Automatically flatten nested tool collections (#508)

Fixes issue #50

Customers naturally want to pass nested collections of tools - the above issue has gathered enough data points proving that.

Author: zastrowm

src/strands/tools/registry.py

@@ -11,7 +11,7 @@
 from importlib import import_module, util
 from os.path import expanduser
 from pathlib import Path
-from typing import Any, Dict, List, Optional
+from typing import Any, Dict, Iterable, List, Optional
 
 from typing_extensions import TypedDict, cast
 
@@ -54,7 +54,7 @@ def process_tools(self, tools: List[Any]) -> List[str]:
         """
         tool_names = []
 
-        for tool in tools:
+        def add_tool(tool: Any) -> None:
             # Case 1: String file path
             if isinstance(tool, str):
                 # Extract tool name from path
@@ -97,9 +97,16 @@ def process_tools(self, tools: List[Any]) -> List[str]:
             elif isinstance(tool, AgentTool):
                 self.register_tool(tool)
                 tool_names.append(tool.tool_name)
+            # Case 6: Nested iterable (list, tuple, etc.) - add each sub-tool
+            elif isinstance(tool, Iterable) and not isinstance(tool, (str, bytes, bytearray)):
+                for t in tool:
+                    add_tool(t)
             else:
                 logger.warning("tool=<%s> | unrecognized tool specification", tool)
 
+        for a_tool in tools:
+            add_tool(a_tool)
+
         return tool_names
 
     def load_tool_from_filepath(self, tool_name: str, tool_path: str) -> None:

tests/strands/agent/test_agent.py

@@ -231,6 +231,25 @@ def test_agent__init__with_string_model_id():
     assert agent.model.config["model_id"] == "nonsense"
 
 
+def test_agent__init__nested_tools_flattening(tool_decorated, tool_module, tool_imported, tool_registry):
+    _ = tool_registry
+    # Nested structure: [tool_decorated, [tool_module, [tool_imported]]]
+    agent = Agent(tools=[tool_decorated, [tool_module, [tool_imported]]])
+    tru_tool_names = sorted(agent.tool_names)
+    exp_tool_names = ["tool_decorated", "tool_imported", "tool_module"]
+    assert tru_tool_names == exp_tool_names
+
+
+def test_agent__init__deeply_nested_tools(tool_decorated, tool_module, tool_imported, tool_registry):
+    _ = tool_registry
+    # Deeply nested structure
+    nested_tools = [[[[tool_decorated]], [[tool_module]], tool_imported]]
+    agent = Agent(tools=nested_tools)
+    tru_tool_names = sorted(agent.tool_names)
+    exp_tool_names = ["tool_decorated", "tool_imported", "tool_module"]
+    assert tru_tool_names == exp_tool_names
+
+
 def test_agent__call__(
     mock_model,
     system_prompt,

tests/strands/tools/test_registry.py

@@ -93,3 +93,30 @@ def tool_function_4(d):
 
     assert len(tools) == 2
     assert all(isinstance(tool, DecoratedFunctionTool) for tool in tools)
+
+
+def test_process_tools_flattens_lists_and_tuples_and_sets():
+    def function() -> str:
+        return "done"
+
+    tool_a = tool(name="tool_a")(function)
+    tool_b = tool(name="tool_b")(function)
+    tool_c = tool(name="tool_c")(function)
+    tool_d = tool(name="tool_d")(function)
+    tool_e = tool(name="tool_e")(function)
+    tool_f = tool(name="tool_f")(function)
+
+    registry = ToolRegistry()
+
+    all_tools = [tool_a, (tool_b, tool_c), [{tool_d, tool_e}, [tool_f]]]
+
+    tru_tool_names = sorted(registry.process_tools(all_tools))
+    exp_tool_names = [
+        "tool_a",
+        "tool_b",
+        "tool_c",
+        "tool_d",
+        "tool_e",
+        "tool_f",
+    ]
+    assert tru_tool_names == exp_tool_names