feat: serialize tool results as JSON when possible (#1752)

Author: clareliguori

src/strands/tools/decorator.py

@@ -43,6 +43,7 @@ def my_tool(param1: str, param2: int = 42) -> dict:
 import asyncio
 import functools
 import inspect
+import json
 import logging
 from collections.abc import Callable
 from typing import (
@@ -61,6 +62,7 @@ def my_tool(param1: str, param2: int = 42) -> dict:
 import docstring_parser
 from pydantic import BaseModel, Field, create_model
 from pydantic.fields import FieldInfo
+from pydantic_core import PydanticSerializationError
 from typing_extensions import override
 
 from ..interrupt import InterruptException
@@ -644,12 +646,25 @@ def _wrap_tool_result(self, tool_use_d: str, result: Any, exception: Exception |
             return ToolResultEvent(cast(ToolResult, result), exception=exception)
         else:
             # Wrap any other return value in the standard format
-            # Always include at least one content item for consistency
+            # Serialize to JSON for consistent, parseable output (except strings)
+            if isinstance(result, str):
+                text = result
+            elif isinstance(result, BaseModel):
+                try:
+                    text = result.model_dump_json()
+                except PydanticSerializationError:
+                    text = str(result)
+            else:
+                try:
+                    text = json.dumps(result)
+                except (TypeError, ValueError):
+                    text = str(result)
+
             return ToolResultEvent(
                 {
                     "toolUseId": tool_use_d,
                     "status": "success",
-                    "content": [{"text": str(result)}],
+                    "content": [{"text": text}],
                 },
                 exception=exception,
             )

tests/strands/tools/test_decorator.py

@@ -136,7 +136,7 @@ def identity(a: int, agent: dict = None):
 
     tru_events = await alist(stream)
     exp_events = [
-        ToolResultEvent({"toolUseId": "unknown", "status": "success", "content": [{"text": "(2, {'state': 1})"}]})
+        ToolResultEvent({"toolUseId": "unknown", "status": "success", "content": [{"text": '[2, {"state": 1}]'}]})
     ]
     assert tru_events == exp_events
 
@@ -595,12 +595,12 @@ def none_return_tool(param: str) -> None:
     assert result["tool_result"]["status"] == "success"
     assert result["tool_result"]["content"][0]["text"] == "Result: test"
 
-    # Test None return - should still create valid ToolResult with "None" text
+    # Test None return - should still create valid ToolResult with "null"
     stream = none_return_tool.stream(tool_use, {})
 
     result = (await alist(stream))[-1]
     assert result["tool_result"]["status"] == "success"
-    assert result["tool_result"]["content"][0]["text"] == "None"
+    assert result["tool_result"]["content"][0]["text"] == "null"
 
 
 @pytest.mark.asyncio
@@ -861,7 +861,7 @@ def int_return_tool(param: str) -> int:
 
     result = (await alist(stream))[-1]
     assert result["tool_result"]["status"] == "success"
-    assert result["tool_result"]["content"][0]["text"] == "None"
+    assert result["tool_result"]["content"][0]["text"] == "null"
 
     # Define tool with Union return type
     @strands.tool
@@ -884,10 +884,7 @@ def union_return_tool(param: str) -> dict[str, Any] | str | None:
 
     result = (await alist(stream))[-1]
     assert result["tool_result"]["status"] == "success"
-    assert (
-        "{'key': 'value'}" in result["tool_result"]["content"][0]["text"]
-        or '{"key": "value"}' in result["tool_result"]["content"][0]["text"]
-    )
+    assert result["tool_result"]["content"][0]["text"] == '{"key": "value"}'
 
     tool_use = {"toolUseId": "test-id", "input": {"param": "str"}}
     stream = union_return_tool.stream(tool_use, {})
@@ -901,7 +898,7 @@ def union_return_tool(param: str) -> dict[str, Any] | str | None:
 
     result = (await alist(stream))[-1]
     assert result["tool_result"]["status"] == "success"
-    assert result["tool_result"]["content"][0]["text"] == "None"
+    assert result["tool_result"]["content"][0]["text"] == "null"
 
 
 @pytest.mark.asyncio
@@ -992,6 +989,132 @@ def custom_result_tool(param: str) -> dict[str, Any]:
     assert result["tool_result"]["content"][1]["type"] == "markdown"
 
 
+@pytest.mark.asyncio
+async def test_tool_result_json_serialization_dict(alist):
+    """Test that dict results are serialized as JSON."""
+
+    @strands.tool
+    def dict_tool() -> dict:
+        """Returns a dict."""
+        return {"key": "value", "number": 42}
+
+    tool_use = {"toolUseId": "test-id", "input": {}}
+    stream = dict_tool.stream(tool_use, {})
+
+    result = (await alist(stream))[-1]
+    text = result["tool_result"]["content"][0]["text"]
+
+    assert text == '{"key": "value", "number": 42}'
+
+
+@pytest.mark.asyncio
+async def test_tool_result_json_serialization_list(alist):
+    """Test that list results are serialized as JSON."""
+
+    @strands.tool
+    def list_tool() -> list:
+        """Returns a list."""
+        return [1, "two", {"three": 3}]
+
+    tool_use = {"toolUseId": "test-id", "input": {}}
+    stream = list_tool.stream(tool_use, {})
+
+    result = (await alist(stream))[-1]
+    text = result["tool_result"]["content"][0]["text"]
+
+    assert text == '[1, "two", {"three": 3}]'
+
+
+@pytest.mark.asyncio
+async def test_tool_result_json_serialization_pydantic(alist):
+    """Test that Pydantic model results are serialized as JSON."""
+    from pydantic import BaseModel
+
+    class MyModel(BaseModel):
+        name: str
+        count: int
+
+    @strands.tool
+    def pydantic_tool() -> MyModel:
+        """Returns a Pydantic model."""
+        return MyModel(name="test", count=5)
+
+    tool_use = {"toolUseId": "test-id", "input": {}}
+    stream = pydantic_tool.stream(tool_use, {})
+
+    result = (await alist(stream))[-1]
+    text = result["tool_result"]["content"][0]["text"]
+
+    assert text == '{"name":"test","count":5}'
+
+
+@pytest.mark.asyncio
+async def test_tool_result_json_serialization_pydantic_non_serializable(alist):
+    """Test that Pydantic models with non-serializable fields fall back to str()."""
+    from pydantic import BaseModel
+
+    class NonSerializable:
+        def __repr__(self):
+            return "NonSerializable()"
+
+    class MyModel(BaseModel):
+        model_config = {"arbitrary_types_allowed": True}
+        data: NonSerializable
+
+    @strands.tool
+    def pydantic_tool() -> MyModel:
+        """Returns a Pydantic model with non-serializable field."""
+        return MyModel(data=NonSerializable())
+
+    tool_use = {"toolUseId": "test-id", "input": {}}
+    stream = pydantic_tool.stream(tool_use, {})
+
+    result = (await alist(stream))[-1]
+    text = result["tool_result"]["content"][0]["text"]
+
+    assert text == "data=NonSerializable()"
+
+
+@pytest.mark.asyncio
+async def test_tool_result_json_serialization_non_serializable(alist):
+    """Test that non-JSON-serializable results fall back to str()."""
+
+    class CustomClass:
+        def __str__(self):
+            return "custom_str_repr"
+
+    @strands.tool
+    def custom_tool() -> Any:
+        """Returns a non-serializable object."""
+        return CustomClass()
+
+    tool_use = {"toolUseId": "test-id", "input": {}}
+    stream = custom_tool.stream(tool_use, {})
+
+    result = (await alist(stream))[-1]
+    text = result["tool_result"]["content"][0]["text"]
+
+    assert text == "custom_str_repr"
+
+
+@pytest.mark.asyncio
+async def test_tool_result_string_not_json_encoded(alist):
+    """Test that string results are NOT JSON-encoded (no extra quotes)."""
+
+    @strands.tool
+    def string_tool() -> str:
+        """Returns a string."""
+        return "hello world"
+
+    tool_use = {"toolUseId": "test-id", "input": {}}
+    stream = string_tool.stream(tool_use, {})
+
+    result = (await alist(stream))[-1]
+    text = result["tool_result"]["content"][0]["text"]
+
+    assert text == "hello world"
+
+
 def test_docstring_parsing():
     """Test that function docstring is correctly parsed into tool spec."""