feat: add public setter methods for ClientSession callbacks#2407
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dgenio wants to merge 1 commit intomodelcontextprotocol:mainfrom
Open
feat: add public setter methods for ClientSession callbacks#2407dgenio wants to merge 1 commit intomodelcontextprotocol:mainfrom
dgenio wants to merge 1 commit intomodelcontextprotocol:mainfrom
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Add set_sampling_callback(), set_elicitation_callback(), and set_list_roots_callback() methods to ClientSession, allowing callbacks to be updated at runtime after initialization without mutating private attributes directly. Also removes the # pragma: no cover from _default_elicitation_callback and adds coverage via the new test for set_elicitation_callback(None). Reported-by: dgenio Github-Issue: modelcontextprotocol#2379
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What changed
src/mcp/client/session.py: added three public methods toClientSession:set_sampling_callback(callback)set_elicitation_callback(callback)set_list_roots_callback(callback)# pragma: no coverfrom_default_elicitation_callback(now covered by test)tests/client/test_list_roots_callback.py: addedtest_set_list_roots_callbacktests/client/test_sampling_callback.py: addedtest_set_sampling_callbacktests/client/test_elicitation_callback.py: new file withtest_set_elicitation_callbackWhy
ClientSessionaccepts callback parameters at initialization but provided no public API to update them at runtime. Any client needing to change callbacks (e.g., updating roots after a user changes working directory) was forced to mutate private attributes like_list_roots_callbackdirectly — coupling consumers to implementation details. This was the pattern used in PrefectHQ/fastmcp#3714.Each setter accepts
callback | None. PassingNonerestores the default behaviour (returning an error to the server), consistent with the__init__fallback pattern. The docstrings note that capabilities are advertised duringinitialize()and won't be re-negotiated by calling these setters.How verified
Tradeoffs / risks
initialize(), the server won't know about the capability until a new session is created. This is documented in each setter's docstring.logging_callback/message_handler: not included — the issue only names the three callbacks above. Can be added as a follow-up if desired.strict-no-covercannot run on Windows; CI will verify on Ubuntu.Scope notes
Changes are strictly limited to the three callbacks named in #2379. No refactoring of existing code.