diff --git a/langgraph_swarm/handoff.py b/langgraph_swarm/handoff.py
index 620ee51..0eb30cc 100644
--- a/langgraph_swarm/handoff.py
+++ b/langgraph_swarm/handoff.py
@@ -32,12 +32,17 @@ def _get_field(obj: Any, key: str) -> Any:
 
 
 WHITESPACE_RE = re.compile(r"\s+")
+NON_ALNUM_UNDERSCORE_RE = re.compile(r"[^a-z0-9_]+")
+MULTI_UNDERSCORE_RE = re.compile(r"_+")
 METADATA_KEY_HANDOFF_DESTINATION = "__handoff_destination"
 
 
 def _normalize_agent_name(agent_name: str) -> str:
     """Normalize an agent name to be used inside the tool name."""
-    return WHITESPACE_RE.sub("_", agent_name.strip()).lower()
+    normalized = WHITESPACE_RE.sub("_", agent_name.strip()).lower()
+    normalized = NON_ALNUM_UNDERSCORE_RE.sub("_", normalized)
+    normalized = MULTI_UNDERSCORE_RE.sub("_", normalized).strip("_")
+    return normalized or "agent"
 
 
 def create_handoff_tool(
diff --git a/langgraph_swarm/swarm.py b/langgraph_swarm/swarm.py
index c6b0609..f2d7d3e 100644
--- a/langgraph_swarm/swarm.py
+++ b/langgraph_swarm/swarm.py
@@ -161,7 +161,18 @@ def add(a: int, b: int) -> int:
         )
 
     def route_to_active_agent(state: dict) -> str:
-        return cast("str", state.get("active_agent", default_active_agent))
+        active_agent = state.get("active_agent", default_active_agent)
+        if not active_agent:
+            return default_active_agent
+        if active_agent not in route_to:
+            warn(
+                f"Active agent '{active_agent}' not found in routes {route_to}. "
+                f"Falling back to '{default_active_agent}'.",
+                RuntimeWarning,
+                stacklevel=2,
+            )
+            return default_active_agent
+        return cast("str", active_agent)
 
     builder.add_conditional_edges(START, route_to_active_agent, path_map=route_to)
     return builder
diff --git a/tests/test_swarm.py b/tests/test_swarm.py
index 912d9a4..defab7a 100644
--- a/tests/test_swarm.py
+++ b/tests/test_swarm.py
@@ -1,6 +1,7 @@
 from collections.abc import Callable, Sequence
 from typing import TYPE_CHECKING, Any
 
+import pytest
 from langchain.agents import AgentState, create_agent
 from langchain.chat_models import BaseChatModel
 from langchain.messages import AIMessage
@@ -45,6 +46,18 @@ def bind_tools(
         return self
 
 
+def test_handoff_tool_default_name_sanitizes_special_characters() -> None:
+    handoff_tool = create_handoff_tool(agent_name="R&D Agent / v2")
+
+    assert handoff_tool.name == "transfer_to_r_d_agent_v2"
+
+
+def test_handoff_tool_default_name_falls_back_when_empty_after_sanitize() -> None:
+    handoff_tool = create_handoff_tool(agent_name="!!!")
+
+    assert handoff_tool.name == "transfer_to_agent"
+
+
 def test_basic_swarm() -> None:
     # Create fake responses for the model
     recorded_messages = [
@@ -152,6 +165,63 @@ def add(a: int, b: int) -> int:
     assert turn_2["active_agent"] == "Alice"
 
 
+def test_swarm_falls_back_on_unknown_active_agent() -> None:
+    recorded_messages = [
+        AIMessage(
+            content="",
+            name="Alice",
+            tool_calls=[
+                {
+                    "name": "transfer_to_bob",
+                    "args": {},
+                    "id": "call_1LlFyjm6iIhDjdn7juWuPYr4",
+                },
+            ],
+        ),
+        AIMessage(
+            content="Ahoy, matey! Bob the pirate be at yer service.",
+            name="Bob",
+        ),
+    ]
+
+    model = FakeChatModel(responses=recorded_messages)  # type: ignore[arg-type]
+
+    def add(a: int, b: int) -> int:
+        """Add two numbers."""
+        return a + b
+
+    alice: Any = create_agent(
+        model,
+        tools=[add, create_handoff_tool(agent_name="Bob")],
+        system_prompt="You are Alice, an addition expert.",
+        name="Alice",
+    )
+
+    bob: Any = create_agent(
+        model,
+        tools=[create_handoff_tool(agent_name="Alice")],
+        system_prompt="You are Bob, you speak like a pirate.",
+        name="Bob",
+    )
+
+    checkpointer = MemorySaver()
+    workflow = create_swarm([alice, bob], default_active_agent="Alice")  # type: ignore[list-item]
+    app = workflow.compile(checkpointer=checkpointer)
+
+    config: RunnableConfig = {"configurable": {"thread_id": "fallback-1"}}
+    with pytest.warns(RuntimeWarning, match="Active agent 'Charlie'"):
+        turn_1 = app.invoke(
+            {  # type: ignore[arg-type]
+                "messages": [{"role": "user", "content": "i'd like to speak to Bob"}],
+                "active_agent": "Charlie",
+            },
+            config,
+        )
+
+    assert turn_1["active_agent"] == "Bob"
+    assert turn_1["messages"][-2].content == "Successfully transferred to Bob"
+
+
 def test_basic_swarm_pydantic() -> None:
     """Test a basic swarm with Pydantic state schema."""