diff --git a/.changeset/flat-radios-cheer.md b/.changeset/flat-radios-cheer.md
new file mode 100644
index 000000000..ab01d34dd
--- /dev/null
+++ b/.changeset/flat-radios-cheer.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+`--experimental-ssr` now is only required when using a 3rd-party adapter
diff --git a/examples/ssr/package.json b/examples/ssr/package.json
index 401f7e06a..370e79972 100644
--- a/examples/ssr/package.json
+++ b/examples/ssr/package.json
@@ -3,9 +3,9 @@
   "version": "0.0.1",
   "private": true,
   "scripts": {
-    "dev": "astro dev --experimental-ssr",
+    "dev": "astro dev",
     "start": "astro dev",
-    "build": "astro build --experimental-ssr",
+    "build": "astro build",
     "server": "node server/server.mjs"
   },
   "devDependencies": {
diff --git a/packages/astro/src/@types/astro.ts b/packages/astro/src/@types/astro.ts
index 33b03a775..800ab9b23 100644
--- a/packages/astro/src/@types/astro.ts
+++ b/packages/astro/src/@types/astro.ts
@@ -345,7 +345,8 @@ export interface AstroUserConfig {
 		 */
 		experimentalStaticBuild?: boolean;
 		/**
-		 * Enable a build for SSR support.
+		 * Enable SSR support for 3rd-party adapters.
+		 * Not required when using a built-in adapter.
 		 * Default: false
 		 */
 		experimentalSsr?: boolean;
diff --git a/packages/astro/src/cli/index.ts b/packages/astro/src/cli/index.ts
index 534937d8c..c68a97a74 100644
--- a/packages/astro/src/cli/index.ts
+++ b/packages/astro/src/cli/index.ts
@@ -39,7 +39,7 @@ function printAstroHelp() {
 			['--project-root <path>', 'Specify the path to the project root folder.'],
 			['--no-sitemap', 'Disable sitemap generation (build only).'],
 			['--legacy-build', 'Use the build strategy prior to 0.24.0'],
-			['--experimental-ssr', 'Enable SSR compilation.'],
+			['--experimental-ssr', 'Enable SSR compilation fot 3rd-party adapters.'],
 			['--drafts', 'Include markdown draft pages in the build.'],
 			['--verbose', 'Enable verbose logging'],
 			['--silent', 'Disable logging'],
diff --git a/packages/astro/src/core/build/generate.ts b/packages/astro/src/core/build/generate.ts
index 7c8e92ce5..5d0c70a41 100644
--- a/packages/astro/src/core/build/generate.ts
+++ b/packages/astro/src/core/build/generate.ts
@@ -12,7 +12,7 @@ import { BEFORE_HYDRATION_SCRIPT_ID } from '../../vite-plugin-scripts/index.js';
 import { call as callEndpoint } from '../endpoint/index.js';
 import { render } from '../render/core.js';
 import { createLinkStylesheetElementSet, createModuleScriptElementWithSrcSet } from '../render/ssr-element.js';
-import { getOutputFilename } from '../util.js';
+import { getOutputFilename, isBuildingToSSR } from '../util.js';
 import { getOutFile, getOutFolder } from './common.js';
 import { eachPageData, getPageDataByComponent } from './internal.js';
 import type { PageBuildData, SingleFileBuiltModule, StaticBuildOptions } from './types';
@@ -71,7 +71,7 @@ export async function generatePages(result: RollupOutput, opts: StaticBuildOptio
 	const timer = performance.now();
 	info(opts.logging, null, `\n${bgGreen(black(' generating static routes '))}`);
 
-	const ssr = !!opts.astroConfig._ctx.adapter?.serverEntrypoint;
+	const ssr = isBuildingToSSR(opts.astroConfig);
 	const serverEntry = opts.buildConfig.serverEntry;
 	const outFolder = ssr ? opts.buildConfig.server : opts.astroConfig.dist;
 	const ssrEntryURL = new URL('./' + serverEntry + `?time=${Date.now()}`, outFolder);
@@ -197,7 +197,7 @@ async function generatePath(pathname: string, opts: StaticBuildOptions, gopts: G
 		route: pageData.route,
 		routeCache,
 		site: astroConfig.buildOptions.site,
-		ssr: opts.astroConfig.buildOptions.experimentalSsr,
+		ssr: isBuildingToSSR(opts.astroConfig),
 	};
 
 	let body: string;
diff --git a/packages/astro/src/core/build/index.ts b/packages/astro/src/core/build/index.ts
index 02e1898f8..db876176a 100644
--- a/packages/astro/src/core/build/index.ts
+++ b/packages/astro/src/core/build/index.ts
@@ -17,7 +17,7 @@ import { staticBuild } from './static-build.js';
 import { RouteCache } from '../render/route-cache.js';
 import { runHookBuildDone, runHookBuildStart, runHookConfigDone, runHookConfigSetup } from '../../integrations/index.js';
 import { getTimeStat } from './util.js';
-import { createSafeError } from '../util.js';
+import { createSafeError, isBuildingToSSR } from '../util.js';
 import { fixViteErrorMessage } from '../errors.js';
 
 export interface BuildOptions {
@@ -101,7 +101,7 @@ class AstroBuilder {
 			origin,
 			routeCache: this.routeCache,
 			viteServer,
-			ssr: this.config.buildOptions.experimentalSsr,
+			ssr: isBuildingToSSR(this.config),
 		});
 
 		// Filter pages by using conditions based on their frontmatter.
@@ -182,7 +182,7 @@ class AstroBuilder {
 		await runHookBuildDone({ config: this.config, pages: pageNames, routes: Object.values(allPages).map((pd) => pd.route) });
 
 		if (this.logging.level && levels[this.logging.level] <= levels['info']) {
-			const buildMode = this.config.buildOptions.experimentalSsr ? 'ssr' : 'static';
+			const buildMode = isBuildingToSSR(this.config) ? 'ssr' : 'static';
 			await this.printStats({ logging: this.logging, timeStart: this.timer.init, pageCount: pageNames.length, buildMode });
 		}
 	}
diff --git a/packages/astro/src/core/build/page-data.ts b/packages/astro/src/core/build/page-data.ts
index 707c89fd7..c84e29588 100644
--- a/packages/astro/src/core/build/page-data.ts
+++ b/packages/astro/src/core/build/page-data.ts
@@ -10,6 +10,7 @@ import { debug } from '../logger/core.js';
 import { preload as ssrPreload } from '../render/dev/index.js';
 import { generateRssFunction } from '../render/rss.js';
 import { callGetStaticPaths, RouteCache, RouteCacheEntry } from '../render/route-cache.js';
+import { isBuildingToSSR } from '../util.js';
 
 export interface CollectPagesDataOptions {
 	astroConfig: AstroConfig;
@@ -33,7 +34,7 @@ export async function collectPagesData(opts: CollectPagesDataOptions): Promise<C
 	const assets: Record<string, string> = {};
 	const allPages: AllPagesData = {};
 
-	const buildMode = astroConfig.buildOptions.experimentalSsr ? 'ssr' : 'static';
+	const buildMode = isBuildingToSSR(astroConfig) ? 'ssr' : 'static';
 
 	const dataCollectionLogTimeout = setInterval(() => {
 		info(opts.logging, 'build', 'The data collection step may take longer for larger projects...');
diff --git a/packages/astro/src/core/build/static-build.ts b/packages/astro/src/core/build/static-build.ts
index 481d51cfa..4e475afb9 100644
--- a/packages/astro/src/core/build/static-build.ts
+++ b/packages/astro/src/core/build/static-build.ts
@@ -111,7 +111,7 @@ export async function staticBuild(opts: StaticBuildOptions) {
 
 async function ssrBuild(opts: StaticBuildOptions, internals: BuildInternals, input: Set<string>) {
 	const { astroConfig, viteConfig } = opts;
-	const ssr = astroConfig.buildOptions.experimentalSsr;
+	const ssr = isBuildingToSSR(astroConfig);
 	const out = ssr ? opts.buildConfig.server : astroConfig.dist;
 
 	const viteBuildConfig = {
diff --git a/packages/astro/src/core/config.ts b/packages/astro/src/core/config.ts
index 394c1c42d..97b467e9d 100644
--- a/packages/astro/src/core/config.ts
+++ b/packages/astro/src/core/config.ts
@@ -261,12 +261,7 @@ function mergeCLIFlags(astroConfig: AstroUserConfig, flags: CLIFlags) {
 	if (typeof flags.host === 'string' || typeof flags.host === 'boolean') astroConfig.devOptions.host = flags.host;
 	if (typeof flags.hostname === 'string') astroConfig.devOptions.hostname = flags.hostname;
 	if (typeof flags.legacyBuild === 'boolean') astroConfig.buildOptions.legacyBuild = flags.legacyBuild;
-	if (typeof flags.experimentalSsr === 'boolean') {
-		astroConfig.buildOptions.experimentalSsr = flags.experimentalSsr;
-		if (flags.experimentalSsr) {
-			astroConfig.buildOptions.legacyBuild = false;
-		}
-	}
+	if (typeof flags.experimentalSsr === 'boolean') astroConfig.buildOptions.experimentalSsr = flags.experimentalSsr;
 	if (typeof flags.experimentalIntegrations === 'boolean') astroConfig.experimentalIntegrations = flags.experimentalIntegrations;
 	if (typeof flags.drafts === 'boolean') astroConfig.buildOptions.drafts = flags.drafts;
 	return astroConfig;
diff --git a/packages/astro/src/core/endpoint/dev/index.ts b/packages/astro/src/core/endpoint/dev/index.ts
index da3671bc0..1e7abe8df 100644
--- a/packages/astro/src/core/endpoint/dev/index.ts
+++ b/packages/astro/src/core/endpoint/dev/index.ts
@@ -1,12 +1,13 @@
 import type { EndpointHandler } from '../../../@types/astro';
 import type { SSROptions } from '../../render/dev';
 import { preload } from '../../render/dev/index.js';
+import { isBuildingToSSR } from '../../util.js';
 import { call as callEndpoint } from '../index.js';
 
 export async function call(ssrOpts: SSROptions) {
 	const [, mod] = await preload(ssrOpts);
 	return await callEndpoint(mod as unknown as EndpointHandler, {
 		...ssrOpts,
-		ssr: ssrOpts.astroConfig.buildOptions.experimentalSsr,
+		ssr: isBuildingToSSR(ssrOpts.astroConfig),
 	});
 }
diff --git a/packages/astro/src/core/render/dev/index.ts b/packages/astro/src/core/render/dev/index.ts
index 1ced45a40..b81ef6863 100644
--- a/packages/astro/src/core/render/dev/index.ts
+++ b/packages/astro/src/core/render/dev/index.ts
@@ -9,6 +9,8 @@ import { createModuleScriptElementWithSrcSet } from '../ssr-element.js';
 import { getStylesForURL } from './css.js';
 import { getHmrScript } from './hmr.js';
 import { injectTags } from './html.js';
+import { isBuildingToSSR } from '../../util.js';
+
 export interface SSROptions {
 	/** an instance of the AstroConfig */
 	astroConfig: AstroConfig;
@@ -146,7 +148,7 @@ export async function render(renderers: SSRLoadedRenderer[], mod: ComponentInsta
 		route,
 		routeCache,
 		site: astroConfig.buildOptions.site,
-		ssr: astroConfig.buildOptions.experimentalSsr,
+		ssr: isBuildingToSSR(astroConfig),
 	});
 
 	if (route?.type === 'endpoint' || content.type === 'response') {
diff --git a/packages/astro/src/core/util.ts b/packages/astro/src/core/util.ts
index 45885e9e8..2ec2a1744 100644
--- a/packages/astro/src/core/util.ts
+++ b/packages/astro/src/core/util.ts
@@ -139,7 +139,27 @@ export function emptyDir(_dir: URL, skip?: Set<string>): void {
 }
 
 export function isBuildingToSSR(config: AstroConfig): boolean {
-	return !!config._ctx.adapter?.serverEntrypoint;
+	const adapter = config._ctx.adapter;
+	if (!adapter) return false;
+
+	if (typeof adapter.serverEntrypoint === 'string') {
+		if (!adapter.name.startsWith('@astrojs/') && !config.buildOptions.experimentalSsr) {
+			throw new Error(
+				[
+					`Server-side rendering (SSR) is still experimental.`,
+					``,
+					`Only official "@astrojs/*" adapters are currently supported.`,
+					`To enable SSR for 3rd-party adapters, use the "--experimental-ssr" flag.`,
+					`Breaking changes may occur in this API before Astro v1.0 is released.`,
+					``,
+				].join('\n')
+			);
+		} else {
+			return true;
+		}
+	} else {
+		return false;
+	}
 }
 
 export function emoji(char: string, fallback: string) {
diff --git a/packages/astro/src/vite-plugin-astro-server/index.ts b/packages/astro/src/vite-plugin-astro-server/index.ts
index 67df56aeb..5d85555af 100644
--- a/packages/astro/src/vite-plugin-astro-server/index.ts
+++ b/packages/astro/src/vite-plugin-astro-server/index.ts
@@ -6,7 +6,7 @@ import { debug, info, warn, error, LogOptions } from '../core/logger/core.js';
 import { getParamsAndProps, GetParamsAndPropsError } from '../core/render/core.js';
 import { createRouteManifest, matchRoute } from '../core/routing/index.js';
 import stripAnsi from 'strip-ansi';
-import { createSafeError } from '../core/util.js';
+import { createSafeError, isBuildingToSSR } from '../core/util.js';
 import { ssr, preload } from '../core/render/dev/index.js';
 import { call as callEndpoint } from '../core/endpoint/dev/index.js';
 import * as msg from '../core/messages.js';
@@ -123,7 +123,7 @@ async function handleRequest(
 	const site = config.buildOptions.site ? new URL(config.buildOptions.site) : undefined;
 	const devRoot = site ? site.pathname : '/';
 	const origin = `${viteServer.config.server.https ? 'https' : 'http'}://${req.headers.host}`;
-	const buildingToSSR = !!config._ctx.adapter?.serverEntrypoint;
+	const buildingToSSR = isBuildingToSSR(config);
 	const url = new URL(origin + req.url);
 	const pathname = decodeURI(url.pathname);
 	const rootRelativeUrl = pathname.substring(devRoot.length - 1);
@@ -185,7 +185,7 @@ async function handleRequest(
 			routeCache,
 			pathname: rootRelativeUrl,
 			logging,
-			ssr: config.buildOptions.experimentalSsr,
+			ssr: isBuildingToSSR(config),
 		});
 		if (paramsAndPropsRes === GetParamsAndPropsError.NoMatchingStaticPath) {
 			warn(logging, 'getStaticPaths', `Route pattern matched, but no matching static path found. (${pathname})`);
diff --git a/packages/astro/test/ssr-api-route.test.js b/packages/astro/test/ssr-api-route.test.js
index 54991e80a..6bf8fcd20 100644
--- a/packages/astro/test/ssr-api-route.test.js
+++ b/packages/astro/test/ssr-api-route.test.js
@@ -10,9 +10,7 @@ describe('API routes in SSR', () => {
 	before(async () => {
 		fixture = await loadFixture({
 			projectRoot: './fixtures/ssr-api-route/',
-			buildOptions: {
-				experimentalSsr: true,
-			},
+			buildOptions: { experimentalSsr: true },
 			adapter: testAdapter(),
 		});
 		await fixture.build();
diff --git a/packages/astro/test/ssr-dynamic.test.js b/packages/astro/test/ssr-dynamic.test.js
index d938e5c95..98d73b98e 100644
--- a/packages/astro/test/ssr-dynamic.test.js
+++ b/packages/astro/test/ssr-dynamic.test.js
@@ -11,9 +11,7 @@ describe('Dynamic pages in SSR', () => {
 	before(async () => {
 		fixture = await loadFixture({
 			projectRoot: './fixtures/ssr-dynamic/',
-			buildOptions: {
-				experimentalSsr: true,
-			},
+			buildOptions: { experimentalSsr: true },
 			adapter: testAdapter(),
 		});
 		await fixture.build();
diff --git a/packages/integrations/deno/test/fixtures/basics/astro.config.mjs b/packages/integrations/deno/test/fixtures/basics/astro.config.mjs
index dfdf9e182..38490926b 100644
--- a/packages/integrations/deno/test/fixtures/basics/astro.config.mjs
+++ b/packages/integrations/deno/test/fixtures/basics/astro.config.mjs
@@ -2,5 +2,6 @@ import { defineConfig } from 'astro/config';
 import deno from '@astrojs/deno';
 
 export default defineConfig({
-	adapter: deno()
+	adapter: deno(),
+	buildOptions: { experimentalSsr: true }
 })
diff --git a/packages/integrations/deno/test/helpers.js b/packages/integrations/deno/test/helpers.js
index 659d24d5e..bc2bc0622 100644
--- a/packages/integrations/deno/test/helpers.js
+++ b/packages/integrations/deno/test/helpers.js
@@ -13,7 +13,7 @@ export async function runBuild(fixturePath) {
 export async function runBuildAndStartApp(fixturePath, cb) {
 	const url = new URL(fixturePath, dir);
 	const close = await runBuild(fixturePath);
-	const mod = await import(new URL('./dist/entry.mjs', url));
+	const mod = await import(new URL('./dist/server/entry.mjs', url));
 	await cb();
 	await mod.stop();
 	await close();