From 8b84dc0decfaf319f57ca8ea9fc881a945ff212c Mon Sep 17 00:00:00 2001
From: Sohum Trivedi <trivsohum@gmail.com>
Date: Fri, 17 Jul 2026 10:16:22 -0700
Subject: [PATCH] Python: fix FunctionResult.__str__ rendering falsy values (0,
 False, 0.0) as empty string

FunctionResult.__str__ used a truthiness check (if self.value:) where a
None-check was intended, so kernel functions returning falsy-but-meaningful
scalars (0, 0.0, False) rendered as an empty string when interpolated into
a prompt template, silently corrupting the prompt sent to the model.

Change the check to 'if self.value is not None:' and add explicit guards
for empty list/dict values so they still render as an empty string (an
empty dict would otherwise raise FunctionResultError when indexing).

Adds unit tests for 0, 0.0, False, '', [], {} and a KernelPromptTemplate
render test covering falsy function results.
---
 .../functions/function_result.py              |  6 ++++-
 .../unit/functions/test_function_result.py    | 19 +++++++++++++++
 .../test_kernel_prompt_template.py            | 24 +++++++++++++++++++
 3 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/python/semantic_kernel/functions/function_result.py b/python/semantic_kernel/functions/function_result.py
index 1d3dd0c29f59..38e988846294 100644
--- a/python/semantic_kernel/functions/function_result.py
+++ b/python/semantic_kernel/functions/function_result.py
@@ -37,9 +37,11 @@ class FunctionResult(KernelBaseModel):
 
     def __str__(self) -> str:
         """Get the string representation of the result."""
-        if self.value:
+        if self.value is not None:
             try:
                 if isinstance(self.value, list):
+                    if not self.value:
+                        return ""
                     return (
                         str(self.value[0])
                         if isinstance(self.value[0], KernelContent)
@@ -48,6 +50,8 @@ def __str__(self) -> str:
                 if isinstance(self.value, dict):
                     # TODO (eavanvalkenburg): remove this once function result doesn't include input args
                     # This is so an integration test can pass.
+                    if not self.value:
+                        return ""
                     return str(list(self.value.values())[-1])
                 return str(self.value)
             except Exception as e:
diff --git a/python/tests/unit/functions/test_function_result.py b/python/tests/unit/functions/test_function_result.py
index a8f686e9648b..7f29e82b548f 100644
--- a/python/tests/unit/functions/test_function_result.py
+++ b/python/tests/unit/functions/test_function_result.py
@@ -63,6 +63,25 @@ def test_function_result_str_empty_value():
     assert str(result) == ""
 
 
+@pytest.mark.parametrize(
+    ("value", "expected"),
+    [
+        (0, "0"),
+        (0.0, "0.0"),
+        (False, "False"),
+        ("", ""),
+        ([], ""),
+        ({}, ""),
+    ],
+)
+def test_function_result_str_with_falsy_value(value, expected):
+    result = FunctionResult(
+        function=KernelFunctionMetadata(name="test_function", is_prompt=False, is_asynchronous=False),
+        value=value,
+    )
+    assert str(result) == expected
+
+
 def test_function_result_str_with_conversion_error():
     class Unconvertible:
         def __str__(self):
diff --git a/python/tests/unit/prompt_template/test_kernel_prompt_template.py b/python/tests/unit/prompt_template/test_kernel_prompt_template.py
index 6d8b1a1b2bf0..0bf2367a5e7f 100644
--- a/python/tests/unit/prompt_template/test_kernel_prompt_template.py
+++ b/python/tests/unit/prompt_template/test_kernel_prompt_template.py
@@ -121,6 +121,30 @@ async def my_function(myVar: str) -> str:
     assert result == "foo-BAR-baz"
 
 
+async def test_it_renders_code_with_falsy_results(kernel: Kernel):
+    arguments = KernelArguments()
+
+    @kernel_function(name="subtract")
+    def subtract(a: int, b: int) -> int:
+        return a - b
+
+    @kernel_function(name="is_even")
+    def is_even(a: int) -> bool:
+        return a % 2 == 0
+
+    kernel.add_function("math", KernelFunction.from_method(subtract, "math"))
+    kernel.add_function("math", KernelFunction.from_method(is_even, "math"))
+
+    arguments["x"] = 5
+    arguments["y"] = 5
+    arguments["z"] = 3
+    template = "The result is: {{math.subtract a=$x b=$y}} and even={{math.is_even a=$z}}"
+    target = create_kernel_prompt_template(template, allow_dangerously_set_content=True)
+    result = await target.render(kernel, arguments)
+
+    assert result == "The result is: 0 and even=False"
+
+
 async def test_it_renders_code_error(kernel: Kernel):
     arguments = KernelArguments()