fix(predict): reject ReActV2 input fields named 'history' or 'tools' (#10464)

ReActV2's inner signature injects internal 'history' and 'tools' input
fields, and forward()/aforward() pop 'history' from the caller's kwargs.
A user signature declaring an input with either name was silently
overwritten (type replaced by dspy.History / list[dspy.Tool]) and the
user's value never reached the task. Output fields were already
validated in __init__; this adds the matching input-field check.

Fixes #10336

Author: chenlichao

dspy/predict/react_v2.py

@@ -20,6 +20,11 @@
 # expansion at the `Prediction(**final_outputs, history=..., termination_reason=...)` call sites.
 _RESERVED_PREDICTION_KEYS = frozenset({"history", "termination_reason"})
 
+# Internal input field names ReActV2 injects into its inner Predict signature. A user
+# input with one of these names would be silently overwritten by `_make_react_signature`
+# and popped out of the caller's kwargs by `forward`/`aforward`, so it must be rejected.
+_RESERVED_INPUT_KEYS = frozenset({"history", "tools"})
+
 if TYPE_CHECKING:
     from dspy.signatures.signature import Signature
 
@@ -52,6 +57,14 @@ def __init__(self, signature: type[Signature], tools: list[Callable | Tool], max
                 "returned Prediction. Rename these output fields on your signature."
             )
 
+        reserved_inputs = _RESERVED_INPUT_KEYS.intersection(self.signature.input_fields)
+        if reserved_inputs:
+            names = ", ".join(f"`{name}`" for name in sorted(reserved_inputs))
+            raise ValueError(
+                f"Input field name(s) {names} are reserved by ReActV2 for its internal "
+                "conversation history and tool list. Rename these input fields on your signature."
+            )
+
         user_tools = [tool if isinstance(tool, Tool) else Tool(tool) for tool in tools]
         self.tools = {tool.name: tool for tool in user_tools}
         if "submit" in self.tools:

tests/predict/test_react_v2.py

@@ -362,6 +362,12 @@ def test_react_v2_rejects_reserved_output_field_names(reserved):
         dspy.ReActV2(f"question -> answer, {reserved}: str", tools=[])
 
 
+@pytest.mark.parametrize("reserved", ["history", "tools"])
+def test_react_v2_rejects_reserved_input_field_names(reserved):
+    with pytest.raises(ValueError, match=reserved):
+        dspy.ReActV2(f"{reserved}: str -> answer: str", tools=[])
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize("use_async", [False, True])
 @pytest.mark.parametrize("break_reason", ["max_iters", "empty_tool_calls", "parse_error", "context_window_exceeded"])