fix: add ensure_ascii=False to all MCP json.dumps calls (#10234) (#10512)

Python's json.dumps() defaults to ensure_ascii=True, escaping non-ASCII
characters to \uXXXX sequences.  For CJK characters this inflates
token count 3-4x — a single Chinese character like '中' becomes
'\u4e2d' (6 chars vs 3 bytes, ~6 tokens vs ~1 token).

Since MCP tool results feed directly into the model's conversation
context, this silently multiplied API costs for Chinese, Japanese,
and Korean users.

Fix: add ensure_ascii=False to all 20 json.dumps calls in mcp_tool.py.
Raw UTF-8 is valid JSON per RFC 8259 and all downstream consumers
(LLM APIs, display) handle it correctly.

Closes #10234

tools/mcp_tool.py

@@ -506,7 +506,7 @@ class SamplingHandler:
                         "type": "function",
                         "function": {
                             "name": tu.name,
-                            "arguments": json.dumps(tu.input) if isinstance(tu.input, dict) else str(tu.input),
+                            "arguments": json.dumps(tu.input, ensure_ascii=False) if isinstance(tu.input, dict) else str(tu.input),
                         },
                     })
                 msg_dict: dict = {"role": msg.role, "tool_calls": tc_list}
@@ -1274,7 +1274,7 @@ def _interrupted_call_result() -> str:
     """Standardized JSON error for a user-interrupted MCP tool call."""
     return json.dumps({
         "error": "MCP call interrupted: user sent a new message"
-    })
+    }, ensure_ascii=False)
 
 
 # ---------------------------------------------------------------------------
@@ -1361,7 +1361,7 @@ def _make_tool_handler(server_name: str, tool_name: str, tool_timeout: float):
         if not server or not server.session:
             return json.dumps({
                 "error": f"MCP server '{server_name}' is not connected"
-            })
+            }, ensure_ascii=False)
 
         async def _call():
             result = await server.session.call_tool(tool_name, arguments=args)
@@ -1375,7 +1375,7 @@ def _make_tool_handler(server_name: str, tool_name: str, tool_timeout: float):
                     "error": _sanitize_error(
                         error_text or "MCP tool returned an error"
                     )
-                })
+                }, ensure_ascii=False)
 
             # Collect text from content blocks
             parts: List[str] = []
@@ -1394,9 +1394,9 @@ def _make_tool_handler(server_name: str, tool_name: str, tool_timeout: float):
                     return json.dumps({
                         "result": text_result,
                         "structuredContent": structured,
-                    })
-                return json.dumps({"result": structured})
-            return json.dumps({"result": text_result})
+                    }, ensure_ascii=False)
+                return json.dumps({"result": structured}, ensure_ascii=False)
+            return json.dumps({"result": text_result}, ensure_ascii=False)
 
         try:
             return _run_on_mcp_loop(_call(), timeout=tool_timeout)
@@ -1411,7 +1411,7 @@ def _make_tool_handler(server_name: str, tool_name: str, tool_timeout: float):
                 "error": _sanitize_error(
                     f"MCP call failed: {type(exc).__name__}: {exc}"
                 )
-            })
+            }, ensure_ascii=False)
 
     return _handler
 
@@ -1425,7 +1425,7 @@ def _make_list_resources_handler(server_name: str, tool_timeout: float):
         if not server or not server.session:
             return json.dumps({
                 "error": f"MCP server '{server_name}' is not connected"
-            })
+            }, ensure_ascii=False)
 
         async def _call():
             result = await server.session.list_resources()
@@ -1441,7 +1441,7 @@ def _make_list_resources_handler(server_name: str, tool_timeout: float):
                 if hasattr(r, "mimeType") and r.mimeType:
                     entry["mimeType"] = r.mimeType
                 resources.append(entry)
-            return json.dumps({"resources": resources})
+            return json.dumps({"resources": resources}, ensure_ascii=False)
 
         try:
             return _run_on_mcp_loop(_call(), timeout=tool_timeout)
@@ -1455,7 +1455,7 @@ def _make_list_resources_handler(server_name: str, tool_timeout: float):
                 "error": _sanitize_error(
                     f"MCP call failed: {type(exc).__name__}: {exc}"
                 )
-            })
+            }, ensure_ascii=False)
 
     return _handler
 
@@ -1471,7 +1471,7 @@ def _make_read_resource_handler(server_name: str, tool_timeout: float):
         if not server or not server.session:
             return json.dumps({
                 "error": f"MCP server '{server_name}' is not connected"
-            })
+            }, ensure_ascii=False)
 
         uri = args.get("uri")
         if not uri:
@@ -1487,7 +1487,7 @@ def _make_read_resource_handler(server_name: str, tool_timeout: float):
                     parts.append(block.text)
                 elif hasattr(block, "blob"):
                     parts.append(f"[binary data, {len(block.blob)} bytes]")
-            return json.dumps({"result": "\n".join(parts) if parts else ""})
+            return json.dumps({"result": "\n".join(parts) if parts else ""}, ensure_ascii=False)
 
         try:
             return _run_on_mcp_loop(_call(), timeout=tool_timeout)
@@ -1501,7 +1501,7 @@ def _make_read_resource_handler(server_name: str, tool_timeout: float):
                 "error": _sanitize_error(
                     f"MCP call failed: {type(exc).__name__}: {exc}"
                 )
-            })
+            }, ensure_ascii=False)
 
     return _handler
 
@@ -1515,7 +1515,7 @@ def _make_list_prompts_handler(server_name: str, tool_timeout: float):
         if not server or not server.session:
             return json.dumps({
                 "error": f"MCP server '{server_name}' is not connected"
-            })
+            }, ensure_ascii=False)
 
         async def _call():
             result = await server.session.list_prompts()
@@ -1536,7 +1536,7 @@ def _make_list_prompts_handler(server_name: str, tool_timeout: float):
                         for a in p.arguments
                     ]
                 prompts.append(entry)
-            return json.dumps({"prompts": prompts})
+            return json.dumps({"prompts": prompts}, ensure_ascii=False)
 
         try:
             return _run_on_mcp_loop(_call(), timeout=tool_timeout)
@@ -1550,7 +1550,7 @@ def _make_list_prompts_handler(server_name: str, tool_timeout: float):
                 "error": _sanitize_error(
                     f"MCP call failed: {type(exc).__name__}: {exc}"
                 )
-            })
+            }, ensure_ascii=False)
 
     return _handler
 
@@ -1566,7 +1566,7 @@ def _make_get_prompt_handler(server_name: str, tool_timeout: float):
         if not server or not server.session:
             return json.dumps({
                 "error": f"MCP server '{server_name}' is not connected"
-            })
+            }, ensure_ascii=False)
 
         name = args.get("name")
         if not name:
@@ -1593,7 +1593,7 @@ def _make_get_prompt_handler(server_name: str, tool_timeout: float):
             resp = {"messages": messages}
             if hasattr(result, "description") and result.description:
                 resp["description"] = result.description
-            return json.dumps(resp)
+            return json.dumps(resp, ensure_ascii=False)
 
         try:
             return _run_on_mcp_loop(_call(), timeout=tool_timeout)
@@ -1607,7 +1607,7 @@ def _make_get_prompt_handler(server_name: str, tool_timeout: float):
                 "error": _sanitize_error(
                     f"MCP call failed: {type(exc).__name__}: {exc}"
                 )
-            })
+            }, ensure_ascii=False)
 
     return _handler