diff --git a/tests/integration/linking.test.ts b/tests/integration/linking.test.ts
new file mode 100644
index 0000000..19319e9
--- /dev/null
+++ b/tests/integration/linking.test.ts
@@ -0,0 +1,131 @@
+import { describe, it, expect, beforeAll, afterAll } from "vitest";
+import PocketBase from "pocketbase";
+
+import { relinkProject, linkCommitManually } from "@/lib/linking/engine";
+import { DEMO, adminPb, requireServices } from "./helpers";
+
+let admin: PocketBase;
+let projectId: string;
+let columnId: string;
+const created: Array<{ collection: string; id: string }> = [];
+
+async function makeCard(fields: Record<string, unknown>) {
+  const numbered = await admin
+    .collection("cards")
+    .getFullList({ filter: `project = "${projectId}"`, fields: "number" });
+  const next =
+    Math.max(0, ...numbered.map((c) => Number(c.number) || 0)) + 1;
+
+  const card = await admin.collection("cards").create({
+    project: projectId,
+    column: columnId,
+    title: "Linking probe",
+    position: 1_000_000 + next,
+    priority: "medium",
+    created_by_role: "owner",
+    accepted: true,
+    archived: false,
+    number: next,
+    ...fields,
+  });
+  created.push({ collection: "cards", id: card.id });
+  return card;
+}
+
+async function makeCommit(fields: Record<string, unknown>) {
+  const commit = await admin.collection("commits").create({
+    project: projectId,
+    sha: `probe${Date.now()}${Math.random().toString(36).slice(2, 8)}`,
+    short_sha: "probe00",
+    message: "feat(probe): a thing",
+    subject: "a thing",
+    type: "feat",
+    scope: "probe",
+    author_name: "Dave",
+    author_email: "dave@example.com",
+    authored_at: new Date().toISOString(),
+    is_merge: false,
+    link_source: "none",
+    ...fields,
+  });
+  created.push({ collection: "commits", id: commit.id });
+  return commit;
+}
+
+beforeAll(async () => {
+  await requireServices();
+  admin = await adminPb();
+
+  const project = await admin
+    .collection("projects")
+    .getFirstListItem(`slug = "${DEMO.projectSlug}"`);
+  projectId = project.id;
+
+  columnId = (
+    await admin
+      .collection("columns")
+      .getFirstListItem(`project = "${projectId}" && is_inbox = true`)
+  ).id;
+});
+
+afterAll(async () => {
+  for (const item of created.reverse()) {
+    await admin.collection(item.collection).delete(item.id).catch(() => {});
+  }
+});
+
+describe("relinking is idempotent and preserves manual links", () => {
+  it("links by scope, and running twice changes nothing", async () => {
+    const card = await makeCard({ scope_rule: "probe" });
+    const commit = await makeCommit({});
+
+    await relinkProject(projectId);
+    const first = await admin.collection("commits").getOne(commit.id);
+    expect(first.card).toBe(card.id);
+    expect(first.link_source).toBe("scope");
+
+    await relinkProject(projectId);
+    const second = await admin.collection("commits").getOne(commit.id);
+    expect(second.card).toBe(card.id);
+    // `updated` must not move: re-linking writes only where the answer changed.
+    expect(second.updated).toBe(first.updated);
+  });
+
+  it("never overwrites a manual link", async () => {
+    const scoped = await makeCard({ scope_rule: "probe" });
+    const other = await makeCard({ scope_rule: "" });
+    const commit = await makeCommit({});
+
+    await linkCommitManually(commit.id, other.id);
+    await relinkProject(projectId);
+
+    const after = await admin.collection("commits").getOne(commit.id);
+    expect(after.card).toBe(other.id);
+    expect(after.card).not.toBe(scoped.id);
+    expect(after.link_source).toBe("manual");
+  });
+
+  it("does not let a card claim commits from before it existed", async () => {
+    const old = await makeCommit({
+      scope: "backdated",
+      authored_at: "2020-01-01T00:00:00.000Z",
+    });
+    await makeCard({ scope_rule: "backdated" });
+
+    await relinkProject(projectId);
+
+    const after = await admin.collection("commits").getOne(old.id);
+    expect(after.card).toBe("");
+    expect(after.link_source).toBe("none");
+  });
+
+  it("never links a merge commit", async () => {
+    await makeCard({ scope_rule: "mergeprobe" });
+    const merge = await makeCommit({ scope: "mergeprobe", is_merge: true });
+
+    await relinkProject(projectId);
+
+    const after = await admin.collection("commits").getOne(merge.id);
+    expect(after.card).toBe("");
+  });
+});
\ No newline at end of file
diff --git a/vitest.config.ts b/vitest.config.ts
index b7c483e..e83ba24 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -4,6 +4,10 @@ import path from "node:path";
 export default defineConfig({
   resolve: {
     alias: { "@": path.resolve(__dirname, "./src") },
+    // Used by every project — including the unit project, where pure modules
+    // must stay free of `import "server-only"`. The integration project adds
+    // `react-server` so its test files can import server-only modules.
+    conditions: ["node", "import"],
   },
   test: {
     projects: [
@@ -16,14 +20,17 @@ export default defineConfig({
         },
       },
       {
-        resolve: { alias: { "@": path.resolve(__dirname, "./src") } },
+        resolve: {
+          alias: { "@": path.resolve(__dirname, "./src") },
+          conditions: ["react-server", "node", "import"],
+        },
         test: {
           name: "integration",
           environment: "node",
           include: ["tests/integration/**/*.test.ts"],
           hookTimeout: 30_000,
           testTimeout: 30_000,
-          poolOptions: { forks: { singleFork: true } },
+          fileParallelism: false,
         },
       },
     ],