From 0a4e8d4e17a51772f564c6e804e589061d4f7636 Mon Sep 17 00:00:00 2001
From: Giuseppe Coviello <gcov@protonmail.com>
Date: Thu, 10 Jul 2025 17:36:45 -0400
Subject: [PATCH] Avoiding to parse string arguments containing valid JSON if
 the argument annotation is str (#1113)

---
 src/mcp/server/fastmcp/utilities/func_metadata.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/mcp/server/fastmcp/utilities/func_metadata.py b/src/mcp/server/fastmcp/utilities/func_metadata.py
index a8f9652d..3c9506fb 100644
--- a/src/mcp/server/fastmcp/utilities/func_metadata.py
+++ b/src/mcp/server/fastmcp/utilities/func_metadata.py
@@ -130,7 +130,7 @@ class FuncMetadata(BaseModel):
         for field_name in self.arg_model.model_fields.keys():
             if field_name not in data.keys():
                 continue
-            if isinstance(data[field_name], str):
+            if isinstance(data[field_name], str) and self.arg_model.model_fields[field_name].annotation is not str:
                 try:
                     pre_parsed = json.loads(data[field_name])
                 except json.JSONDecodeError: