Keep unknown keys on client and server capabilities

Author: Kludex

scripts/gen_surface_types.py

@@ -106,12 +106,24 @@
 }
 
 # Classes the spec defines as open key-value bags: `_meta` content, the
-# JSON-Schema-document fields on `Tool`, and the schemas with explicit
-# `additionalProperties: {}`. These keep `extra="allow"` so the sieve preserves
-# arbitrary keys; every other class ignores extras. Per-version because codegen
-# reuses class names across versions for unrelated schemas (e.g. `Data`).
+# JSON-Schema-document fields on `Tool`, the capability objects (documented as
+# "not a closed set"), and the schemas with explicit `additionalProperties: {}`.
+# These keep `extra="allow"` so the sieve preserves arbitrary keys; every other
+# class ignores extras. Per-version because codegen reuses class names across
+# versions for unrelated schemas (e.g. `Data`).
 OPEN_CLASSES: dict[str, frozenset[str]] = {
-    "2025-11-25": frozenset({"Meta", "InputSchema", "OutputSchema", "Result", "GetTaskPayloadResult", "Data"}),
+    "2025-11-25": frozenset(
+        {
+            "Meta",
+            "InputSchema",
+            "OutputSchema",
+            "Result",
+            "GetTaskPayloadResult",
+            "Data",
+            "ClientCapabilities",
+            "ServerCapabilities",
+        }
+    ),
     "2026-07-28": frozenset(
         {
             "MetaObject",
@@ -122,6 +134,8 @@
             "InputSchema",
             "OutputSchema",
             "Result",
+            "ClientCapabilities",
+            "ServerCapabilities",
         }
     ),
 }

src/mcp-types/mcp_types/_types.py

@@ -393,6 +393,8 @@ class ClientCapabilities(MCPModel):
     `initialize` through 2025-11-25; per-request in `_meta` on 2026-07-28.
     """
 
+    model_config = ConfigDict(extra="allow")
+
     experimental: dict[str, dict[str, Any]] | None = None
     """Experimental, non-standard capabilities that the client supports."""
     sampling: SamplingCapability | None = None
@@ -485,6 +487,8 @@ class ServerTasksCapability(MCPModel):
 class ServerCapabilities(MCPModel):
     """Capabilities that a server may support. Not a closed set."""
 
+    model_config = ConfigDict(extra="allow")
+
     experimental: dict[str, dict[str, Any]] | None = None
     """Experimental, non-standard capabilities that the server supports."""
 

src/mcp-types/mcp_types/_v2025_11_25/__init__.py

@@ -196,7 +196,7 @@ class ClientCapabilities(WireModel):
     """
 
     model_config = ConfigDict(
-        extra="ignore",
+        extra="allow",
     )
     elicitation: Elicitation | None = None
     """
@@ -1106,7 +1106,7 @@ class ServerCapabilities(WireModel):
     """
 
     model_config = ConfigDict(
-        extra="ignore",
+        extra="allow",
     )
     completions: dict[str, Any] | None = None
     """

src/mcp-types/mcp_types/_v2026_07_28/__init__.py

@@ -2041,7 +2041,7 @@ class ServerCapabilities(WireModel):
     """
 
     model_config = ConfigDict(
-        extra="ignore",
+        extra="allow",
     )
     completions: JSONObject | None = None
     """
@@ -2287,7 +2287,7 @@ class ClientCapabilities(WireModel):
     """
 
     model_config = ConfigDict(
-        extra="ignore",
+        extra="allow",
     )
     elicitation: Elicitation | None = None
     """

tests/types/test_methods.py

@@ -1008,6 +1008,30 @@ def test_serialize_server_result_drops_top_level_server_info_on_discover_but_kee
     assert sieved["_meta"] == {types.SERVER_INFO_META_KEY: stamp}
 
 
+def test_discover_round_trips_unknown_server_capabilities():
+    capabilities = types.ServerCapabilities.model_validate({"tools": {}, "events": {"listChanged": False}})
+    result = types.DiscoverResult(supported_versions=["2026-07-28"], capabilities=capabilities)
+    dumped = result.model_dump(by_alias=True, mode="json", exclude_none=True)
+    sieved = methods.serialize_server_result("server/discover", "2026-07-28", dumped)
+    assert sieved["capabilities"] == {"tools": {}, "events": {"listChanged": False}}
+    parsed = methods.parse_server_result("server/discover", "2026-07-28", sieved)
+    assert isinstance(parsed, types.DiscoverResult)
+    assert parsed.capabilities.model_dump(by_alias=True, exclude_none=True) == sieved["capabilities"]
+
+
+@pytest.mark.parametrize("version", PRE_2026)
+def test_initialize_round_trips_unknown_capabilities_at_pre_2026_versions(version: str):
+    capabilities: dict[str, Any] = {"tools": {}, "events": {}}
+    result = {"protocolVersion": version, "capabilities": capabilities, "serverInfo": {"name": "S", "version": "1"}}
+    sieved = methods.serialize_server_result("initialize", version, result)
+    assert sieved["capabilities"] == capabilities
+    client_info = {"name": "C", "version": "1"}
+    params = {"protocolVersion": version, "capabilities": {"events": {"enabled": True}}, "clientInfo": client_info}
+    request = methods.parse_client_request("initialize", version, params)
+    assert isinstance(request, types.InitializeRequest)
+    assert request.params.capabilities.model_dump(exclude_none=True) == {"events": {"enabled": True}}
+
+
 def test_serialize_server_result_drops_an_unknown_nested_tool_field():
     tool = {"name": "echo", "inputSchema": {"type": "object"}, "unknownField": 1}
     sieved = methods.serialize_server_result("tools/list", "2025-11-25", {"tools": [tool], "resultType": "complete"})