diff --git a/src/mcp/client/session.py b/src/mcp/client/session.py index 097ade1c9..171dfce15 100644 --- a/src/mcp/client/session.py +++ b/src/mcp/client/session.py @@ -6,7 +6,7 @@ from functools import reduce from operator import or_ from types import TracebackType -from typing import Annotated, Any, Final, Literal, Protocol, cast, overload +from typing import TYPE_CHECKING, Annotated, Any, Final, Literal, Protocol, cast, overload import anyio import anyio.abc @@ -56,6 +56,11 @@ from mcp.shared.subscriptions import SUBSCRIPTION_ID_META_KEY, event_from_wire from mcp.shared.transport_context import TransportContext +if TYPE_CHECKING: + # `jsonschema` is imported lazily inside `validate_tool_result`: pulling it (and its + # `attrs`/`referencing` tree) in at module scope costs every client that never validates. + from jsonschema.protocols import Validator + DEFAULT_CLIENT_INFO = types.Implementation(name="mcp", version="0.1.0") DISCOVER_TIMEOUT_SECONDS = 10.0 _NOTIFICATION_QUEUE_SIZE: Final = 256 @@ -357,6 +362,9 @@ def __init__( self._logging_callback = logging_callback or _default_logging_callback self._message_handler = message_handler or _default_message_handler self._tool_output_schemas: dict[str, dict[str, Any] | None] = {} + # Compiled output-schema validators, each paired with the schema object it was built + # from so a re-listed tool can't be served a validator for its previous schema. + self._tool_output_validators: dict[str, tuple[dict[str, Any], Validator]] = {} self._x_mcp_header_maps: dict[str, dict[tuple[str, ...], str]] = {} self._initialize_result: types.InitializeResult | None = None self._discover_result: types.DiscoverResult | None = None @@ -1032,16 +1040,51 @@ async def validate_tool_result(self, name: str, result: types.CallToolResult) -> logger.warning(f"Tool {name} not listed by server, cannot validate any structured content") if output_schema is not None: - from jsonschema import SchemaError, ValidationError, validate + from jsonschema import exceptions as jsonschema_exceptions if result.structured_content is None: raise RuntimeError(f"Tool {name} has an output schema but did not return structured content") - try: - validate(result.structured_content, output_schema) - except ValidationError as e: - raise RuntimeError(f"Invalid structured content returned by tool {name}: {e}") - except SchemaError as e: # pragma: no cover - raise RuntimeError(f"Invalid schema for tool {name}: {e}") # pragma: no cover + validator = self._output_schema_validator(name, output_schema) + # `best_match` picks the same error the previous `jsonschema.validate()` call raised, + # so the message a caller sees is unchanged. It is untyped upstream. + errors = validator.iter_errors(result.structured_content) + error = cast( + "Exception | None", + jsonschema_exceptions.best_match(errors), # pyright: ignore[reportUnknownMemberType] + ) + if error is not None: + raise RuntimeError(f"Invalid structured content returned by tool {name}: {error}") + + def _output_schema_validator(self, name: str, output_schema: dict[str, Any]) -> Validator: + """Compiled validator for `output_schema`, built once and reused across calls. + + Compiling is ~60x the cost of validating, so a one-shot `jsonschema.validate()` per + result dominates `call_tool`. The cache holds the schema object it compiled alongside + the validator and only reuses it on an identity match: `_absorb_tool_listing` assigns a + freshly parsed schema object on every listing, so a server that changes a tool's schema + forces a rebuild rather than reusing the stale validator. Holding the reference also + keeps the allocator from recycling that identity behind our back. + + Raises: + RuntimeError: The schema is not a valid JSON Schema. Raised on every call, since a + failed compile is never cached. + """ + from jsonschema import SchemaError + from jsonschema.validators import validator_for + + cached = self._tool_output_validators.get(name) + if cached is not None and cached[0] is output_schema: + return cached[1] + + validator_cls = validator_for(output_schema) + try: + validator_cls.check_schema(output_schema) + except SchemaError as e: + raise RuntimeError(f"Invalid schema for tool {name}: {e}") + # The `Validator` protocol declares `registry` without a default; concrete classes have one. + validator = cast("Callable[[dict[str, Any]], Validator]", validator_cls)(output_schema) + self._tool_output_validators[name] = (output_schema, validator) + return validator async def list_prompts(self, *, params: types.PaginatedRequestParams | None = None) -> types.ListPromptsResult: """Send a prompts/list request. diff --git a/tests/client/test_session_promotions.py b/tests/client/test_session_promotions.py index e4a62732b..b6b82ba7c 100644 --- a/tests/client/test_session_promotions.py +++ b/tests/client/test_session_promotions.py @@ -64,3 +64,48 @@ async def test_validate_tool_result_raises_on_schema_mismatch() -> None: # Stable SDK prefix only: the message tail is jsonschema text that shifts with the dependency. with pytest.raises(RuntimeError, match="Invalid structured content returned by tool t"): await client.session.validate_tool_result("t", CallToolResult(content=[], structured_content={"x": "no"})) + + +@pytest.mark.anyio +async def test_validate_tool_result_raises_on_an_unusable_output_schema() -> None: + """A schema that isn't valid JSON Schema is reported as such, on every call.""" + server = _make_server({"type": "not-a-json-schema-type"}) + async with Client(server) as client: + result = CallToolResult(content=[], structured_content={"x": 1}) + for _ in range(2): + # Compiling is never cached on failure, so the second call raises like the first. + with pytest.raises(RuntimeError, match="Invalid schema for tool t"): + await client.session.validate_tool_result("t", result) + + +@pytest.mark.anyio +async def test_validate_tool_result_compiles_the_output_schema_once_per_tool() -> None: + """Regression guard: compiling dominates validating, so the validator must outlive one call.""" + server = _make_server({"type": "object", "properties": {"x": {"type": "integer"}}, "required": ["x"]}) + async with Client(server) as client: + result = CallToolResult(content=[], structured_content={"x": 1}) + await client.session.validate_tool_result("t", result) + compiled = client.session._tool_output_validators["t"] + await client.session.validate_tool_result("t", result) + assert client.session._tool_output_validators["t"] is compiled + + +@pytest.mark.anyio +async def test_validate_tool_result_recompiles_when_the_server_changes_the_schema() -> None: + """A relisted tool must not be validated against the schema it used to declare.""" + schemas = [ + {"type": "object", "properties": {"x": {"type": "integer"}}, "required": ["x"]}, + {"type": "object", "properties": {"x": {"type": "string"}}, "required": ["x"]}, + ] + + async def on_list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult: + return ListToolsResult(tools=[Tool(name="t", input_schema={"type": "object"}, output_schema=schemas.pop(0))]) + + server = Server("test-server", on_list_tools=on_list_tools) + async with Client(server) as client: + integer_result = CallToolResult(content=[], structured_content={"x": 1}) + await client.session.validate_tool_result("t", integer_result) + + await client.session.list_tools() + with pytest.raises(RuntimeError, match="Invalid structured content returned by tool t"): + await client.session.validate_tool_result("t", integer_result)