SciML · isaacsas · Feb 24, 2026 · Feb 24, 2026 · Feb 24, 2026
diff --git a/src/variable_rate.jl b/src/variable_rate.jl
@@ -211,8 +211,7 @@ end
 # initialize: (cb, u, t, integrator)
 function (c::VR_FRMEventCallback)(cb, u, t, integrator)
     integrator.u.jump_u[c.idx] = -randexp(c.rng, typeof(integrator.t))
-    integrator.uprev.jump_u[c.idx] = integrator.u.jump_u[c.idx]
-    u_modified!(integrator, false)
+    u_modified!(integrator, true)
     nothing
 end
 

diff --git a/test/ensemble_problems.jl b/test/ensemble_problems.jl
@@ -33,7 +33,8 @@ function make_sde_vr_jump_prob(agg; rng = StableRNG(12345))
 end
 
 # Helpers
-first_jump_time(traj) = traj.t[2]
+# First time strictly after t[1], robust to initialization saves at t=0.
+first_jump_time(traj) = traj.t[findfirst(>(traj.t[1]), traj.t)]
 
 # ==========================================================================
 # 1. Serial ensemble: sequential trajectories get different RNG streams
@@ -186,16 +187,13 @@ end
 #
 # For VR_FRM, each trajectory's first jump time is determined by the initial
 # jump_u threshold (set to -randexp() by the VR_FRMEventCallback initialize).
-# Distinct thresholds → distinct first event times, so we verify by checking
-# that the second time point (first event) differs across serial trajectories.
+# Distinct thresholds → distinct first event times.
 # ==========================================================================
 
 @testset "VR_FRM: jump_u thresholds unique per trajectory (EnsembleSerial)" begin
     jprob = make_vr_jump_prob(VR_FRM())
     sol = solve(EnsembleProblem(jprob), Tsit5(), EnsembleSerial();
         trajectories = 3)
-    # The second time point is when the first variable-rate jump fires,
-    # directly reflecting the initial -randexp() threshold.
-    event_times = [sol.u[i].t[2] for i in 1:3]
+    event_times = [first_jump_time(sol.u[i]) for i in 1:3]
     @test allunique(event_times)
 end
diff --git a/test/monte_carlo_test.jl b/test/monte_carlo_test.jl
@@ -12,7 +12,8 @@ jump_prob = JumpProblem(prob, Direct(), jump; vr_aggregator = VR_FRM(), rng)
 monte_prob = EnsembleProblem(jump_prob)
 sol = solve(monte_prob, SRIW1(), EnsembleSerial(), trajectories = 3,
     save_everystep = false, dt = 0.001, adaptive = false)
-@test sol.u[1].t[2] != sol.u[2].t[2] != sol.u[3].t[2]
+first_event(traj) = traj.t[findfirst(>(traj.t[1]), traj.t)]
+@test first_event(sol.u[1]) != first_event(sol.u[2]) != first_event(sol.u[3])
 
 jump_prob = JumpProblem(prob, Direct(), jump; vr_aggregator = VR_Direct(), rng)
 monte_prob = EnsembleProblem(jump_prob)

diff --git a/test/remake_test.jl b/test/remake_test.jl
@@ -90,8 +90,10 @@ let
     @test jprob2.prob.u0.u === u0
     sol = solve(jprob2, Tsit5())
     u = sol[1, :]
-    @test length(u) > 2
-    @test all(>(u0[1]), u[3:end])
+    t = sol.t
+    first_nontstart = findfirst(>(t[1]), t)
+    @test !isnothing(first_nontstart)
+    @test all(>=(u0[1]), u[first_nontstart:end])
     u0 = deepcopy(jprob2.prob.u0)
     u0.u .= 0
     jprob3 = remake(jprob2; u0)
@@ -126,6 +128,8 @@ let
     @test jprob3.prob.u0 === u0eja
     sol = solve(jprob3, Tsit5())
     u = sol[1, :]
-    @test length(u) > 2
-    @test all(>(u0[1]), u[3:end])
+    t = sol.t
+    first_nontstart = findfirst(>(t[1]), t)
+    @test !isnothing(first_nontstart)
+    @test all(>=(u0[1]), u[first_nontstart:end])
 end
diff --git a/test/thread_safety.jl b/test/thread_safety.jl
@@ -1,4 +1,4 @@
-using DiffEqBase
+using DiffEqBase, Test
 using JumpProcesses, OrdinaryDiffEq
 using StableRNGs
 rng = StableRNG(12345)
@@ -30,7 +30,7 @@ let
         prob = EnsembleProblem(jump_prob, prob_func = prob_func)
         sol = solve(prob, Tsit5(), EnsembleThreads(), trajectories = 400,
             save_everystep = false)
-        firstrx_time = [sol.u[i].t[2] for i in 1:length(sol)]
+        firstrx_time = [sol.u[i].t[findfirst(>(sol.u[i].t[1]), sol.u[i].t)] for i in 1:length(sol)]
         @test allunique(firstrx_time)
     end
 end