fix: actions wrong return 415

.changeset/honest-terms-help.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: correctly return 415 when unexpected content types are submitted to actions

packages/kit/src/runtime/server/page/actions.js

@@ -220,7 +220,8 @@ async function call_action(event, actions) {
 	}
 
 	if (!is_form_content_type(event.request)) {
-		throw new Error(
+		throw error(
+			415,
 			`Actions expect form-encoded data (received ${event.request.headers.get('content-type')})`
 		);
 	}

packages/kit/test/apps/basics/test/test.js

@@ -1220,6 +1220,21 @@ test.describe('Actions', () => {
 
 		await expect(page.locator('pre')).toHaveText('something went wrong');
 	});
+
+	test('submitting application/json should return http status code 415', async ({ baseURL }) => {
+		const response = await fetch(`${baseURL}/actions/form-errors`, {
+			method: 'POST',
+			body: JSON.stringify({ foo: 'bar' }),
+			headers: {
+				'Content-Type': 'application/json',
+				Origin: `${baseURL}`
+			}
+		});
+		const { type, error } = await response.json();
+		expect(type).toBe('error');
+		expect(error.message).toBe('Actions expect form-encoded data (received application/json)');
+		expect(response.status).toBe(415);
+	});
 });
 
 // Run in serial to not pollute the log with (correct) cookie warnings