chore: Lazy import jsonschema library (#1596)

Co-authored-by: Max Isbey <[email protected]>

Author: wuliang229

src/mcp/client/session.py

@@ -4,7 +4,6 @@
 
 importanyio.lowlevel
 fromanyio.streams.memoryimportMemoryObjectReceiveStream, MemoryObjectSendStream
-fromjsonschemaimportSchemaError, ValidationError, validate
 frompydanticimportAnyUrl, TypeAdapter
 fromtyping_extensionsimportdeprecated
 
@@ -376,6 +375,8 @@ 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")
 
 ifoutput_schemaisnotNone:
+fromjsonschemaimportSchemaError, ValidationError, validate
+
 ifresult.structuredContentisNone:
 raiseRuntimeError(
 f"Tool {name} has an output schema but did not return structured content"

tests/client/test_output_schema_validation.py

@@ -19,9 +19,27 @@ def bypass_server_output_validation():
  This simulates a malicious or non-compliant server that doesn't validate
  its outputs, allowing us to test client-side validation.
  """
-# Patch jsonschema.validate in the server module to disable all validation
-withpatch("mcp.server.lowlevel.server.jsonschema.validate"):
-# The mock will simply return None (do nothing) for all validation calls
+importjsonschema
+
+# Save the original validate function
+original_validate=jsonschema.validate
+
+# Create a mock that tracks which module is calling it
+defselective_mock(instance: Any=None, schema: Any=None, *args: Any, **kwargs: Any) ->None:
+importinspect
+
+# Check the call stack to see where this is being called from
+forframe_infoininspect.stack():
+# If called from the server module, skip validation
+# TODO: fix this as it's a rather gross workaround and will eventually break
+# Normalize path separators for cross-platform compatibility
+normalized_path=frame_info.filename.replace("\\", "/")
+if"mcp/server/lowlevel/server.py"innormalized_path:
+returnNone
+# Otherwise, use the real validation (for client-side)
+returnoriginal_validate(instance=instance, schema=schema, *args, **kwargs)
+
+withpatch("jsonschema.validate", selective_mock):
 yield