diff --git a/src/ai/types/tools.py b/src/ai/types/tools.py
index 3016909d..a83e5974 100644
--- a/src/ai/types/tools.py
+++ b/src/ai/types/tools.py
@@ -20,9 +20,26 @@ class FunctionToolArgs(pydantic.BaseModel):
 class Tool(pydantic.BaseModel):
     kind: Literal["function", "provider"]
     name: str
-    args: pydantic.BaseModel
+    args: pydantic.SerializeAsAny[pydantic.BaseModel]
     require_approval: bool = False
 
+    # we're using the same type for regular and provider-side tools.
+    # because of that args can be either FunctionToolArgs or some
+    # provider-specific type.
+    @pydantic.model_validator(mode="before")
+    @classmethod
+    def validate_args_input(cls, data: Any) -> Any:
+        if (
+            isinstance(data, dict)
+            and data.get("kind") == "function"
+            and isinstance(data.get("args"), dict)
+        ):
+            return {
+                **data,
+                "args": FunctionToolArgs.model_validate(data["args"]),
+            }
+        return data
+
     @pydantic.model_validator(mode="after")
     def validate_args_shape(self) -> Self:
         match self.kind:
diff --git a/tests/types/test_tools.py b/tests/types/test_tools.py
new file mode 100644
index 00000000..5933107f
--- /dev/null
+++ b/tests/types/test_tools.py
@@ -0,0 +1,32 @@
+"""Focused tests for tool model serialization."""
+
+from __future__ import annotations
+
+from ai.types import tools
+
+
+def test_function_tool_args_round_trip_through_json_dump() -> None:
+    tool = tools.Tool(
+        kind="function",
+        name="weather",
+        args=tools.FunctionToolArgs(
+            description="Get weather",
+            params={
+                "type": "object",
+                "properties": {"city": {"type": "string"}},
+            },
+        ),
+    )
+
+    data = tool.model_dump(mode="json")
+    restored = tools.Tool.model_validate(data)
+
+    assert data["args"] == {
+        "description": "Get weather",
+        "params": {
+            "type": "object",
+            "properties": {"city": {"type": "string"}},
+        },
+    }
+    assert isinstance(restored.args, tools.FunctionToolArgs)
+    assert restored.args.description == "Get weather"