diff --git a/packages/core-eslint/rules/usecase-must-have-test-file.js b/packages/core-eslint/rules/usecase-must-have-test-file.js
new file mode 100644
index 0000000..c7e3b71
--- /dev/null
+++ b/packages/core-eslint/rules/usecase-must-have-test-file.js
@@ -0,0 +1,32 @@
+import fs from "node:fs";
+
+/** @type {import("eslint").Rule.RuleModule} */
+export default {
+  meta: {
+    type: "problem",
+    docs: {
+      description:
+        "Every *.use-case.ts file must have a sibling *.use-case.test.ts (TDD discipline).",
+    },
+    schema: [],
+    messages: {
+      missingTestFile:
+        "Use case {{filename}} has no sibling test file at {{expected}}. Write the red test first.",
+    },
+  },
+  create(context) {
+    return {
+      Program(node) {
+        const filename = context.filename;
+        if (!filename.endsWith(".use-case.ts")) return;
+        const expected = filename.replace(/\.use-case\.ts$/, ".use-case.test.ts");
+        if (fs.existsSync(expected)) return;
+        context.report({
+          node,
+          messageId: "missingTestFile",
+          data: { filename: filename.split("/").pop(), expected: expected.split("/").pop() },
+        });
+      },
+    };
+  },
+};
diff --git a/packages/core-eslint/rules/usecase-must-have-test-file.test.js b/packages/core-eslint/rules/usecase-must-have-test-file.test.js
new file mode 100644
index 0000000..fc870f1
--- /dev/null
+++ b/packages/core-eslint/rules/usecase-must-have-test-file.test.js
@@ -0,0 +1,42 @@
+import { describe, it } from "vitest";
+import { RuleTester } from "eslint";
+import path from "node:path";
+import os from "node:os";
+import fs from "node:fs";
+import rule from "./usecase-must-have-test-file.js";
+
+function makeUseCaseFixture({ withTest }) {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "umht-"));
+  const useCase = path.join(dir, "sign-in.use-case.ts");
+  fs.writeFileSync(useCase, `export const signInUseCase = () => async () => {};`);
+  if (withTest) {
+    fs.writeFileSync(path.join(dir, "sign-in.use-case.test.ts"), `import { it } from "vitest"; it("works", () => {});`);
+  }
+  return { useCase };
+}
+
+const tester = new RuleTester({ languageOptions: { ecmaVersion: "latest", sourceType: "module" } });
+
+describe("usecase-must-have-test-file", () => {
+  it("passes when a sibling .test.ts exists", () => {
+    const { useCase } = makeUseCaseFixture({ withTest: true });
+    tester.run("usecase-must-have-test-file", rule, {
+      valid: [{ filename: useCase, code: fs.readFileSync(useCase, "utf8") }],
+      invalid: [],
+    });
+  });
+
+  it("fires when no sibling test file exists", () => {
+    const { useCase } = makeUseCaseFixture({ withTest: false });
+    tester.run("usecase-must-have-test-file", rule, {
+      valid: [],
+      invalid: [
+        {
+          filename: useCase,
+          code: fs.readFileSync(useCase, "utf8"),
+          errors: [{ messageId: "missingTestFile" }],
+        },
+      ],
+    });
+  });
+});