diff --git a/langchain/agents/agent.py b/langchain/agents/agent.py
index c675b558..0099a035 100644
--- a/langchain/agents/agent.py
+++ b/langchain/agents/agent.py
@@ -454,7 +454,11 @@ class Agent(BaseSingleActionAgent):
     @classmethod
     def _validate_tools(cls, tools: Sequence[BaseTool]) -> None:
         """Validate that appropriate tools are passed in."""
-        pass
+        for tool in tools:
+            if not tool.is_single_input:
+                raise ValueError(
+                    f"{cls.__name__} does not support multi-input tool {tool.name}."
+                )
 
     @classmethod
     @abstractmethod
diff --git a/langchain/agents/mrkl/base.py b/langchain/agents/mrkl/base.py
index 4bb1d519..60f7c981 100644
--- a/langchain/agents/mrkl/base.py
+++ b/langchain/agents/mrkl/base.py
@@ -122,6 +122,7 @@ class ZeroShotAgent(Agent):
 
     @classmethod
     def _validate_tools(cls, tools: Sequence[BaseTool]) -> None:
+        super()._validate_tools(tools)
         for tool in tools:
             if tool.description is None:
                 raise ValueError(
diff --git a/langchain/agents/react/base.py b/langchain/agents/react/base.py
index 08d27d4d..afb199c2 100644
--- a/langchain/agents/react/base.py
+++ b/langchain/agents/react/base.py
@@ -37,6 +37,7 @@ class ReActDocstoreAgent(Agent):
 
     @classmethod
     def _validate_tools(cls, tools: Sequence[BaseTool]) -> None:
+        super()._validate_tools(tools)
         if len(tools) != 2:
             raise ValueError(f"Exactly two tools must be specified, but got {tools}")
         tool_names = {tool.name for tool in tools}
@@ -119,6 +120,7 @@ class ReActTextWorldAgent(ReActDocstoreAgent):
 
     @classmethod
     def _validate_tools(cls, tools: Sequence[BaseTool]) -> None:
+        super()._validate_tools(tools)
         if len(tools) != 1:
             raise ValueError(f"Exactly one tool must be specified, but got {tools}")
         tool_names = {tool.name for tool in tools}
diff --git a/langchain/agents/self_ask_with_search/base.py b/langchain/agents/self_ask_with_search/base.py
index 297b0345..5e1905b5 100644
--- a/langchain/agents/self_ask_with_search/base.py
+++ b/langchain/agents/self_ask_with_search/base.py
@@ -36,6 +36,7 @@ class SelfAskWithSearchAgent(Agent):
 
     @classmethod
     def _validate_tools(cls, tools: Sequence[BaseTool]) -> None:
+        super()._validate_tools(tools)
         if len(tools) != 1:
             raise ValueError(f"Exactly one tool must be specified, but got {tools}")
         tool_names = {tool.name for tool in tools}
diff --git a/langchain/tools/base.py b/langchain/tools/base.py
index 54d8db5d..95aae536 100644
--- a/langchain/tools/base.py
+++ b/langchain/tools/base.py
@@ -115,6 +115,11 @@ class BaseTool(ABC, BaseModel, metaclass=ToolMetaclass):
         extra = Extra.forbid
         arbitrary_types_allowed = True
 
+    @property
+    def is_single_input(self) -> bool:
+        """Whether the tool only accepts a single input."""
+        return len(self.args) == 1
+
     @property
     def args(self) -> dict:
         if self.args_schema is not None:
@@ -148,11 +153,11 @@ class BaseTool(ABC, BaseModel, metaclass=ToolMetaclass):
         return callback_manager or get_callback_manager()
 
     @abstractmethod
-    def _run(self, *args: Any, **kwargs: Any) -> str:
+    def _run(self, *args: Any, **kwargs: Any) -> Any:
         """Use the tool."""
 
     @abstractmethod
-    async def _arun(self, *args: Any, **kwargs: Any) -> str:
+    async def _arun(self, *args: Any, **kwargs: Any) -> Any:
         """Use the tool asynchronously."""
 
     def run(
@@ -183,7 +188,7 @@ class BaseTool(ABC, BaseModel, metaclass=ToolMetaclass):
             self.callback_manager.on_tool_error(e, verbose=verbose_)
             raise e
         self.callback_manager.on_tool_end(
-            observation, verbose=verbose_, color=color, name=self.name, **kwargs
+            str(observation), verbose=verbose_, color=color, name=self.name, **kwargs
         )
         return observation
 
@@ -194,7 +199,7 @@ class BaseTool(ABC, BaseModel, metaclass=ToolMetaclass):
         start_color: Optional[str] = "green",
         color: Optional[str] = "green",
         **kwargs: Any,
-    ) -> str:
+    ) -> Any:
         """Run the tool asynchronously."""
         self._parse_input(tool_input)
         if not self.verbose and verbose is not None:
@@ -229,7 +234,11 @@ class BaseTool(ABC, BaseModel, metaclass=ToolMetaclass):
             raise e
         if self.callback_manager.is_async:
             await self.callback_manager.on_tool_end(
-                observation, verbose=verbose_, color=color, name=self.name, **kwargs
+                str(observation),
+                verbose=verbose_,
+                color=color,
+                name=self.name,
+                **kwargs,
             )
         else:
             self.callback_manager.on_tool_end(
@@ -237,6 +246,6 @@ class BaseTool(ABC, BaseModel, metaclass=ToolMetaclass):
             )
         return observation
 
-    def __call__(self, tool_input: str) -> str:
+    def __call__(self, tool_input: Union[str, dict]) -> Any:
         """Make tool callable."""
         return self.run(tool_input)
diff --git a/tests/unit_tests/agents/test_tools.py b/tests/unit_tests/agents/test_tools.py
index d76011dc..6cac896f 100644
--- a/tests/unit_tests/agents/test_tools.py
+++ b/tests/unit_tests/agents/test_tools.py
@@ -2,11 +2,19 @@
 from datetime import datetime
 from functools import partial
 from typing import Optional, Type, Union
+from unittest.mock import MagicMock
 
 import pydantic
 import pytest
 from pydantic import BaseModel
 
+from langchain.agents.agent import Agent
+from langchain.agents.chat.base import ChatAgent
+from langchain.agents.conversational.base import ConversationalAgent
+from langchain.agents.conversational_chat.base import ConversationalChatAgent
+from langchain.agents.mrkl.base import ZeroShotAgent
+from langchain.agents.react.base import ReActDocstoreAgent, ReActTextWorldAgent
+from langchain.agents.self_ask_with_search.base import SelfAskWithSearchAgent
 from langchain.agents.tools import Tool, tool
 from langchain.tools.base import BaseTool, SchemaAnnotationError
 
@@ -152,6 +160,7 @@ def test_decorated_function_schema_equivalent() -> None:
         return f"{arg1} {arg2} {arg3}"
 
     assert isinstance(structured_tool_input, Tool)
+    assert structured_tool_input.args_schema is not None
     assert (
         structured_tool_input.args_schema.schema()["properties"]
         == _MockSchema.schema()["properties"]
@@ -309,33 +318,38 @@ def test_tool_with_kwargs() -> None:
 
     @tool(return_direct=True)
     def search_api(
-        arg_1: float,
+        arg_0: str,
+        arg_1: float = 4.3,
         ping: str = "hi",
     ) -> str:
         """Search the API for the query."""
-        return f"arg_1={arg_1}, ping={ping}"
+        return f"arg_0={arg_0}, arg_1={arg_1}, ping={ping}"
 
     assert isinstance(search_api, Tool)
     result = search_api.run(
         tool_input={
+            "arg_0": "foo",
             "arg_1": 3.2,
             "ping": "pong",
         }
     )
-    assert result == "arg_1=3.2, ping=pong"
+    assert result == "arg_0=foo, arg_1=3.2, ping=pong"
 
     result = search_api.run(
         tool_input={
-            "arg_1": 3.2,
+            "arg_0": "foo",
         }
     )
-    assert result == "arg_1=3.2, ping=hi"
+    assert result == "arg_0=foo, arg_1=4.3, ping=hi"
+    # For backwards compatibility, we still accept a single str arg
+    result = search_api.run("foobar")
+    assert result == "arg_0=foobar, arg_1=4.3, ping=hi"
 
 
 def test_missing_docstring() -> None:
     """Test error is raised when docstring is missing."""
     # expect to throw a value error if theres no docstring
-    with pytest.raises(AssertionError):
+    with pytest.raises(AssertionError, match="Function must have a docstring"):
 
         @tool
         def search_api(query: str) -> str:
@@ -348,11 +362,13 @@ def test_create_tool_positional_args() -> None:
     assert test_tool("foo") == "foo"
     assert test_tool.name == "test_name"
     assert test_tool.description == "test_description"
+    assert test_tool.is_single_input
 
 
 def test_create_tool_keyword_args() -> None:
     """Test that keyword arguments are allowed."""
     test_tool = Tool(name="test_name", func=lambda x: x, description="test_description")
+    assert test_tool.is_single_input
     assert test_tool("foo") == "foo"
     assert test_tool.name == "test_name"
     assert test_tool.description == "test_description"
@@ -371,8 +387,39 @@ async def test_create_async_tool() -> None:
         description="test_description",
         coroutine=_test_func,
     )
+    assert test_tool.is_single_input
     assert test_tool("foo") == "foo"
     assert test_tool.name == "test_name"
     assert test_tool.description == "test_description"
     assert test_tool.coroutine is not None
     assert await test_tool.arun("foo") == "foo"
+
+
+@pytest.mark.parametrize(
+    "agent_cls",
+    [
+        ChatAgent,
+        ZeroShotAgent,
+        ConversationalChatAgent,
+        ConversationalAgent,
+        ReActDocstoreAgent,
+        ReActTextWorldAgent,
+        SelfAskWithSearchAgent,
+    ],
+)
+def test_single_input_agent_raises_error_on_structured_tool(
+    agent_cls: Type[Agent],
+) -> None:
+    """Test that older agents raise errors on older tools."""
+
+    @tool
+    def the_tool(foo: str, bar: str) -> str:
+        """Return the concat of foo and bar."""
+        return foo + bar
+
+    with pytest.raises(
+        ValueError,
+        match=f"{agent_cls.__name__} does not support"  # type: ignore
+        f" multi-input tool {the_tool.name}.",
+    ):
+        agent_cls.from_llm_and_tools(MagicMock(), [the_tool])  # type: ignore