fix: audit round 5 — 3 bugs (edge cases + missed copilot.py int() cast)

B1: prepare_treatment raises ValueError for single-value treatment
    (was IndexError on unique_vals[1] when nunique < 2)
B2: copilot.py matching removed int() cast on control/treatment values
    (Round 3 B3 only fixed server.py, missed copilot.py)
B3: server.py T0/T1 parsing handles categorical string values
    (float("A") crashed with ValueError for categorical treatments)

Audit confirmed: 0 GlobalState field gaps, 0 scientific rigor issues,
full main.py parity. 134 tests pass (3 new).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cauchyturing

causal_copilot/copilot.py

@@ -820,8 +820,8 @@ def estimate_effect(
                     treatment,
                     outcome,
                     match_conf,
-                    int(control_value),
-                    int(treatment_value),
+                    control_value,
+                    treatment_value,
                 )
             elif selected_method == "dml":
                 X_col = [c for c in names if c != treatment and c != outcome and c not in conf_list]

causal_copilot/mcp/offline.py

@@ -77,6 +77,12 @@ def prepare_treatment(
     """
     treatment_col = data[treatment]
 
+    if treatment_col.nunique() < 2:
+        raise ValueError(
+            f"Treatment '{treatment}' has {treatment_col.nunique()} unique value(s). "
+            "Need at least 2 distinct values for causal effect estimation."
+        )
+
     # Case 1: String / object / category
     if treatment_col.dtype == "object" or treatment_col.dtype.name == "category":
         unique_vals = sorted(treatment_col.unique().tolist())

causal_copilot/mcp/server.py

@@ -490,8 +490,17 @@ def estimate_effect(
 
     # Parse user-provided T0/T1 or leave as None for auto-detection.
     # Auto-detect: binary/discrete → min/max, continuous → 10th/90th percentile.
-    T0_input = float(control_value) if control_value else None
-    T1_input = float(treatment_value) if treatment_value else None
+    # Use try/except for float() to support categorical string values (e.g., "A", "B").
+    def _parse_tv(val):
+        if not val:
+            return None
+        try:
+            return float(val)
+        except (ValueError, TypeError):
+            return val  # categorical string
+
+    T0_input = _parse_tv(control_value)
+    T1_input = _parse_tv(treatment_value)
     _, T0_computed, T1_computed, treatment_kind = prepare_treatment(df, treatment, T0=T0_input, T1=T1_input)
     control_value = T0_computed
     treatment_value = T1_computed

tests/test_mcp.py

@@ -2653,6 +2653,42 @@ def test_b8_copilot_extracts_lagged_graph(self):
         assert "lagged_graph" in source
 
 
+# ── Audit Round 5 — Bug Fixes ──────────────────────────────────────────
+
+
+class TestAuditRound5Bugs:
+    """Tests for bugs found in the fifth comprehensive audit."""
+
+    def test_b1_prepare_treatment_single_value_raises(self):
+        """B1: prepare_treatment must raise ValueError for single-value treatment."""
+        from causal_copilot.mcp.offline import prepare_treatment
+
+        df = pd.DataFrame({"T": [1, 1, 1, 1], "Y": [2, 3, 4, 5]})
+        with pytest.raises(ValueError, match="unique value"):
+            prepare_treatment(df, "T")
+
+    def test_b2_copilot_matching_no_int_cast(self):
+        """B2: copilot.py matching must NOT int()-cast control/treatment values."""
+        import inspect
+
+        from causal_copilot.copilot import CausalCopilot
+
+        source = inspect.getsource(CausalCopilot)
+        # The estimate_matching call should NOT have int() wrapping
+        assert "int(control_value)" not in source
+        assert "int(treatment_value)" not in source
+
+    def test_b3_server_categorical_t0_t1(self):
+        """B3: server.py T0/T1 parsing handles categorical string values."""
+        import inspect
+
+        from causal_copilot.mcp.server import estimate_effect
+
+        source = inspect.getsource(estimate_effect)
+        # Must have try/except or other handling for non-numeric T0/T1
+        assert "_parse_tv" in source or "except" in source
+
+
 # ── MCP CLI ────────────────────────────────────────────────────────────