diff --git a/docs/fixes/payment-hardening.md b/docs/fixes/payment-hardening.md
new file mode 100644
index 00000000..78a358e9
--- /dev/null
+++ b/docs/fixes/payment-hardening.md
@@ -0,0 +1,61 @@
+# Payment hardening fixes
+
+Summary of four issues fixed on branch `fix/payment-index-validation-tsconfig-strictness`.
+
+## 1. Missing indexes on `payments.merchantId` (bug, tech-debt)
+
+`src/payments/entities/payment.entity.ts` had no `@Index` decorators. Since
+`merchantId` is the WHERE clause for `PaymentsService.findAll()`,
+`findOne()`, and `getStats()` (`src/payments/payments.service.ts`), and is
+also the AML velocity-check join key (`AmlService.checkAndFlag`), every
+merchant-scoped query was a sequential scan.
+
+**Fix:** added `@Index()` on `merchantId`, plus composite
+`@Index(['merchantId', 'status'])` and `@Index(['merchantId', 'createdAt'])`
+on the `Payment` entity to match actual query patterns (status filtering,
+date-range/list queries). A migration will need to be generated/run against
+the target database to materialize these indexes.
+
+## 2. Unbounded `amountUsd` and `metadata` on payment creation (bug, security)
+
+`src/payments/dto/create-payment.dto.ts` validated `amountUsd` with only
+`@IsNumber()`/`@IsPositive()` (no ceiling) and `metadata` with only
+`@IsObject()` (no size limit), storing directly into a `jsonb` column.
+
+**Fix:**
+- Added `@Max(1_000_000)` to `amountUsd` in both `CreatePaymentDto` and
+  `BatchPaymentItemDto`.
+- Added a new reusable custom validator,
+  `src/common/decorators/max-json-size.decorator.ts` (`@MaxJsonSize`), which
+  rejects a field whose serialized JSON size exceeds a configured byte
+  limit. Applied `@MaxJsonSize(4096)` to `metadata` on both the single and
+  batch payment DTOs.
+
+## 3. `expiryMinutes` had no bounds on single-payment creation (bug)
+
+`CreatePaymentDto.expiryMinutes` only had `@IsNumber()`, unlike
+`BatchPaymentItemDto.expiryMinutes` which already had `@IsPositive()`. This
+allowed `expiryMinutes: 0`/negative (payment expired at creation) or an
+arbitrarily large value (never-expiring payment).
+
+**Fix:** added `@IsPositive()` and `@Max(1440)` (24h cap) to
+`CreatePaymentDto.expiryMinutes`, aligning it with the batch DTO (which also
+got the same `@Max(1440)` cap for consistency).
+
+## 4. `strictNullChecks` / `noImplicitAny` disabled (tech-debt)
+
+`tsconfig.json` had both flags set to `false`, disabling two of
+TypeScript's most important safety nets for a codebase full of
+`nullable: true` entity columns and optional DTO fields.
+
+**Fix:** flipped `strictNullChecks` and `noImplicitAny` to `true` in
+`tsconfig.json`.
+
+**Follow-up required:** enabling these flags project-wide will surface
+existing null/undefined and implicit-`any` type errors across services
+(e.g. `payment.customerWalletAddress`, `merchant.customFeeRate`,
+`catch (err) { err.message }` patterns). Those call sites will need to be
+fixed incrementally (nullish checks, explicit typing on catch clauses,
+etc.) before this change can compile cleanly in CI. This commit intentionally
+does not attempt that codebase-wide migration — it only flips the compiler
+flags as requested, per the issue's own suggested incremental approach.
diff --git a/src/common/decorators/max-json-size.decorator.ts b/src/common/decorators/max-json-size.decorator.ts
new file mode 100644
index 00000000..b03a5406
--- /dev/null
+++ b/src/common/decorators/max-json-size.decorator.ts
@@ -0,0 +1,34 @@
+import { registerDecorator, ValidationOptions, ValidationArguments } from 'class-validator';
+
+/**
+ * Validates that the JSON-serialized size of a property does not exceed
+ * maxBytes. Used to cap arbitrary/unbounded object fields (e.g. metadata)
+ * stored directly into jsonb columns, preventing oversized payloads.
+ */
+export function MaxJsonSize(maxBytes: number, validationOptions?: ValidationOptions) {
+  return (object: object, propertyName: string) => {
+    registerDecorator({
+      name: 'maxJsonSize',
+      target: object.constructor,
+      propertyName,
+      constraints: [maxBytes],
+      options: validationOptions,
+      validator: {
+        validate(value: unknown, args: ValidationArguments) {
+          if (value === undefined || value === null) return true;
+          const [limit] = args.constraints;
+          try {
+            const size = Buffer.byteLength(JSON.stringify(value), 'utf8');
+            return size <= limit;
+          } catch {
+            return false;
+          }
+        },
+        defaultMessage(args: ValidationArguments) {
+          const [limit] = args.constraints;
+          return `${args.property} must not exceed ${limit} bytes when serialized`;
+        },
+      },
+    });
+  };
+}
diff --git a/src/payments/dto/batch-create-payment.dto.ts b/src/payments/dto/batch-create-payment.dto.ts
index fc58f8af..f855b89b 100644
--- a/src/payments/dto/batch-create-payment.dto.ts
+++ b/src/payments/dto/batch-create-payment.dto.ts
@@ -11,14 +11,20 @@ import {
   ValidateNested,
   IsNotEmpty,
   MinLength,
+  Max,
 } from 'class-validator';
 import { Type, Transform } from 'class-transformer';
 import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
+import { MaxJsonSize } from '../../common/decorators/max-json-size.decorator';
+
+const MAX_AMOUNT_USD = 1_000_000;
+const MAX_METADATA_BYTES = 4096;
 
 export class BatchPaymentItemDto {
   @ApiProperty({ example: 50.0, description: 'Amount in USD — must be greater than 0' })
   @IsNumber()
   @IsPositive()
+  @Max(MAX_AMOUNT_USD)
   amountUsd: number;
 
   @ApiProperty({ example: 'Order #123', description: 'Non-empty memo for this payment' })
@@ -37,12 +43,16 @@ export class BatchPaymentItemDto {
   @ApiPropertyOptional()
   @IsOptional()
   @IsObject()
+  @MaxJsonSize(MAX_METADATA_BYTES, {
+    message: `metadata must not exceed ${MAX_METADATA_BYTES} bytes when serialized`,
+  })
   metadata?: Record<string, any>;
 
-  @ApiPropertyOptional({ example: 30, description: 'Expiry in minutes (default 30)' })
+  @ApiPropertyOptional({ example: 30, description: 'Expiry in minutes (default 30, max 1440)' })
   @IsOptional()
   @IsNumber()
   @IsPositive()
+  @Max(1440)
   expiryMinutes?: number;
 }
 
diff --git a/src/payments/dto/create-payment.dto.ts b/src/payments/dto/create-payment.dto.ts
index 1b1c8c12..17261903 100644
--- a/src/payments/dto/create-payment.dto.ts
+++ b/src/payments/dto/create-payment.dto.ts
@@ -1,11 +1,16 @@
-import { IsNumber, IsPositive, IsString, IsOptional, IsEmail, IsObject } from 'class-validator';
+import { IsNumber, IsPositive, IsString, IsOptional, IsEmail, IsObject, Max } from 'class-validator';
 import { Transform } from 'class-transformer';
 import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
+import { MaxJsonSize } from '../../common/decorators/max-json-size.decorator';
+
+const MAX_AMOUNT_USD = 1_000_000;
+const MAX_METADATA_BYTES = 4096;
 
 export class CreatePaymentDto {
   @ApiProperty({ example: 50.0 })
   @IsNumber()
   @IsPositive()
+  @Max(MAX_AMOUNT_USD)
   amountUsd: number;
 
   @ApiPropertyOptional({ example: 'Payment for order #123' })
@@ -23,10 +28,15 @@ export class CreatePaymentDto {
   @ApiPropertyOptional()
   @IsOptional()
   @IsObject()
+  @MaxJsonSize(MAX_METADATA_BYTES, {
+    message: `metadata must not exceed ${MAX_METADATA_BYTES} bytes when serialized`,
+  })
   metadata?: Record<string, any>;
 
-  @ApiPropertyOptional({ example: 30, description: 'Expiry in minutes (default 30)' })
+  @ApiPropertyOptional({ example: 30, description: 'Expiry in minutes (default 30, max 1440)' })
   @IsOptional()
   @IsNumber()
+  @IsPositive()
+  @Max(1440)
   expiryMinutes?: number;
 }
diff --git a/src/payments/entities/payment.entity.ts b/src/payments/entities/payment.entity.ts
index 91444f33..9efaee0e 100644
--- a/src/payments/entities/payment.entity.ts
+++ b/src/payments/entities/payment.entity.ts
@@ -7,6 +7,7 @@ import {
   DeleteDateColumn,
   ManyToOne,
   JoinColumn,
+  Index,
 } from 'typeorm';
 import { Merchant } from '../../merchants/entities/merchant.entity';
 import { Settlement } from '../../settlements/entities/settlement.entity';
@@ -33,6 +34,8 @@ export enum PaymentNetwork {
 }
 
 @Entity('payments')
+@Index(['merchantId', 'status'])
+@Index(['merchantId', 'createdAt'])
 export class Payment {
   @PrimaryGeneratedColumn('uuid')
   id: string;
@@ -44,6 +47,7 @@ export class Payment {
   @JoinColumn({ name: 'merchantId' })
   merchant: Merchant;
 
+  @Index()
   @Column()
   merchantId: string;
 
diff --git a/tsconfig.json b/tsconfig.json
index 95f5641c..c5555e22 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -12,8 +12,8 @@
     "baseUrl": "./",
     "incremental": true,
     "skipLibCheck": true,
-    "strictNullChecks": false,
-    "noImplicitAny": false,
+    "strictNullChecks": true,
+    "noImplicitAny": true,
     "strictBindCallApply": false,
     "forceConsistentCasingInFileNames": false,
     "noFallthroughCasesInSwitch": false