diff --git a/backend/libs/typescript-sdk/src/exception.ts b/backend/libs/typescript-sdk/src/exception.ts
new file mode 100644
index 0000000..1f06e24
--- /dev/null
+++ b/backend/libs/typescript-sdk/src/exception.ts
@@ -0,0 +1,15 @@
+export class Exception extends Error {
+  constructor(message: string, private readonly causation: Error | undefined = undefined) {
+    super(message);
+    this.stack = causation ? Exception.errorCausedBy(this, causation).stack : this.stack;
+  }
+
+  causedBy(causation: Error): Exception {
+    return new Exception(Exception.errorCausedBy(this, causation).message, causation);
+  }
+
+  private static errorCausedBy(error: Error, causation: Error): Error {
+    error.stack += '\nCaused by: \n' + causation.message + '\n' + causation.stack;
+    return error;
+  }
+}
diff --git a/backend/libs/typescript-sdk/src/index.ts b/backend/libs/typescript-sdk/src/index.ts
new file mode 100644
index 0000000..3d5b914
--- /dev/null
+++ b/backend/libs/typescript-sdk/src/index.ts
@@ -0,0 +1 @@
+export * from './exception';
diff --git a/backend/libs/typescript-sdk/tsconfig.lib.json b/backend/libs/typescript-sdk/tsconfig.lib.json
new file mode 100644
index 0000000..960f5d0
--- /dev/null
+++ b/backend/libs/typescript-sdk/tsconfig.lib.json
@@ -0,0 +1,9 @@
+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "declaration": true,
+    "outDir": "../../dist/libs/typescript-sdk"
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist", "test", "**/*spec.ts"]
+}
diff --git a/backend/nest-cli.json b/backend/nest-cli.json
index 11d886a..0917a12 100755
--- a/backend/nest-cli.json
+++ b/backend/nest-cli.json
@@ -55,6 +55,15 @@
       "compilerOptions": {
         "tsConfigPath": "libs/eventstore-projections/tsconfig.lib.json"
       }
+    },
+    "typescript-sdk": {
+      "type": "library",
+      "root": "libs/typescript-sdk",
+      "entryFile": "index",
+      "sourceRoot": "libs/typescript-sdk/src",
+      "compilerOptions": {
+        "tsConfigPath": "libs/typescript-sdk/tsconfig.lib.json"
+      }
     }
   },
   "compilerOptions": {
diff --git a/backend/package-lock.json b/backend/package-lock.json
index d8e9bf7..7e56d42 100755
--- a/backend/package-lock.json
+++ b/backend/package-lock.json
@@ -8190,6 +8190,11 @@
       "resolved": "https://registry.npmjs.org/moment/-/moment-2.25.3.tgz",
       "integrity": "sha512-PuYv0PHxZvzc15Sp8ybUCoQ+xpyPWvjOuK72a5ovzp2LI32rJXOiIfyoFoYvG3s6EwwrdkMyWuRiEHSZRLJNdg=="
     },
+    "monet": {
+      "version": "0.9.1",
+      "resolved": "https://registry.npmjs.org/monet/-/monet-0.9.1.tgz",
+      "integrity": "sha512-GaZw308g6t0WD+U2mGujwJckGp2b8AGGVuB6PiIwkXbq6rJeiqddre2mdbO4PxjyILPi+7GgtRkkfr6dBYXWtA=="
+    },
     "move-concurrently": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/move-concurrently/-/move-concurrently-1.0.1.tgz",
diff --git a/backend/package.json b/backend/package.json
index 4bfcfd9..23495f6 100755
--- a/backend/package.json
+++ b/backend/package.json
@@ -51,6 +51,7 @@
     "class-transformer": "^0.2.3",
     "class-validator": "^0.12.2",
     "moment": "^2.25.3",
+    "monet": "^0.9.1",
     "pg": "^8.2.0",
     "postgres": "^1.0.2",
     "reflect-metadata": "^0.1.13",
@@ -115,10 +116,12 @@
       "@coders-board-library/axios-utils/(.*)": "<rootDir>/libs/axios-utils/src/$1",
       "@coders-board-library/axios-utils": "<rootDir>/libs/axios-utils/src",
       "@coders-board-library/eventstore-projections/(.*)": "<rootDir>/libs/eventstore-projections/src/$1",
-      "@coders-board-library/eventstore-projections": "<rootDir>/libs/eventstore-projections/src"
+      "@coders-board-library/eventstore-projections": "<rootDir>/libs/eventstore-projections/src",
+      "@coders-board-library/typescript-sdk/(.*)": "<rootDir>/libs/typescript-sdk/src/$1",
+      "@coders-board-library/typescript-sdk": "<rootDir>/libs/typescript-sdk/src"
     }
   },
   "eslintIgnore": [
     "**/*.projection.*"
   ]
-}
+}
\ No newline at end of file
diff --git a/backend/src/inviting-applicants/write-side/presentation/rest-api/v1/applicant-invitation.v1.write-side-controller.ts b/backend/src/inviting-applicants/write-side/presentation/rest-api/v1/applicant-invitation.v1.write-side-controller.ts
index 90056ec..e39f5ef 100644
--- a/backend/src/inviting-applicants/write-side/presentation/rest-api/v1/applicant-invitation.v1.write-side-controller.ts
+++ b/backend/src/inviting-applicants/write-side/presentation/rest-api/v1/applicant-invitation.v1.write-side-controller.ts
@@ -9,6 +9,7 @@ import { ApiCreatedResponse, ApiNoContentResponse, ApiTags } from '@nestjs/swagg
 import { InviteApplicant } from '../../../application/internal-command/invite-applicant.internal-command';
 import { CancelApplicantInvitation } from '../../../application/internal-command/cancel-applicant-invitation.internal-command';
 import { v4 as uuid } from 'uuid';
+import { CancelApplicantInvitationRequestParams } from './request-params/cancel-applicant-invitation.request-params';
 
 @ApiTags('inviting-applicants')
 @Controller('/rest-api/v1/applicant-invitations')
@@ -36,7 +37,7 @@ export class ApplicantInvitationV1WriteSideController {
   })
   @HttpCode(204)
   @Post(':invitationId/cancellation')
-  postApplicantInvitationCancellation(@Param('invitationId') invitationId: string) {
-    return this.internalCommandBus.sendAndWait(new CancelApplicantInvitation(invitationId));
+  postApplicantInvitationCancellation(@Param() params: CancelApplicantInvitationRequestParams) {
+    return this.internalCommandBus.sendAndWait(new CancelApplicantInvitation(params.invitationId));
   }
 }
diff --git a/backend/src/inviting-applicants/write-side/presentation/rest-api/v1/request/cancel-applicant-invitation.request-body.ts b/backend/src/inviting-applicants/write-side/presentation/rest-api/v1/request-params/cancel-applicant-invitation.request-params.ts
similarity index 61%
rename from backend/src/inviting-applicants/write-side/presentation/rest-api/v1/request/cancel-applicant-invitation.request-body.ts
rename to backend/src/inviting-applicants/write-side/presentation/rest-api/v1/request-params/cancel-applicant-invitation.request-params.ts
index b8b5564..00c4c22 100644
--- a/backend/src/inviting-applicants/write-side/presentation/rest-api/v1/request/cancel-applicant-invitation.request-body.ts
+++ b/backend/src/inviting-applicants/write-side/presentation/rest-api/v1/request-params/cancel-applicant-invitation.request-params.ts
@@ -1,8 +1,9 @@
-import { IsDefined, IsNotEmpty } from 'class-validator';
+import { IsDefined, IsNotEmpty, IsUUID } from 'class-validator';
 import { ApiProperty } from '@nestjs/swagger';
 
-export class CancelApplicantInvitationRequestBody {
+export class CancelApplicantInvitationRequestParams {
   @ApiProperty()
+  @IsUUID()
   @IsDefined()
   @IsNotEmpty()
   readonly invitationId: string;
diff --git a/backend/src/main.ts b/backend/src/main.ts
index ba5994e..b03627b 100755
--- a/backend/src/main.ts
+++ b/backend/src/main.ts
@@ -2,6 +2,7 @@ import { NestFactory } from '@nestjs/core';
 import { AppModule } from './app.module';
 import { DocumentBuilder, SwaggerModule } from '@nestjs/swagger';
 import { INestApplication, ValidationPipe } from '@nestjs/common';
+import { InternalCommandInvalidSchemaExceptionFilter } from './shared-kernel/write-side/presentation/rest-api/nestjs-exception-filter/internal-command-invalid-schema.exception-filter';
 
 function setupSwagger(app: INestApplication) {
   const options = new DocumentBuilder()
@@ -21,6 +22,7 @@ async function bootstrap() {
   const app = await NestFactory.create(AppModule);
   setupValidationPipe(app);
   setupSwagger(app);
+  app.useGlobalFilters(new InternalCommandInvalidSchemaExceptionFilter());
   await app.listen(process.env.CODERSBOARD_SERVER_PORT || 4000);
 }
 
diff --git a/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-invalid-schema.exception.ts b/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-invalid-schema.exception.ts
new file mode 100644
index 0000000..6a0cf91
--- /dev/null
+++ b/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-invalid-schema.exception.ts
@@ -0,0 +1,15 @@
+import { InternalCommand } from './internal-command';
+import { Exception } from '@coders-board-library/typescript-sdk/exception';
+import { ValidationError } from 'class-validator';
+
+export class InternalCommandInvalidSchemaException extends Exception {
+  constructor(command: InternalCommand, readonly validationErrors: ValidationError[]) {
+    super(
+      `Internal command ${
+        command.constructor.name
+      } rejected! Schema doesn't match. \n Validation errors: ${validationErrors.map(
+        (it, index) => `\n${index + 1}. ${it}`,
+      )}`,
+    );
+  }
+}
diff --git a/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-rejected.exception.ts b/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-rejected.exception.ts
new file mode 100644
index 0000000..74bbeb9
--- /dev/null
+++ b/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-rejected.exception.ts
@@ -0,0 +1,8 @@
+import { InternalCommand } from './internal-command';
+import { Exception } from '@coders-board-library/typescript-sdk/exception';
+
+export class InternalCommandRejectedException extends Exception {
+  constructor(command: InternalCommand, causation: Error | undefined) {
+    super(`Internal command ${command.constructor.name} rejected!`, causation);
+  }
+}
diff --git a/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-sender.ts b/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-sender.ts
index ed25053..8a6cf1d 100644
--- a/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-sender.ts
+++ b/backend/src/shared-kernel/write-side/application/internal-command-sender/internal-command-sender.ts
@@ -6,5 +6,5 @@ export interface InternalCommandSender {
   sendAndWait<R = void>(command: InternalCommand): Promise<R>;
 
   //TODO: Add inbox and ack
-  sendAndForget(command: InternalCommand): void;
+  sendAndForget(command: InternalCommand): Promise<void>;
 }
diff --git a/backend/src/shared-kernel/write-side/infrastructure/internal-command-sender/class-validatior-command-sender.ts b/backend/src/shared-kernel/write-side/infrastructure/internal-command-sender/class-validatior-command-sender.ts
index cec82ea..d995a77 100644
--- a/backend/src/shared-kernel/write-side/infrastructure/internal-command-sender/class-validatior-command-sender.ts
+++ b/backend/src/shared-kernel/write-side/infrastructure/internal-command-sender/class-validatior-command-sender.ts
@@ -1,15 +1,25 @@
 import { InternalCommandSender } from '../../application/internal-command-sender/internal-command-sender';
 import { InternalCommand } from '../../application/internal-command-sender/internal-command';
 import { validateOrReject } from 'class-validator';
+import { InternalCommandRejectedException } from '../../application/internal-command-sender/internal-command-rejected.exception';
+import { InternalCommandInvalidSchemaException } from '../../application/internal-command-sender/internal-command-invalid-schema.exception';
 
 export class ClassValidatorInternalCommandSender implements InternalCommandSender {
   constructor(private readonly commandSender: InternalCommandSender) {}
 
   sendAndWait<R>(command: InternalCommand): Promise<R> {
-    return validateOrReject(command).then(() => this.commandSender.sendAndWait<R>(command));
+    return validateOrReject(command)
+      .catch(validationError => {
+        throw new InternalCommandInvalidSchemaException(command, validationError);
+      })
+      .then(() => this.commandSender.sendAndWait<R>(command));
   }
 
-  sendAndForget<T extends InternalCommand>(command: T) {
-    return validateOrReject(command).then(() => this.commandSender.sendAndForget(command));
+  sendAndForget<T extends InternalCommand>(command: T): Promise<void> {
+    return validateOrReject(command)
+      .catch(validationError => {
+        throw new InternalCommandInvalidSchemaException(command, validationError);
+      })
+      .then(() => this.commandSender.sendAndForget(command));
   }
 }
diff --git a/backend/src/shared-kernel/write-side/infrastructure/internal-command-sender/nest-js-internal-command-sender.ts b/backend/src/shared-kernel/write-side/infrastructure/internal-command-sender/nest-js-internal-command-sender.ts
index f8b9a2a..529d56e 100644
--- a/backend/src/shared-kernel/write-side/infrastructure/internal-command-sender/nest-js-internal-command-sender.ts
+++ b/backend/src/shared-kernel/write-side/infrastructure/internal-command-sender/nest-js-internal-command-sender.ts
@@ -9,7 +9,8 @@ export class NestJsInternalCommandSender implements InternalCommandSender {
     return this.commandBus.execute(command);
   }
 
-  sendAndForget<T extends InternalCommand>(command: T) {
+  sendAndForget<T extends InternalCommand>(command: T): Promise<void> {
     this.commandBus.execute(command).then();
+    return Promise.resolve();
   }
 }
diff --git a/backend/src/shared-kernel/write-side/presentation/rest-api/nestjs-exception-filter/internal-command-invalid-schema.exception-filter.ts b/backend/src/shared-kernel/write-side/presentation/rest-api/nestjs-exception-filter/internal-command-invalid-schema.exception-filter.ts
new file mode 100644
index 0000000..817e064
--- /dev/null
+++ b/backend/src/shared-kernel/write-side/presentation/rest-api/nestjs-exception-filter/internal-command-invalid-schema.exception-filter.ts
@@ -0,0 +1,18 @@
+import { ExceptionFilter, Catch, ArgumentsHost, HttpStatus } from '@nestjs/common';
+import { InternalCommandInvalidSchemaException } from '../../../application/internal-command-sender/internal-command-invalid-schema.exception';
+
+@Catch(InternalCommandInvalidSchemaException)
+export class InternalCommandInvalidSchemaExceptionFilter implements ExceptionFilter {
+  catch(exception: InternalCommandInvalidSchemaException, host: ArgumentsHost) {
+    const ctx = host.switchToHttp();
+    const response = ctx.getResponse();
+    const request = ctx.getRequest();
+
+    response.status(HttpStatus.BAD_REQUEST).json({
+      statusCode: HttpStatus.BAD_REQUEST,
+      timestamp: new Date().toISOString(),
+      path: request.url,
+      message: exception.validationErrors, //TODO: Flatten from NestJS validation pipe!
+    });
+  }
+}
diff --git a/backend/test-integration/jest-integration.json b/backend/test-integration/jest-integration.json
index d4b6f89..0bcbd04 100755
--- a/backend/test-integration/jest-integration.json
+++ b/backend/test-integration/jest-integration.json
@@ -28,6 +28,8 @@
     "@coders-board-library/axios-utils/(.*)": "<rootDir>/../libs/axios-utils/src/$1",
     "@coders-board-library/axios-utils": "<rootDir>/../libs/axios-utils/src",
     "@coders-board-library/eventstore-projections/(.*)": "<rootDir>/../libs/eventstore-projections/src/$1",
-    "@coders-board-library/eventstore-projections": "<rootDir>/../libs/eventstore-projections/src"
+    "@coders-board-library/eventstore-projections": "<rootDir>/../libs/eventstore-projections/src",
+    "@coders-board-library/typescript-sdk/(.*)": "<rootDir>/../libs/typescript-sdk/src/$1",
+    "@coders-board-library/typescript-sdk": "<rootDir>/../libs/typescript-sdk/src"
   }
 }
\ No newline at end of file
diff --git a/backend/test-module/jest-module.json b/backend/test-module/jest-module.json
index 881706b..2cf7446 100755
--- a/backend/test-module/jest-module.json
+++ b/backend/test-module/jest-module.json
@@ -28,6 +28,8 @@
     "@coders-board-library/axios-utils/(.*)": "<rootDir>/../libs/axios-utils/src/$1",
     "@coders-board-library/axios-utils": "<rootDir>/../libs/axios-utils/src",
     "@coders-board-library/eventstore-projections/(.*)": "<rootDir>/../libs/eventstore-projections/src/$1",
-    "@coders-board-library/eventstore-projections": "<rootDir>/../libs/eventstore-projections/src"
+    "@coders-board-library/eventstore-projections": "<rootDir>/../libs/eventstore-projections/src",
+    "@coders-board-library/typescript-sdk/(.*)": "<rootDir>/../libs/typescript-sdk/src/$1",
+    "@coders-board-library/typescript-sdk": "<rootDir>/../libs/typescript-sdk/src"
   }
 }
\ No newline at end of file
diff --git a/backend/test-unit/jest-unit.json b/backend/test-unit/jest-unit.json
index 31493dd..f65cff0 100755
--- a/backend/test-unit/jest-unit.json
+++ b/backend/test-unit/jest-unit.json
@@ -28,6 +28,8 @@
     "@coders-board-library/axios-utils/(.*)": "<rootDir>/../libs/axios-utils/src/$1",
     "@coders-board-library/axios-utils": "<rootDir>/../libs/axios-utils/src",
     "@coders-board-library/eventstore-projections/(.*)": "<rootDir>/../libs/eventstore-projections/src/$1",
-    "@coders-board-library/eventstore-projections": "<rootDir>/../libs/eventstore-projections/src"
+    "@coders-board-library/eventstore-projections": "<rootDir>/../libs/eventstore-projections/src",
+    "@coders-board-library/typescript-sdk/(.*)": "<rootDir>/../libs/typescript-sdk/src/$1",
+    "@coders-board-library/typescript-sdk": "<rootDir>/../libs/typescript-sdk/src"
   }
 }
\ No newline at end of file
diff --git a/backend/tsconfig.json b/backend/tsconfig.json
index 5d4f92e..2fe8a57 100755
--- a/backend/tsconfig.json
+++ b/backend/tsconfig.json
@@ -46,6 +46,12 @@
       ],
       "@coders-board-library/eventstore-projections/*": [
         "libs/eventstore-projections/src/*"
+      ],
+      "@coders-board-library/typescript-sdk": [
+        "libs/typescript-sdk/src"
+      ],
+      "@coders-board-library/typescript-sdk/*": [
+        "libs/typescript-sdk/src/*"
       ]
     }
   },