create a setting for legacy parsing

docs/servers/tools.mdx

@@ -708,3 +708,13 @@ The duplicate behavior options are:
 -   `"error"`: Raises a `ValueError`, preventing the duplicate registration.
 -   `"replace"`: Silently replaces the existing tool with the new one.
 -   `"ignore"`: Keeps the original tool and ignores the new registration attempt.
+
+### Legacy JSON Parsing
+
+<VersionBadge version="2.2.10" />
+
+FastMCP 1.0 and < 2.2.10 relied on a crutch that attempted to work around LLM limitations by automatically parsing stringified JSON in tool arguments (e.g., converting `"[1,2,3]"` to `[1,2,3]`). As of FastMCP 2.2.10, this behavior is disabled by default because it circumvents type validation and can lead to unexpected type coercion issues (e.g. parsing "true" as a bool and attempting to call a tool that expected a string, which would fail type validation).
+
+Most modern LLMs correctly format JSON, but if working with models that unnecessarily stringify JSON (as was the case with Claude Desktop in late 2024), you can re-enable this behavior on your server by setting the environment variable `FASTMCP_TOOL_ATTEMPT_PARSE_JSON_ARGS=1`.
+
+We strongly recommend leaving this disabled unless necessary.

src/fastmcp/settings.py

@@ -27,6 +27,16 @@ class Settings(BaseSettings):
 
     test_mode: bool = False
     log_level: LOG_LEVEL = "INFO"
+    tool_attempt_parse_json_args: bool = Field(
+        default=False,
+        description="""
+        Note: this enables a legacy behavior. If True, will attempt to parse
+        stringified JSON lists and objects strings in tool arguments before
+        passing them to the tool. This is an old behavior that can create
+        unexpected type coercion issues, but may be helpful for less powerful
+        LLMs that stringify JSON instead of passing actual lists and objects.
+        Defaults to False.""",
+    )
 
 
 class ServerSettings(BaseSettings):
@@ -83,3 +93,6 @@ class ClientSettings(BaseSettings):
     )
 
     log_level: LOG_LEVEL = Field(default_factory=lambda: Settings().log_level)
+
+
+settings = Settings()

src/fastmcp/tools/tool.py

@@ -10,6 +10,7 @@ from mcp.types import EmbeddedResource, ImageContent, TextContent, ToolAnnotatio
 from mcp.types import Tool as MCPTool
 from pydantic import BaseModel, BeforeValidator, Field
 
+import fastmcp
 from fastmcp.exceptions import ToolError
 from fastmcp.utilities.json_schema import prune_params
 from fastmcp.utilities.logging import get_logger
@@ -107,6 +108,7 @@ class Tool(BaseModel):
         context: Context[ServerSessionT, LifespanContextT] | None = None,
     ) -> list[TextContent | ImageContent | EmbeddedResource]:
         """Run the tool with arguments."""
+
         try:
             injected_args = (
                 {self.context_kwarg: context} if self.context_kwarg is not None else {}
@@ -114,22 +116,29 @@ class Tool(BaseModel):
 
             parsed_args = arguments.copy()
 
-            # Pre-parse data from JSON in order to handle cases like `["a", "b", "c"]`
-            # being passed in as JSON inside a string rather than an actual list.
-            #
-            # Claude desktop is prone to this - in fact it seems incapable of NOT doing
-            # this. For sub-models, it tends to pass dicts (JSON objects) as JSON strings,
-            # which can be pre-parsed here.
-            for param_name in self.parameters["properties"]:
-                if isinstance(parsed_args.get(param_name, None), str):
-                    try:
-                        parsed_args[param_name] = json.loads(parsed_args[param_name])
-                    except json.JSONDecodeError:
-                        pass
-
-            type_adapter = get_cached_typeadapter(
-                self.fn, config=frozenset([("coerce_numbers_to_str", True)])
-            )
+            if fastmcp.settings.settings.tool_attempt_parse_json_args:
+                # Pre-parse data from JSON in order to handle cases like `["a", "b", "c"]`
+                # being passed in as JSON inside a string rather than an actual list.
+                #
+                # Claude desktop is prone to this - in fact it seems incapable of NOT doing
+                # this. For sub-models, it tends to pass dicts (JSON objects) as JSON strings,
+                # which can be pre-parsed here.
+                signature = inspect.signature(self.fn)
+                for param_name in self.parameters["properties"]:
+                    if param_name not in signature.parameters:
+                        continue
+                    arg = parsed_args.get(param_name, None)
+                    if isinstance(arg, str) and signature.parameters[
+                        param_name
+                    ].annotation not in (int, float, bool):
+                        # if arg.strip().startswith("{") or arg.strip().startswith("["):
+                        try:
+                            parsed_args[param_name] = json.loads(arg)
+
+                        except json.JSONDecodeError:
+                            pass
+
+            type_adapter = get_cached_typeadapter(self.fn)
             result = type_adapter.validate_python(parsed_args | injected_args)
             if inspect.isawaitable(result):
                 result = await result

src/fastmcp/utilities/tests.py

@@ -0,0 +1,41 @@
+import copy
+from contextlib import contextmanager
+from typing import Any
+
+from fastmcp.settings import settings
+
+
+@contextmanager
+def temporary_settings(**kwargs: Any):
+    """
+    Temporarily override ControlFlow setting values.
+
+    Args:
+        **kwargs: The settings to override, including nested settings.
+
+    Example:
+        Temporarily override a setting:
+        ```python
+        import fastmcp
+        from fastmcp.utilities.tests import temporary_settings
+
+        with temporary_settings(log_level='DEBUG'):
+            assert fastmcp.settings.settings.log_level == 'DEBUG'
+        assert fastmcp.settings.settings.log_level == 'INFO'
+        ```
+    """
+    old_settings = copy.deepcopy(settings.model_dump())
+
+    try:
+        # apply the new settings
+        for attr, value in kwargs.items():
+            if not hasattr(settings, attr):
+                raise AttributeError(f"Setting {attr} does not exist.")
+            setattr(settings, attr, value)
+        yield
+
+    finally:
+        # restore the old settings
+        for attr in kwargs:
+            if hasattr(settings, attr):
+                setattr(settings, attr, old_settings[attr])

tests/server/test_server_interactions.py

@@ -349,81 +349,6 @@ class TestToolParameters:
             assert isinstance(result[0], TextContent)
             assert result[0].text == "true"
 
-    async def test_tool_list_coercion(self):
-        """Test JSON string to collection type coercion."""
-        mcp = FastMCP()
-
-        @mcp.tool()
-        def process_list(items: list[int]) -> int:
-            return sum(items)
-
-        async with Client(mcp) as client:
-            # JSON array string should be coerced to list
-            result = await client.call_tool(
-                "process_list", {"items": "[1, 2, 3, 4, 5]"}
-            )
-            assert isinstance(result[0], TextContent)
-            assert result[0].text == "15"
-
-    async def test_tool_list_coercion_error(self):
-        """Test that a list coercion error is raised if the input is not a valid list."""
-        mcp = FastMCP()
-
-        @mcp.tool()
-        def process_list(items: list[int]) -> int:
-            return sum(items)
-
-        async with Client(mcp) as client:
-            with pytest.raises(
-                ClientError,
-                match="Input should be a valid list",
-            ):
-                await client.call_tool("process_list", {"items": "['a', 'b', 3]"})
-
-    async def test_tool_dict_coercion(self):
-        """Test JSON string to dict type coercion."""
-        mcp = FastMCP()
-
-        @mcp.tool()
-        def process_dict(data: dict[str, int]) -> int:
-            return sum(data.values())
-
-        async with Client(mcp) as client:
-            # JSON object string should be coerced to dict
-            result = await client.call_tool(
-                "process_dict", {"data": '{"a": 1, "b": "2", "c": 3}'}
-            )
-            assert isinstance(result[0], TextContent)
-            assert result[0].text == "6"
-
-    async def test_tool_set_coercion(self):
-        """Test JSON string to set type coercion."""
-        mcp = FastMCP()
-
-        @mcp.tool()
-        def process_set(items: set[int]) -> int:
-            assert isinstance(items, set)
-            return sum(items)
-
-        async with Client(mcp) as client:
-            result = await client.call_tool("process_set", {"items": "[1, 2, 3, 4, 5]"})
-            assert isinstance(result[0], TextContent)
-            assert result[0].text == "15"
-
-    async def test_tool_tuple_coercion(self):
-        """Test JSON string to tuple type coercion."""
-        mcp = FastMCP()
-
-        @mcp.tool()
-        def process_tuple(items: tuple[int, str]) -> int:
-            assert isinstance(items, tuple)
-            return items[0] + len(items[1])
-
-        async with Client(mcp) as client:
-            result = await client.call_tool("process_tuple", {"items": '["1", "two"]'})
-            assert isinstance(result[0], TextContent)
-            assert result[0].text == "4"
-
     async def test_annotated_field_validation(self):
         mcp = FastMCP()
 

tests/tools/test_tool.py

@@ -2,8 +2,11 @@ import pytest
 from mcp.types import ImageContent, TextContent
 from pydantic import BaseModel
 
-from fastmcp import Image
+from fastmcp import FastMCP, Image
+from fastmcp.client import Client
+from fastmcp.exceptions import ClientError
 from fastmcp.tools.tool import Tool
+from fastmcp.utilities.tests import temporary_settings
 
 
 class TestToolFromFunction:
@@ -150,9 +153,14 @@ class TestToolFromFunction:
             x: int = 10
 
 
-class TestToolJsonParsing:
+class TestLegacyToolJsonParsing:
     """Tests for Tool's JSON pre-parsing functionality."""
 
+    @pytest.fixture(autouse=True)
+    def enable_legacy_json_parsing(self):
+        with temporary_settings(tool_attempt_parse_json_args=True):
+            yield
+
     async def test_json_string_arguments(self):
         """Test that JSON string arguments are parsed and validated correctly"""
 
@@ -264,3 +272,78 @@ class TestToolJsonParsing:
         invalid_json = '{"x": 1, "y": {"invalid": "hello"}}'
         with pytest.raises(Exception):
             await tool.run({"data": invalid_json})
+
+    async def test_tool_list_coercion(self):
+        """Test JSON string to collection type coercion."""
+        mcp = FastMCP()
+
+        @mcp.tool()
+        def process_list(items: list[int]) -> int:
+            return sum(items)
+
+        async with Client(mcp) as client:
+            # JSON array string should be coerced to list
+            result = await client.call_tool(
+                "process_list", {"items": "[1, 2, 3, 4, 5]"}
+            )
+            assert isinstance(result[0], TextContent)
+            assert result[0].text == "15"
+
+    async def test_tool_list_coercion_error(self):
+        """Test that a list coercion error is raised if the input is not a valid list."""
+        mcp = FastMCP()
+
+        @mcp.tool()
+        def process_list(items: list[int]) -> int:
+            return sum(items)
+
+        async with Client(mcp) as client:
+            with pytest.raises(
+                ClientError,
+                match="Input should be a valid list",
+            ):
+                await client.call_tool("process_list", {"items": "['a', 'b', 3]"})
+
+    async def test_tool_dict_coercion(self):
+        """Test JSON string to dict type coercion."""
+        mcp = FastMCP()
+
+        @mcp.tool()
+        def process_dict(data: dict[str, int]) -> int:
+            return sum(data.values())
+
+        async with Client(mcp) as client:
+            # JSON object string should be coerced to dict
+            result = await client.call_tool(
+                "process_dict", {"data": '{"a": 1, "b": "2", "c": 3}'}
+            )
+            assert isinstance(result[0], TextContent)
+            assert result[0].text == "6"
+
+    async def test_tool_set_coercion(self):
+        """Test JSON string to set type coercion."""
+        mcp = FastMCP()
+
+        @mcp.tool()
+        def process_set(items: set[int]) -> int:
+            assert isinstance(items, set)
+            return sum(items)
+
+        async with Client(mcp) as client:
+            result = await client.call_tool("process_set", {"items": "[1, 2, 3, 4, 5]"})
+            assert isinstance(result[0], TextContent)
+            assert result[0].text == "15"
+
+    async def test_tool_tuple_coercion(self):
+        """Test JSON string to tuple type coercion."""
+        mcp = FastMCP()
+
+        @mcp.tool()
+        def process_tuple(items: tuple[int, str]) -> int:
+            assert isinstance(items, tuple)
+            return items[0] + len(items[1])
+
+        async with Client(mcp) as client:
+            result = await client.call_tool("process_tuple", {"items": '["1", "two"]'})
+            assert isinstance(result[0], TextContent)
+            assert result[0].text == "4"

tests/tools/test_tool_manager.py

@@ -14,6 +14,7 @@ from fastmcp import Context, FastMCP, Image
 from fastmcp.exceptions import NotFoundError, ToolError
 from fastmcp.tools import ToolManager
 from fastmcp.tools.tool import Tool
+from fastmcp.utilities.tests import temporary_settings
 
 
 class TestAddTools:
@@ -320,14 +321,6 @@ class TestCallTools:
 
         manager = ToolManager()
         manager.add_tool_from_fn(sum_vals)
-        # Try both with plain list and with JSON list
-
-        result = await manager.call_tool("sum_vals", {"vals": "[1, 2, 3]"})
-        assert isinstance(result, list)
-        assert len(result) == 1
-        assert isinstance(result[0], TextContent)
-        assert result[0].text == "6"
-        assert json.loads(result[0].text) == 6
 
         result = await manager.call_tool("sum_vals", {"vals": [1, 2, 3]})
         assert isinstance(result, list)
@@ -336,6 +329,24 @@ class TestCallTools:
         assert result[0].text == "6"
         assert json.loads(result[0].text) == 6
 
+    async def test_call_tool_with_list_int_input_legacy_behavior(self):
+        """Legacy behavior -- parse a stringified JSON object"""
+
+        def sum_vals(vals: list[int]) -> int:
+            return sum(vals)
+
+        manager = ToolManager()
+        manager.add_tool_from_fn(sum_vals)
+        # Try both with plain list and with JSON list
+
+        with temporary_settings(tool_attempt_parse_json_args=True):
+            result = await manager.call_tool("sum_vals", {"vals": "[1, 2, 3]"})
+            assert isinstance(result, list)
+            assert len(result) == 1
+            assert isinstance(result[0], TextContent)
+            assert result[0].text == "6"
+            assert json.loads(result[0].text) == 6
+
     async def test_call_tool_with_list_str_or_str_input(self):
         def concat_strs(vals: list[str] | str) -> str:
             return vals if isinstance(vals, str) else "".join(vals)
@@ -350,23 +361,33 @@ class TestCallTools:
         assert isinstance(result[0], TextContent)
         assert result[0].text == "abc"
 
-        result = await manager.call_tool("concat_strs", {"vals": '["a", "b", "c"]'})
-        assert isinstance(result, list)
-        assert len(result) == 1
-        assert isinstance(result[0], TextContent)
-        assert result[0].text == "abc"
-
         result = await manager.call_tool("concat_strs", {"vals": "a"})
         assert isinstance(result, list)
         assert len(result) == 1
         assert isinstance(result[0], TextContent)
         assert result[0].text == "a"
 
-        result = await manager.call_tool("concat_strs", {"vals": '"a"'})
-        assert isinstance(result, list)
-        assert len(result) == 1
-        assert isinstance(result[0], TextContent)
-        assert result[0].text == "a"
+    async def test_call_tool_with_list_str_or_str_input_legacy_behavior(self):
+        """Legacy behavior -- parse a stringified JSON object"""
+
+        def concat_strs(vals: list[str] | str) -> str:
+            return vals if isinstance(vals, str) else "".join(vals)
+
+        manager = ToolManager()
+        manager.add_tool_from_fn(concat_strs)
+
+        with temporary_settings(tool_attempt_parse_json_args=True):
+            result = await manager.call_tool("concat_strs", {"vals": '["a", "b", "c"]'})
+            assert isinstance(result, list)
+            assert len(result) == 1
+            assert isinstance(result[0], TextContent)
+            assert result[0].text == "abc"
+
+            result = await manager.call_tool("concat_strs", {"vals": '"a"'})
+            assert isinstance(result, list)
+            assert len(result) == 1
+            assert isinstance(result[0], TextContent)
+            assert result[0].text == "a"
 
     async def test_call_tool_with_complex_model(self):
         class MyShrimpTank(BaseModel):

tests/utilities/test_tests.py

@@ -0,0 +1,9 @@
+import fastmcp
+from fastmcp.utilities.tests import temporary_settings
+
+
+class TestTemporarySettings:
+    def test_temporary_settings(self):
+        with temporary_settings(log_level="DEBUG"):
+            assert fastmcp.settings.settings.log_level == "DEBUG"
+        assert fastmcp.settings.settings.log_level == "INFO"