diff --git a/packages/backend/src/server/oauth/OAuth2ProviderService.ts b/packages/backend/src/server/oauth/OAuth2ProviderService.ts
index da84d876a..5b954cbf6 100644
--- a/packages/backend/src/server/oauth/OAuth2ProviderService.ts
+++ b/packages/backend/src/server/oauth/OAuth2ProviderService.ts
@@ -18,11 +18,11 @@ import type { Config } from '@/config.js';
 import { DI } from '@/di-symbols.js';
 import { bindThis } from '@/decorators.js';
 import type { AccessTokensRepository, UsersRepository } from '@/models/index.js';
-import type { IdService } from '@/core/IdService.js';
-import type { CacheService } from '@/core/CacheService.js';
+import { IdService } from '@/core/IdService.js';
+import { CacheService } from '@/core/CacheService.js';
 import type { LocalUser } from '@/models/entities/User.js';
 import { MemoryKVCache } from '@/misc/cache.js';
-import type { LoggerService } from '@/core/LoggerService.js';
+import { LoggerService } from '@/core/LoggerService.js';
 import Logger from '@/logger.js';
 import type { ServerResponse } from 'node:http';
 import type { FastifyInstance } from 'fastify';
@@ -376,9 +376,9 @@ export class OAuth2ProviderService {
 					}
 					areq.scope = scopes;
 
-					if (type !== 'code') {
-						throw new AuthorizationError('`response_type` parameter must be set as "code"', 'invalid_request');
-					}
+					// Require PKCE parameters.
+					// Recommended by https://indieauth.spec.indieweb.org/#authorization-request, but also prevents downgrade attack:
+					// https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics#name-pkce-downgrade-attack
 					if (typeof codeChallenge !== 'string') {
 						throw new AuthorizationError('`code_challenge` parameter is required', 'invalid_request');
 					}
diff --git a/packages/backend/test/e2e/oauth.ts b/packages/backend/test/e2e/oauth.ts
index dbef0e457..b515eabac 100644
--- a/packages/backend/test/e2e/oauth.ts
+++ b/packages/backend/test/e2e/oauth.ts
@@ -6,7 +6,7 @@
 process.env.NODE_ENV = 'test';
 
 import * as assert from 'assert';
-import { AuthorizationCode, type AuthorizationTokenConfig } from 'simple-oauth2';
+import { AuthorizationCode, ResourceOwnerPassword, type AuthorizationTokenConfig, ClientCredentials } from 'simple-oauth2';
 import pkceChallenge from 'pkce-challenge';
 import { JSDOM } from 'jsdom';
 import type * as misskey from 'misskey-js';
@@ -375,7 +375,10 @@ describe('OAuth', () => {
 						code,
 						redirect_uri,
 						code_verifier: wrong_verifier,
-					} as AuthorizationTokenConfigExtended));
+					} as AuthorizationTokenConfigExtended), (err: any) => {
+						assert.strictEqual(err.data.payload.error, 'invalid_grant');
+						return true;
+					});
 				});
 			}
 		});
@@ -400,20 +403,29 @@ describe('OAuth', () => {
 				code,
 				redirect_uri,
 				code_verifier,
-			} as AuthorizationTokenConfigExtended));
+			} as AuthorizationTokenConfigExtended), (err: any) => {
+				assert.strictEqual(err.data.payload.error, 'invalid_grant');
+				return true;
+			});
 		});
 
 		test('On failure', async () => {
 			const { code_challenge, code_verifier } = await pkceChallenge(128);
 			const { client, code } = await fetchAuthorizationCode(alice, 'write:notes', code_challenge);
 
-			await assert.rejects(client.getToken({ code, redirect_uri }));
+			await assert.rejects(client.getToken({ code, redirect_uri }), (err: any) => {
+				assert.strictEqual(err.data.payload.error, 'invalid_grant');
+				return true;
+			});
 
 			await assert.rejects(client.getToken({
 				code,
 				redirect_uri,
 				code_verifier,
-			} as AuthorizationTokenConfigExtended));
+			} as AuthorizationTokenConfigExtended), (err: any) => {
+				assert.strictEqual(err.data.payload.error, 'invalid_grant');
+				return true;
+			});
 		});
 	});
 
@@ -660,7 +672,10 @@ describe('OAuth', () => {
 				code,
 				redirect_uri: 'http://127.0.0.2/',
 				code_verifier,
-			} as AuthorizationTokenConfigExtended));
+			} as AuthorizationTokenConfigExtended), (err: any) => {
+				assert.strictEqual(err.data.payload.error, 'invalid_grant');
+				return true;
+			});
 		});
 
 		test('Invalid redirect_uri including the valid one at token endpoint', async () => {
@@ -672,7 +687,10 @@ describe('OAuth', () => {
 				code,
 				redirect_uri: 'http://127.0.0.1/redirection',
 				code_verifier,
-			} as AuthorizationTokenConfigExtended));
+			} as AuthorizationTokenConfigExtended), (err: any) => {
+				assert.strictEqual(err.data.payload.error, 'invalid_grant');
+				return true;
+			});
 		});
 
 		test('No redirect_uri at token endpoint', async () => {
@@ -683,7 +701,10 @@ describe('OAuth', () => {
 			await assert.rejects(client.getToken({
 				code,
 				code_verifier,
-			} as AuthorizationTokenConfigExtended));
+			} as AuthorizationTokenConfigExtended), (err: any) => {
+				assert.strictEqual(err.data.payload.error, 'invalid_grant');
+				return true;
+			});
 		});
 	});
 
@@ -752,6 +773,61 @@ describe('OAuth', () => {
 		});
 	});
 
+	// Only authorization code grant is supported
+	describe('Grant type', () => {
+		test('Implicit grant is not supported', async () => {
+			const url = new URL('/oauth/authorize', host);
+			url.searchParams.append('response_type', 'token');
+			const response = await fetch(url);
+			assertDirectError(response, 501, 'unsupported_response_type');
+		});
+
+		test('Resource owner grant is not supported', async () => {
+			const client = new ResourceOwnerPassword({
+				client: {
+					id: `http://127.0.0.1:${clientPort}/`,
+					secret: '',
+				},
+				auth: {
+					tokenHost: host,
+					tokenPath: '/oauth/token',
+				},
+				options: {
+					authorizationMethod: 'body',
+				},
+			});
+
+			await assert.rejects(client.getToken({
+				username: 'alice',
+				password: 'test',
+			}), (err: any) => {
+				assert.strictEqual(err.data.payload.error, 'unsupported_grant_type');
+				return true;
+			});
+		});
+
+		test('Client credential grant is not supported', async () => {
+			const client = new ClientCredentials({
+				client: {
+					id: `http://127.0.0.1:${clientPort}/`,
+					secret: '',
+				},
+				auth: {
+					tokenHost: host,
+					tokenPath: '/oauth/token',
+				},
+				options: {
+					authorizationMethod: 'body',
+				},
+			});
+
+			await assert.rejects(client.getToken({}), (err: any) => {
+				assert.strictEqual(err.data.payload.error, 'unsupported_grant_type');
+				return true;
+			});
+		});
+	});
+
 	// https://indieauth.spec.indieweb.org/#client-information-discovery
 	describe('Client Information Discovery', () => {
 		describe('Redirection', () => {