diff --git a/ai_docs/agent/backend/agent_state.json b/ai_docs/agent/backend/agent_state.json
index 1eff644..5c4bfb7 100644
--- a/ai_docs/agent/backend/agent_state.json
+++ b/ai_docs/agent/backend/agent_state.json
@@ -1,7 +1,7 @@
 {
   "component": "backend",
-  "current_task_id": "BACKEND-007",
-  "iteration_count": 5,
+  "current_task_id": "BACKEND-008",
+  "iteration_count": 6,
   "max_iterations": 10,
   "started_at": "2025-11-21T01:06:15.789515+00:00",
   "last_commit": null,
diff --git a/ai_docs/agent/backend/task_list.json b/ai_docs/agent/backend/task_list.json
index 093eafa..084fe6b 100644
--- a/ai_docs/agent/backend/task_list.json
+++ b/ai_docs/agent/backend/task_list.json
@@ -173,7 +173,7 @@
       "id": "BACKEND-008",
       "title": "Update OpenAPI Specification for Statistics Endpoints",
       "priority": "medium",
-      "status": "pending",
+      "status": "in_progress",
       "estimated_hours": 2.0,
       "description": "Update public/open_api.yaml to include all statistics endpoints (detailed, packs, words, timeline, sessions, achievements) with proper request/response schemas, query parameters, and examples.",
       "acceptance_criteria": [
diff --git a/mnemo_cards_backend/public/open_api.yaml b/mnemo_cards_backend/public/open_api.yaml
index 899cdc2..51591ff 100644
--- a/mnemo_cards_backend/public/open_api.yaml
+++ b/mnemo_cards_backend/public/open_api.yaml
@@ -117,61 +117,472 @@ paths:
       tags:
         - UsersApiV2
       summary: getDetailedStatistics
-      description: "GET /api/v2/users/me/statistics/detailed\nReturns detailed user statistics including streaks, study time, achievements."
+      description: "GET /api/v2/users/me/statistics/detailed\nReturns detailed user statistics including streaks, study time, achievements, word statistics, and pack progress."
       operationId: getDetailedStatistics
+      security:
+        - bearerAuth: []
       responses:
         200:
-          description: "Operation completed!"
+          description: Detailed user statistics
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/UserDataDto'
+              example:
+                allWordsStatistics:
+                  words:
+                    - word: "hello"
+                      correct: 10.0
+                      incorrect: 2.0
+                      skipped: 0.0
+                      questionTypes: ["translation", "pronunciation"]
+                  correct: 150.0
+                  incorrect: 30.0
+                  skipped: 5.0
+                allTestsStatistics:
+                  tests: []
+                  totalAttempts: 25
+                  averageScore: 0.85
+                lastTimeOnline: "2024-01-15T10:30:00Z"
+                totalStudyTimeMinutes: 1200
+                currentStreak: 7
+                longestStreak: 15
+                packProgress:
+                  basic_pack:
+                    packId: "basic_pack"
+                    totalCards: 100
+                    learnedCards: 45
+                    studyTimeMinutes: 300
+                    lastStudyDate: "2024-01-15T09:00:00Z"
+                    firstStudyDate: "2024-01-01T08:00:00Z"
+                    cardAttempts: {}
+                    averageAccuracy: 0.82
+                studyDates:
+                  - "2024-01-15T09:00:00Z"
+                  - "2024-01-14T10:00:00Z"
+                categoryMinutes:
+                  basic: 300
+                achievements:
+                  - id: "streak_7"
+                    title: "Week Warrior"
+                    description: "Study for 7 consecutive days"
+                    type: "streak7Days"
+                    unlockedAt: "2024-01-15T09:00:00Z"
+                    progress: 1.0
+        401:
+          description: Unauthorized - Authentication required
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "unauthorized"
+                message: "Authentication required"
+        404:
+          description: User data not found
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "user_data_not_found"
+        500:
+          description: Internal server error
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "internal_server_error"
+                message: "An unexpected error occurred"
   /users/me/statistics/packs:
     get:
       tags:
         - UsersApiV2
       summary: getPacksStatistics
-      description: "GET /api/v2/users/me/statistics/packs\nReturns statistics for all user packs or specific pack if packId provided.\nQuery parameters: ?packId=<packId>"
+      description: "GET /api/v2/users/me/statistics/packs\nReturns statistics for all user packs or specific pack if packId provided."
       operationId: getPacksStatistics
+      security:
+        - bearerAuth: []
+      parameters:
+        - name: packId
+          in: query
+          description: Filter by specific pack ID
+          required: false
+          schema:
+            type: string
+          example: "basic_pack"
       responses:
         200:
-          description: "Operation completed!"
+          description: List of pack progress statistics
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/PackProgressDto'
+              example:
+                - packId: "basic_pack"
+                  totalCards: 100
+                  learnedCards: 45
+                  studyTimeMinutes: 300
+                  lastStudyDate: "2024-01-15T09:00:00Z"
+                  firstStudyDate: "2024-01-01T08:00:00Z"
+                  cardAttempts: {}
+                  averageAccuracy: 0.82
+                - packId: "advanced_pack"
+                  totalCards: 200
+                  learnedCards: 120
+                  studyTimeMinutes: 600
+                  lastStudyDate: "2024-01-14T15:00:00Z"
+                  firstStudyDate: "2023-12-01T10:00:00Z"
+                  cardAttempts: {}
+                  averageAccuracy: 0.75
+        401:
+          description: Unauthorized - Authentication required
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "unauthorized"
+                message: "Authentication required"
+        500:
+          description: Internal server error
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "internal_server_error"
+                message: "An unexpected error occurred"
   /users/me/statistics/words:
     get:
       tags:
         - UsersApiV2
       summary: getWordsStatistics
-      description: "GET /api/v2/users/me/statistics/words\nReturns paginated word statistics with optional filtering.\nQuery parameters:\n- packId: filter by specific pack\n- limit: number of results (default 50, max 100)\n- offset: pagination offset (default 0)\n- sortBy: 'difficulty', 'accuracy', 'recent' (default 'difficulty')\n- needsReview: 'true' to show only words needing review"
+      description: "GET /api/v2/users/me/statistics/words\nReturns paginated word statistics with optional filtering."
       operationId: getWordsStatistics
+      security:
+        - bearerAuth: []
+      parameters:
+        - name: packId
+          in: query
+          description: Filter by specific pack ID
+          required: false
+          schema:
+            type: string
+          example: "basic_pack"
+        - name: limit
+          in: query
+          description: Number of results per page (default 50, max 100)
+          required: false
+          schema:
+            type: integer
+            minimum: 1
+            maximum: 100
+            default: 50
+          example: 50
+        - name: offset
+          in: query
+          description: Pagination offset (default 0)
+          required: false
+          schema:
+            type: integer
+            minimum: 0
+            default: 0
+          example: 0
+        - name: sortBy
+          in: query
+          description: Sort order - 'difficulty', 'accuracy', 'recent', or 'alphabetical' (default 'difficulty')
+          required: false
+          schema:
+            type: string
+            enum: [difficulty, accuracy, recent, alphabetical]
+            default: difficulty
+          example: "difficulty"
+        - name: needsReview
+          in: query
+          description: Filter to show only words needing review (set to 'true')
+          required: false
+          schema:
+            type: string
+            enum: ["true", "false"]
+          example: "false"
       responses:
         200:
-          description: "Operation completed!"
+          description: Paginated word statistics
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/WordStatisticsPaginatedResponse'
+              example:
+                words:
+                  - word: "hello"
+                    correct: 10.0
+                    incorrect: 2.0
+                    skipped: 0.0
+                    questionTypes: ["translation"]
+                    lastReviewed: "2024-01-15T09:00:00Z"
+                    firstLearned: "2024-01-01T08:00:00Z"
+                    recentAttempts: []
+                    difficultyScore: 0.17
+                    needsReview: false
+                    packId: "basic_pack"
+                  - word: "world"
+                    correct: 5.0
+                    incorrect: 8.0
+                    skipped: 1.0
+                    questionTypes: ["translation", "pronunciation"]
+                    lastReviewed: "2024-01-14T10:00:00Z"
+                    firstLearned: "2024-01-01T08:00:00Z"
+                    recentAttempts: []
+                    difficultyScore: 0.57
+                    needsReview: true
+                    packId: "basic_pack"
+                totalCount: 45
+                page: 0
+                pageSize: 50
+                hasMore: false
+        400:
+          description: Bad request - Invalid query parameters
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "bad_request"
+                message: "Invalid limit parameter. Must be between 1 and 100."
+        401:
+          description: Unauthorized - Authentication required
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "unauthorized"
+                message: "Authentication required"
+        500:
+          description: Internal server error
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "internal_server_error"
+                message: "An unexpected error occurred"
   /users/me/statistics/timeline:
     get:
       tags:
         - UsersApiV2
       summary: getTimelineStatistics
-      description: "GET /api/v2/users/me/statistics/timeline\nReturns timeline statistics for study activity.\nQuery parameters:\n- period: 'day', 'week', 'month', 'year' (default 'month')\n- from: ISO date string for start date\n- to: ISO date string for end date"
+      description: "GET /api/v2/users/me/statistics/timeline\nReturns timeline statistics for study activity over a specified period."
       operationId: getTimelineStatistics
+      security:
+        - bearerAuth: []
+      parameters:
+        - name: period
+          in: query
+          description: Time period - 'day', 'week', 'month', or 'year' (default 'month')
+          required: false
+          schema:
+            type: string
+            enum: [day, week, month, year]
+            default: month
+          example: "month"
+        - name: from
+          in: query
+          description: Start date in ISO 8601 format (overrides period if provided)
+          required: false
+          schema:
+            type: string
+            format: date-time
+          example: "2024-01-01T00:00:00Z"
+        - name: to
+          in: query
+          description: End date in ISO 8601 format (defaults to now if not provided)
+          required: false
+          schema:
+            type: string
+            format: date-time
+          example: "2024-01-31T23:59:59Z"
       responses:
         200:
-          description: "Operation completed!"
+          description: Timeline statistics for the specified period
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/TimelineStatisticsResponse'
+              example:
+                period: "month"
+                startDate: "2024-01-01T00:00:00Z"
+                endDate: "2024-01-31T23:59:59Z"
+                totalDays: 31
+                activeDays: 20
+                totalMinutes: 1200
+                averageDailyMinutes: 60.0
+                currentStreak: 7
+                dailyActivity:
+                  "2024-01-15T00:00:00Z": 60
+                  "2024-01-14T00:00:00Z": 45
+                  "2024-01-13T00:00:00Z": 30
+                studyDates:
+                  - "2024-01-15T09:00:00Z"
+                  - "2024-01-14T10:00:00Z"
+                  - "2024-01-13T08:00:00Z"
+        400:
+          description: Bad request - Invalid date format
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "bad_request"
+                message: "Invalid date format. Use ISO 8601 format (YYYY-MM-DDTHH:mm:ssZ)."
+        401:
+          description: Unauthorized - Authentication required
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "unauthorized"
+                message: "Authentication required"
+        500:
+          description: Internal server error
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "internal_server_error"
+                message: "An unexpected error occurred"
   /users/me/sessions:
     post:
       tags:
         - UsersApiV2
       summary: recordStudySession
-      description: POST /api/v2/users/me/sessions\nRecords a study session for the user.
+      description: "POST /api/v2/users/me/sessions\nRecords a study session for the user. Used to track study activity and update statistics."
       operationId: recordStudySession
+      security:
+        - bearerAuth: []
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/StudySessionDto'
+            example:
+              sessionId: "session_1234567890"
+              startTime: "2024-01-15T09:00:00Z"
+              endTime: "2024-01-15T09:30:00Z"
+              wordsLearned: 10
+              testsCompleted: 2
+              accuracy: 0.85
+              packId: "basic_pack"
+              testId: null
       responses:
         200:
-          description: "Operation completed!"
+          description: Session recorded successfully
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  result:
+                    type: boolean
+                    example: true
+                  sessionId:
+                    type: string
+                    example: "session_1234567890"
+              example:
+                result: true
+                sessionId: "session_1234567890"
+        400:
+          description: Bad request - Invalid session data
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "bad_request"
+                message: "Invalid session data: startTime is required"
+        401:
+          description: Unauthorized - Authentication required
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "unauthorized"
+                message: "Authentication required"
+        500:
+          description: Internal server error
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "internal_server_error"
+                message: "An unexpected error occurred"
   /users/me/achievements:
     get:
       tags:
         - UsersApiV2
       summary: getAchievements
-      description: GET /api/v2/users/me/achievements\nReturns user's achievements and progress.
+      description: "GET /api/v2/users/me/achievements\nReturns user's achievements and progress. Includes both unlocked and locked achievements with progress indicators."
       operationId: getAchievements
+      security:
+        - bearerAuth: []
       responses:
         200:
-          description: "Operation completed!"
+          description: List of user achievements
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/AchievementDto'
+              example:
+                - id: "streak_7"
+                  title: "Week Warrior"
+                  description: "Study for 7 consecutive days"
+                  iconUrl: null
+                  unlockedAt: "2024-01-15T09:00:00Z"
+                  type: "streak7Days"
+                  progress: 1.0
+                - id: "words_10"
+                  title: "Word Explorer"
+                  description: "Learn 10 words"
+                  iconUrl: null
+                  unlockedAt: "2024-01-10T08:00:00Z"
+                  type: "words10Learned"
+                  progress: 1.0
+                - id: "streak_30"
+                  title: "Monthly Master"
+                  description: "Study for 30 consecutive days"
+                  iconUrl: null
+                  unlockedAt: null
+                  type: "streak30Days"
+                  progress: 0.23
+        401:
+          description: Unauthorized - Authentication required
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "unauthorized"
+                message: "Authentication required"
+        500:
+          description: Internal server error
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              example:
+                error: "internal_server_error"
+                message: "An unexpected error occurred"
   /admin/users:
     get:
       tags:
@@ -760,7 +1171,447 @@ paths:
       responses:
         200:
           description: "Operation completed!"
-components: { }
+components:
+  securitySchemes:
+    bearerAuth:
+      type: http
+      scheme: bearer
+      bearerFormat: JWT
+      description: JWT Bearer token authentication
+  schemas:
+    ErrorResponse:
+      type: object
+      properties:
+        error:
+          type: string
+          description: Error code identifier
+          example: "bad_request"
+        message:
+          type: string
+          description: Human-readable error message
+          example: "Invalid request parameters"
+      required:
+        - error
+    UserDataDto:
+      type: object
+      description: Comprehensive user statistics and data
+      properties:
+        allWordsStatistics:
+          $ref: '#/components/schemas/AllWordsStatisticsDto'
+        allTestsStatistics:
+          $ref: '#/components/schemas/AllTestsStatisticsDto'
+        lastTimeOnline:
+          type: string
+          format: date-time
+          nullable: true
+          description: When user was last online
+        totalStudyTimeMinutes:
+          type: integer
+          description: Total study time in minutes
+          example: 1200
+        currentStreak:
+          type: integer
+          description: Current consecutive days streak
+          example: 7
+        longestStreak:
+          type: integer
+          description: Longest streak ever achieved
+          example: 15
+        packProgress:
+          type: object
+          additionalProperties:
+            $ref: '#/components/schemas/PackProgressDto'
+          description: Map of pack ID to pack progress statistics
+        studyDates:
+          type: array
+          items:
+            type: string
+            format: date-time
+          description: List of dates when user studied
+        categoryMinutes:
+          type: object
+          additionalProperties:
+            type: integer
+          description: Study time by category/language in minutes
+        achievements:
+          type: array
+          items:
+            $ref: '#/components/schemas/AchievementDto'
+          description: User's achievements
+    AllWordsStatisticsDto:
+      type: object
+      description: Aggregated word statistics
+      properties:
+        words:
+          type: array
+          items:
+            $ref: '#/components/schemas/WordStatisticsDto'
+        correct:
+          type: number
+          format: double
+          description: Total correct answers
+        incorrect:
+          type: number
+          format: double
+          description: Total incorrect answers
+        skipped:
+          type: number
+          format: double
+          description: Total skipped answers
+    WordStatisticsDto:
+      type: object
+      description: Basic word statistics
+      properties:
+        word:
+          type: string
+          description: The word being tracked
+          example: "hello"
+        correct:
+          type: number
+          format: double
+          description: Number of correct answers
+          example: 10.0
+        incorrect:
+          type: number
+          format: double
+          description: Number of incorrect answers
+          example: 2.0
+        skipped:
+          type: number
+          format: double
+          description: Number of skipped answers
+          example: 0.0
+        questionTypes:
+          type: array
+          items:
+            type: string
+          description: Types of questions attempted
+          example: ["translation", "pronunciation"]
+    AllTestsStatisticsDto:
+      type: object
+      description: Aggregated test statistics
+      properties:
+        tests:
+          type: array
+          items:
+            type: object
+        totalAttempts:
+          type: integer
+          description: Total number of test attempts
+        averageScore:
+          type: number
+          format: double
+          description: Average test score (0.0 to 1.0)
+    PackProgressDto:
+      type: object
+      description: Statistics about user's progress on a specific pack
+      properties:
+        packId:
+          type: string
+          description: Pack identifier
+          example: "basic_pack"
+        totalCards:
+          type: integer
+          description: Total number of cards in the pack
+          example: 100
+        learnedCards:
+          type: integer
+          description: Number of cards learned by the user
+          example: 45
+        studyTimeMinutes:
+          type: integer
+          description: Total study time spent on this pack in minutes
+          example: 300
+        lastStudyDate:
+          type: string
+          format: date-time
+          nullable: true
+          description: Date when user last studied this pack
+          example: "2024-01-15T09:00:00Z"
+        firstStudyDate:
+          type: string
+          format: date-time
+          nullable: true
+          description: Date when user first started studying this pack
+          example: "2024-01-01T08:00:00Z"
+        cardAttempts:
+          type: object
+          additionalProperties:
+            type: integer
+          description: Map of card ID to number of attempts
+          example: {}
+        averageAccuracy:
+          type: number
+          format: double
+          description: Average accuracy across all attempts (0.0 to 1.0)
+          example: 0.82
+      required:
+        - packId
+        - totalCards
+    DetailedWordStatisticsDto:
+      allOf:
+        - $ref: '#/components/schemas/WordStatisticsDto'
+        - type: object
+          properties:
+            lastReviewed:
+              type: string
+              format: date-time
+              nullable: true
+              description: When this word was last reviewed
+            firstLearned:
+              type: string
+              format: date-time
+              nullable: true
+              description: When this word was first learned
+            recentAttempts:
+              type: array
+              items:
+                $ref: '#/components/schemas/WordAttemptDto'
+              description: Recent attempts (last 10)
+            difficultyScore:
+              type: number
+              format: double
+              description: Difficulty score (0.0 = easy, 1.0 = hard)
+              example: 0.57
+            needsReview:
+              type: boolean
+              description: Whether this word needs review
+              example: true
+            packId:
+              type: string
+              nullable: true
+              description: Which pack this word belongs to
+              example: "basic_pack"
+    WordAttemptDto:
+      type: object
+      description: Individual word attempt data
+      properties:
+        timestamp:
+          type: string
+          format: date-time
+          description: When the attempt happened
+        wasCorrect:
+          type: boolean
+          description: Whether the answer was correct
+        questionType:
+          type: string
+          description: Type of question asked
+          example: "translation"
+        wasSkipped:
+          type: boolean
+          description: Whether the question was skipped
+          default: false
+      required:
+        - timestamp
+        - wasCorrect
+        - questionType
+    WordStatisticsPaginatedResponse:
+      type: object
+      description: Paginated response for word statistics
+      properties:
+        words:
+          type: array
+          items:
+            $ref: '#/components/schemas/DetailedWordStatisticsDto'
+          description: List of word statistics
+        totalCount:
+          type: integer
+          description: Total number of words matching the filter
+          example: 45
+        page:
+          type: integer
+          description: Current page number (0-based)
+          example: 0
+        pageSize:
+          type: integer
+          description: Number of results per page
+          example: 50
+        hasMore:
+          type: boolean
+          description: Whether there are more results available
+          example: false
+      required:
+        - words
+        - totalCount
+        - page
+        - pageSize
+        - hasMore
+    TimelineStatisticsResponse:
+      type: object
+      description: Timeline statistics for study activity
+      properties:
+        period:
+          type: string
+          description: Time period used
+          enum: [day, week, month, year]
+          example: "month"
+        startDate:
+          type: string
+          format: date-time
+          description: Start date of the period
+          example: "2024-01-01T00:00:00Z"
+        endDate:
+          type: string
+          format: date-time
+          description: End date of the period
+          example: "2024-01-31T23:59:59Z"
+        totalDays:
+          type: integer
+          description: Total days in the period
+          example: 31
+        activeDays:
+          type: integer
+          description: Number of days with study activity
+          example: 20
+        totalMinutes:
+          type: integer
+          description: Total study time in minutes
+          example: 1200
+        averageDailyMinutes:
+          type: number
+          format: double
+          description: Average study time per active day in minutes
+          example: 60.0
+        currentStreak:
+          type: integer
+          description: Current streak within the period
+          example: 7
+        dailyActivity:
+          type: object
+          additionalProperties:
+            type: integer
+          description: Map of date (ISO string) to study minutes for that day
+          example:
+            "2024-01-15T00:00:00Z": 60
+            "2024-01-14T00:00:00Z": 45
+        studyDates:
+          type: array
+          items:
+            type: string
+            format: date-time
+          description: List of dates when user studied
+          example:
+            - "2024-01-15T09:00:00Z"
+            - "2024-01-14T10:00:00Z"
+      required:
+        - period
+        - startDate
+        - endDate
+        - totalDays
+        - activeDays
+        - totalMinutes
+        - averageDailyMinutes
+        - currentStreak
+        - dailyActivity
+        - studyDates
+    StudySessionDto:
+      type: object
+      description: Study session data for tracking learning activity
+      properties:
+        sessionId:
+          type: string
+          nullable: true
+          description: Unique session identifier
+          example: "session_1234567890"
+        startTime:
+          type: string
+          format: date-time
+          description: When the session started
+          example: "2024-01-15T09:00:00Z"
+        endTime:
+          type: string
+          format: date-time
+          nullable: true
+          description: When the session ended (null if still active)
+          example: "2024-01-15T09:30:00Z"
+        wordsLearned:
+          type: integer
+          description: Number of words learned during this session
+          default: 0
+          example: 10
+        testsCompleted:
+          type: integer
+          description: Number of tests completed during this session
+          default: 0
+          example: 2
+        accuracy:
+          type: number
+          format: double
+          description: Overall accuracy during this session (0.0 to 1.0)
+          default: 0.0
+          example: 0.85
+        packId:
+          type: string
+          nullable: true
+          description: Pack being studied (if focused on specific pack)
+          example: "basic_pack"
+        testId:
+          type: string
+          nullable: true
+          description: Test being taken (if part of a test)
+      required:
+        - startTime
+    AchievementDto:
+      type: object
+      description: Achievement data structure
+      properties:
+        id:
+          type: string
+          description: Unique achievement identifier
+          example: "streak_7"
+        title:
+          type: string
+          description: Achievement title
+          example: "Week Warrior"
+        description:
+          type: string
+          description: Achievement description
+          example: "Study for 7 consecutive days"
+        iconUrl:
+          type: string
+          nullable: true
+          description: URL to achievement icon/badge image
+        unlockedAt:
+          type: string
+          format: date-time
+          nullable: true
+          description: When this achievement was unlocked (null if locked)
+          example: "2024-01-15T09:00:00Z"
+        type:
+          type: string
+          description: Achievement type for categorization
+          enum:
+            - firstWordLearned
+            - firstTestCompleted
+            - firstPackCompleted
+            - streak3Days
+            - streak7Days
+            - streak30Days
+            - streak100Days
+            - words10Learned
+            - words50Learned
+            - words100Learned
+            - words500Learned
+            - words1000Learned
+            - perfectTestScore
+            - speedLearner
+            - dedicatedLearner
+            - nightOwl
+            - earlyBird
+            - consistentLearner
+            - languageMaster
+          example: "streak7Days"
+        progress:
+          type: number
+          format: double
+          description: Progress towards unlocking (0.0 to 1.0 for locked achievements)
+          default: 0.0
+          example: 1.0
+      required:
+        - id
+        - title
+        - description
+        - type
 tags:
   - name: PurchasesApiV2
     description: Purchases API v2\n\nRESTful endpoints for managing purchases and payments