From 886776f5fced4b312154bf770b0662128b23936d Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Tue, 4 Aug 2026 15:25:46 -0400
Subject: [PATCH] Canonicalize response cache arguments (#4753)

Co-authored-by: LHMQ878 <LHMQ878@users.noreply.github.com>
---
 .../fastmcp/server/middleware/caching.py      | 11 ++++-
 tests/server/middleware/test_caching.py       | 49 +++++++++++++++++++
 2 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/fastmcp_slim/fastmcp/server/middleware/caching.py b/fastmcp_slim/fastmcp/server/middleware/caching.py
index d470f1b90..e96948fb0 100644
--- a/fastmcp_slim/fastmcp/server/middleware/caching.py
+++ b/fastmcp_slim/fastmcp/server/middleware/caching.py
@@ -1,6 +1,7 @@
 """A middleware for response caching."""
 
 import hashlib
+import json
 from collections.abc import Sequence
 from logging import Logger
 from typing import Any, TypedDict
@@ -593,13 +594,19 @@ class ResponseCachingMiddleware(Middleware):
 
 
 def _get_arguments_str(arguments: dict[str, Any] | None) -> str:
-    """Get a string representation of the arguments."""
+    """Get a canonical string representation of the arguments."""
 
     if arguments is None:
         return "null"
 
     try:
-        return pydantic_core.to_json(value=arguments, fallback=str).decode()
+        return json.dumps(
+            pydantic_core.to_jsonable_python(arguments, fallback=str),
+            ensure_ascii=False,
+            separators=(",", ":"),
+            sort_keys=True,
+            default=str,
+        )
 
     except TypeError:
         return repr(arguments)
diff --git a/tests/server/middleware/test_caching.py b/tests/server/middleware/test_caching.py
index b45961bcf..fc7c1ffc2 100644
--- a/tests/server/middleware/test_caching.py
+++ b/tests/server/middleware/test_caching.py
@@ -4,6 +4,7 @@ import sys
 import tempfile
 import warnings
 from pathlib import Path
+from typing import Any
 from unittest.mock import AsyncMock, MagicMock
 
 import mcp_types
@@ -284,6 +285,42 @@ class TestResponseCachingMiddleware:
         )
         assert middleware1._matches_tool_cache_settings(tool_name=tool_name) is result
 
+    @pytest.mark.parametrize(
+        ("first", "second"),
+        [
+            ({"a": 5, "b": 3}, {"b": 3, "a": 5}),
+            ({"q": {"x": 1, "y": 2}}, {"q": {"y": 2, "x": 1}}),
+            ({"items": [{"x": 1, "y": 2}]}, {"items": [{"y": 2, "x": 1}]}),
+        ],
+        ids=["top level", "nested dict", "dict inside a list"],
+    )
+    def test_call_tool_cache_key_ignores_argument_order(
+        self, first: dict[str, Any], second: dict[str, Any]
+    ):
+        assert _make_call_tool_cache_key(
+            mcp_types.CallToolRequestParams(name="tool", arguments=first)
+        ) == _make_call_tool_cache_key(
+            mcp_types.CallToolRequestParams(name="tool", arguments=second)
+        )
+
+    def test_get_prompt_cache_key_ignores_argument_order(self):
+        assert _make_get_prompt_cache_key(
+            mcp_types.GetPromptRequestParams(
+                name="prompt", arguments={"a": "5", "b": "3"}
+            )
+        ) == _make_get_prompt_cache_key(
+            mcp_types.GetPromptRequestParams(
+                name="prompt", arguments={"b": "3", "a": "5"}
+            )
+        )
+
+    def test_call_tool_cache_key_distinguishes_arguments(self):
+        assert _make_call_tool_cache_key(
+            mcp_types.CallToolRequestParams(name="tool", arguments={"a": 5, "b": 3})
+        ) != _make_call_tool_cache_key(
+            mcp_types.CallToolRequestParams(name="tool", arguments={"a": 3, "b": 5})
+        )
+
 
 @pytest.mark.skipif(
     sys.platform == "win32",
@@ -424,6 +461,18 @@ class TestResponseCachingMiddlewareIntegration:
             )
             assert call_tool_result_one == call_tool_result_two
 
+    async def test_call_tool_with_reordered_arguments_hits_cache(
+        self,
+        caching_server: FastMCP,
+        tracking_calculator: TrackingCalculator,
+    ):
+        async with Client[FastMCPTransport](transport=caching_server) as client:
+            first = await client.call_tool("add", {"a": 5, "b": 3})
+            second = await client.call_tool("add", {"b": 3, "a": 5})
+
+        assert first == second
+        assert tracking_calculator.add_calls == 1
+
     async def test_call_tool_very_large_value(
         self,
         caching_server: FastMCP,