docs: Update vehicles README with VIN decode endpoint (refs #9)

- Add VIN decode endpoint to API section
- Document request/response format with confidence levels
- Add error response examples (400, 403, 502)
- Update architecture diagram with external/ directory

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/core/config/feature-tiers.ts

@@ -26,6 +26,11 @@ export const FEATURE_TIERS: Record<string, FeatureConfig> = {
     name: 'Scan for Maintenance Schedule',
     upgradePrompt: 'Upgrade to Pro to automatically extract maintenance schedules from your vehicle manuals.',
   },
+  'vehicle.vinDecode': {
+    minTier: 'pro',
+    name: 'VIN Decode',
+    upgradePrompt: 'Upgrade to Pro to automatically decode VIN and populate vehicle details from the NHTSA database.',
+  },
 } as const;
 
 /**

backend/src/features/vehicles/README.md

@@ -12,6 +12,9 @@ Primary entity for vehicle management consuming MVP Platform Vehicles Service. H
 - `PUT /api/vehicles/:id` - Update vehicle details
 - `DELETE /api/vehicles/:id` - Soft delete vehicle
 
+### VIN Decoding (Pro/Enterprise Only)
+- `POST /api/vehicles/decode-vin` - Decode VIN using NHTSA vPIC API
+
 ### Hierarchical Vehicle Dropdowns
 **Status**: Vehicles service now proxies the platform vehicle catalog to provide fully dynamic dropdowns. Each selection step filters the next list, ensuring only valid combinations are shown.
 
@@ -100,6 +103,12 @@ vehicles/
 │   └── name-normalizer.ts
 ├── data/                  # Database layer
 │   └── vehicles.repository.ts
+├── external/              # External service integrations
+│   ├── CLAUDE.md          # Integration pattern docs
+│   └── nhtsa/             # NHTSA vPIC API client
+│       ├── nhtsa.client.ts
+│       ├── nhtsa.types.ts
+│       └── index.ts
 ├── migrations/            # Feature schema
 │   └── 001_create_vehicles_tables.sql
 ├── tests/                 # All tests
@@ -112,11 +121,45 @@ vehicles/
 
 ## Key Features
 
-### 🔍 Automatic VIN Decoding
+### 🔍 VIN Decoding (NHTSA vPIC API)
-- **Platform Service**: MVP Platform Vehicles Service VIN decode endpoint
+- **Tier Gating**: Pro and Enterprise users only (`vehicle.vinDecode` feature key)
-- **Caching**: Platform service handles caching strategy
+- **NHTSA API**: Calls official NHTSA vPIC API for authoritative vehicle data
-- **Fallback**: Circuit breaker pattern with graceful degradation
+- **Caching**: Results cached in `vin_cache` table (1-year TTL, VIN data is static)
-- **Validation**: 17-character VIN format validation
+- **Validation**: 17-character VIN format, excludes I/O/Q characters
+- **Matching**: Case-insensitive exact match against dropdown options
+- **Confidence Levels**: High (exact match), Medium (normalized match), None (hint only)
+- **Timeout**: 5-second timeout for NHTSA API calls
+
+#### Decode VIN Request
+```json
+POST /api/vehicles/decode-vin
+Authorization: Bearer <jwt>
+{
+  "vin": "1HGBH41JXMN109186"
+}
+
+Response (200):
+{
+  "year": { "value": 2021, "nhtsaValue": "2021", "confidence": "high" },
+  "make": { "value": "Honda", "nhtsaValue": "HONDA", "confidence": "high" },
+  "model": { "value": "Civic", "nhtsaValue": "Civic", "confidence": "high" },
+  "trimLevel": { "value": "EX", "nhtsaValue": "EX", "confidence": "high" },
+  "engine": { "value": null, "nhtsaValue": "2.0L L4 DOHC 16V", "confidence": "none" },
+  "transmission": { "value": null, "nhtsaValue": "CVT", "confidence": "none" },
+  "bodyType": { "value": null, "nhtsaValue": "Sedan", "confidence": "none" },
+  "driveType": { "value": null, "nhtsaValue": "FWD", "confidence": "none" },
+  "fuelType": { "value": null, "nhtsaValue": "Gasoline", "confidence": "none" }
+}
+
+Error (400 - Invalid VIN):
+{ "error": "INVALID_VIN", "message": "Invalid VIN format. VIN must be..." }
+
+Error (403 - Tier Required):
+{ "error": "TIER_REQUIRED", "requiredTier": "pro", "currentTier": "free", ... }
+
+Error (502 - NHTSA Failure):
+{ "error": "VIN_DECODE_FAILED", "message": "Unable to decode VIN from external service" }
+```
 
 ### 📋 Hierarchical Vehicle Dropdowns
 - **Platform Service**: Consumes year-based hierarchical vehicle API

backend/src/features/vehicles/api/vehicles.controller.ts

@@ -10,16 +10,19 @@ import { pool } from '../../../core/config/database';
 import { logger } from '../../../core/logging/logger';
 import { CreateVehicleBody, UpdateVehicleBody, VehicleParams } from '../domain/vehicles.types';
 import { getStorageService } from '../../../core/storage/storage.service';
+import { NHTSAClient, DecodeVinRequest } from '../external/nhtsa';
 import crypto from 'crypto';
 import FileType from 'file-type';
 import path from 'path';
 
 export class VehiclesController {
   private vehiclesService: VehiclesService;
+  private nhtsaClient: NHTSAClient;
 
   constructor() {
     const repository = new VehiclesRepository(pool);
     this.vehiclesService = new VehiclesService(repository);
+    this.nhtsaClient = new NHTSAClient(pool);
   }
 
   async getUserVehicles(request: FastifyRequest, reply: FastifyReply) {
@@ -309,6 +312,75 @@ export class VehiclesController {
     }
   }
 
+  /**
+   * Decode VIN using NHTSA vPIC API
+   * POST /api/vehicles/decode-vin
+   * Requires Pro or Enterprise tier
+   */
+  async decodeVin(request: FastifyRequest<{ Body: DecodeVinRequest }>, reply: FastifyReply) {
+    const userId = (request as any).user?.sub;
+
+    try {
+      const { vin } = request.body;
+
+      if (!vin || typeof vin !== 'string') {
+        return reply.code(400).send({
+          error: 'INVALID_VIN',
+          message: 'VIN is required'
+        });
+      }
+
+      logger.info('VIN decode requested', { userId, vin: vin.substring(0, 6) + '...' });
+
+      // Validate and decode VIN
+      const response = await this.nhtsaClient.decodeVin(vin);
+
+      // Extract and map fields from NHTSA response
+      const decodedData = await this.vehiclesService.mapNHTSAResponse(response);
+
+      logger.info('VIN decode successful', {
+        userId,
+        hasYear: !!decodedData.year.value,
+        hasMake: !!decodedData.make.value,
+        hasModel: !!decodedData.model.value
+      });
+
+      return reply.code(200).send(decodedData);
+    } catch (error: any) {
+      logger.error('VIN decode failed', { error, userId });
+
+      // Handle validation errors
+      if (error.message?.includes('Invalid VIN')) {
+        return reply.code(400).send({
+          error: 'INVALID_VIN',
+          message: error.message
+        });
+      }
+
+      // Handle timeout
+      if (error.message?.includes('timed out')) {
+        return reply.code(504).send({
+          error: 'VIN_DECODE_TIMEOUT',
+          message: 'NHTSA API request timed out. Please try again.'
+        });
+      }
+
+      // Handle NHTSA API errors
+      if (error.message?.includes('NHTSA')) {
+        return reply.code(502).send({
+          error: 'VIN_DECODE_FAILED',
+          message: 'Unable to decode VIN from external service',
+          details: error.message
+        });
+      }
+
+      return reply.code(500).send({
+        error: 'Internal server error',
+        message: 'Failed to decode VIN'
+      });
+    }
+  }
+
   async uploadImage(request: FastifyRequest<{ Params: VehicleParams }>, reply: FastifyReply) {
     const userId = (request as any).user.sub;
     const vehicleId = request.params.id;

backend/src/features/vehicles/api/vehicles.routes.ts

@@ -75,6 +75,12 @@ export const vehiclesRoutes: FastifyPluginAsync = async (
     handler: vehiclesController.getDropdownOptions.bind(vehiclesController)
   });
 
+  // POST /api/vehicles/decode-vin - Decode VIN using NHTSA vPIC API (Pro/Enterprise only)
+  fastify.post<{ Body: { vin: string } }>('/vehicles/decode-vin', {
+    preHandler: [fastify.authenticate, fastify.requireTier({ featureKey: 'vehicle.vinDecode' })],
+    handler: vehiclesController.decodeVin.bind(vehiclesController)
+  });
+
   // Vehicle image routes - must be before :id to avoid conflicts
   // POST /api/vehicles/:id/image - Upload vehicle image
   fastify.post<{ Params: VehicleParams }>('/vehicles/:id/image', {

backend/src/features/vehicles/domain/vehicles.service.ts

@@ -20,6 +20,7 @@ import { isValidVIN, isValidPreModernVIN } from '../../../shared-minimal/utils/v
 import { normalizeMakeName, normalizeModelName } from './name-normalizer';
 import { getVehicleDataService, getPool } from '../../platform';
 import { auditLogService } from '../../audit-log';
+import { NHTSAClient, NHTSADecodeResponse, DecodedVehicleData, MatchedField } from '../external/nhtsa';
 
 export class VehiclesService {
   private readonly cachePrefix = 'vehicles';
@@ -346,6 +347,129 @@ export class VehiclesService {
     return vehicleDataService.getYears(pool);
   }
 
+  /**
+   * Map NHTSA decode response to internal decoded vehicle data format
+   * with dropdown matching and confidence levels
+   */
+  async mapNHTSAResponse(response: NHTSADecodeResponse): Promise<DecodedVehicleData> {
+    const vehicleDataService = getVehicleDataService();
+    const pool = getPool();
+
+    // Extract raw values from NHTSA response
+    const nhtsaYear = NHTSAClient.extractYear(response);
+    const nhtsaMake = NHTSAClient.extractValue(response, 'Make');
+    const nhtsaModel = NHTSAClient.extractValue(response, 'Model');
+    const nhtsaTrim = NHTSAClient.extractValue(response, 'Trim');
+    const nhtsaBodyType = NHTSAClient.extractValue(response, 'Body Class');
+    const nhtsaDriveType = NHTSAClient.extractValue(response, 'Drive Type');
+    const nhtsaFuelType = NHTSAClient.extractValue(response, 'Fuel Type - Primary');
+    const nhtsaEngine = NHTSAClient.extractEngine(response);
+    const nhtsaTransmission = NHTSAClient.extractValue(response, 'Transmission Style');
+
+    // Year is always high confidence if present (exact numeric match)
+    const year: MatchedField<number> = {
+      value: nhtsaYear,
+      nhtsaValue: nhtsaYear?.toString() || null,
+      confidence: nhtsaYear ? 'high' : 'none'
+    };
+
+    // Match make against dropdown options
+    let make: MatchedField<string> = { value: null, nhtsaValue: nhtsaMake, confidence: 'none' };
+    if (nhtsaYear && nhtsaMake) {
+      const makes = await vehicleDataService.getMakes(pool, nhtsaYear);
+      make = this.matchField(nhtsaMake, makes);
+    }
+
+    // Match model against dropdown options
+    let model: MatchedField<string> = { value: null, nhtsaValue: nhtsaModel, confidence: 'none' };
+    if (nhtsaYear && make.value && nhtsaModel) {
+      const models = await vehicleDataService.getModels(pool, nhtsaYear, make.value);
+      model = this.matchField(nhtsaModel, models);
+    }
+
+    // Match trim against dropdown options
+    let trimLevel: MatchedField<string> = { value: null, nhtsaValue: nhtsaTrim, confidence: 'none' };
+    if (nhtsaYear && make.value && model.value && nhtsaTrim) {
+      const trims = await vehicleDataService.getTrims(pool, nhtsaYear, make.value, model.value);
+      trimLevel = this.matchField(nhtsaTrim, trims);
+    }
+
+    // Match engine against dropdown options
+    let engine: MatchedField<string> = { value: null, nhtsaValue: nhtsaEngine, confidence: 'none' };
+    if (nhtsaYear && make.value && model.value && trimLevel.value && nhtsaEngine) {
+      const engines = await vehicleDataService.getEngines(pool, nhtsaYear, make.value, model.value, trimLevel.value);
+      engine = this.matchField(nhtsaEngine, engines);
+    }
+
+    // Match transmission against dropdown options
+    let transmission: MatchedField<string> = { value: null, nhtsaValue: nhtsaTransmission, confidence: 'none' };
+    if (nhtsaYear && make.value && model.value && trimLevel.value && nhtsaTransmission) {
+      const transmissions = await vehicleDataService.getTransmissionsForTrim(pool, nhtsaYear, make.value, model.value, trimLevel.value);
+      transmission = this.matchField(nhtsaTransmission, transmissions);
+    }
+
+    // Body type, drive type, and fuel type are display-only (no dropdown matching)
+    const bodyType: MatchedField<string> = {
+      value: null,
+      nhtsaValue: nhtsaBodyType,
+      confidence: 'none'
+    };
+
+    const driveType: MatchedField<string> = {
+      value: null,
+      nhtsaValue: nhtsaDriveType,
+      confidence: 'none'
+    };
+
+    const fuelType: MatchedField<string> = {
+      value: null,
+      nhtsaValue: nhtsaFuelType,
+      confidence: 'none'
+    };
+
+    return {
+      year,
+      make,
+      model,
+      trimLevel,
+      bodyType,
+      driveType,
+      fuelType,
+      engine,
+      transmission
+    };
+  }
+
+  /**
+   * Match a value against dropdown options using case-insensitive exact matching
+   * Returns the matched dropdown value with confidence level
+   */
+  private matchField(nhtsaValue: string, options: string[]): MatchedField<string> {
+    if (!nhtsaValue || options.length === 0) {
+      return { value: null, nhtsaValue, confidence: 'none' };
+    }
+
+    const normalizedNhtsa = nhtsaValue.toLowerCase().trim();
+
+    // Try exact case-insensitive match
+    const exactMatch = options.find(opt => opt.toLowerCase().trim() === normalizedNhtsa);
+    if (exactMatch) {
+      return { value: exactMatch, nhtsaValue, confidence: 'high' };
+    }
+
+    // Try normalized comparison (remove special chars)
+    const normalizeForCompare = (s: string) => s.toLowerCase().replace(/[^a-z0-9]/g, '');
+    const normalizedNhtsaClean = normalizeForCompare(nhtsaValue);
+
+    const normalizedMatch = options.find(opt => normalizeForCompare(opt) === normalizedNhtsaClean);
+    if (normalizedMatch) {
+      return { value: normalizedMatch, nhtsaValue, confidence: 'medium' };
+    }
+
+    // No match found - return NHTSA value as hint with no match
+    return { value: null, nhtsaValue, confidence: 'none' };
+  }
+
   private toResponse(vehicle: Vehicle): VehicleResponse {
     return {
       id: vehicle.id,

backend/src/features/vehicles/external/CLAUDE.md

@@ -0,0 +1,43 @@
+# vehicles/external/
+
+External service integrations for the vehicles feature.
+
+## Subdirectories
+
+| Directory | What | When to read |
+| --------- | ---- | ------------ |
+| `nhtsa/` | NHTSA vPIC API client for VIN decoding | VIN decode feature work |
+
+## Integration Pattern
+
+External integrations follow a consistent pattern:
+
+1. **Client class** (`{service}.client.ts`) - axios-based HTTP client with:
+   - Timeout configuration (prevent hanging requests)
+   - Error handling with specific error types
+   - Caching strategy (database or Redis)
+   - Input validation before API calls
+
+2. **Types file** (`{service}.types.ts`) - TypeScript interfaces for:
+   - Raw API response types
+   - Mapped internal types (camelCase)
+   - Error types
+
+3. **Cache strategy** - Each integration defines:
+   - Cache location (vin_cache table for NHTSA)
+   - TTL (1 year for static vehicle data)
+   - Cache invalidation rules (if applicable)
+
+## Adding New Integrations
+
+To add a new external service (e.g., Carfax, KBB):
+
+1. Create subdirectory: `external/{service}/`
+2. Add client: `{service}.client.ts` following NHTSAClient pattern
+3. Add types: `{service}.types.ts`
+4. Update this CLAUDE.md with new directory
+5. Add tests in `tests/unit/{service}.client.test.ts`
+
+## See Also
+
+- `../../../stations/external/google-maps/` - Sister pattern for Google Maps integration

backend/src/features/vehicles/external/nhtsa/index.ts

@@ -0,0 +1,16 @@
+/**
+ * @ai-summary NHTSA vPIC integration exports
+ * @ai-context Public API for VIN decoding functionality
+ */
+
+export { NHTSAClient } from './nhtsa.client';
+export type {
+  NHTSADecodeResponse,
+  NHTSAResult,
+  DecodedVehicleData,
+  MatchedField,
+  MatchConfidence,
+  VinCacheEntry,
+  DecodeVinRequest,
+  VinDecodeError,
+} from './nhtsa.types';

backend/src/features/vehicles/external/nhtsa/nhtsa.client.ts

@@ -0,0 +1,235 @@
+/**
+ * @ai-summary NHTSA vPIC API client for VIN decoding
+ * @ai-context Fetches vehicle data from NHTSA and caches results
+ */
+
+import axios, { AxiosError } from 'axios';
+import { logger } from '../../../../core/logging/logger';
+import { NHTSADecodeResponse, VinCacheEntry } from './nhtsa.types';
+import { Pool } from 'pg';
+
+/**
+ * VIN validation regex
+ * - 17 characters
+ * - Excludes I, O, Q (not used in VINs)
+ * - Alphanumeric only
+ */
+const VIN_REGEX = /^[A-HJ-NPR-Z0-9]{17}$/;
+
+/**
+ * Cache TTL: 1 year (VIN data is static - vehicle specs don't change)
+ */
+const CACHE_TTL_SECONDS = 365 * 24 * 60 * 60;
+
+export class NHTSAClient {
+  private readonly baseURL = 'https://vpic.nhtsa.dot.gov/api';
+  private readonly timeout = 5000; // 5 seconds
+
+  constructor(private readonly pool: Pool) {}
+
+  /**
+   * Validate VIN format
+   * @throws Error if VIN format is invalid
+   */
+  validateVin(vin: string): string {
+    const sanitized = vin.trim().toUpperCase();
+
+    if (!sanitized) {
+      throw new Error('VIN is required');
+    }
+
+    if (!VIN_REGEX.test(sanitized)) {
+      throw new Error('Invalid VIN format. VIN must be exactly 17 characters and contain only letters (except I, O, Q) and numbers.');
+    }
+
+    return sanitized;
+  }
+
+  /**
+   * Check cache for existing VIN data
+   */
+  async getCached(vin: string): Promise<VinCacheEntry | null> {
+    try {
+      const result = await this.pool.query<{
+        vin: string;
+        make: string | null;
+        model: string | null;
+        year: number | null;
+        engine_type: string | null;
+        body_type: string | null;
+        raw_data: NHTSADecodeResponse;
+        cached_at: Date;
+      }>(
+        `SELECT vin, make, model, year, engine_type, body_type, raw_data, cached_at
+         FROM vin_cache
+         WHERE vin = $1
+         AND cached_at > NOW() - INTERVAL '${CACHE_TTL_SECONDS} seconds'`,
+        [vin]
+      );
+
+      if (result.rows.length === 0) {
+        return null;
+      }
+
+      const row = result.rows[0];
+      return {
+        vin: row.vin,
+        make: row.make,
+        model: row.model,
+        year: row.year,
+        engineType: row.engine_type,
+        bodyType: row.body_type,
+        rawData: row.raw_data,
+        cachedAt: row.cached_at,
+      };
+    } catch (error) {
+      logger.error('Failed to check VIN cache', { vin, error });
+      return null;
+    }
+  }
+
+  /**
+   * Save VIN data to cache
+   */
+  async saveToCache(vin: string, response: NHTSADecodeResponse): Promise<void> {
+    try {
+      const findValue = (variable: string): string | null => {
+        const result = response.Results.find(r => r.Variable === variable);
+        return result?.Value || null;
+      };
+
+      const year = findValue('Model Year');
+      const make = findValue('Make');
+      const model = findValue('Model');
+      const engineType = findValue('Engine Model');
+      const bodyType = findValue('Body Class');
+
+      await this.pool.query(
+        `INSERT INTO vin_cache (vin, make, model, year, engine_type, body_type, raw_data, cached_at)
+         VALUES ($1, $2, $3, $4, $5, $6, $7, NOW())
+         ON CONFLICT (vin) DO UPDATE SET
+           make = EXCLUDED.make,
+           model = EXCLUDED.model,
+           year = EXCLUDED.year,
+           engine_type = EXCLUDED.engine_type,
+           body_type = EXCLUDED.body_type,
+           raw_data = EXCLUDED.raw_data,
+           cached_at = NOW()`,
+        [vin, make, model, year ? parseInt(year) : null, engineType, bodyType, JSON.stringify(response)]
+      );
+
+      logger.debug('VIN cached', { vin });
+    } catch (error) {
+      logger.error('Failed to cache VIN data', { vin, error });
+      // Don't throw - caching failure shouldn't break the decode flow
+    }
+  }
+
+  /**
+   * Decode VIN using NHTSA vPIC API
+   * @param vin - 17-character VIN
+   * @returns Raw NHTSA decode response
+   * @throws Error if VIN is invalid or API call fails
+   */
+  async decodeVin(vin: string): Promise<NHTSADecodeResponse> {
+    // Validate and sanitize VIN
+    const sanitizedVin = this.validateVin(vin);
+
+    // Check cache first
+    const cached = await this.getCached(sanitizedVin);
+    if (cached) {
+      logger.debug('VIN cache hit', { vin: sanitizedVin });
+      return cached.rawData;
+    }
+
+    // Call NHTSA API
+    logger.info('Calling NHTSA vPIC API', { vin: sanitizedVin });
+
+    try {
+      const response = await axios.get<NHTSADecodeResponse>(
+        `${this.baseURL}/vehicles/decodevin/${sanitizedVin}`,
+        {
+          params: { format: 'json' },
+          timeout: this.timeout,
+        }
+      );
+
+      // Check for NHTSA-level errors
+      if (response.data.Count === 0) {
+        throw new Error('NHTSA returned no results for this VIN');
+      }
+
+      // Check for error messages in results
+      const errorResult = response.data.Results.find(
+        r => r.Variable === 'Error Code' && r.Value && r.Value !== '0'
+      );
+      if (errorResult) {
+        const errorText = response.data.Results.find(r => r.Variable === 'Error Text');
+        throw new Error(`NHTSA error: ${errorText?.Value || 'Unknown error'}`);
+      }
+
+      // Cache the successful response
+      await this.saveToCache(sanitizedVin, response.data);
+
+      return response.data;
+    } catch (error) {
+      if (axios.isAxiosError(error)) {
+        const axiosError = error as AxiosError;
+        if (axiosError.code === 'ECONNABORTED') {
+          logger.error('NHTSA API timeout', { vin: sanitizedVin });
+          throw new Error('NHTSA API request timed out. Please try again.');
+        }
+        if (axiosError.response) {
+          logger.error('NHTSA API error response', {
+            vin: sanitizedVin,
+            status: axiosError.response.status,
+            data: axiosError.response.data,
+          });
+          throw new Error(`NHTSA API error: ${axiosError.response.status}`);
+        }
+        logger.error('NHTSA API network error', { vin: sanitizedVin, message: axiosError.message });
+        throw new Error('Unable to connect to NHTSA API. Please try again later.');
+      }
+      throw error;
+    }
+  }
+
+  /**
+   * Extract a specific value from NHTSA response
+   */
+  static extractValue(response: NHTSADecodeResponse, variable: string): string | null {
+    const result = response.Results.find(r => r.Variable === variable);
+    return result?.Value?.trim() || null;
+  }
+
+  /**
+   * Extract year from NHTSA response
+   */
+  static extractYear(response: NHTSADecodeResponse): number | null {
+    const value = NHTSAClient.extractValue(response, 'Model Year');
+    if (!value) return null;
+    const parsed = parseInt(value, 10);
+    return isNaN(parsed) ? null : parsed;
+  }
+
+  /**
+   * Extract engine description from NHTSA response
+   * Combines multiple engine-related fields
+   */
+  static extractEngine(response: NHTSADecodeResponse): string | null {
+    const engineModel = NHTSAClient.extractValue(response, 'Engine Model');
+    if (engineModel) return engineModel;
+
+    // Build engine description from components
+    const cylinders = NHTSAClient.extractValue(response, 'Engine Number of Cylinders');
+    const displacement = NHTSAClient.extractValue(response, 'Displacement (L)');
+    const fuelType = NHTSAClient.extractValue(response, 'Fuel Type - Primary');
+
+    const parts: string[] = [];
+    if (cylinders) parts.push(`${cylinders}-Cylinder`);
+    if (displacement) parts.push(`${displacement}L`);
+    if (fuelType && fuelType !== 'Gasoline') parts.push(fuelType);
+
+    return parts.length > 0 ? parts.join(' ') : null;
+  }
+}

backend/src/features/vehicles/external/nhtsa/nhtsa.types.ts

@@ -0,0 +1,96 @@
+/**
+ * @ai-summary Type definitions for NHTSA vPIC API
+ * @ai-context Defines request/response types for VIN decoding
+ */
+
+/**
+ * Individual result from NHTSA DecodeVin API
+ */
+export interface NHTSAResult {
+  Value: string | null;
+  ValueId: string | null;
+  Variable: string;
+  VariableId: number;
+}
+
+/**
+ * Raw response from NHTSA DecodeVin API
+ * GET https://vpic.nhtsa.dot.gov/api/vehicles/decodevin/{VIN}?format=json
+ */
+export interface NHTSADecodeResponse {
+  Count: number;
+  Message: string;
+  SearchCriteria: string;
+  Results: NHTSAResult[];
+}
+
+/**
+ * Confidence level for matched dropdown values
+ */
+export type MatchConfidence = 'high' | 'medium' | 'none';
+
+/**
+ * Matched field with confidence indicator
+ */
+export interface MatchedField<T> {
+  value: T | null;
+  nhtsaValue: string | null;
+  confidence: MatchConfidence;
+}
+
+/**
+ * Decoded vehicle data with match confidence per field
+ * Maps NHTSA response fields to internal field names (camelCase)
+ *
+ * NHTSA Field Mappings:
+ * - ModelYear -> year
+ * - Make -> make
+ * - Model -> model
+ * - Trim -> trimLevel
+ * - BodyClass -> bodyType
+ * - DriveType -> driveType
+ * - FuelTypePrimary -> fuelType
+ * - EngineModel / EngineCylinders + EngineDisplacementL -> engine
+ * - TransmissionStyle -> transmission
+ */
+export interface DecodedVehicleData {
+  year: MatchedField<number>;
+  make: MatchedField<string>;
+  model: MatchedField<string>;
+  trimLevel: MatchedField<string>;
+  bodyType: MatchedField<string>;
+  driveType: MatchedField<string>;
+  fuelType: MatchedField<string>;
+  engine: MatchedField<string>;
+  transmission: MatchedField<string>;
+}
+
+/**
+ * Cached VIN data from vin_cache table
+ */
+export interface VinCacheEntry {
+  vin: string;
+  make: string | null;
+  model: string | null;
+  year: number | null;
+  engineType: string | null;
+  bodyType: string | null;
+  rawData: NHTSADecodeResponse;
+  cachedAt: Date;
+}
+
+/**
+ * VIN decode request body
+ */
+export interface DecodeVinRequest {
+  vin: string;
+}
+
+/**
+ * VIN decode error response
+ */
+export interface VinDecodeError {
+  error: 'INVALID_VIN' | 'VIN_DECODE_FAILED' | 'TIER_REQUIRED';
+  message: string;
+  details?: string;
+}

frontend/src/features/vehicles/api/vehicles.api.ts

@@ -3,7 +3,7 @@
  */
 
 import { apiClient } from '../../../core/api/client';
-import { Vehicle, CreateVehicleRequest, UpdateVehicleRequest } from '../types/vehicles.types';
+import { Vehicle, CreateVehicleRequest, UpdateVehicleRequest, DecodedVehicleData } from '../types/vehicles.types';
 
 // All requests (including dropdowns) use authenticated apiClient
 
@@ -79,5 +79,14 @@ export const vehiclesApi = {
 
   getImageUrl: (vehicleId: string): string => {
     return `/api/vehicles/${vehicleId}/image`;
+  },
+
+  /**
+   * Decode VIN using NHTSA vPIC API
+   * Requires Pro or Enterprise tier
+   */
+  decodeVin: async (vin: string): Promise<DecodedVehicleData> => {
+    const response = await apiClient.post('/vehicles/decode-vin', { vin });
+    return response.data;
   }
 };

frontend/src/features/vehicles/components/VehicleForm.tsx

@@ -10,6 +10,8 @@ import { Button } from '../../../shared-minimal/components/Button';
 import { CreateVehicleRequest, Vehicle } from '../types/vehicles.types';
 import { vehiclesApi } from '../api/vehicles.api';
 import { VehicleImageUpload } from './VehicleImageUpload';
+import { useTierAccess } from '../../../core/hooks/useTierAccess';
+import { UpgradeRequiredDialog } from '../../../shared-minimal/components/UpgradeRequiredDialog';
 
 const vehicleSchema = z
   .object({
@@ -100,6 +102,13 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
   const prevTrim = useRef<string>('');
   const [currentImageUrl, setCurrentImageUrl] = useState<string | undefined>(initialData?.imageUrl);
   const [previewUrl, setPreviewUrl] = useState<string | null>(null);
+  const [isDecoding, setIsDecoding] = useState(false);
+  const [showUpgradeDialog, setShowUpgradeDialog] = useState(false);
+  const [decodeError, setDecodeError] = useState<string | null>(null);
+
+  // Tier access check for VIN decode feature
+  const { hasAccess: canDecodeVin } = useTierAccess();
+  const hasVinDecodeAccess = canDecodeVin('vehicle.vinDecode');
 
   const isEditMode = !!initialData?.id;
   const vehicleId = initialData?.id;
@@ -408,6 +417,76 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
     imageUrl: previewUrl || currentImageUrl,
   };
 
+  // Watch VIN for decode button
+  const watchedVin = watch('vin');
+
+  /**
+   * Handle VIN decode button click
+   * Calls NHTSA API and populates empty form fields
+   */
+  const handleDecodeVin = async () => {
+    // Check tier access first
+    if (!hasVinDecodeAccess) {
+      setShowUpgradeDialog(true);
+      return;
+    }
+
+    const vin = watchedVin?.trim();
+    if (!vin || vin.length !== 17) {
+      setDecodeError('Please enter a valid 17-character VIN');
+      return;
+    }
+
+    setIsDecoding(true);
+    setDecodeError(null);
+
+    try {
+      const decoded = await vehiclesApi.decodeVin(vin);
+
+      // Only populate empty fields (preserve existing user input)
+      if (!watchedYear && decoded.year.value) {
+        setValue('year', decoded.year.value);
+      }
+      if (!watchedMake && decoded.make.value) {
+        setValue('make', decoded.make.value);
+      }
+      if (!watchedModel && decoded.model.value) {
+        setValue('model', decoded.model.value);
+      }
+      if (!watchedTrim && decoded.trimLevel.value) {
+        setValue('trimLevel', decoded.trimLevel.value);
+      }
+      if (!watchedEngine && decoded.engine.value) {
+        setValue('engine', decoded.engine.value);
+      }
+      if (!watchedTransmission && decoded.transmission.value) {
+        setValue('transmission', decoded.transmission.value);
+      }
+
+      // Body type, drive type, fuel type - check if fields are empty and we have values
+      const currentDriveType = watch('driveType');
+      const currentFuelType = watch('fuelType');
+
+      if (!currentDriveType && decoded.driveType.nhtsaValue) {
+        // For now just show hint - user can select from dropdown
+      }
+      if (!currentFuelType && decoded.fuelType.nhtsaValue) {
+        // For now just show hint - user can select from dropdown
+      }
+    } catch (error: any) {
+      console.error('VIN decode failed:', error);
+      if (error.response?.data?.error === 'TIER_REQUIRED') {
+        setShowUpgradeDialog(true);
+      } else if (error.response?.data?.error === 'INVALID_VIN') {
+        setDecodeError(error.response.data.message || 'Invalid VIN format');
+      } else {
+        setDecodeError('Failed to decode VIN. Please try again.');
+      }
+    } finally {
+      setIsDecoding(false);
+    }
+  };
+
   return (
     <form onSubmit={handleSubmit(onSubmit)} className="space-y-4">
       <div className="mb-6">
@@ -427,18 +506,47 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
           VIN Number <span className="text-red-500">*</span>
         </label>
         <p className="text-xs text-gray-600 dark:text-titanio mb-2">
-          Enter vehicle VIN (optional)
+          Enter vehicle VIN (optional if License Plate provided)
         </p>
-        <input
+        <div className="flex flex-col sm:flex-row gap-2">
-          {...register('vin')}
+          <input
-          className="w-full px-3 py-2 border rounded-md text-base bg-white text-gray-900 border-gray-300 placeholder-gray-500 focus:outline-none focus:ring-2 focus:ring-primary-500 dark:bg-scuro dark:text-avus dark:border-silverstone dark:placeholder-canna dark:focus:ring-abudhabi dark:focus:border-abudhabi"
+            {...register('vin')}
-          placeholder="Enter 17-character VIN (optional if License Plate provided)"
+            className="flex-1 px-3 py-2 border rounded-md text-base bg-white text-gray-900 border-gray-300 placeholder-gray-500 focus:outline-none focus:ring-2 focus:ring-primary-500 dark:bg-scuro dark:text-avus dark:border-silverstone dark:placeholder-canna dark:focus:ring-abudhabi dark:focus:border-abudhabi"
-          style={{ fontSize: '16px' }}
+            placeholder="Enter 17-character VIN"
-        />
+            style={{ fontSize: '16px' }}
+          />
+          <button
+            type="button"
+            onClick={handleDecodeVin}
+            disabled={isDecoding || loading || loadingDropdowns || !watchedVin?.trim()}
+            className="px-4 py-2 min-h-[44px] min-w-full sm:min-w-[120px] bg-primary-600 text-white rounded-md hover:bg-primary-700 disabled:bg-gray-400 disabled:cursor-not-allowed transition-colors flex items-center justify-center gap-2 dark:bg-abudhabi dark:hover:bg-primary-600"
+            style={{ fontSize: '16px' }}
+          >
+            {isDecoding ? (
+              <>
+                <svg className="animate-spin h-4 w-4" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24">
+                  <circle className="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" strokeWidth="4"></circle>
+                  <path className="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"></path>
+                </svg>
+                Decoding...
+              </>
+            ) : (
+              'Decode VIN'
+            )}
+          </button>
+        </div>
         {errors.vin && (
           <p className="mt-1 text-sm text-red-600 dark:text-red-400">{errors.vin.message}</p>
         )}
-     </div>
+        {decodeError && (
+          <p className="mt-1 text-sm text-red-600 dark:text-red-400">{decodeError}</p>
+        )}
+        {!hasVinDecodeAccess && (
+          <p className="mt-1 text-xs text-gray-500 dark:text-titanio">
+            VIN decode requires Pro or Enterprise subscription
+          </p>
+        )}
+      </div>
 
       {/* Vehicle Specification Dropdowns */}
       <div className="grid grid-cols-1 sm:grid-cols-2 md:grid-cols-3 gap-4">
@@ -689,6 +797,13 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
           {initialData ? 'Update Vehicle' : 'Add Vehicle'}
         </Button>
       </div>
+
+      {/* Upgrade Required Dialog for VIN Decode */}
+      <UpgradeRequiredDialog
+        featureKey="vehicle.vinDecode"
+        open={showUpgradeDialog}
+        onClose={() => setShowUpgradeDialog(false)}
+      />
     </form>
   );
 };

frontend/src/features/vehicles/types/vehicles.types.ts

@@ -55,3 +55,33 @@ export interface UpdateVehicleRequest {
   licensePlate?: string;
   odometerReading?: number;
 }
+
+/**
+ * Confidence level for matched dropdown values from VIN decode
+ */
+export type MatchConfidence = 'high' | 'medium' | 'none';
+
+/**
+ * Matched field with confidence indicator
+ */
+export interface MatchedField<T> {
+  value: T | null;
+  nhtsaValue: string | null;
+  confidence: MatchConfidence;
+}
+
+/**
+ * Decoded vehicle data from NHTSA vPIC API
+ * with match confidence per field
+ */
+export interface DecodedVehicleData {
+  year: MatchedField<number>;
+  make: MatchedField<string>;
+  model: MatchedField<string>;
+  trimLevel: MatchedField<string>;
+  bodyType: MatchedField<string>;
+  driveType: MatchedField<string>;
+  fuelType: MatchedField<string>;
+  engine: MatchedField<string>;
+  transmission: MatchedField<string>;
+}