Fix visualization recursion with cycle detection (#737)

## Summary
- avoid infinite recursion in visualization by tracking visited agents
- test cycle detection in graph utility

## Testing
- `make mypy`
- `make tests` 

Resolves #668

src/agents/extensions/visualization.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from __future__ import annotations
 
 import graphviz  # type: ignore
 
@@ -31,7 +31,9 @@ def get_main_graph(agent: Agent) -> str:
     return "".join(parts)
 
 
-def get_all_nodes(agent: Agent, parent: Optional[Agent] = None) -> str:
+def get_all_nodes(
+    agent: Agent, parent: Agent | None = None, visited: set[str] | None = None
+) -> str:
     """
     Recursively generates the nodes for the given agent and its handoffs in DOT format.
 
@@ -41,17 +43,23 @@ def get_all_nodes(agent: Agent, parent: Optional[Agent] = None) -> str:
     Returns:
         str: The DOT format string representing the nodes.
     """
+    if visited is None:
+        visited = set()
+    if agent.name in visited:
+        return ""
+    visited.add(agent.name)
+
     parts = []
 
     # Start and end the graph
-    parts.append(
-        '"__start__" [label="__start__", shape=ellipse, style=filled, '
-        "fillcolor=lightblue, width=0.5, height=0.3];"
-        '"__end__" [label="__end__", shape=ellipse, style=filled, '
-        "fillcolor=lightblue, width=0.5, height=0.3];"
-    )
-    # Ensure parent agent node is colored
     if not parent:
+        parts.append(
+            '"__start__" [label="__start__", shape=ellipse, style=filled, '
+            "fillcolor=lightblue, width=0.5, height=0.3];"
+            '"__end__" [label="__end__", shape=ellipse, style=filled, '
+            "fillcolor=lightblue, width=0.5, height=0.3];"
+        )
+        # Ensure parent agent node is colored
         parts.append(
             f'"{agent.name}" [label="{agent.name}", shape=box, style=filled, '
             "fillcolor=lightyellow, width=1.5, height=0.8];"
@@ -71,17 +79,20 @@ def get_all_nodes(agent: Agent, parent: Optional[Agent] = None) -> str:
                 f"fillcolor=lightyellow, width=1.5, height=0.8];"
             )
         if isinstance(handoff, Agent):
-            parts.append(
-                f'"{handoff.name}" [label="{handoff.name}", '
-                f"shape=box, style=filled, style=rounded, "
-                f"fillcolor=lightyellow, width=1.5, height=0.8];"
-            )
-            parts.append(get_all_nodes(handoff))
+            if handoff.name not in visited:
+                parts.append(
+                    f'"{handoff.name}" [label="{handoff.name}", '
+                    f"shape=box, style=filled, style=rounded, "
+                    f"fillcolor=lightyellow, width=1.5, height=0.8];"
+                )
+            parts.append(get_all_nodes(handoff, agent, visited))
 
     return "".join(parts)
 
 
-def get_all_edges(agent: Agent, parent: Optional[Agent] = None) -> str:
+def get_all_edges(
+    agent: Agent, parent: Agent | None = None, visited: set[str] | None = None
+) -> str:
     """
     Recursively generates the edges for the given agent and its handoffs in DOT format.
 
@@ -92,6 +103,12 @@ def get_all_edges(agent: Agent, parent: Optional[Agent] = None) -> str:
     Returns:
         str: The DOT format string representing the edges.
     """
+    if visited is None:
+        visited = set()
+    if agent.name in visited:
+        return ""
+    visited.add(agent.name)
+
     parts = []
 
     if not parent:
@@ -109,7 +126,7 @@ def get_all_edges(agent: Agent, parent: Optional[Agent] = None) -> str:
         if isinstance(handoff, Agent):
             parts.append(f"""
             "{agent.name}" -> "{handoff.name}";""")
-            parts.append(get_all_edges(handoff, agent))
+            parts.append(get_all_edges(handoff, agent, visited))
 
     if not agent.handoffs and not isinstance(agent, Tool):  # type: ignore
         parts.append(f'"{agent.name}" -> "__end__";')
@@ -117,7 +134,7 @@ def get_all_edges(agent: Agent, parent: Optional[Agent] = None) -> str:
     return "".join(parts)
 
 
-def draw_graph(agent: Agent, filename: Optional[str] = None) -> graphviz.Source:
+def draw_graph(agent: Agent, filename: str | None = None) -> graphviz.Source:
     """
     Draws the graph for the given agent and optionally saves it as a PNG file.
 

tests/test_visualization.py

@@ -134,3 +134,18 @@ def test_draw_graph(mock_agent):
         '"Handoff1" [label="Handoff1", shape=box, style=filled, style=rounded, '
         "fillcolor=lightyellow, width=1.5, height=0.8];" in graph.source
     )
+
+
+def test_cycle_detection():
+    agent_a = Agent(name="A")
+    agent_b = Agent(name="B")
+    agent_a.handoffs.append(agent_b)
+    agent_b.handoffs.append(agent_a)
+
+    nodes = get_all_nodes(agent_a)
+    edges = get_all_edges(agent_a)
+
+    assert nodes.count('"A" [label="A"') == 1
+    assert nodes.count('"B" [label="B"') == 1
+    assert '"A" -> "B"' in edges
+    assert '"B" -> "A"' in edges