diff --git a/server/test/unit/services/tagsService.test.ts b/server/test/unit/services/tagsService.test.ts
new file mode 100644
index 000000000..d611d641e
--- /dev/null
+++ b/server/test/unit/services/tagsService.test.ts
@@ -0,0 +1,124 @@
+import { describe, expect, it, jest } from "@jest/globals";
+import { TagsService } from "../../../src/service/infrastructure/tagService.ts";
+import type { Tag } from "../../../src/types/index.ts";
+
+// ── Helpers ──────────────────────────────────────────────────────────────────
+
+const createTagsRepo = () => ({
+	create: jest.fn(),
+	findById: jest.fn(),
+	findByTeamId: jest.fn(),
+	updateById: jest.fn(),
+	deleteById: jest.fn(),
+});
+
+const createMonitorsRepo = () => ({
+	removeTagFromMonitors: jest.fn(),
+});
+
+const createService = (overrides?: Record<string, unknown>) => {
+	const tagsRepository = createTagsRepo();
+	const monitorsRepository = createMonitorsRepo();
+
+	const defaults = {
+		tagsRepository,
+		monitorsRepository,
+		...overrides,
+	};
+
+	const service = new TagsService(defaults.tagsRepository as any, defaults.monitorsRepository as any);
+
+	return { service, ...defaults };
+};
+
+const makeTag = (overrides?: Partial<Tag>): Tag =>
+	({
+		id: "tag-1",
+		teamId: "team-1",
+		name: "Production",
+		color: "#13715B",
+		createdAt: "2026-01-01T00:00:00Z",
+		updatedAt: "2026-01-01T00:00:00Z",
+		...overrides,
+	}) as Tag;
+
+// ── Tests ────────────────────────────────────────────────────────────────────
+
+describe("TagsService", () => {
+	describe("createTag", () => {
+		it("sets teamId and delegates to repository", async () => {
+			const created = makeTag();
+			const { service, tagsRepository } = createService();
+			(tagsRepository.create as jest.Mock).mockResolvedValue(created);
+
+			const result = await service.createTag({ name: "Production", color: "#13715B" }, "team-1");
+
+			expect(result).toBe(created);
+			expect(tagsRepository.create).toHaveBeenCalledWith(expect.objectContaining({ teamId: "team-1", name: "Production", color: "#13715B" }));
+		});
+	});
+
+	describe("getTag", () => {
+		it("delegates to repository", async () => {
+			const tag = makeTag();
+			const { service, tagsRepository } = createService();
+			(tagsRepository.findById as jest.Mock).mockResolvedValue(tag);
+
+			const result = await service.getTag("tag-1", "team-1");
+
+			expect(result).toBe(tag);
+			expect(tagsRepository.findById).toHaveBeenCalledWith("tag-1", "team-1");
+		});
+	});
+
+	describe("getTagsByTeamId", () => {
+		it("delegates to repository", async () => {
+			const tags = [makeTag()];
+			const { service, tagsRepository } = createService();
+			(tagsRepository.findByTeamId as jest.Mock).mockResolvedValue(tags);
+
+			const result = await service.getTagsByTeamId("team-1");
+
+			expect(result).toBe(tags);
+			expect(tagsRepository.findByTeamId).toHaveBeenCalledWith("team-1");
+		});
+	});
+
+	describe("updateTag", () => {
+		it("delegates to repository", async () => {
+			const updated = makeTag({ name: "Staging" });
+			const { service, tagsRepository } = createService();
+			(tagsRepository.updateById as jest.Mock).mockResolvedValue(updated);
+
+			const result = await service.updateTag("tag-1", "team-1", { name: "Staging" });
+
+			expect(result).toBe(updated);
+			expect(tagsRepository.updateById).toHaveBeenCalledWith("tag-1", "team-1", { name: "Staging" });
+		});
+	});
+
+	describe("deleteTag", () => {
+		it("removes the tag from monitors and deletes it", async () => {
+			const deleted = makeTag();
+			const { service, tagsRepository, monitorsRepository } = createService();
+			(tagsRepository.deleteById as jest.Mock).mockResolvedValue(deleted);
+
+			const result = await service.deleteTag("tag-1", "team-1");
+
+			expect(result).toBe(deleted);
+			expect(monitorsRepository.removeTagFromMonitors).toHaveBeenCalledWith("tag-1");
+			expect(tagsRepository.deleteById).toHaveBeenCalledWith("tag-1", "team-1");
+		});
+
+		it("detaches the tag from monitors before deleting it", async () => {
+			const { service, tagsRepository, monitorsRepository } = createService();
+			(tagsRepository.deleteById as jest.Mock).mockResolvedValue(makeTag());
+
+			await service.deleteTag("tag-1", "team-1");
+
+			const removeOrder = (monitorsRepository.removeTagFromMonitors as jest.Mock).mock.invocationCallOrder[0];
+			const deleteOrder = (tagsRepository.deleteById as jest.Mock).mock.invocationCallOrder[0];
+			expect(removeOrder).toBeLessThan(deleteOrder);
+		});
+	});
+});