diff --git a/tests/tools/test_code_execution.py b/tests/tools/test_code_execution.py
index 6f6260ffe24..a5806046583 100644
--- a/tests/tools/test_code_execution.py
+++ b/tests/tools/test_code_execution.py
@@ -114,14 +114,30 @@ class TestHermesToolsGeneration(unittest.TestCase):
         self.assertIn("def json_parse(", src)
         self.assertIn("def shell_quote(", src)
         self.assertIn("def retry(", src)
-        self.assertIn("import json, os, socket, shlex, time", src)
+        self.assertIn("import json, os, socket, shlex, threading, time", src)
 
     def test_file_transport_uses_tempfile_fallback_for_rpc_dir(self):
         src = generate_hermes_tools_module(["terminal"], transport="file")
-        self.assertIn("import json, os, shlex, tempfile, time", src)
+        self.assertIn("import json, os, shlex, tempfile, threading, time", src)
         self.assertIn("os.path.join(tempfile.gettempdir(), \"hermes_rpc\")", src)
         self.assertNotIn('os.environ.get("HERMES_RPC_DIR", "/tmp/hermes_rpc")', src)
 
+    def test_uds_transport_serializes_concurrent_calls(self):
+        """Regression: UDS _call() must hold a lock across send+recv so that
+        concurrent tool calls from multiple threads don't interleave on the
+        shared socket and receive each other's responses."""
+        src = generate_hermes_tools_module(["terminal"], transport="uds")
+        self.assertIn("_call_lock = threading.Lock()", src)
+        self.assertIn("with _call_lock:", src)
+
+    def test_file_transport_serializes_seq_allocation(self):
+        """Regression: file transport _call() must allocate `_seq` under a
+        lock, otherwise concurrent threads can pick the same seq and clobber
+        each other's request files."""
+        src = generate_hermes_tools_module(["terminal"], transport="file")
+        self.assertIn("_seq_lock = threading.Lock()", src)
+        self.assertIn("with _seq_lock:", src)
+
 
 class TestExecuteCodeRemoteTempDir(unittest.TestCase):
     def test_execute_remote_uses_backend_temp_dir_for_sandbox(self):
@@ -226,6 +242,64 @@ print(f"file lines: {r2['total_lines']}")
         result = self._run("raise ValueError('test error')")
         self.assertEqual(result["status"], "error")
 
+    def test_concurrent_tool_calls_match_responses(self):
+        """Regression for the UDS RPC race: multiple threads inside the
+        sandbox calling terminal() concurrently must each receive their own
+        response, not another thread's.
+
+        Before the fix, `_sock` and the recv-loop were shared without a
+        lock, so responses (written FIFO by the single-threaded server)
+        got delivered to whichever client thread happened to win the
+        recv() race. That surfaced as each thread seeing another thread's
+        output.
+
+        The mock dispatcher sleeps briefly to guarantee the requests
+        overlap on the socket.
+        """
+        code = '''
+import threading
+from concurrent.futures import ThreadPoolExecutor
+from hermes_tools import terminal
+
+N = 10
+
+def call(i):
+    r = terminal(f"echo TAG-{i}")
+    return i, r.get("output", "")
+
+with ThreadPoolExecutor(max_workers=N) as ex:
+    results = list(ex.map(call, range(N)))
+
+mismatches = [(i, out) for i, out in results if f"TAG-{i}" not in out]
+if mismatches:
+    print(f"MISMATCH {len(mismatches)}/{N}: {mismatches[:3]}")
+else:
+    print(f"OK {N}/{N}")
+'''
+
+        def slow_mock(function_name, function_args, task_id=None, user_task=None):
+            import time as _t
+            if function_name == "terminal":
+                _t.sleep(0.05)  # ensure requests overlap on the socket
+                cmd = function_args.get("command", "")
+                # Echo semantics: strip leading "echo " and return the rest
+                out = cmd[5:] if cmd.startswith("echo ") else f"mock: {cmd}"
+                return json.dumps({"output": out, "exit_code": 0})
+            return _mock_handle_function_call(
+                function_name, function_args, task_id=task_id, user_task=user_task
+            )
+
+        with patch("model_tools.handle_function_call", side_effect=slow_mock):
+            raw = execute_code(
+                code=code,
+                task_id="test-concurrent",
+                enabled_tools=list(SANDBOX_ALLOWED_TOOLS),
+            )
+        result = json.loads(raw)
+        self.assertEqual(result["status"], "success", msg=result)
+        self.assertIn("OK 10/10", result["output"],
+                      msg=f"Concurrent tool calls mismatched: {result['output']!r}")
+
     def test_excluded_tool_returns_error(self):
         """Script calling a tool not in the allow-list gets an error from RPC."""
         code = """
diff --git a/tools/code_execution_tool.py b/tools/code_execution_tool.py
index c91907c4d12..ffcf726fcd5 100644
--- a/tools/code_execution_tool.py
+++ b/tools/code_execution_tool.py
@@ -224,9 +224,14 @@ def retry(fn, max_attempts=3, delay=2):
 
 _UDS_TRANSPORT_HEADER = '''\
 """Auto-generated Hermes tools RPC stubs."""
-import json, os, socket, shlex, time
+import json, os, socket, shlex, threading, time
 
 _sock = None
+# The RPC server handles a single client connection serially and has no
+# request-id in the protocol, so concurrent _call() invocations from multiple
+# threads (e.g. ThreadPoolExecutor) would race on the shared socket and get
+# each other's responses. Serialize the entire send+recv round-trip.
+_call_lock = threading.Lock()
 ''' + _COMMON_HELPERS + '''\
 
 def _connect():
@@ -239,17 +244,18 @@ def _connect():
 
 def _call(tool_name, args):
     """Send a tool call to the parent process and return the parsed result."""
-    conn = _connect()
     request = json.dumps({"tool": tool_name, "args": args}) + "\\n"
-    conn.sendall(request.encode())
-    buf = b""
-    while True:
-        chunk = conn.recv(65536)
-        if not chunk:
-            raise RuntimeError("Agent process disconnected")
-        buf += chunk
-        if buf.endswith(b"\\n"):
-            break
+    with _call_lock:
+        conn = _connect()
+        conn.sendall(request.encode())
+        buf = b""
+        while True:
+            chunk = conn.recv(65536)
+            if not chunk:
+                raise RuntimeError("Agent process disconnected")
+            buf += chunk
+            if buf.endswith(b"\\n"):
+                break
     raw = buf.decode().strip()
     result = json.loads(raw)
     if isinstance(result, str):
@@ -265,24 +271,30 @@ def _call(tool_name, args):
 
 _FILE_TRANSPORT_HEADER = '''\
 """Auto-generated Hermes tools RPC stubs (file-based transport)."""
-import json, os, shlex, tempfile, time
+import json, os, shlex, tempfile, threading, time
 
 _RPC_DIR = os.environ.get("HERMES_RPC_DIR") or os.path.join(tempfile.gettempdir(), "hermes_rpc")
 _seq = 0
+# `_seq += 1` is not atomic (read-modify-write), so concurrent _call()
+# invocations from multiple threads could allocate the same sequence number
+# and clobber each other's request files. Guard seq allocation with a lock.
+_seq_lock = threading.Lock()
 ''' + _COMMON_HELPERS + '''\
 
 def _call(tool_name, args):
     """Send a tool call request via file-based RPC and wait for response."""
     global _seq
-    _seq += 1
-    seq_str = f"{_seq:06d}"
+    with _seq_lock:
+        _seq += 1
+        seq = _seq
+    seq_str = f"{seq:06d}"
     req_file = os.path.join(_RPC_DIR, f"req_{seq_str}")
     res_file = os.path.join(_RPC_DIR, f"res_{seq_str}")
 
     # Write request atomically (write to .tmp, then rename)
     tmp = req_file + ".tmp"
     with open(tmp, "w") as f:
-        json.dump({"tool": tool_name, "args": args, "seq": _seq}, f)
+        json.dump({"tool": tool_name, "args": args, "seq": seq}, f)
     os.rename(tmp, req_file)
 
     # Wait for response with adaptive polling