fix: validate ReceiptBuilder ngram and threshold

CHANGELOG.md

@@ -15,6 +15,8 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Fixed
 
+- `ReceiptBuilder` now rejects `ngram < 1` and out-of-range `threshold` values at construction so degenerate n-gram sets cannot produce a perfect grounding score.
+
 - Explicitly supplied source contents must now cover every source in the
   receipt; empty and partial mappings report the missing source IDs and fail
   verification.

src/answerproof/builder.py

@@ -36,9 +36,15 @@ class ReceiptBuilder:
     """Collects the facts of one RAG request and produces a signed receipt."""
 
     def __init__(self, signing_key: SigningKey, *, ngram: int = 3, threshold: float = 0.5):
+        if not isinstance(ngram, int) or isinstance(ngram, bool) or ngram < 1:
+            raise ValueError("ngram must be an integer >= 1")
+        if not isinstance(threshold, (int, float)) or isinstance(threshold, bool):
+            raise ValueError("threshold must be a number between 0 and 1 inclusive")
+        if threshold < 0 or threshold > 1:
+            raise ValueError("threshold must be a number between 0 and 1 inclusive")
         self._signing_key = signing_key
         self._ngram = ngram
-        self._threshold = threshold
+        self._threshold = float(threshold)
         self._query: str | None = None
         self._answer: str | None = None
         self._principal = Principal(id="anonymous")

tests/test_builder.py

@@ -93,3 +93,28 @@ def test_source_order_is_preserved(sk):
         b.add_source(f"s{i}", content=f"content {i}")
     receipt = b.finalize()
     assert [s.id for s in receipt.payload.sources] == [f"s{i}" for i in range(5)]
+
+
+def test_rejects_zero_ngram(sk):
+    with pytest.raises(ValueError, match="ngram"):
+        ReceiptBuilder(sk, ngram=0)
+
+
+def test_rejects_negative_ngram(sk):
+    with pytest.raises(ValueError, match="ngram"):
+        ReceiptBuilder(sk, ngram=-1)
+
+
+def test_rejects_threshold_above_one(sk):
+    with pytest.raises(ValueError, match="threshold"):
+        ReceiptBuilder(sk, threshold=1.5)
+
+
+def test_rejects_threshold_below_zero(sk):
+    with pytest.raises(ValueError, match="threshold"):
+        ReceiptBuilder(sk, threshold=-0.1)
+
+
+def test_accepts_boundary_threshold(sk):
+    ReceiptBuilder(sk, ngram=1, threshold=0.0)
+    ReceiptBuilder(sk, ngram=1, threshold=1.0)