From 98bf8557d510943d84568b5ca170c6a8ae87c6bc Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Wed, 5 Aug 2026 18:24:52 -0400
Subject: [PATCH] Align dataclass output schemas

---
 .../fastmcp/tools/function_parsing.py         | 11 ++++
 tests/tools/tool/test_results.py              | 63 +++++++++++++++++++
 2 files changed, 74 insertions(+)

diff --git a/fastmcp_slim/fastmcp/tools/function_parsing.py b/fastmcp_slim/fastmcp/tools/function_parsing.py
index cb2eee19f..72d594794 100644
--- a/fastmcp_slim/fastmcp/tools/function_parsing.py
+++ b/fastmcp_slim/fastmcp/tools/function_parsing.py
@@ -164,6 +164,15 @@ class _ToolOutputSchemaGenerator(GenerateJsonSchema):
         finally:
             self.by_alias = previous_by_alias
 
+    def dataclass_schema(self, schema: core_schema.DataclassSchema) -> JsonSchemaValue:
+        previous_by_alias = self.by_alias
+        configured = (schema.get("config") or {}).get("serialize_by_alias")
+        self.by_alias = False if configured is None else configured
+        try:
+            return super().dataclass_schema(schema)
+        finally:
+            self.by_alias = previous_by_alias
+
 
 T = TypeVarExt("T", default=Any)
 
@@ -424,6 +433,7 @@ class ParsedFunction:
                 type_adapter = get_cached_typeadapter(clean_output_type)
                 base_schema = type_adapter.json_schema(
                     mode="serialization",
+                    by_alias=False,
                     schema_generator=_ToolOutputSchemaGenerator,
                 )
 
@@ -437,6 +447,7 @@ class ParsedFunction:
                     wrapped_adapter = get_cached_typeadapter(wrapped_type)
                     output_schema = wrapped_adapter.json_schema(
                         mode="serialization",
+                        by_alias=False,
                         schema_generator=_ToolOutputSchemaGenerator,
                     )
                     output_schema["x-fastmcp-wrap-result"] = True
diff --git a/tests/tools/tool/test_results.py b/tests/tools/tool/test_results.py
index dce6b8a12..2165070c7 100644
--- a/tests/tools/tool/test_results.py
+++ b/tests/tools/tool/test_results.py
@@ -5,6 +5,8 @@ from typing import Annotated, Any
 import pytest
 from mcp_types import CallToolResult, TextContent
 from pydantic import BaseModel, ConfigDict, Field
+from pydantic.dataclasses import dataclass as pydantic_dataclass
+from typing_extensions import TypedDict
 
 from fastmcp import Client, FastMCP
 from fastmcp.tools.base import Tool, ToolResult
@@ -437,6 +439,67 @@ class TestSerializeByAlias:
             "aliased": {"aliasedValue": "aliased"},
         }
 
+    async def test_dataclass_uses_pydantic_alias_default(self):
+        """A dataclass schema uses field names when aliases are not enabled."""
+
+        @dataclass
+        class Output:
+            value: Annotated[str, Field(serialization_alias="dataValue")]
+
+        mcp = FastMCP()
+
+        @mcp.tool
+        def get_output() -> Output:
+            return Output(value="data")
+
+        async with Client(mcp) as client:
+            tools = {tool.name: tool for tool in await client.list_tools()}
+            result = await client.call_tool("get_output", {})
+
+        assert set(tools["get_output"].output_schema["properties"]) == {"value"}  # type: ignore[index]
+        assert result.structured_content == {"value": "data"}
+
+    async def test_pydantic_dataclass_can_enable_aliases(self):
+        """A Pydantic dataclass can opt in to serialization aliases."""
+
+        @pydantic_dataclass(config=ConfigDict(serialize_by_alias=True))
+        class Output:
+            value: Annotated[str, Field(serialization_alias="dataValue")]
+
+        mcp = FastMCP()
+
+        @mcp.tool
+        def get_output() -> Output:
+            return Output(value="data")
+
+        async with Client(mcp) as client:
+            tools = {tool.name: tool for tool in await client.list_tools()}
+            result = await client.call_tool("get_output", {})
+
+        assert set(tools["get_output"].output_schema["properties"]) == {  # type: ignore[index]
+            "dataValue"
+        }
+        assert result.structured_content == {"dataValue": "data"}
+
+    async def test_typed_dict_uses_pydantic_alias_default(self):
+        """A TypedDict schema uses the field names emitted by Pydantic."""
+
+        class Output(TypedDict):
+            value: Annotated[str, Field(serialization_alias="dataValue")]
+
+        mcp = FastMCP()
+
+        @mcp.tool
+        def get_output() -> Output:
+            return {"value": "data"}
+
+        async with Client(mcp) as client:
+            tools = {tool.name: tool for tool in await client.list_tools()}
+            result = await client.call_tool("get_output", {})
+
+        assert set(tools["get_output"].output_schema["properties"]) == {"value"}  # type: ignore[index]
+        assert result.structured_content == {"value": "data"}
+
     async def test_serialize_by_alias_true_uses_alias(self):
         """serialize_by_alias=True emits aliases."""