From a1a867de376a0d896add776f1c774ff8bcfc4643 Mon Sep 17 00:00:00 2001
From: Guillaume Raille <guillaume.raille@gmail.com>
Date: Wed, 17 Sep 2025 10:05:39 +0200
Subject: [PATCH 1/2] add a test reproducing issue #71

---
 tests/test_crewai_adapter.py | 66 ++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/tests/test_crewai_adapter.py b/tests/test_crewai_adapter.py
index 06281c6..aee8150 100644
--- a/tests/test_crewai_adapter.py
+++ b/tests/test_crewai_adapter.py
@@ -430,3 +430,69 @@ def test_multiple_content_response_handling(multiple_content_server_script):
         # Should return string representation of list of text contents
         expected = str(["First response", "Second response", "Third response"])
         assert result == expected
+
+
+@pytest.fixture
+def empty_union_server_script():
+    """Server that returns a tool with a schema containing only null types in anyOf."""
+    return dedent(
+        """
+        import mcp.types as types
+        from mcp.server.lowlevel import Server
+        from mcp.server.stdio import stdio_server
+        import anyio
+
+        app = Server("empty-union-server")
+
+        @app.list_tools()
+        async def list_tools() -> list[types.Tool]:
+            return [
+                types.Tool(
+                    name="problematic_tool",
+                    description="Tool with empty union after null filtering",
+                    inputSchema={
+                        "type": "object",
+                        "properties": {
+                            "nullable_only": {
+                                "anyOf": [
+                                    {"type": "null"}
+                                ]
+                            },
+                            "type_array_null_only": {
+                                "type": ["null"]
+                            }
+                        },
+                    },
+                )
+            ]
+
+        @app.call_tool()
+        async def call_tool(name: str, arguments: dict | None) -> list[types.TextContent]:
+            return [types.TextContent(type="text", text="Success")]
+
+        async def arun():
+            async with stdio_server() as streams:
+                await app.run(
+                    streams[0], streams[1], app.create_initialization_options()
+                )
+
+        anyio.run(arun)
+        """
+    )
+
+
+def test_empty_union_type_error(empty_union_server_script):
+    """Test that empty unions (anyOf with only null types) raise TypeError.
+
+    This test reproduces issue #71 where schemas with anyOf containing only
+    null types cause 'Cannot take a Union of no types' error.
+    """
+    with MCPAdapt(
+        StdioServerParameters(
+            command="uv",
+            args=["run", "python", "-c", empty_union_server_script],
+        ),
+        CrewAIAdapter(),
+    ) as tools:
+        # Should raise TypeError before reaching here
+        pass

From c1e11e846c1caf5150f2e5ada946b51fd8cf9159 Mon Sep 17 00:00:00 2001
From: Guillaume Raille <guillaume.raille@gmail.com>
Date: Wed, 17 Sep 2025 10:10:06 +0200
Subject: [PATCH 2/2] fixes Cannot take a Union of no types. Fixes #71

---
 src/mcpadapt/utils/modeling.py | 15 +++++++++++----
 tests/test_crewai_adapter.py   |  7 +++----
 2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/src/mcpadapt/utils/modeling.py b/src/mcpadapt/utils/modeling.py
index 49fbf76..6ac704c 100644
--- a/src/mcpadapt/utils/modeling.py
+++ b/src/mcpadapt/utils/modeling.py
@@ -132,7 +132,12 @@ def get_field_type(field_name: str, field_schema: Dict[str, Any], required: set)
 
                     types.append(created_models[ref_name])
 
-            field_type = types[0] if len(types) == 1 else Union[tuple(types)]  # type: ignore
+            if len(types) == 0:
+                field_type = Any
+            elif len(types) == 1:
+                field_type = types[0]
+            else:
+                field_type = Union[tuple(types)]  # type: ignore
             default = field_schema.get("default")
             is_required = field_name in required and default is None
 
@@ -158,10 +163,12 @@ def get_field_type(field_name: str, field_schema: Dict[str, Any], required: set)
                         mapped_type = json_type_mapping.get(t, Any)
                         types.append(mapped_type)
 
-                if len(types) > 1:
-                    field_type = Union[tuple(types)]  # type: ignore
+                if len(types) == 0:
+                    field_type = Any
+                elif len(types) == 1:
+                    field_type = types[0]
                 else:
-                    field_type = types[0] if types else Any
+                    field_type = Union[tuple(types)]  # type: ignore
             else:
                 # Original code for simple types
                 field_type = json_type_mapping.get(json_type, Any)  # type: ignore
diff --git a/tests/test_crewai_adapter.py b/tests/test_crewai_adapter.py
index aee8150..679e6d7 100644
--- a/tests/test_crewai_adapter.py
+++ b/tests/test_crewai_adapter.py
@@ -482,9 +482,9 @@ async def arun():
 
 
 def test_empty_union_type_error(empty_union_server_script):
-    """Test that empty unions (anyOf with only null types) raise TypeError.
+    """Test that empty unions (anyOf with only null types) do not raise TypeError.
 
-    This test reproduces issue #71 where schemas with anyOf containing only
+    This test reproduced issue #71 where schemas with anyOf containing only
     null types cause 'Cannot take a Union of no types' error.
     """
     with MCPAdapt(
@@ -494,5 +494,4 @@ def test_empty_union_type_error(empty_union_server_script):
         ),
         CrewAIAdapter(),
     ) as tools:
-        # Should raise TypeError before reaching here
-        pass
+        assert len(tools) == 1