diff --git a/requirements.txt b/requirements.txt
index be5f5d450..3c5114f53 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -38,7 +38,10 @@ python-dateutil
 caldav
 cryptography
 bcrypt
-mcp
+# Built-in servers use the v1 low-level Server decorator API. MCP SDK v2 is a
+# breaking rewrite, so keep fresh installs on the maintained v1 line until the
+# servers are migrated together.
+mcp<2
 pyotp
 qrcode[pil]
 croniter
diff --git a/tests/test_mcp_dependency_compatibility.py b/tests/test_mcp_dependency_compatibility.py
new file mode 100644
index 000000000..9efefe4fe
--- /dev/null
+++ b/tests/test_mcp_dependency_compatibility.py
@@ -0,0 +1,15 @@
+"""Regression coverage for the built-in MCP servers' SDK compatibility line."""
+
+from pathlib import Path
+
+
+REQUIREMENTS = Path(__file__).resolve().parents[1] / "requirements.txt"
+
+
+def test_mcp_requirement_excludes_breaking_v2_sdk():
+    requirements = [
+        line.split("#", 1)[0].strip().replace(" ", "")
+        for line in REQUIREMENTS.read_text(encoding="utf-8").splitlines()
+    ]
+
+    assert "mcp<2" in requirements