diff --git a/src/sdk/agent.py b/src/sdk/agent.py
index e1dc49de1..6499a51bf 100644
--- a/src/sdk/agent.py
+++ b/src/sdk/agent.py
@@ -45,6 +45,8 @@ def stop(self) -> None:
         self._running = False
 
     def set_metadata(self, key: str, value: Any) -> None:
+        if not isinstance(key, str) or not key.strip():
+            raise ValueError("metadata key must be a non-empty string")
         self._metadata[key] = value
 
     def get_metadata(self, key: str, default: Any = None) -> Any:
diff --git a/tests/test_sdk_agent_metadata.py b/tests/test_sdk_agent_metadata.py
new file mode 100644
index 000000000..e23d1a555
--- /dev/null
+++ b/tests/test_sdk_agent_metadata.py
@@ -0,0 +1,26 @@
+import pytest
+
+from src.sdk.agent import BaseAgent
+
+
+class _Agent(BaseAgent):
+    async def setup(self) -> None:
+        return None
+
+    async def handle_task(self, task):
+        return task
+
+    async def cleanup(self) -> None:
+        return None
+
+
+def test_set_metadata_rejects_empty_key():
+    agent = _Agent(agent_id="a1", name="agent")
+    with pytest.raises(ValueError):
+        agent.set_metadata("", "value")
+
+
+def test_set_metadata_accepts_non_empty_key():
+    agent = _Agent(agent_id="a1", name="agent")
+    agent.set_metadata("env", "prod")
+    assert agent.get_metadata("env") == "prod"