diff --git a/backend/src/milestone/controllers/milestone.controller.ts b/backend/src/milestone/controllers/milestone.controller.ts
index 851864a2..5431a98e 100644
--- a/backend/src/milestone/controllers/milestone.controller.ts
+++ b/backend/src/milestone/controllers/milestone.controller.ts
@@ -1,77 +1,83 @@
 import {
+  Body,
   Controller,
   Get,
-  Post,
-  Param,
   HttpCode,
   HttpStatus,
-  Body,
+  Param,
+  Post,
+  UseGuards,
 } from '@nestjs/common';
+import { JwtAuthGuard } from '../../auth/guards/jwt-auth.guard';
+import { CurrentUser } from '../../auth/decorators/current-user.decorator';
+import { APIKeyGuard } from '../../api-key/api-key.guard';
 import type { MilestoneService } from '../services/milestone.service';
 
+/**
+ * Identity rules (issue #482):
+ * - The `/my` routes resolve the user id from the verified JWT principal
+ *   (`req.user.id` via the JwtStrategy validate() hook) — never from a
+ *   placeholder or any client-supplied value. Unauthenticated calls are
+ *   rejected with 401.
+ * - The `/trigger/*` routes are server-to-server endpoints that mint
+ *   milestone awards for the user named in the body, so they sit behind
+ *   the API key guard (`x-api-key` header) which a browser user cannot
+ *   satisfy. They intentionally do NOT accept JWT auth.
+ */
 @Controller('milestones')
 export class MilestoneController {
   constructor(private readonly milestoneService: MilestoneService) {}
 
   @Get('my')
-  async getMyMilestones() {
-    // In a real app, you'd get userId from JWT token
-    const req = { user: { id: 'user-id-placeholder' } }; // Mock request object
-    const userId = req.user.id;
-
+  @UseGuards(JwtAuthGuard)
+  async getMyMilestones(@CurrentUser('id') userId: string) {
     return this.milestoneService.getUserMilestones(userId);
   }
 
   @Get('my/stats')
-  async getMyMilestoneStats() {
-    // In a real app, you'd get userId from JWT token
-    const req = { user: { id: 'user-id-placeholder' } }; // Mock request object
-    const userId = req.user.id;
-
+  @UseGuards(JwtAuthGuard)
+  async getMyMilestoneStats(@CurrentUser('id') userId: string) {
     return this.milestoneService.getUserMilestoneStats(userId);
   }
 
   @Get('my/next')
-  async getMyNextMilestones() {
-    // In a real app, you'd get userId from JWT token
-    const req = { user: { id: 'user-id-placeholder' } }; // Mock request object
-    const userId = req.user.id;
-
+  @UseGuards(JwtAuthGuard)
+  async getMyNextMilestones(@CurrentUser('id') userId: string) {
     return this.milestoneService.getNextMilestones(userId);
   }
 
+  /**
+   * Marks a milestone as viewed for the *calling* user only: the update is
+   * scoped by `{ id, userId }` in the assignment service, so one user can
+   * never mutate another user's unread state.
+   */
   @Post('my/:milestoneId/view')
   @HttpCode(HttpStatus.OK)
-  async markMilestoneAsViewed(@Param('milestoneId') milestoneId: string) {
-    // In a real app, you'd get userId from JWT token
-    const req = { user: { id: 'user-id-placeholder' } }; // Mock request object
-    const userId = req.user.id;
-
+  @UseGuards(JwtAuthGuard)
+  async markMilestoneAsViewed(
+    @CurrentUser('id') userId: string,
+    @Param('milestoneId') milestoneId: string,
+  ) {
     await this.milestoneService.markMilestoneAsViewed(userId, milestoneId);
     return { success: true };
   }
 
-  // Endpoint for external systems to trigger milestone checks
+  // Server-to-server endpoints for external systems to trigger milestone
+  // checks. Guarded by API key auth, not JWT (see class docblock).
   @Post('trigger/puzzle-completed')
   @HttpCode(HttpStatus.OK)
+  @UseGuards(APIKeyGuard)
   async triggerPuzzleCompleted(
     @Body() body: { userId: string; puzzleData?: any },
   ) {
-    return this.milestoneService.onPuzzleCompleted(
-      body.userId,
-      body.puzzleData,
-    );
+    return this.milestoneService.onPuzzleCompleted(body.userId, body.puzzleData);
   }
 
   @Post('trigger/streak-updated')
   @HttpCode(HttpStatus.OK)
+  @UseGuards(APIKeyGuard)
   async triggerStreakUpdated(
-    @Body()
-    body: {
-      userId: string;
-      currentStreak: number;
-      longestStreak: number;
-    },
+    @Body() body: { userId: string; currentStreak: number; longestStreak: number },
   ) {
     return this.milestoneService.onStreakUpdated(
       body.userId,
diff --git a/backend/src/milestone/milestone.module.ts b/backend/src/milestone/milestone.module.ts
index 14aec13a..9d0ea731 100644
--- a/backend/src/milestone/milestone.module.ts
+++ b/backend/src/milestone/milestone.module.ts
@@ -9,10 +9,14 @@ import { MilestoneAssignmentService } from './services/milestone-assignment.serv
 import { MilestoneService } from './services/milestone.service';
 import { MilestoneController } from './controllers/milestone.controller';
 import { UserMilestoneController } from './controllers/user-milestone.controller';
+import { ApiKeyModule } from '../api-key/api-key.module';
 
 @Module({
   imports: [
     TypeOrmModule.forFeature([MilestoneTemplate, UserMilestone, UserProgress]),
+    // Provides ApiKeyService for the APIKeyGuard that protects the
+    // server-to-server `/milestones/trigger/*` routes (issue #482).
+    ApiKeyModule,
   ],
   controllers: [MilestoneController, UserMilestoneController],
   providers: [