Fix/agent run trigger

VERSION.txt

@@ -1 +1 @@
-2.27.0-rc0
+2.27.0

haystack/components/agents/agent.py

@@ -481,6 +481,44 @@ def from_dict(cls, data: dict[str, Any]) -> "Agent":
 
         return default_from_dict(cls, data)
 
+    def can_run(self, inputs: dict[str, Any]) -> bool:
+        """
+        Check if the agent can run with the given inputs.
+
+        If there's a snapshot input, the agent can always run because it can resume from the snapshot state without
+        needing any of the other inputs. Otherwise, if any of the main inputs (messages, user_prompt, system_prompt)
+        are connected as input, they must be provided at runtime.
+
+        For the special case where none of the main inputs are connected, nor provided, and no prompts are set via
+        initialization, we return True, to raise an exception later in the run method.
+
+        :param inputs: Inputs for the agent.
+        :returns: True if the agent can run, False otherwise.
+        """
+        # If there's a snapshot, we can always run (we can resume from the snapshot state without needing any of the
+        # other inputs)
+        if inputs.get("snapshot") is not None:
+            return True
+
+        # messages, user_prompt, or system_prompt are the main inputs that can trigger the agent to run.
+        # If any of them is connected as input it must be provided at runtime, otherwise the agent won't be triggered.
+        main_inputs = ["messages", "user_prompt", "system_prompt"]
+        for main_input in main_inputs:
+            if self.is_socket_connected(main_input) and inputs.get(main_input) is None:
+                return False
+
+        # Either none of the main inputs are connected, or all connected inputs have a value at runtime
+        return True
+
+    def is_socket_connected(self, socket_name: str) -> bool:
+        """
+        Check if a socket is connected to any sender.
+
+        :param socket_name: The name of the socket to check.
+        :returns: True if the socket is connected to at least one sender, False otherwise.
+        """
+        return bool(self.__haystack_input__[socket_name].senders)
+
     def _create_agent_span(self) -> Any:
         """
         Create a span for the agent run.

haystack/core/pipeline/component_checks.py

@@ -19,6 +19,10 @@ def can_component_run(component: dict, inputs: dict) -> bool:
     :param component: Component metadata and the component instance.
     :param inputs: Inputs for the component.
     """
+    instance = component["instance"]
+    if hasattr(instance, "can_run") and callable(instance.can_run):
+        return instance.can_run(inputs)
+
     received_all_mandatory_inputs = are_all_sockets_ready(component, inputs, only_check_mandatory=True)
     received_trigger = has_any_trigger(component, inputs)