diff --git a/packages/backend/src/server/ActivityPubServerService.ts b/packages/backend/src/server/ActivityPubServerService.ts
index bdd2e97508..632a2ccb2e 100644
--- a/packages/backend/src/server/ActivityPubServerService.ts
+++ b/packages/backend/src/server/ActivityPubServerService.ts
@@ -1,3 +1,4 @@
+import { IncomingMessage } from 'node:http';
 import { Inject, Injectable } from '@nestjs/common';
 import fastifyAccepts from '@fastify/accepts';
 import httpSignature from '@peertube/http-signature';
@@ -19,6 +20,7 @@ import { QueryService } from '@/core/QueryService.js';
 import { UtilityService } from '@/core/UtilityService.js';
 import { UserEntityService } from '@/core/entities/UserEntityService.js';
 import { bindThis } from '@/decorators.js';
+import { IActivity } from '@/core/activitypub/type.js';
 import type { FastifyInstance, FastifyRequest, FastifyReply, FastifyPluginOptions } from 'fastify';
 import type { FindOptionsWhere } from 'typeorm';
 
@@ -97,7 +99,8 @@ export class ActivityPubServerService {
 			return;
 		}
 
-		this.queueService.inbox(request.body, signature);
+		// TODO: request.bodyのバリデーション?
+		this.queueService.inbox(request.body as IActivity, signature);
 
 		reply.code(202);
 	}
@@ -413,20 +416,21 @@ export class ActivityPubServerService {
 
 	@bindThis
 	public createServer(fastify: FastifyInstance, options: FastifyPluginOptions, done: (err?: Error) => void) {
-		fastify.addConstraintStrategy({
+		// addConstraintStrategy の型定義がおかしいため
+		(fastify.addConstraintStrategy as any)({
 			name: 'apOrHtml',
 			storage() {
-				const store = {};
+				const store = {} as any;
 				return {
-					get(key) {
+					get(key: string) {
 						return store[key] ?? null;
 					},
-					set(key, value) {
+					set(key: string, value: any) {
 						store[key] = value;
 					},
 				};
 			},
-			deriveConstraint(request, ctx) {
+			deriveConstraint(request: IncomingMessage) {
 				const accepted = accepts(request).type(['html', ACTIVITY_JSON, LD_JSON]);
 				const isAp = typeof accepted === 'string' && !accepted.match(/html/);
 				return isAp ? 'ap' : 'html';
diff --git a/packages/backend/src/server/api/SigninApiService.ts b/packages/backend/src/server/api/SigninApiService.ts
index 10f8423d44..d490097dea 100644
--- a/packages/backend/src/server/api/SigninApiService.ts
+++ b/packages/backend/src/server/api/SigninApiService.ts
@@ -10,9 +10,9 @@ import { getIpHash } from '@/misc/get-ip-hash.js';
 import type { ILocalUser } from '@/models/entities/User.js';
 import { IdService } from '@/core/IdService.js';
 import { TwoFactorAuthenticationService } from '@/core/TwoFactorAuthenticationService.js';
+import { bindThis } from '@/decorators.js';
 import { RateLimiterService } from './RateLimiterService.js';
 import { SigninService } from './SigninService.js';
-import { bindThis } from '@/decorators.js';
 import type { FastifyRequest, FastifyReply } from 'fastify';
 
 @Injectable()
@@ -131,7 +131,7 @@ export class SigninApiService {
 				createdAt: new Date(),
 				userId: user.id,
 				ip: request.ip,
-				headers: request.headers,
+				headers: request.headers as any,
 				success: false,
 			});
 
diff --git a/packages/backend/src/server/api/SigninService.ts b/packages/backend/src/server/api/SigninService.ts
index 89a8a9ff16..655bf51a16 100644
--- a/packages/backend/src/server/api/SigninService.ts
+++ b/packages/backend/src/server/api/SigninService.ts
@@ -33,7 +33,7 @@ export class SigninService {
 				createdAt: new Date(),
 				userId: user.id,
 				ip: request.ip,
-				headers: request.headers,
+				headers: request.headers as any,
 				success: true,
 			}).then(x => this.signinsRepository.findOneByOrFail(x.identifiers[0]));
 	
diff --git a/packages/backend/src/server/api/SignupApiService.ts b/packages/backend/src/server/api/SignupApiService.ts
index 4b676bb8b9..c46f71d934 100644
--- a/packages/backend/src/server/api/SignupApiService.ts
+++ b/packages/backend/src/server/api/SignupApiService.ts
@@ -162,7 +162,7 @@ export class SignupApiService {
 					token: secret,
 				};
 			} catch (err) {
-				throw new FastifyReplyError(400, err);
+				throw new FastifyReplyError(400, typeof err === 'string' ? err : (err as Error).toString());
 			}
 		}
 	}
@@ -195,7 +195,7 @@ export class SignupApiService {
 
 			return this.signinService.signin(request, reply, account as ILocalUser);
 		} catch (err) {
-			throw new FastifyReplyError(400, err);
+			throw new FastifyReplyError(400, typeof err === 'string' ? err : (err as Error).toString());
 		}
 	}
 }
diff --git a/packages/backend/src/server/api/endpoints/endpoint.ts b/packages/backend/src/server/api/endpoints/endpoint.ts
index a337a05f8c..13b91685a4 100644
--- a/packages/backend/src/server/api/endpoints/endpoint.ts
+++ b/packages/backend/src/server/api/endpoints/endpoint.ts
@@ -27,7 +27,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> {
 			return {
 				params: Object.entries(ep.params.properties ?? {}).map(([k, v]) => ({
 					name: k,
-					type: v.type.charAt(0).toUpperCase() + v.type.slice(1),
+					type: v.type ? v.type.charAt(0).toUpperCase() + v.type.slice(1) : 'string',
 				})),
 			};
 		});
diff --git a/packages/backend/src/server/web/ClientServerService.ts b/packages/backend/src/server/web/ClientServerService.ts
index 2a764a25b0..044cba8d1e 100644
--- a/packages/backend/src/server/web/ClientServerService.ts
+++ b/packages/backend/src/server/web/ClientServerService.ts
@@ -155,7 +155,7 @@ export class ClientServerService {
 		});
 
 		serverAdapter.setBasePath(bullBoardPath);
-		fastify.register(serverAdapter.registerPlugin(), { prefix: bullBoardPath });
+		(fastify.register as any)(serverAdapter.registerPlugin(), { prefix: bullBoardPath });
 		//#endregion
 
 		fastify.register(fastifyView, {