diff --git a/packages/backend/src/lib/errors.ts b/packages/backend/src/lib/errors.ts
new file mode 100644
index 0000000..bd45d19
--- /dev/null
+++ b/packages/backend/src/lib/errors.ts
@@ -0,0 +1,47 @@
+export class AppError extends Error {
+  statusCode: number
+
+  constructor(message: string, statusCode: number) {
+    super(message)
+    this.name = 'AppError'
+    this.statusCode = statusCode
+  }
+}
+
+export class ValidationError extends AppError {
+  details?: unknown
+
+  constructor(message: string, details?: unknown) {
+    super(message, 400)
+    this.name = 'ValidationError'
+    this.details = details
+  }
+}
+
+export class NotFoundError extends AppError {
+  constructor(entity: string) {
+    super(`${entity} not found`, 404)
+    this.name = 'NotFoundError'
+  }
+}
+
+export class ForbiddenError extends AppError {
+  constructor(message = 'Insufficient permissions') {
+    super(message, 403)
+    this.name = 'ForbiddenError'
+  }
+}
+
+export class ConflictError extends AppError {
+  constructor(message: string) {
+    super(message, 409)
+    this.name = 'ConflictError'
+  }
+}
+
+export class StorageError extends AppError {
+  constructor(message: string) {
+    super(message, 500)
+    this.name = 'StorageError'
+  }
+}
diff --git a/packages/backend/src/plugins/error-handler.ts b/packages/backend/src/plugins/error-handler.ts
index a0aba3c..6d86cc5 100644
--- a/packages/backend/src/plugins/error-handler.ts
+++ b/packages/backend/src/plugins/error-handler.ts
@@ -1,23 +1,47 @@
 import fp from 'fastify-plugin'
+import { AppError, ValidationError } from '../lib/errors.js'
 
 export const errorHandlerPlugin = fp(async (app) => {
   app.setErrorHandler((error, request, reply) => {
-    const statusCode = error.statusCode ?? 500
+    // Use AppError statusCode if available, else Fastify's, else 500
+    const statusCode = error instanceof AppError
+      ? error.statusCode
+      : (error.statusCode ?? 500)
 
-    request.log.error({
-      err: error,
+    // Only log 5xx as errors, 4xx as warnings
+    const logData = {
+      err: statusCode >= 500 ? error : undefined,
       statusCode,
       url: request.url,
       method: request.method,
-    })
+      requestId: request.id,
+      message: error.message,
+    }
 
-    reply.status(statusCode).send({
+    if (statusCode >= 500) {
+      request.log.error(logData)
+    } else {
+      request.log.warn(logData)
+    }
+
+    const response: Record<string, unknown> = {
       error: {
         message: statusCode >= 500 ? 'Internal Server Error' : error.message,
         statusCode,
-        ...(process.env.NODE_ENV === 'development' ? { stack: error.stack } : {}),
       },
-    })
+    }
+
+    // Include validation details in non-production
+    if (error instanceof ValidationError && error.details) {
+      (response.error as Record<string, unknown>).details = error.details
+    }
+
+    // Stack trace only in development
+    if (process.env.NODE_ENV === 'development' && statusCode >= 500) {
+      (response.error as Record<string, unknown>).stack = error.stack
+    }
+
+    reply.status(statusCode).send(response)
   })
 
   app.setNotFoundHandler((request, reply) => {
diff --git a/packages/backend/src/routes/v1/files.ts b/packages/backend/src/routes/v1/files.ts
index 3a2499c..f0ae5c8 100644
--- a/packages/backend/src/routes/v1/files.ts
+++ b/packages/backend/src/routes/v1/files.ts
@@ -1,11 +1,12 @@
 import type { FastifyPluginAsync } from 'fastify'
 import multipart from '@fastify/multipart'
 import { FileService } from '../../services/file.service.js'
+import { ValidationError } from '../../lib/errors.js'
 
 export const fileRoutes: FastifyPluginAsync = async (app) => {
   await app.register(multipart, {
     limits: {
-      fileSize: 25 * 1024 * 1024, // 25 MB max
+      fileSize: 25 * 1024 * 1024,
       files: 1,
     },
   })
@@ -14,11 +15,10 @@ export const fileRoutes: FastifyPluginAsync = async (app) => {
   app.get('/files', { preHandler: [app.authenticate] }, async (request, reply) => {
     const { entityType, entityId } = request.query as { entityType?: string; entityId?: string }
     if (!entityType || !entityId) {
-      return reply.status(400).send({
-        error: { message: 'entityType and entityId query params required', statusCode: 400 },
-      })
+      throw new ValidationError('entityType and entityId query params required')
     }
 
+    // Files are company-scoped in the service — companyId from JWT ensures access control
     const fileRecords = await FileService.listByEntity(app.db, request.companyId, entityType, entityId)
     const data = await Promise.all(
       fileRecords.map(async (f) => ({ ...f, url: await app.storage.getUrl(f.path) })),
@@ -30,7 +30,7 @@ export const fileRoutes: FastifyPluginAsync = async (app) => {
   app.post('/files', { preHandler: [app.authenticate] }, async (request, reply) => {
     const data = await request.file()
     if (!data) {
-      return reply.status(400).send({ error: { message: 'No file provided', statusCode: 400 } })
+      throw new ValidationError('No file provided')
     }
 
     const entityType = (data.fields.entityType as { value?: string })?.value
@@ -38,38 +38,54 @@ export const fileRoutes: FastifyPluginAsync = async (app) => {
     const category = (data.fields.category as { value?: string })?.value
 
     if (!entityType || !entityId || !category) {
-      return reply.status(400).send({
-        error: { message: 'entityType, entityId, and category are required', statusCode: 400 },
-      })
+      throw new ValidationError('entityType, entityId, and category are required')
+    }
+
+    // Validate entityType is a known type
+    const allowedEntityTypes = ['member', 'member_identifier', 'product', 'rental_agreement', 'repair_ticket']
+    if (!allowedEntityTypes.includes(entityType)) {
+      throw new ValidationError(`Invalid entityType: ${entityType}`)
+    }
+
+    // Validate entityId format (UUID)
+    if (!/^[0-9a-f]{8}-[0-9a-f]{4}-[1-8][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i.test(entityId)) {
+      throw new ValidationError('entityId must be a valid UUID')
+    }
+
+    // Validate category (alphanumeric + underscore only)
+    if (!/^[a-z0-9_]+$/.test(category)) {
+      throw new ValidationError('category must be lowercase alphanumeric with underscores')
     }
 
     const buffer = await data.toBuffer()
 
-    try {
-      const file = await FileService.upload(app.db, app.storage, request.companyId, {
-        data: buffer,
-        filename: data.filename,
-        contentType: data.mimetype,
-        entityType,
-        entityId,
-        category,
-        uploadedBy: request.user.id,
-      })
-      const url = await app.storage.getUrl(file.path)
-      return reply.status(201).send({ ...file, url })
-    } catch (err) {
-      if (err instanceof Error && (err.message.includes('not allowed') || err.message.includes('too large') || err.message.includes('Maximum'))) {
-        return reply.status(400).send({ error: { message: err.message, statusCode: 400 } })
-      }
-      throw err
-    }
+    const file = await FileService.upload(app.db, app.storage, request.companyId, {
+      data: buffer,
+      filename: data.filename,
+      contentType: data.mimetype,
+      entityType,
+      entityId,
+      category,
+      uploadedBy: request.user.id,
+    })
+
+    request.log.info({ fileId: file.id, entityType, entityId, category, sizeBytes: file.sizeBytes }, 'File uploaded')
+
+    const url = await app.storage.getUrl(file.path)
+    return reply.status(201).send({ ...file, url })
   })
 
   // Serve file content (for local provider)
+  // Path traversal protection: validate the path starts with the requesting company's ID
   app.get('/files/serve/*', { preHandler: [app.authenticate] }, async (request, reply) => {
     const filePath = (request.params as { '*': string })['*']
     if (!filePath) {
-      return reply.status(400).send({ error: { message: 'Path required', statusCode: 400 } })
+      throw new ValidationError('Path required')
+    }
+
+    // Path traversal protection: must start with company ID, no '..' allowed
+    if (filePath.includes('..') || !filePath.startsWith(request.companyId)) {
+      return reply.status(403).send({ error: { message: 'Access denied', statusCode: 403 } })
     }
 
     try {
@@ -101,6 +117,7 @@ export const fileRoutes: FastifyPluginAsync = async (app) => {
     const { id } = request.params as { id: string }
     const file = await FileService.delete(app.db, app.storage, request.companyId, id)
     if (!file) return reply.status(404).send({ error: { message: 'File not found', statusCode: 404 } })
+    request.log.info({ fileId: id, path: file.path }, 'File deleted')
     return reply.send(file)
   })
 }
diff --git a/packages/backend/src/routes/v1/health.ts b/packages/backend/src/routes/v1/health.ts
index 6073a97..cbcf41d 100644
--- a/packages/backend/src/routes/v1/health.ts
+++ b/packages/backend/src/routes/v1/health.ts
@@ -2,6 +2,7 @@ import type { FastifyPluginAsync } from 'fastify'
 import { sql } from 'drizzle-orm'
 
 export const healthRoutes: FastifyPluginAsync = async (app) => {
+  // Intentionally public — no auth. Load balancers, Docker health checks, and monitoring need this.
   app.get('/health', async (request, reply) => {
     let dbStatus = 'disconnected'
     let redisStatus = 'disconnected'
diff --git a/packages/backend/src/routes/v1/lookups.ts b/packages/backend/src/routes/v1/lookups.ts
index c0400e0..e77533a 100644
--- a/packages/backend/src/routes/v1/lookups.ts
+++ b/packages/backend/src/routes/v1/lookups.ts
@@ -1,6 +1,7 @@
 import type { FastifyPluginAsync } from 'fastify'
 import { LookupCreateSchema, LookupUpdateSchema } from '@forte/shared/schemas'
 import { UnitStatusService, ItemConditionService } from '../../services/lookup.service.js'
+import { ConflictError, ValidationError } from '../../lib/errors.js'
 
 function createLookupRoutes(prefix: string, service: typeof UnitStatusService) {
   const routes: FastifyPluginAsync = async (app) => {
@@ -12,13 +13,12 @@ function createLookupRoutes(prefix: string, service: typeof UnitStatusService) {
     app.post(`/${prefix}`, { preHandler: [app.authenticate] }, async (request, reply) => {
       const parsed = LookupCreateSchema.safeParse(request.body)
       if (!parsed.success) {
-        return reply.status(400).send({ error: { message: 'Validation failed', details: parsed.error.flatten(), statusCode: 400 } })
+        throw new ValidationError('Validation failed', parsed.error.flatten())
       }
 
-      // Check slug uniqueness
       const existing = await service.getBySlug(app.db, request.companyId, parsed.data.slug)
       if (existing) {
-        return reply.status(409).send({ error: { message: `Slug "${parsed.data.slug}" already exists`, statusCode: 409 } })
+        throw new ConflictError(`Slug "${parsed.data.slug}" already exists`)
       }
 
       const row = await service.create(app.db, request.companyId, parsed.data)
@@ -29,32 +29,18 @@ function createLookupRoutes(prefix: string, service: typeof UnitStatusService) {
       const { id } = request.params as { id: string }
       const parsed = LookupUpdateSchema.safeParse(request.body)
       if (!parsed.success) {
-        return reply.status(400).send({ error: { message: 'Validation failed', details: parsed.error.flatten(), statusCode: 400 } })
-      }
-      try {
-        const row = await service.update(app.db, request.companyId, id, parsed.data)
-        if (!row) return reply.status(404).send({ error: { message: 'Not found', statusCode: 404 } })
-        return reply.send(row)
-      } catch (err) {
-        if (err instanceof Error && err.message.includes('system')) {
-          return reply.status(403).send({ error: { message: err.message, statusCode: 403 } })
-        }
-        throw err
+        throw new ValidationError('Validation failed', parsed.error.flatten())
       }
+      const row = await service.update(app.db, request.companyId, id, parsed.data)
+      if (!row) return reply.status(404).send({ error: { message: 'Not found', statusCode: 404 } })
+      return reply.send(row)
     })
 
     app.delete(`/${prefix}/:id`, { preHandler: [app.authenticate] }, async (request, reply) => {
       const { id } = request.params as { id: string }
-      try {
-        const row = await service.delete(app.db, request.companyId, id)
-        if (!row) return reply.status(404).send({ error: { message: 'Not found', statusCode: 404 } })
-        return reply.send(row)
-      } catch (err) {
-        if (err instanceof Error && err.message.includes('system')) {
-          return reply.status(403).send({ error: { message: err.message, statusCode: 403 } })
-        }
-        throw err
-      }
+      const row = await service.delete(app.db, request.companyId, id)
+      if (!row) return reply.status(404).send({ error: { message: 'Not found', statusCode: 404 } })
+      return reply.send(row)
     })
   }
   return routes
diff --git a/packages/backend/src/routes/v1/products.ts b/packages/backend/src/routes/v1/products.ts
index 93398ff..4757fc9 100644
--- a/packages/backend/src/routes/v1/products.ts
+++ b/packages/backend/src/routes/v1/products.ts
@@ -59,15 +59,8 @@ export const productRoutes: FastifyPluginAsync = async (app) => {
     if (!parsed.success) {
       return reply.status(400).send({ error: { message: 'Validation failed', details: parsed.error.flatten(), statusCode: 400 } })
     }
-    try {
-      const unit = await InventoryUnitService.create(app.db, request.companyId, parsed.data)
-      return reply.status(201).send(unit)
-    } catch (err) {
-      if (err instanceof Error && (err.message.includes('Invalid condition') || err.message.includes('Invalid status'))) {
-        return reply.status(400).send({ error: { message: err.message, statusCode: 400 } })
-      }
-      throw err
-    }
+    const unit = await InventoryUnitService.create(app.db, request.companyId, parsed.data)
+    return reply.status(201).send(unit)
   })
 
   app.get('/products/:productId/units', { preHandler: [app.authenticate] }, async (request, reply) => {
@@ -90,15 +83,8 @@ export const productRoutes: FastifyPluginAsync = async (app) => {
     if (!parsed.success) {
       return reply.status(400).send({ error: { message: 'Validation failed', details: parsed.error.flatten(), statusCode: 400 } })
     }
-    try {
-      const unit = await InventoryUnitService.update(app.db, request.companyId, id, parsed.data)
-      if (!unit) return reply.status(404).send({ error: { message: 'Unit not found', statusCode: 404 } })
-      return reply.send(unit)
-    } catch (err) {
-      if (err instanceof Error && (err.message.includes('Invalid condition') || err.message.includes('Invalid status'))) {
-        return reply.status(400).send({ error: { message: err.message, statusCode: 400 } })
-      }
-      throw err
-    }
+    const unit = await InventoryUnitService.update(app.db, request.companyId, id, parsed.data)
+    if (!unit) return reply.status(404).send({ error: { message: 'Unit not found', statusCode: 404 } })
+    return reply.send(unit)
   })
 }
diff --git a/packages/backend/src/services/file.service.ts b/packages/backend/src/services/file.service.ts
index abc55e5..fb277ca 100644
--- a/packages/backend/src/services/file.service.ts
+++ b/packages/backend/src/services/file.service.ts
@@ -3,6 +3,7 @@ import type { PostgresJsDatabase } from 'drizzle-orm/postgres-js'
 import { files } from '../db/schema/files.js'
 import type { StorageProvider } from '../storage/index.js'
 import { randomUUID } from 'crypto'
+import { ValidationError } from '../lib/errors.js'
 
 const ALLOWED_IMAGE_TYPES = ['image/jpeg', 'image/png', 'image/webp']
 const ALLOWED_PDF_TYPES = ['application/pdf']
@@ -38,13 +39,13 @@ export const FileService = {
   ) {
     // Validate content type
     if (!ALLOWED_TYPES.includes(input.contentType)) {
-      throw new Error(`File type not allowed: ${input.contentType}`)
+      throw new ValidationError(`File type not allowed: ${input.contentType}`)
     }
 
     // Validate size
     const maxSize = ALLOWED_IMAGE_TYPES.includes(input.contentType) ? MAX_IMAGE_SIZE : MAX_PDF_SIZE
     if (input.data.length > maxSize) {
-      throw new Error(`File too large: ${input.data.length} bytes (max ${maxSize})`)
+      throw new ValidationError(`File too large: ${input.data.length} bytes (max ${maxSize})`)
     }
 
     // Check per-entity limit
@@ -59,7 +60,7 @@ export const FileService = {
         ),
       )
     if (existing.total >= MAX_FILES_PER_ENTITY) {
-      throw new Error(`Maximum ${MAX_FILES_PER_ENTITY} files per entity`)
+      throw new ValidationError(`Maximum ${MAX_FILES_PER_ENTITY} files per entity`)
     }
 
     // Generate path
diff --git a/packages/backend/src/services/lookup.service.ts b/packages/backend/src/services/lookup.service.ts
index 38a4d00..d4155c2 100644
--- a/packages/backend/src/services/lookup.service.ts
+++ b/packages/backend/src/services/lookup.service.ts
@@ -1,5 +1,6 @@
 import { eq, and } from 'drizzle-orm'
 import type { PostgresJsDatabase } from 'drizzle-orm/postgres-js'
+import { ForbiddenError } from '../lib/errors.js'
 import {
   inventoryUnitStatuses,
   itemConditions,
@@ -73,7 +74,7 @@ function createLookupService(
 
       if (!existing[0]) return null
       if (existing[0].isSystem && input.isActive === false) {
-        throw new Error('Cannot deactivate a system status')
+        throw new ForbiddenError('Cannot deactivate a system status')
       }
 
       const [row] = await db
@@ -93,7 +94,7 @@ function createLookupService(
 
       if (!existing[0]) return null
       if (existing[0].isSystem) {
-        throw new Error('Cannot delete a system status')
+        throw new ForbiddenError('Cannot delete a system status')
       }
 
       const [row] = await db
diff --git a/packages/backend/src/services/product.service.ts b/packages/backend/src/services/product.service.ts
index 77401a6..2ee0790 100644
--- a/packages/backend/src/services/product.service.ts
+++ b/packages/backend/src/services/product.service.ts
@@ -1,6 +1,7 @@
 import { eq, and, count } from 'drizzle-orm'
 import type { PostgresJsDatabase } from 'drizzle-orm/postgres-js'
 import { products, inventoryUnits, priceHistory } from '../db/schema/inventory.js'
+import { ValidationError } from '../lib/errors.js'
 import type {
   ProductCreateInput,
   ProductUpdateInput,
@@ -119,11 +120,11 @@ export const InventoryUnitService = {
   async create(db: PostgresJsDatabase, companyId: string, input: InventoryUnitCreateInput) {
     if (input.condition) {
       const valid = await ItemConditionService.validateSlug(db, companyId, input.condition)
-      if (!valid) throw new Error(`Invalid condition: "${input.condition}"`)
+      if (!valid) throw new ValidationError(`Invalid condition: "${input.condition}"`)
     }
     if (input.status) {
       const valid = await UnitStatusService.validateSlug(db, companyId, input.status)
-      if (!valid) throw new Error(`Invalid status: "${input.status}"`)
+      if (!valid) throw new ValidationError(`Invalid status: "${input.status}"`)
     }
 
     const [unit] = await db
@@ -190,11 +191,11 @@ export const InventoryUnitService = {
   ) {
     if (input.condition) {
       const valid = await ItemConditionService.validateSlug(db, companyId, input.condition)
-      if (!valid) throw new Error(`Invalid condition: "${input.condition}"`)
+      if (!valid) throw new ValidationError(`Invalid condition: "${input.condition}"`)
     }
     if (input.status) {
       const valid = await UnitStatusService.validateSlug(db, companyId, input.status)
-      if (!valid) throw new Error(`Invalid status: "${input.status}"`)
+      if (!valid) throw new ValidationError(`Invalid status: "${input.status}"`)
     }
 
     const updates: Record<string, unknown> = { ...input }