From cb808e7897d6173edea45ef9a8fd2124d45a79c3 Mon Sep 17 00:00:00 2001
From: Liang Wu <18244712+wuliang229@users.noreply.github.com>
Date: Fri, 7 Nov 2025 23:57:05 -0800
Subject: [PATCH 1/2] Lazy import `jsonschema`

The import of `jsonschema` takes up almost 2s at cold start, which dramatically slow down the cold start in a containerized environment. Changing it to lazy import within the function that uses it will save those 2s at startup. As MCP is getting used more in production, this time-saving is critical.
---
 src/mcp/client/session.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/mcp/client/session.py b/src/mcp/client/session.py
index 8f071021d..7254cef61 100644
--- a/src/mcp/client/session.py
+++ b/src/mcp/client/session.py
@@ -4,7 +4,6 @@
 
 import anyio.lowlevel
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
-from jsonschema import SchemaError, ValidationError, validate
 from pydantic import AnyUrl, TypeAdapter
 from typing_extensions import deprecated
 
@@ -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")
 
         if output_schema is not None:
+            from jsonschema import SchemaError, ValidationError, validate
+            
             if result.structuredContent is None:
                 raise RuntimeError(f"Tool {name} has an output schema but did not return structured content")
             try:

From 3bd6dfcdafae99a18377e19bae65e0f84abc8f95 Mon Sep 17 00:00:00 2001
From: Max Isbey <224885523+maxisbey@users.noreply.github.com>
Date: Wed, 19 Nov 2025 15:18:57 +0000
Subject: [PATCH 2/2] fix: Update test mock to work with lazy jsonschema import

The lazy import change caused the test's `bypass_server_output_validation()`
to inadvertently mock the client's validation as well. This updates the mock
to selectively bypass only server-side validation by checking the call stack.
---
 src/mcp/client/session.py                     |  2 +-
 tests/client/test_output_schema_validation.py | 24 ++++++++++++++++---
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/src/mcp/client/session.py b/src/mcp/client/session.py
index d205c3126..3817ca6b5 100644
--- a/src/mcp/client/session.py
+++ b/src/mcp/client/session.py
@@ -376,7 +376,7 @@ async def _validate_tool_result(self, name: str, result: types.CallToolResult) -
 
         if output_schema is not None:
             from jsonschema import SchemaError, ValidationError, validate
-            
+
             if result.structuredContent is None:
                 raise RuntimeError(
                     f"Tool {name} has an output schema but did not return structured content"
diff --git a/tests/client/test_output_schema_validation.py b/tests/client/test_output_schema_validation.py
index 4e649b0eb..e4a06b7f8 100644
--- a/tests/client/test_output_schema_validation.py
+++ b/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
-    with patch("mcp.server.lowlevel.server.jsonschema.validate"):
-        # The mock will simply return None (do nothing) for all validation calls
+    import jsonschema
+
+    # Save the original validate function
+    original_validate = jsonschema.validate
+
+    # Create a mock that tracks which module is calling it
+    def selective_mock(instance: Any = None, schema: Any = None, *args: Any, **kwargs: Any) -> None:
+        import inspect
+
+        # Check the call stack to see where this is being called from
+        for frame_info in inspect.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" in normalized_path:
+                return None
+        # Otherwise, use the real validation (for client-side)
+        return original_validate(instance=instance, schema=schema, *args, **kwargs)
+
+    with patch("jsonschema.validate", selective_mock):
         yield