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[Bug] check_capability does not validate elicitation sub-capabilities (form/url) #2965
Description
Summary
Connection.check_capability() returns True when a client has URL-mode elicitation but the caller checks for form-mode elicitation — the sub-capability (ElicitationCapability.form / .url) is never inspected.
Root cause
In src/mcp/server/connection.py:340-341:
if capability.elicitation is not None and have.elicitation is None:
return FalseThis only returns False when the client has no elicitation at all. It does not check individual sub-capabilities (form / url). Compare with sampling, which correctly checks sub-capabilities:
if capability.sampling is not None:
if have.sampling is None:
return False
if capability.sampling.context is not None and have.sampling.context is None:
return False
if capability.sampling.tools is not None and have.sampling.tools is None:
return FalseReproduction
# Client supports URL elicitation only
have = ClientCapabilities(elicitation=ElicitationCapability(url=UrlElicitationCapability()))
# Check for form elicitation — should be False (client does not have form)
want = ClientCapabilities(elicitation=ElicitationCapability(form=FormElicitationCapability()))
Connection.from_envelope("2025-11-25", client_info, have).check_capability(want) # Returns True (BUG)Impact
check_capabilityis public API (viaServerSession.check_client_capability)- Currently no production callers use elicitation sub-capability checks — low immediate impact
- But once someone relies on it (e.g., checking if the client supports form elicitation before calling
elicit_form), it will return wrong results
Additional gaps (lower priority)
extensions—ClientCapabilities.extensionsis not checked at alltasks—ClientCapabilities.tasksand sub-capabilities entirely unhandled
Proposed fix
Add form / url sub-capability checks, matching the sampling pattern:
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 FalseAI assistance: Bug discovered and analyzed with AI assistance (opencode).
Activity
syf2211 commentedon Jun 26, 2026
Rodrigo-Palma commentedon Jul 3, 2026
I ran into this as well and opened #3061 with a fix. It mirrors the existing
samplingsub-capability handling forelicitation.form/elicitation.url: when a specific mode is requested, the client must declare that mode; a bare elicitation request is still satisfied by any elicitation support.The PR extends the existing
test_check_capability_per_field_branchesparametrization with the two bug cases (URL-only client vs. form request, and form-only vs. URL) plus the positive/bare cases — the bug cases fail onmainand pass with the change.pytest,pyright, andruffare all green.I scoped it to elicitation only:
extensionsalready appears handled onmain(SEP-2133), and I lefttasksout to keep the PR small — happy to follow up separately. Glad to be assigned if this looks good.(AI-assisted analysis and drafting; the change is reviewed and understood by me.)