core: prompt variable error msg (#25787)

libs/core/langchain_core/prompts/base.py

@@ -142,11 +142,18 @@ class BasePromptTemplate(
                 )
         missing = set(self.input_variables).difference(inner_input)
         if missing:
-            raise KeyError(
+            msg = (
                 f"Input to {self.__class__.__name__} is missing variables {missing}. "
                 f" Expected: {self.input_variables}"
                 f" Received: {list(inner_input.keys())}"
             )
+            example_key = missing.pop()
+            msg += (
+                f"\nNote: if you intended {{{example_key}}} to be part of the string"
+                " and not a variable, please escape it with double curly braces like: "
+                f"'{{{{{example_key}}}}}'."
+            )
+            raise KeyError(msg)
         return inner_input
 
     def _format_prompt_with_error_handling(self, inner_input: Dict) -> PromptValue:

libs/core/tests/unit_tests/prompts/test_prompt.py

@@ -702,3 +702,15 @@ def test_prompt_falsy_vars(
         expected if not isinstance(expected, dict) else expected[template_format]
     )
     assert result.to_string() == expected_output
+
+
+def test_prompt_missing_vars_error() -> None:
+    prompt = PromptTemplate.from_template("This is a {foo} {goingtobemissing} test.")
+    with pytest.raises(KeyError) as e:
+        prompt.invoke({"foo": "bar"})
+
+    # Check that the error message contains the missing variable
+    assert "{'goingtobemissing'}" in str(e.value.args[0])
+
+    # Check helper text has right number of braces
+    assert "'{{goingtobemissing}}'" in str(e.value.args[0])