diff --git a/apps/meteor/app/api/server/definition.spec.ts b/apps/meteor/app/api/server/definition.spec.ts
new file mode 100644
index 0000000000000..3c659820b34b3
--- /dev/null
+++ b/apps/meteor/app/api/server/definition.spec.ts
@@ -0,0 +1,73 @@
+import type { IUser } from '@rocket.chat/core-typings';
+import type { ValidateFunction } from 'ajv';
+
+import type { TypedThis, TypedOptions, Action, InnerAction } from './definition';
+
+// --- Utilities ---
+type IsTypeEqual<A, B> = (<T>() => T extends A ? 1 : 2) extends <T>() => T extends B ? 1 : 2 ? true : false;
+type ExtractThis<T> = T extends (this: infer U, ...args: any[]) => any ? U : never;
+type Mutable<T> = { -readonly [K in keyof T]: T[K] };
+
+// --- Scenario: POST /v1/chat.postMessage ---
+interface IChatPostMessageBody {
+	channal: string;
+	text: string;
+	attachments?: Array<{
+		text: string;
+		color?: string;
+		image_url?: string;
+	}>;
+}
+type ChatPostMessageBodyValidator = ValidateFunction<IChatPostMessageBody>;
+
+type ChatPostMessageOptions = TypedOptions & {
+	body: ChatPostMessageBodyValidator;
+	authRequired: true;
+};
+
+type Post = 'POST';
+type Path = '/v1/chat.postMessage';
+type Options = ChatPostMessageOptions;
+
+// --- Expected ActionThis shape (single source of truth) ---
+type ExpectedActionThis = {
+	route: string;
+	readonly requestIp: string;
+	urlParams: never;
+	readonly response: Response;
+	readonly queryParams: Record<string, string>;
+	readonly bodyParams: ChatPostMessageBodyValidator;
+	readonly request: Request;
+	readonly queryOperations: never;
+	parseJsonQuery: () => Promise<{
+		sort: Record<string, 1 | -1>;
+		fields: Record<string, 0 | 1>;
+		query: Record<string, unknown>;
+	}>;
+	readonly connection: {
+		token: string;
+		id: string;
+		close: () => void;
+		clientAddress: string;
+		httpHeaders: Record<string, any>;
+	};
+	user: IUser;
+	userId: string;
+	readonly token: string;
+};
+
+// --- Type-level assertions ---
+type ChatPostMessageThis = TypedThis<Options, Path>;
+type TestChatPostMessageBodyParams = IsTypeEqual<ChatPostMessageThis['bodyParams'], IChatPostMessageBody>;
+true satisfies TestChatPostMessageBodyParams;
+
+type ActionType = Action<Post, Path, Options>;
+type ActionThisType = ExtractThis<Exclude<ActionType, undefined>>;
+type TestActionBodyParams = IsTypeEqual<ActionThisType['bodyParams'], ExpectedActionThis['bodyParams']>;
+true satisfies TestActionBodyParams;
+
+type InnerActionType = InnerAction<Post, Path, Options>;
+type ExpectedInnerActionThis = Mutable<ExpectedActionThis>;
+type InnerActionThisType = ExtractThis<Exclude<InnerActionType, undefined>>;
+type TestInnerActionBodyParams = IsTypeEqual<InnerActionThisType['bodyParams'], ExpectedInnerActionThis['bodyParams']>;
+true satisfies TestInnerActionBodyParams;