diff --git a/artifacts/api-server/src/lib/setupService.ts b/artifacts/api-server/src/lib/setupService.ts
index 86bc543a..2c406028 100644
--- a/artifacts/api-server/src/lib/setupService.ts
+++ b/artifacts/api-server/src/lib/setupService.ts
@@ -253,18 +253,31 @@ export async function completeInstall(
       return { ok: false, status: 409, error: "already_installed" };
     }
 
-    // Username/email uniqueness re-check inside the txn.
-    const dup = await client.query<{ id: number }>(
-      `SELECT id FROM users WHERE username = $1 OR email = $2 LIMIT 1`,
+    // Username/email uniqueness re-check inside the txn. We map
+    // collisions back to per-field keys so the wizard can highlight
+    // the offending input precisely.
+    const dup = await client.query<{
+      id: number;
+      username: string;
+      email: string;
+    }>(
+      `SELECT id, username, email FROM users WHERE username = $1 OR email = $2`,
       [input.admin.username, input.admin.email],
     );
     if ((dup.rowCount ?? 0) > 0) {
       await client.query("ROLLBACK");
+      const errors: ValidationErrors = {};
+      for (const row of dup.rows) {
+        if (row.username === input.admin.username)
+          errors["admin.username"] = "username_taken";
+        if (row.email === input.admin.email)
+          errors["admin.email"] = "email_taken";
+      }
       return {
         ok: false,
         status: 400,
         error: "validation_failed",
-        errors: { "admin.username": "taken_or_email_taken" },
+        errors,
       };
     }