputMode: "server", or ssr without outputMode, the build warns that the option is not considered and prerenders to <route>/index.html. The dev server does not prerender and shows no warning.
• It is a top-level option rather than part of prerender: when outputMode is set, the prerender option is not considered (options.ts), and outputMode: "static" is the documented way to build a fully static application.

As suggested by @SanderElias in the issue, the option description states that not all hosting services support this. Happy to rename the option if you prefer a different name.

Does this PR introduce a breaking change?

• Yes
• No

Other information

As a stopgap, I built a builder (@angular-schule/prerender-format) that wraps @angular/build:application. It only works by replacing the internal prerenderPages() export of @angular/build at runtime to rename the output files, which is fragile and can break with any internal refactoring. A built-in option is the clean solution.

The option description in schema.json feeds the ng build reference on angular.dev. If this lands, I'm happy to follow up with a short section in the SSR guide ("Generate a fully static application") in angular/angular.

[gemini-code-assist]

Code Review

This pull request introduces the "prerenderFormat" option to the Angular application builder, allowing prerendered pages to be output as ".html" files (using the "file" format) instead of the default "/index.html" structure. It includes robust conflict resolution logic for routes that cannot be safely written to ".html" (such as routes named "index" or matching the application's index file), falling back to the directory format with a warning. The review feedback suggests a performance optimization in "prerender.ts" to pre-lowercase the "indexOutput" variable once outside the route rendering loop, rather than performing redundant string operations inside "getFileFormatConflict" for every route.