From c3dbc22d605dbb1de1d007af8755e30fd3de5ec0 Mon Sep 17 00:00:00 2001
From: Robin1987China <41602358+Robin1987China@users.noreply.github.com>
Date: Thu, 25 Jun 2026 18:17:53 +0800
Subject: [PATCH] fix: check_capability now validates elicitation
 sub-capabilities (form/url)

Schema has ElicitationCapability.form and .url sub-fields but
check_capability only validated the top-level elicitation key.
A client that supports URL elicitation but not form elicitation
would incorrectly return True when the caller checks for form.

Fix matches the existing sampling sub-capability pattern.
---
 src/mcp/server/connection.py    |  9 +++++++--
 tests/server/test_connection.py | 24 ++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/src/mcp/server/connection.py b/src/mcp/server/connection.py
index f5bfc18df..7786b5520 100644
--- a/src/mcp/server/connection.py
+++ b/src/mcp/server/connection.py
@@ -337,8 +337,13 @@ 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
diff --git a/tests/server/test_connection.py b/tests/server/test_connection.py
index 8ca1ae8a7..5b465b40e 100644
--- a/tests/server/test_connection.py
+++ b/tests/server/test_connection.py
@@ -27,6 +27,7 @@
     CreateMessageRequestParams,
     ElicitationCapability,
     EmptyResult,
+    FormElicitationCapability,
     Implementation,
     ListRootsRequest,
     ListRootsResult,
@@ -37,6 +38,7 @@
     SamplingCapability,
     SamplingContextCapability,
     SamplingToolsCapability,
+    UrlElicitationCapability,
 )
 
 _CLIENT_INFO = Implementation(name="t", version="0")
@@ -365,6 +367,28 @@ 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),
+        # Elicitation sub-capability checks (form / url)
+        (ClientCapabilities(elicitation=None), ClientCapabilities(elicitation=ElicitationCapability()), False),
+        (
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability())),
+            ClientCapabilities(elicitation=ElicitationCapability(url=UrlElicitationCapability())),
+            False,
+        ),
+        (
+            ClientCapabilities(elicitation=ElicitationCapability(url=UrlElicitationCapability())),
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability())),
+            False,
+        ),
+        (
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability(), url=UrlElicitationCapability())),
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability())),
+            True,
+        ),
+        (
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability())),
+            ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability())),
+            True,
+        ),
     ],
 )
 def test_check_capability_per_field_branches(have: ClientCapabilities, want: ClientCapabilities, expected: bool):