diff --git a/backend/src/middleware/validateRequest.js b/backend/src/middleware/validateRequest.js
index 35b92094..37763e18 100644
--- a/backend/src/middleware/validateRequest.js
+++ b/backend/src/middleware/validateRequest.js
@@ -2,19 +2,41 @@
 import { formatZodErrors } from '../schemas.js';
 
 /**
+ * Validates incoming request payload against Zod schema.
+ * Enforces explicit validation, type coercion, and uniform error responses.
+ * Returns error boundaries with structured details for client debugging.
+ *
  * Creates middleware to validate `req.body` against a Zod schema.
  * Returns uniform 400 error shape on validation failure:
  * { error: "Validation failed", code: "VALIDATION_ERROR", details: [...] }
  *
  * @param {import('zod').ZodSchema} schema
+ * @param {Object} options
+ * @param {number} [options.maxPayloadSize=10485760] - max payload size in bytes (default 10MB)
  */
-export function validateBody(schema) {
+export function validateBody(schema, options = {}) {
+  const { maxPayloadSize = 10 * 1024 * 1024 } = options;
+
   return (req, res, next) => {
+    if (!req.body) {
+      req.body = {};
+    }
+
+    const payloadSize = JSON.stringify(req.body).length;
+    if (payloadSize > maxPayloadSize) {
+      return res.status(413).json({
+        error: 'Payload too large',
+        code: 'PAYLOAD_TOO_LARGE',
+        details: [{ field: 'body', message: `Payload exceeds ${maxPayloadSize} bytes` }],
+      });
+    }
+
     const result = schema.safeParse(req.body);
     if (!result.success) {
       const details = result.error.issues.map((issue) => ({
-        field: issue.path.join('.'),
+        field: issue.path.join('.') || 'root',
         message: issue.message,
+        code: issue.code,
       }));
       return res.status(400).json({
         error: 'Validation failed',
@@ -29,6 +51,9 @@ export function validateBody(schema) {
 }
 
 /**
+ * Validates query string parameters against Zod schema.
+ * Enforces type coercion and boundary checking.
+ *
  * Creates middleware to validate `req.query` against a Zod schema.
  * @param {import('zod').ZodSchema} schema
  */
@@ -37,8 +62,9 @@ export function validateQuery(schema) {
     const result = schema.safeParse(req.query);
     if (!result.success) {
       const details = result.error.issues.map((issue) => ({
-        field: issue.path.join('.'),
+        field: issue.path.join('.') || 'root',
         message: issue.message,
+        code: issue.code,
       }));
       return res.status(400).json({
         error: 'Validation failed',
@@ -53,6 +79,9 @@ export function validateQuery(schema) {
 }
 
 /**
+ * Validates path parameters against Zod schema.
+ * Enforces defensive checks and type validation on URL segments.
+ *
  * Creates middleware to validate `req.params` against a Zod schema.
  * @param {import('zod').ZodSchema} schema
  */
@@ -61,8 +90,9 @@ export function validateParams(schema) {
     const result = schema.safeParse(req.params);
     if (!result.success) {
       const details = result.error.issues.map((issue) => ({
-        field: issue.path.join('.'),
+        field: issue.path.join('.') || 'root',
         message: issue.message,
+        code: issue.code,
       }));
       return res.status(400).json({
         error: 'Validation failed',
diff --git a/backend/src/schemas/validationRegistry.js b/backend/src/schemas/validationRegistry.js
new file mode 100644
index 00000000..a98109ce
--- /dev/null
+++ b/backend/src/schemas/validationRegistry.js
@@ -0,0 +1,68 @@
+// @ts-check
+import { z } from 'zod';
+
+export const validationRegistry = {
+  request: {
+    pagination: z.object({
+      limit: z.coerce.number().int().min(1).max(1000).default(10),
+      offset: z.coerce.number().int().min(0).default(0),
+      cursor: z.string().optional(),
+    }),
+
+    identifiers: z.object({
+      id: z.string().trim().min(1),
+      userId: z.string().trim().min(1),
+      email: z.string().email('Invalid email format'),
+      campaignId: z.string().trim().min(1),
+    }),
+
+    filters: z.object({
+      status: z.enum(['active', 'inactive', 'archived']).optional(),
+      category: z.string().optional(),
+      search: z.string().max(255).optional(),
+    }),
+  },
+
+  response: {
+    error: z.object({
+      error: z.string(),
+      code: z.string(),
+      details: z.array(z.object({
+        field: z.string().optional(),
+        message: z.string(),
+        code: z.string().optional(),
+      })).optional(),
+    }),
+
+    success: z.object({
+      data: z.unknown(),
+      meta: z.object({
+        timestamp: z.string(),
+        version: z.string().optional(),
+      }).optional(),
+    }),
+  },
+
+  auth: {
+    credentials: z.object({
+      email: z.string().email('Invalid email'),
+      password: z.string().min(8, 'Password must be at least 8 characters'),
+    }),
+
+    token: z.object({
+      token: z.string().min(1),
+      expiresAt: z.string().datetime().optional(),
+      type: z.enum(['Bearer', 'Basic']).default('Bearer'),
+    }),
+  },
+};
+
+export const createValidationSchema = (registry, path) => {
+  const keys = path.split('.');
+  let schema = registry;
+  for (const key of keys) {
+    schema = schema[key];
+    if (!schema) throw new Error(`Schema not found at path: ${path}`);
+  }
+  return schema;
+};