Fix ContextWindowExceededError after 3 retries in react loop

When _call_with_potential_trajectory_truncation exhausts all retry
attempts, it now raises a clear ValueError instead of returning None.
This prevents the AttributeError: 'NoneType' object has no attribute
'next_thought' that occurred when accessing properties on the None
return value.

The ValueError is caught in forward/aforward and causes the loop to
break gracefully, allowing the extract phase to proceed with whatever
trajectory was collected.

Added tests for both sync and async versions of this scenario.

Co-authored-by: TomeHirata <[email protected]>

Author: Copilot

dspy/predict/react.py

@@ -152,7 +152,16 @@ def _call_with_potential_trajectory_truncation(self, module, trajectory, **input
                 )
             except ContextWindowExceededError:
                 logger.warning("Trajectory exceeded the context window, truncating the oldest tool call information.")
-                trajectory = self.truncate_trajectory(trajectory)
+                try:
+                    trajectory = self.truncate_trajectory(trajectory)
+                except ValueError:
+                    # Cannot truncate further, re-raise as a clear error
+                    raise ValueError(
+                        "The context window was exceeded and the trajectory could not be truncated further."
+                    )
+        raise ValueError(
+            "The context window was exceeded after 3 attempts to truncate the trajectory."
+        )
 
     async def _async_call_with_potential_trajectory_truncation(self, module, trajectory, **input_args):
         for _ in range(3):
@@ -163,7 +172,16 @@ async def _async_call_with_potential_trajectory_truncation(self, module, traject
                 )
             except ContextWindowExceededError:
                 logger.warning("Trajectory exceeded the context window, truncating the oldest tool call information.")
-                trajectory = self.truncate_trajectory(trajectory)
+                try:
+                    trajectory = self.truncate_trajectory(trajectory)
+                except ValueError:
+                    # Cannot truncate further, re-raise as a clear error
+                    raise ValueError(
+                        "The context window was exceeded and the trajectory could not be truncated further."
+                    )
+        raise ValueError(
+            "The context window was exceeded after 3 attempts to truncate the trajectory."
+        )
 
     def truncate_trajectory(self, trajectory):
         """Truncates the trajectory so that it fits in the context window.

tests/predict/test_react.py

@@ -204,6 +204,68 @@ def mock_react(**kwargs):
     assert result.output_text == "Final output"
 
 
+def test_context_window_exceeded_after_retries():
+    """Test that context window errors are handled gracefully after exhausting retry attempts.
+
+    This tests the fix for the bug where returning None after exhausting retries
+    caused "'NoneType' object has no attribute 'next_thought'" error.
+
+    The fix raises a ValueError instead, which is caught in forward() and causes
+    the loop to break gracefully.
+    """
+
+    def echo(text: str) -> str:
+        return f"Echoed: {text}"
+
+    react = dspy.ReAct("input_text -> output_text", tools=[echo])
+
+    # Always raise context window exceeded - simulating case where prompt is too large
+    # even on the very first call with empty trajectory
+    def mock_react(**kwargs):
+        raise litellm.ContextWindowExceededError("Context window exceeded", "dummy_model", "dummy_provider")
+
+    react.react = mock_react
+    react.extract = lambda **kwargs: dspy.Prediction(output_text="Fallback output")
+
+    # Call forward - should handle the error gracefully by logging and breaking the loop
+    # This should NOT raise AttributeError: 'NoneType' object has no attribute 'next_thought'
+    result = react(input_text="test input")
+
+    # The trajectory should be empty since the first call failed
+    assert result.trajectory == {}
+    # Extract should still be called and produce output
+    assert result.output_text == "Fallback output"
+
+
+@pytest.mark.asyncio
+async def test_async_context_window_exceeded_after_retries():
+    """Test that context window errors are handled gracefully after exhausting retry attempts in async mode."""
+
+    async def echo(text: str) -> str:
+        return f"Echoed: {text}"
+
+    react = dspy.ReAct("input_text -> output_text", tools=[echo])
+
+    # Always raise context window exceeded
+    async def mock_react(**kwargs):
+        raise litellm.ContextWindowExceededError("Context window exceeded", "dummy_model", "dummy_provider")
+
+    async def mock_extract(**kwargs):
+        return dspy.Prediction(output_text="Fallback output")
+
+    react.react.acall = mock_react
+    react.extract.acall = mock_extract
+
+    # Call forward - should handle the error gracefully
+    # This should NOT raise AttributeError: 'NoneType' object has no attribute 'next_thought'
+    result = await react.acall(input_text="test input")
+
+    # The trajectory should be empty since the first call failed
+    assert result.trajectory == {}
+    # Extract should still be called and produce output
+    assert result.output_text == "Fallback output"
+
+
 def test_error_retry():
     # --- a tiny tool that always fails -------------------------------------
     def foo(a, b):