diff --git a/src/controlflow/agents/agent.py b/src/controlflow/agents/agent.py
index 790d38aa..e96c134c 100644
--- a/src/controlflow/agents/agent.py
+++ b/src/controlflow/agents/agent.py
@@ -97,11 +97,11 @@ class Agent(ControlFlowModel, abc.ABC):
 
     _cm_stack: list[AbstractContextManager] = PrivateAttr(default_factory=list)
 
-    @model_validator(mode="before")
-    def handle_positional_arg(cls, v):
-        if isinstance(v, str):
-            return {"instructions": v}
-        return v
+    def __init__(self, instructions: Optional[str] = None, **kwargs):
+        if instructions is not None:
+            kwargs["instructions"] = instructions
+
+        super().__init__(**kwargs)
 
     @model_validator(mode="before")
     @classmethod