From 6a73faf8171d03bbce4da7df0b166292019fee9f Mon Sep 17 00:00:00 2001
From: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Date: Wed, 15 Jul 2026 04:40:21 -0400
Subject: [PATCH] Fix strict Kantorovich step acceptance

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
---
 lib/ImplicitDiscreteSolve/Project.toml      |  2 +-
 lib/ImplicitDiscreteSolve/src/controller.jl | 10 +++++-----
 lib/ImplicitDiscreteSolve/test/runtests.jl  | 14 ++++++++++++++
 3 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/lib/ImplicitDiscreteSolve/Project.toml b/lib/ImplicitDiscreteSolve/Project.toml
index a12aaced1c5..274e6b949eb 100644
--- a/lib/ImplicitDiscreteSolve/Project.toml
+++ b/lib/ImplicitDiscreteSolve/Project.toml
@@ -1,7 +1,7 @@
 name = "ImplicitDiscreteSolve"
 uuid = "3263718b-31ed-49cf-8a0f-35a466e8af96"
 authors = ["vyudu <vincent.duyuan@gmail.com>"]
-version = "2.1.3"
+version = "2.1.4"
 
 [deps]
 CommonSolve = "38540f10-b2f7-11e9-35d8-d573e4eb0ff2"
diff --git a/lib/ImplicitDiscreteSolve/src/controller.jl b/lib/ImplicitDiscreteSolve/src/controller.jl
index 77ffd180ee6..619212dea03 100644
--- a/lib/ImplicitDiscreteSolve/src/controller.jl
+++ b/lib/ImplicitDiscreteSolve/src/controller.jl
@@ -112,14 +112,14 @@ function OrdinaryDiffEqCore.step_reject_controller!(
     return
 end
 
+function _accept_kantorovich_step(controller, Θks)
+    return !controller.strict || all(Θk -> Θk <= controller.Θreject, Θks)
+end
+
 function OrdinaryDiffEqCore.accept_step_controller(integrator, cache::KantorovichTypeControllerCache, alg)
     (; controller) = cache
     (; Θks) = integrator.cache
-    if controller.strict
-        return all(controller.Θreject .< Θks)
-    else
-        return true
-    end
+    return _accept_kantorovich_step(controller, Θks)
 end
 
 function OrdinaryDiffEqCore.sync_controllers!(cache1::KantorovichTypeControllerCache, cache2::KantorovichTypeControllerCache)
diff --git a/lib/ImplicitDiscreteSolve/test/runtests.jl b/lib/ImplicitDiscreteSolve/test/runtests.jl
index a3ed609a380..1e4562ed34c 100644
--- a/lib/ImplicitDiscreteSolve/test/runtests.jl
+++ b/lib/ImplicitDiscreteSolve/test/runtests.jl
@@ -92,6 +92,20 @@ if TEST_GROUP != "QA"
         @test idsol.retcode == ReturnCode.Success
     end
 
+    @testset "Kantorovich strict acceptance" begin
+        strict = ImplicitDiscreteSolve.KantorovichTypeController(;
+            Θmin = 1 // 8, p = 1
+        )
+        @test ImplicitDiscreteSolve._accept_kantorovich_step(strict, Float64[])
+        @test ImplicitDiscreteSolve._accept_kantorovich_step(strict, [0.2, 0.95])
+        @test !ImplicitDiscreteSolve._accept_kantorovich_step(strict, [0.2, 0.951])
+
+        nonstrict = ImplicitDiscreteSolve.KantorovichTypeController(;
+            Θmin = 1 // 8, p = 1, strict = false
+        )
+        @test ImplicitDiscreteSolve._accept_kantorovich_step(nonstrict, [Inf])
+    end
+
     @testset "Handle nothing in u0" begin
         emptyiip(residual, u_next, u, p, t) = nothing
         emptyoop(u_next, u, p, t) = nothing