Validate elicitation sub-capabilities in check_capability

check_capability only rejected a requested elicitation capability when the
client declared no elicitation support at all, ignoring the form/url
sub-capabilities. A client that advertised only URL-mode elicitation would
incorrectly pass a check for form-mode elicitation (and vice versa).

Mirror the existing sampling sub-capability handling: when a specific
elicitation mode is requested, require the client to declare that mode.
A bare elicitation request is still satisfied by any elicitation support.

Fixes #2965

Author: Rodrigo-Palma

src/mcp/server/connection.py

@@ -388,8 +388,13 @@ def check_capability(self, capability: ClientCapabilities) -> bool:
                 return False
             if capability.sampling.tools is not None and have.sampling.tools is None:
                 return False
-        if capability.elicitation is not None and have.elicitation is None:
-            return False
+        if capability.elicitation is not None:
+            if have.elicitation is None:
+                return False
+            if capability.elicitation.form is not None and have.elicitation.form is None:
+                return False
+            if capability.elicitation.url is not None and have.elicitation.url is None:
+                return False
         if capability.experimental is not None:
             if have.experimental is None:
                 return False

tests/server/test_connection.py

@@ -21,6 +21,7 @@
     CreateMessageRequestParams,
     ElicitationCapability,
     EmptyResult,
+    FormElicitationCapability,
     Implementation,
     ListRootsRequest,
     ListRootsResult,
@@ -31,6 +32,7 @@
     SamplingCapability,
     SamplingContextCapability,
     SamplingToolsCapability,
+    UrlElicitationCapability,
 )
 from mcp_types.version import LATEST_HANDSHAKE_VERSION, LATEST_MODERN_VERSION
 from pydantic import BaseModel, ValidationError
@@ -364,6 +366,35 @@ def test_connection_check_capability_false_when_no_client_params_recorded():
         (ClientCapabilities(experimental={"a": {}}), ClientCapabilities(experimental={"b": {}}), False),
         (ClientCapabilities(experimental={"a": {"x": 1}}), ClientCapabilities(experimental={"a": {"x": 2}}), False),
         (ClientCapabilities(experimental={"a": {}}), ClientCapabilities(experimental={"a": {}}), True),
+        (ClientCapabilities(elicitation=None), ClientCapabilities(elicitation=ElicitationCapability()), False),
+        # The client offers only URL-mode elicitation, but form mode is requested.
+        (
+            ClientCapabilities(elicitation=ElicitationCapability(url=UrlElicitationCapability())),
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability())),
+            False,
+        ),
+        # The client offers only form-mode elicitation, but URL mode is requested.
+        (
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability())),
+            ClientCapabilities(elicitation=ElicitationCapability(url=UrlElicitationCapability())),
+            False,
+        ),
+        (
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability())),
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability())),
+            True,
+        ),
+        (
+            ClientCapabilities(elicitation=ElicitationCapability(url=UrlElicitationCapability())),
+            ClientCapabilities(elicitation=ElicitationCapability(url=UrlElicitationCapability())),
+            True,
+        ),
+        # A bare elicitation request (no sub-capability) is satisfied by any elicitation support.
+        (
+            ClientCapabilities(elicitation=ElicitationCapability(url=UrlElicitationCapability())),
+            ClientCapabilities(elicitation=ElicitationCapability()),
+            True,
+        ),
     ],
 )
 def test_check_capability_per_field_branches(have: ClientCapabilities, want: ClientCapabilities, expected: bool):