diff --git a/src/agents/agent.py b/src/agents/agent.py
index b67a12c0d..5c6037532 100644
--- a/src/agents/agent.py
+++ b/src/agents/agent.py
@@ -221,6 +221,119 @@ class Agent(AgentBase, Generic[TContext]):
     """Whether to reset the tool choice to the default value after a tool has been called. Defaults
     to True. This ensures that the agent doesn't enter an infinite loop of tool usage."""
 
+    def __post_init__(self):
+        from typing import get_origin
+
+        if not isinstance(self.name, str):
+            raise TypeError(f"Agent name must be a string, got {type(self.name).__name__}")
+
+        if self.handoff_description is not None and not isinstance(self.handoff_description, str):
+            raise TypeError(
+                f"Agent handoff_description must be a string or None, "
+                f"got {type(self.handoff_description).__name__}"
+            )
+
+        if not isinstance(self.tools, list):
+            raise TypeError(f"Agent tools must be a list, got {type(self.tools).__name__}")
+
+        if not isinstance(self.mcp_servers, list):
+            raise TypeError(
+                f"Agent mcp_servers must be a list, got {type(self.mcp_servers).__name__}"
+            )
+
+        if not isinstance(self.mcp_config, dict):
+            raise TypeError(
+                f"Agent mcp_config must be a dict, got {type(self.mcp_config).__name__}"
+            )
+
+        if (
+            self.instructions is not None
+            and not isinstance(self.instructions, str)
+            and not callable(self.instructions)
+        ):
+            raise TypeError(
+                f"Agent instructions must be a string, callable, or None, "
+                f"got {type(self.instructions).__name__}"
+            )
+
+        if (
+            self.prompt is not None
+            and not callable(self.prompt)
+            and not hasattr(self.prompt, "get")
+        ):
+            raise TypeError(
+                f"Agent prompt must be a Prompt, DynamicPromptFunction, or None, "
+                f"got {type(self.prompt).__name__}"
+            )
+
+        if not isinstance(self.handoffs, list):
+            raise TypeError(f"Agent handoffs must be a list, got {type(self.handoffs).__name__}")
+
+        if self.model is not None and not isinstance(self.model, str):
+            from .models.interface import Model
+
+            if not isinstance(self.model, Model):
+                raise TypeError(
+                    f"Agent model must be a string, Model, or None, got {type(self.model).__name__}"
+                )
+
+        if not isinstance(self.model_settings, ModelSettings):
+            raise TypeError(
+                f"Agent model_settings must be a ModelSettings instance, "
+                f"got {type(self.model_settings).__name__}"
+            )
+
+        if not isinstance(self.input_guardrails, list):
+            raise TypeError(
+                f"Agent input_guardrails must be a list, got {type(self.input_guardrails).__name__}"
+            )
+
+        if not isinstance(self.output_guardrails, list):
+            raise TypeError(
+                f"Agent output_guardrails must be a list, "
+                f"got {type(self.output_guardrails).__name__}"
+            )
+
+        if self.output_type is not None:
+            from .agent_output import AgentOutputSchemaBase
+
+            if not (
+                isinstance(self.output_type, (type, AgentOutputSchemaBase))
+                or get_origin(self.output_type) is not None
+            ):
+                raise TypeError(
+                    f"Agent output_type must be a type, AgentOutputSchemaBase, or None, "
+                    f"got {type(self.output_type).__name__}"
+                )
+
+        if self.hooks is not None:
+            from .lifecycle import AgentHooksBase
+
+            if not isinstance(self.hooks, AgentHooksBase):
+                raise TypeError(
+                    f"Agent hooks must be an AgentHooks instance or None, "
+                    f"got {type(self.hooks).__name__}"
+                )
+
+        if (
+            not (
+                isinstance(self.tool_use_behavior, str)
+                and self.tool_use_behavior in ["run_llm_again", "stop_on_first_tool"]
+            )
+            and not isinstance(self.tool_use_behavior, dict)
+            and not callable(self.tool_use_behavior)
+        ):
+            raise TypeError(
+                f"Agent tool_use_behavior must be 'run_llm_again', 'stop_on_first_tool', "
+                f"StopAtTools dict, or callable, got {type(self.tool_use_behavior).__name__}"
+            )
+
+        if not isinstance(self.reset_tool_choice, bool):
+            raise TypeError(
+                f"Agent reset_tool_choice must be a boolean, "
+                f"got {type(self.reset_tool_choice).__name__}"
+            )
+
     def clone(self, **kwargs: Any) -> Agent[TContext]:
         """Make a copy of the agent, with the given arguments changed. For example, you could do:
         ```
diff --git a/tests/test_agent_config.py b/tests/test_agent_config.py
index a985fd60d..5b633b70b 100644
--- a/tests/test_agent_config.py
+++ b/tests/test_agent_config.py
@@ -2,6 +2,8 @@
 from pydantic import BaseModel
 
 from agents import Agent, AgentOutputSchema, Handoff, RunContextWrapper, handoff
+from agents.lifecycle import AgentHooksBase
+from agents.model_settings import ModelSettings
 from agents.run import AgentRunner
 
 
@@ -167,3 +169,58 @@ async def test_agent_final_output():
     assert schema.is_strict_json_schema() is True
     assert schema.json_schema() is not None
     assert not schema.is_plain_text()
+
+
+class TestAgentValidation:
+    """Essential validation tests for Agent __post_init__"""
+
+    def test_name_validation_critical_cases(self):
+        """Test name validation - the original issue that started this PR"""
+        # This was the original failing case that caused JSON serialization errors
+        with pytest.raises(TypeError, match="Agent name must be a string, got int"):
+            Agent(name=1)  # type: ignore
+
+        with pytest.raises(TypeError, match="Agent name must be a string, got NoneType"):
+            Agent(name=None)  # type: ignore
+
+    def test_tool_use_behavior_dict_validation(self):
+        """Test tool_use_behavior accepts StopAtTools dict - fixes existing test failures"""
+        # This test ensures the existing failing tests now pass
+        Agent(name="test", tool_use_behavior={"stop_at_tool_names": ["tool1"]})
+
+        # Invalid cases that should fail
+        with pytest.raises(TypeError, match="Agent tool_use_behavior must be"):
+            Agent(name="test", tool_use_behavior=123)  # type: ignore
+
+    def test_hooks_validation_python39_compatibility(self):
+        """Test hooks validation works with Python 3.9 - fixes generic type issues"""
+
+        class MockHooks(AgentHooksBase):
+            pass
+
+        # Valid case
+        Agent(name="test", hooks=MockHooks())  # type: ignore
+
+        # Invalid case
+        with pytest.raises(TypeError, match="Agent hooks must be an AgentHooks instance"):
+            Agent(name="test", hooks="invalid")  # type: ignore
+
+    def test_list_field_validation(self):
+        """Test critical list fields that commonly get wrong types"""
+        # These are the most common mistakes users make
+        with pytest.raises(TypeError, match="Agent tools must be a list"):
+            Agent(name="test", tools="not_a_list")  # type: ignore
+
+        with pytest.raises(TypeError, match="Agent handoffs must be a list"):
+            Agent(name="test", handoffs="not_a_list")  # type: ignore
+
+    def test_model_settings_validation(self):
+        """Test model_settings validation - prevents runtime errors"""
+        # Valid case
+        Agent(name="test", model_settings=ModelSettings())
+
+        # Invalid case that could cause runtime issues
+        with pytest.raises(
+            TypeError, match="Agent model_settings must be a ModelSettings instance"
+        ):
+            Agent(name="test", model_settings={})  # type: ignore