diff --git a/src/rtichoke/calibration/__init__.py b/src/rtichoke/calibration/__init__.py
index b04d51eb..95d013ce 100644
--- a/src/rtichoke/calibration/__init__.py
+++ b/src/rtichoke/calibration/__init__.py
@@ -16,8 +16,25 @@ def create_calibration_curve(*args, **kwargs):
     )
 
 
+def _extract_heuristics_sets(args, kwargs):
+    if "heuristics_sets" in kwargs:
+        return kwargs["heuristics_sets"]
+    if len(args) > 4:
+        return args[4]
+    return None
+
+
 def create_calibration_curve_times(*args, **kwargs):
     """Create an interactive time-dependent calibration plot with a square main panel."""
+    heuristics_sets = _extract_heuristics_sets(args, kwargs)
+    if heuristics_sets is not None and len(heuristics_sets) != 1:
+        raise ValueError(
+            "create_calibration_curve_times() currently supports exactly one "
+            "heuristics set. Multiple heuristic sets would be combined in the "
+            "same plotted calibration curve. Call the function separately for "
+            "each heuristic set."
+        )
+
     return enforce_square_calibration_panel(
         _original_create_calibration_curve_times(*args, **kwargs)
     )
diff --git a/tests/test_calibration_times_heuristics_validation.py b/tests/test_calibration_times_heuristics_validation.py
new file mode 100644
index 00000000..69b0eff0
--- /dev/null
+++ b/tests/test_calibration_times_heuristics_validation.py
@@ -0,0 +1,63 @@
+import numpy as np
+import pytest
+
+from rtichoke import create_calibration_curve_times as create_calibration_curve_times_top_level
+from rtichoke.calibration import create_calibration_curve_times
+from rtichoke.calibration.calibration import (
+    create_calibration_curve_times as create_calibration_curve_times_direct,
+)
+
+
+@pytest.mark.parametrize(
+    "entry_point",
+    [
+        create_calibration_curve_times_top_level,
+        create_calibration_curve_times,
+        create_calibration_curve_times_direct,
+    ],
+)
+def test_create_calibration_curve_times_rejects_multiple_heuristic_sets(entry_point):
+    probs = {"model_1": np.array([0.1, 0.2, 0.3, 0.4])}
+    reals = np.array([0, 1, 0, 1])
+    times = np.array([1.0, 2.0, 3.0, 4.0])
+
+    heuristics_sets = [
+        {
+            "censoring_heuristic": "adjusted",
+            "competing_heuristic": "adjusted_as_negative",
+        },
+        {
+            "censoring_heuristic": "excluded",
+            "competing_heuristic": "excluded",
+        },
+    ]
+
+    with pytest.raises(ValueError, match="exactly one heuristics set"):
+        entry_point(
+            probs,
+            reals,
+            times,
+            fixed_time_horizons=[2.0],
+            heuristics_sets=heuristics_sets,
+        )
+
+
+def test_create_calibration_curve_times_allows_one_heuristic_set():
+    probs = {"model_1": np.array([0.1, 0.2, 0.3, 0.4])}
+    reals = np.array([0, 1, 0, 1])
+    times = np.array([1.0, 2.0, 3.0, 4.0])
+
+    fig = create_calibration_curve_times(
+        probs,
+        reals,
+        times,
+        fixed_time_horizons=[2.0],
+        heuristics_sets=[
+            {
+                "censoring_heuristic": "adjusted",
+                "competing_heuristic": "adjusted_as_negative",
+            }
+        ],
+    )
+
+    assert fig is not None
diff --git a/user_guide/02-curve-api-compatibility.qmd b/user_guide/02-curve-api-compatibility.qmd
index e603b3fd..d2eb52dd 100644
--- a/user_guide/02-curve-api-compatibility.qmd
+++ b/user_guide/02-curve-api-compatibility.qmd
@@ -48,10 +48,11 @@ These statements describe the effect of the heuristics on the estimates. Functio
 
 ## Time-dependent calibration heuristics
 
-`create_calibration_curve_times()` differs from its ROC, precision-recall, Gains, Lift, and decision-curve siblings in two important ways:
+`create_calibration_curve_times()` differs from its ROC, precision-recall, Gains, Lift, and decision-curve siblings in three important ways:
 
 1. `heuristics_sets` is currently required rather than defaulted.
-2. Calibration explicitly rejects unsupported heuristic combinations (specifically `competing_heuristic="adjusted_as_censored"`) with an `Unsupported calibration heuristics` error instead of silently skipping requested horizons.
+2. Calibration currently supports exactly one heuristic set per call. Use separate calls when comparing heuristic choices so distinct calibration estimates are never combined into one plotted curve.
+3. Calibration explicitly rejects unsupported heuristic combinations (specifically `competing_heuristic="adjusted_as_censored"`) with an `Unsupported calibration heuristics` error instead of silently skipping requested horizons.
 
 Pass the calibration heuristic explicitly. For adjusted or exclusion-based paths: