From 69661fc83a2cc04ce511aa5c1191706eb8ce3425 Mon Sep 17 00:00:00 2001
From: Patrick Kidger <33688385+patrick-kidger@users.noreply.github.com>
Date: Fri, 30 Jan 2026 16:31:34 +0100
Subject: [PATCH] Fix #720; bool event + root find + terminate on first step

---
 diffrax/_integrate.py | 11 ++++++++---
 test/test_event.py    | 27 +++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/diffrax/_integrate.py b/diffrax/_integrate.py
index 6fc38ce3..3017e30d 100644
--- a/diffrax/_integrate.py
+++ b/diffrax/_integrate.py
@@ -761,7 +761,11 @@ def _call_real_impl():
             _tfinal = _event_root_find.value
             # TODO: we might need to change the way we evaluate `_yfinal` in order to
             # get more accurate derivatives?
-            _yfinal = _interpolator.evaluate(_tfinal)
+            _yfinal = lax.cond(
+                final_state.num_steps == 0,
+                lambda: final_state.y,
+                lambda: _interpolator.evaluate(_tfinal),
+            )
             _result = RESULTS.where(
                 _event_root_find.result == optx.RESULTS.successful,
                 result,
@@ -1323,7 +1327,7 @@ def _allocate_output(subsaveat: SubSaveAt) -> SaveState:
         event_mask = None
     else:
         event_tprev = tprev
-        event_tnext = tnext
+        event_tnext = tprev
         # Fill the dense-info with dummy values on the first step, when we haven't yet
         # made any steps.
         # Note that we're threading a needle here! What if we terminate on the very
@@ -1334,8 +1338,9 @@ def _allocate_output(subsaveat: SubSaveAt) -> SaveState:
         #   to the end of the interval).
         # - A floating event can't terminate on the first step (it requires a sign
         #   change).
+        # c.f. https://github.com/patrick-kidger/diffrax/issues/720
         event_dense_info = jtu.tree_map(
-            lambda x: jnp.empty(x.shape, x.dtype),
+            lambda x: jnp.zeros(x.shape, x.dtype),
             dense_info_struct,  # pyright: ignore[reportPossiblyUnboundVariable]
         )
 
diff --git a/test/test_event.py b/test/test_event.py
index 5de0ded6..6a6b97e1 100644
--- a/test/test_event.py
+++ b/test/test_event.py
@@ -833,3 +833,30 @@ def run(event):
     t_final, y_final = run(event)
     assert jnp.allclose(t_final, 10.0)
     assert jnp.allclose(y_final, jnp.array([10.0, 11.0]))
+
+
+# https://github.com/patrick-kidger/diffrax/issues/720
+def test_boolean_with_root_find_terminating_on_first_step():
+    controller = diffrax.PIDController(rtol=1e-6, atol=1e-6)
+    steady_state_event = diffrax.steady_state_event(rtol=1e-6, atol=1e-6)
+    root_finder = optx.Newton(atol=1e-4, rtol=1e-4)
+
+    sol = diffrax.diffeqsolve(
+        diffrax.ODETerm(lambda t, y, args: jnp.zeros_like(y)),
+        diffrax.Kvaerno5(),
+        t0=0.0,
+        t1=1.2,
+        dt0=None,
+        y0=jnp.array([10.0]),
+        stepsize_controller=controller,
+        event=diffrax.Event(
+            cond_fn=steady_state_event,
+            root_finder=root_finder,
+        ),
+        saveat=diffrax.SaveAt(t1=True),
+        max_steps=100,
+    )
+    assert sol.ts is not None
+    assert sol.ys is not None
+    assert jnp.allclose(sol.ts, jnp.array([0.0]))
+    assert jnp.allclose(sol.ys, jnp.array([[10.0]]))