diff --git a/.changeset/deep-parks-marry.md b/.changeset/deep-parks-marry.md
new file mode 100644
index 000000000000..6d332012e916
--- /dev/null
+++ b/.changeset/deep-parks-marry.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+chore: improve runtime config parsing
diff --git a/packages/adapter-vercel/index.d.ts b/packages/adapter-vercel/index.d.ts
index 792dfe5b4f00..8d37ce4cc409 100644
--- a/packages/adapter-vercel/index.d.ts
+++ b/packages/adapter-vercel/index.d.ts
@@ -1,5 +1,6 @@
 import { Adapter } from '@sveltejs/kit';
 import './ambient.js';
+import { RuntimeConfigKey } from './utils.js';
 
 export default function plugin(config?: Config): Adapter;
 
@@ -8,7 +9,7 @@ export interface ServerlessConfig {
 	 * Whether to use [Edge Functions](https://vercel.com/docs/concepts/functions/edge-functions) (`'edge'`) or [Serverless Functions](https://vercel.com/docs/concepts/functions/serverless-functions) (`'nodejs18.x'`, `'nodejs20.x'` etc).
 	 * @default Same as the build environment
 	 */
-	runtime?: `nodejs${number}.x` | `experimental_bun1.x`;
+	runtime?: Exclude<RuntimeConfigKey, 'edge'>;
 	/**
 	 * To which regions to deploy the app. A list of regions.
 	 * More info: https://vercel.com/docs/concepts/edge-network/regions
diff --git a/packages/adapter-vercel/index.js b/packages/adapter-vercel/index.js
index 809c6dea3382..b001407c6560 100644
--- a/packages/adapter-vercel/index.js
+++ b/packages/adapter-vercel/index.js
@@ -5,7 +5,7 @@ import process from 'node:process';
 import { fileURLToPath } from 'node:url';
 import { nodeFileTrace } from '@vercel/nft';
 import esbuild from 'esbuild';
-import { get_pathname, parse_isr_expiration, pattern_to_src } from './utils.js';
+import { get_pathname, parse_isr_expiration, pattern_to_src, resolve_runtime } from './utils.js';
 import { VERSION } from '@sveltejs/kit';
 
 /**
@@ -24,30 +24,6 @@ const INTERNAL = '![-]'; // this name is guaranteed not to conflict with user ro
 
 const [kit_major, kit_minor] = VERSION.split('.');
 
-const get_default_runtime = () => {
-	const major = Number(process.version.slice(1).split('.')[0]);
-
-	// If we're building on Vercel, we know that the version will be fine because Vercel
-	// provides Node (and Vercel won't provide something it doesn't support).
-	// Also means we're not on the hook for updating the adapter every time a new Node
-	// version is added to Vercel.
-	if (!process.env.VERCEL) {
-		if (major < 20 || major > 22) {
-			throw new Error(
-				`Building locally with unsupported Node.js version: ${process.version}. Please use Node 20 or 22 to build your project, or explicitly specify a runtime in your adapter configuration.`
-			);
-		}
-
-		if (major % 2 !== 0) {
-			throw new Error(
-				`Unsupported Node.js version: ${process.version}. Please use an even-numbered Node version to build your project, or explicitly specify a runtime in your adapter configuration.`
-			);
-		}
-	}
-
-	return `nodejs${major}.x`;
-};
-
 // https://vercel.com/docs/functions/edge-functions/edge-runtime#compatible-node.js-modules
 const compatible_node_modules = ['async_hooks', 'events', 'buffer', 'assert', 'util'];
 
@@ -294,12 +270,8 @@ const plugin = function (defaults = {}) {
 
 			// group routes by config
 			for (const route of builder.routes) {
-				const runtime = (
-					route.config?.runtime ??
-					defaults?.runtime ??
-					get_default_runtime()
-				).replace('experimental_', '');
-				const config = { runtime, ...defaults, ...route.config };
+				const runtime = resolve_runtime(defaults.runtime, route.config.runtime);
+				const config = { ...defaults, ...route.config, runtime };
 
 				if (is_prerendered(route)) {
 					if (config.isr) {
@@ -308,23 +280,10 @@ const plugin = function (defaults = {}) {
 					continue;
 				}
 
-				const node_runtime = /nodejs([0-9]+)\.x/.exec(runtime);
-				const bun_runtime = /^bun/.exec(runtime);
-				if (
-					runtime !== 'edge' &&
-					!bun_runtime &&
-					(!node_runtime || parseInt(node_runtime[1]) < 20)
-				) {
-					throw new Error(
-						`Invalid runtime '${runtime}' for route ${route.id}. Valid runtimes are 'edge', 'experimental_bun1.x', 'nodejs20.x' or 'nodejs22.x' ` +
-							'(see the Node.js Version section in your Vercel project settings for info on the currently supported versions).'
-					);
-				}
-
 				if (config.isr) {
 					const directory = path.relative('.', builder.config.kit.files.routes + route.id);
 
-					if (!runtime.startsWith('nodejs') && !bun_runtime) {
+					if (runtime === 'edge') {
 						throw new Error(
 							`${directory}: Routes using \`isr\` must use a Node.js or Bun runtime (for example 'nodejs22.x' or 'experimental_bun1.x')`
 						);
@@ -409,13 +368,13 @@ const plugin = function (defaults = {}) {
 				// we need to create a catch-all route so that 404s are handled
 				// by SvelteKit rather than Vercel
 
-				const runtime = (defaults.runtime ?? get_default_runtime()).replace('experimental_', '');
+				const runtime = resolve_runtime(defaults.runtime);
 				const generate_function =
 					runtime === 'edge' ? generate_edge_function : generate_serverless_function;
 
 				await generate_function(
 					`${INTERNAL}/catchall`,
-					/** @type {any} */ ({ runtime, ...defaults }),
+					/** @type {any} */ ({ ...defaults, runtime }),
 					[]
 				);
 			}
diff --git a/packages/adapter-vercel/test/utils.spec.js b/packages/adapter-vercel/test/utils.spec.js
index fbcd95996667..ffee426fa170 100644
--- a/packages/adapter-vercel/test/utils.spec.js
+++ b/packages/adapter-vercel/test/utils.spec.js
@@ -1,5 +1,5 @@
 import { assert, test, describe } from 'vitest';
-import { get_pathname, parse_isr_expiration, pattern_to_src } from '../utils.js';
+import { get_pathname, parse_isr_expiration, pattern_to_src, resolve_runtime } from '../utils.js';
 
 // workaround so that TypeScript doesn't follow that import which makes it pick up that file and then error on missing import aliases
 const { parse_route_id } = await import('../../kit/src/' + 'utils/routing.js');
@@ -171,3 +171,19 @@ describe('parse_isr_expiration', () => {
 		);
 	});
 });
+
+describe('resolve_runtime', () => {
+	test('prefers override_key over default_key', () => {
+		const result = resolve_runtime('nodejs20.x', 'experimental_bun1.x');
+		assert.equal(result, 'bun1.x');
+	});
+
+	test('uses default_key when override_key is undefined', () => {
+		const result = resolve_runtime('experimental_bun1.x');
+		assert.equal(result, 'bun1.x');
+	});
+
+	test('throws an error when resolving to an invalid runtime', () => {
+		assert.throws(() => resolve_runtime('node18.x', undefined), /Unsupported runtime: node18.x/);
+	});
+});
diff --git a/packages/adapter-vercel/utils.js b/packages/adapter-vercel/utils.js
index 0da89f84927b..983b30bbb5be 100644
--- a/packages/adapter-vercel/utils.js
+++ b/packages/adapter-vercel/utils.js
@@ -1,3 +1,5 @@
+import process from 'node:process';
+
 /** @param {import("@sveltejs/kit").RouteDefinition<any>} route */
 export function get_pathname(route) {
 	let i = 1;
@@ -106,3 +108,47 @@ export function parse_isr_expiration(value, route_id) {
 	}
 	return parsed;
 }
+
+/**
+ * @param {string | undefined} default_key
+ * @param {string | undefined} [override_key]
+ * @returns {RuntimeKey}
+ */
+export function resolve_runtime(default_key, override_key) {
+	const key = (override_key ?? default_key ?? get_default_runtime()).replace('experimental_', '');
+	assert_is_valid_runtime(key);
+	return key;
+}
+
+/** @returns {RuntimeKey} */
+function get_default_runtime() {
+	// TODO may someday need to auto-detect Bun, but this will be complicated because you may want to run your build
+	// with Bun but not have your serverless runtime be in Bun. Vercel will likely have to attach something to `globalThis` or similar
+	// to tell us what the bun configuration is.
+	const major = Number(process.version.slice(1).split('.')[0]);
+
+	if (major !== 20 && major !== 22) {
+		throw new Error(
+			`Unsupported Node.js version: ${process.version}. Please use Node 20 or 22 to build your project, or explicitly specify a runtime in your adapter configuration.`
+		);
+	}
+
+	return `nodejs${major}.x`;
+}
+
+const valid_runtimes = /** @type {const} */ (['nodejs20.x', 'nodejs22.x', 'bun1.x', 'edge']);
+
+/**
+ * @param {string} key
+ * @returns {asserts key is RuntimeKey}
+ */
+function assert_is_valid_runtime(key) {
+	if (!valid_runtimes.includes(/** @type {RuntimeKey} */ (key))) {
+		throw new Error(
+			`Unsupported runtime: ${key}. Supported runtimes are: ${valid_runtimes.join(', ')}. See the Node.js Version section in your Vercel project settings for info on the currently supported versions.`
+		);
+	}
+}
+
+/** @typedef {Exclude<RuntimeKey, 'bun1.x'> | 'experimental_bun1.x'} RuntimeConfigKey */
+/** @typedef {typeof valid_runtimes[number]} RuntimeKey */