From 04ad13c9e473d9acb2db5152a06a5c889a0b581f Mon Sep 17 00:00:00 2001
From: vominh1919 <vominh1919@gmail.com>
Date: Sun, 10 May 2026 20:41:59 +0700
Subject: [PATCH] fix: replace mutable default arguments with None in
 StatefulToolEnv and MCPEnv
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Two instances of the classic Python mutable default argument bug:

1. StatefulToolEnv.add_tool(args_to_skip=[]) — the default [] is created
   once at function definition time and shared across all calls. If any
   caller mutates it (e.g., args_to_skip.append(...)), subsequent calls
   without an explicit argument see the mutated list.

2. MCPEnv.__init__(mcp_servers=[]) — same issue. The default list is
   shared across all MCPEnv instances that don't pass mcp_servers explicitly.

Fix: Replace `=[]` with `=None` and initialize inside the function body.
---
 verifiers/envs/experimental/mcp_env.py | 4 ++--
 verifiers/envs/stateful_tool_env.py    | 4 +++-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/verifiers/envs/experimental/mcp_env.py b/verifiers/envs/experimental/mcp_env.py
index 542b0866f..18b09222b 100644
--- a/verifiers/envs/experimental/mcp_env.py
+++ b/verifiers/envs/experimental/mcp_env.py
@@ -148,13 +148,13 @@ def __init__(
         self,
         # MCPEnv is designed for global server processes, not per-rollout,
         # stateful server instances with mutable task-specific data.
-        mcp_servers: List[MCPServerConfig | dict] = [],
+        mcp_servers: List[MCPServerConfig | dict] | None = None,
         max_turns: int = 10,
         error_formatter: Callable[[Exception], str] = lambda e: f"Error: {str(e)}",
         **kwargs,
     ):
         self.mcp_servers: List[MCPServerConfig] = []
-        if mcp_servers:
+        if mcp_servers is not None:
             for server in mcp_servers:
                 if isinstance(server, MCPServerConfig):
                     self.mcp_servers.append(server)
diff --git a/verifiers/envs/stateful_tool_env.py b/verifiers/envs/stateful_tool_env.py
index cb44555a4..37cdc1b52 100644
--- a/verifiers/envs/stateful_tool_env.py
+++ b/verifiers/envs/stateful_tool_env.py
@@ -66,7 +66,7 @@ def __init__(
         self.skipped_args: dict[str, list[str]] = {}
         self.max_turns: int = max_turns
 
-    def add_tool(self, tool: Callable, args_to_skip: list[str] = []):
+    def add_tool(self, tool: Callable, args_to_skip: list[str] | None = None):
         """Add a tool, optionally hiding arguments from the agent's view.
 
         Skipped args are removed from the schema shown to the agent but can be
@@ -75,6 +75,8 @@ def add_tool(self, tool: Callable, args_to_skip: list[str] = []):
 
         Assumes all non-skipped args use standard JSON types (no remaining $ref/$defs).
         """
+        if args_to_skip is None:
+            args_to_skip = []
         self.tools.append(tool)
         tool_def = convert_func_to_tool_def(filter_signature(tool, args_to_skip))
         params = tool_def.parameters