3 files changed, 26 insertions, 28 deletions

diff --git a/packages/astro/src/core/config/config.ts b/packages/astro/src/core/config/config.ts
index dc7de2bc7..b04aae727 100644
--- a/packages/astro/src/core/config/config.ts
+++ b/packages/astro/src/core/config/config.ts
@@ -6,6 +6,7 @@ import * as colors from 'kleur/colors';
 import path from 'path';
 import { fileURLToPath, pathToFileURL } from 'url';
 import { AstroError, AstroErrorData } from '../errors/index.js';
+import { mergeConfig } from './merge.js';
 import { createRelativeSchema } from './schema.js';
 import { loadConfigWithVite } from './vite-load.js';
 
@@ -114,28 +115,18 @@ export function resolveRoot(cwd?: string | URL): string {
 
 /** Merge CLI flags & user config object (CLI flags take priority) */
 function mergeCLIFlags(astroConfig: AstroUserConfig, flags: CLIFlags) {
-	astroConfig.server = astroConfig.server || {};
-	astroConfig.markdown = astroConfig.markdown || {};
-	astroConfig.experimental = astroConfig.experimental || {};
-	if (typeof flags.site === 'string') astroConfig.site = flags.site;
-	if (typeof flags.base === 'string') astroConfig.base = flags.base;
-	if (typeof flags.drafts === 'boolean') astroConfig.markdown.drafts = flags.drafts;
-	if (typeof flags.port === 'number') {
-		// @ts-expect-error astroConfig.server may be a function, but TS doesn't like attaching properties to a function.
-		// TODO: Come back here and refactor to remove this expected error.
-		astroConfig.server.port = flags.port;
-	}
-	if (typeof flags.host === 'string' || typeof flags.host === 'boolean') {
-		// @ts-expect-error astroConfig.server may be a function, but TS doesn't like attaching properties to a function.
-		// TODO: Come back here and refactor to remove this expected error.
-		astroConfig.server.host = flags.host;
-	}
-	if (typeof flags.open === 'boolean') {
-		// @ts-expect-error astroConfig.server may be a function, but TS doesn't like attaching properties to a function.
-		// TODO: Come back here and refactor to remove this expected error.
-		astroConfig.server.open = flags.open;
-	}
-	return astroConfig;
+	return mergeConfig(astroConfig, {
+		site: flags.site,
+		base: flags.base,
+		markdown: {
+			drafts: flags.drafts,
+		},
+		server: {
+			port: flags.port,
+			host: flags.host,
+			open: flags.open,
+		},
+	});
 }
 
 async function search(fsMod: typeof fs, root: string) {
diff --git a/packages/astro/src/core/config/merge.ts b/packages/astro/src/core/config/merge.ts
index 3a498332b..93c386eca 100644
--- a/packages/astro/src/core/config/merge.ts
+++ b/packages/astro/src/core/config/merge.ts
@@ -25,6 +25,18 @@ function mergeConfigRecursively(
 			merged[key] = mergeViteConfig(existing, value);
 			continue;
 		}
+		if (key === 'server' && rootPath === '') {
+			// server config can be a function or an object, if one of the two values is a function,
+			// create a new wrapper function that merges them
+			if (typeof existing === 'function' || typeof value === 'function') {
+				merged[key] = (...args: any[]) => {
+					const existingConfig = typeof existing === 'function' ? existing(...args) : existing;
+					const valueConfig = typeof value === 'function' ? value(...args) : value;
+					return mergeConfigRecursively(existingConfig, valueConfig, key);
+				};
+				continue;
+			}
+		}
 
 		if (Array.isArray(existing) || Array.isArray(value)) {
 			merged[key] = [...arraify(existing ?? []), ...arraify(value ?? [])];
diff --git a/packages/astro/src/core/config/schema.ts b/packages/astro/src/core/config/schema.ts
index ae681a543..5b559e444 100644
--- a/packages/astro/src/core/config/schema.ts
+++ b/packages/astro/src/core/config/schema.ts
@@ -299,12 +299,7 @@ export function createRelativeSchema(cmd: string, fileProtocolRoot: URL) {
 			// preprocess
 			(val) => {
 				if (typeof val === 'function') {
-					const result = val({ command: cmd === 'dev' ? 'dev' : 'preview' });
-					// @ts-expect-error revive attached prop added from CLI flags
-					if (val.port) result.port = val.port;
-					// @ts-expect-error revive attached prop added from CLI flags
-					if (val.host) result.host = val.host;
-					return result;
+					return val({ command: cmd === 'dev' ? 'dev' : 'preview' });
 				} else {
 					return val;
 				}