From 358994846488dcb4d3a159ef3024530415de79ef Mon Sep 17 00:00:00 2001
From: Kristoffer Carlsson <kcarlsson89@gmail.com>
Date: Thu, 23 Apr 2026 15:52:45 +0200
Subject: [PATCH 1/2] add HyperHessian definition

ref https://github.com/JuliaDiff/DifferentiationInterface.jl/pull/940
---
 src/ADTypes.jl   |  1 +
 src/dense.jl     | 32 ++++++++++++++++++++++++++++++++
 src/symbols.jl   |  4 ++--
 test/dense.jl    | 14 ++++++++++++++
 test/runtests.jl |  3 +++
 test/symbols.jl  |  1 +
 6 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/src/ADTypes.jl b/src/ADTypes.jl
index f53cee1..de5f756 100644
--- a/src/ADTypes.jl
+++ b/src/ADTypes.jl
@@ -36,6 +36,7 @@ export AutoChainRules,
     AutoFiniteDifferences,
     AutoForwardDiff,
     AutoGTPSA,
+    AutoHyperHessians,
     AutoModelingToolkit,
     AutoMooncake,
     AutoMooncakeForward,
diff --git a/src/dense.jl b/src/dense.jl
index 6f7944d..a7ff514 100644
--- a/src/dense.jl
+++ b/src/dense.jl
@@ -562,6 +562,38 @@ struct AutoZygote <: AbstractADType end
 
 mode(::AutoZygote) = ReverseMode()
 
+"""
+    AutoHyperHessians{chunksize}
+
+Struct used to select the [HyperHessians.jl](https://github.com/KristofferC/HyperHessians.jl) backend for automatic differentiation.
+
+Defined by [ADTypes.jl](https://github.com/SciML/ADTypes.jl).
+
+# Constructors
+
+    AutoHyperHessians(; chunksize=nothing)
+
+# Type parameters
+
+  - `chunksize`: the preferred chunk size to evaluate several derivatives at once. If `nothing`, HyperHessians chooses automatically.
+"""
+struct AutoHyperHessians{chunksize} <: AbstractADType end
+
+function AutoHyperHessians(; chunksize::Union{Nothing, Int} = nothing)
+    if chunksize isa Int
+        chunksize > 0 || throw(ArgumentError("chunksize must be positive, got $chunksize"))
+    end
+    return AutoHyperHessians{chunksize}()
+end
+
+mode(::AutoHyperHessians) = ForwardMode()
+
+function Base.show(io::IO, ::AutoHyperHessians{chunksize}) where {chunksize}
+    print(io, AutoHyperHessians, "(")
+    chunksize !== nothing && print(io, "chunksize=", repr(chunksize; context = io))
+    return print(io, ")")
+end
+
 """
     NoAutoDiff
 
diff --git a/src/symbols.jl b/src/symbols.jl
index 11d27ca..afa5a22 100644
--- a/src/symbols.jl
+++ b/src/symbols.jl
@@ -24,8 +24,8 @@ Auto(package::Symbol, args...; kws...) = Auto(Val(package), args...; kws...)
 
 for backend in (
         :ChainRules, :Diffractor, :Enzyme, :Reactant, :FastDifferentiation,
-        :FiniteDiff, :FiniteDifferences, :ForwardDiff, :GTPSA, :Mooncake, :PolyesterForwardDiff,
-        :ReverseDiff, :Symbolics, :Tapir, :TaylorDiff, :Tracker, :Zygote,
+        :FiniteDiff, :FiniteDifferences, :ForwardDiff, :GTPSA, :HyperHessians, :Mooncake,
+        :PolyesterForwardDiff, :ReverseDiff, :Symbolics, :Tapir, :TaylorDiff, :Tracker, :Zygote,
     )
     @eval Auto(::Val{$(QuoteNode(backend))}, args...; kws...) = $(Symbol(:Auto, backend))(
         args...; kws...
diff --git a/test/dense.jl b/test/dense.jl
index 932b8ef..1948b4d 100644
--- a/test/dense.jl
+++ b/test/dense.jl
@@ -160,6 +160,20 @@ end
     @test ad.descriptor == Val(:descriptor)
 end
 
+@testset "AutoHyperHessians" begin
+    ad = AutoHyperHessians()
+    @test ad isa AbstractADType
+    @test ad isa AutoHyperHessians{nothing}
+    @test mode(ad) isa ForwardMode
+
+    ad = AutoHyperHessians(; chunksize = 8)
+    @test ad isa AbstractADType
+    @test ad isa AutoHyperHessians{8}
+    @test mode(ad) isa ForwardMode
+
+    @test_throws ArgumentError AutoHyperHessians(; chunksize = -1)
+end
+
 @testset "AutoMooncake" begin
     ad = AutoMooncake(; config = :config)
     @test ad isa AbstractADType
diff --git a/test/runtests.jl b/test/runtests.jl
index c04d2ac..c77d322 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -46,6 +46,7 @@ function every_ad()
         AutoFiniteDifferences(; fdm = :fdm),
         AutoForwardDiff(),
         AutoGTPSA(),
+        AutoHyperHessians(),
         AutoPolyesterForwardDiff(),
         AutoReverseDiff(),
         AutoSymbolics(),
@@ -72,6 +73,8 @@ function every_ad_with_options()
         AutoForwardDiff(chunksize = 3, tag = :tag),
         AutoGTPSA(),
         AutoGTPSA(descriptor = Val(:descriptor)),
+        AutoHyperHessians(),
+        AutoHyperHessians(chunksize = 8),
         AutoMooncake(; config = :config),
         AutoMooncakeForward(; config = :config),
         AutoPolyesterForwardDiff(),
diff --git a/test/symbols.jl b/test/symbols.jl
index 10802cc..90d330c 100644
--- a/test/symbols.jl
+++ b/test/symbols.jl
@@ -8,6 +8,7 @@ using Test
 @test ADTypes.Auto(:FiniteDiff) isa AutoFiniteDiff
 @test ADTypes.Auto(:FiniteDifferences, 1.0) isa AutoFiniteDifferences{Float64}
 @test ADTypes.Auto(:ForwardDiff) isa AutoForwardDiff
+@test ADTypes.Auto(:HyperHessians) isa AutoHyperHessians
 @test ADTypes.Auto(:Mooncake) isa AutoMooncake
 @test ADTypes.Auto(:PolyesterForwardDiff) isa AutoPolyesterForwardDiff
 @test ADTypes.Auto(:ReverseDiff) isa AutoReverseDiff

From d1a196b29311dae64feb5a8ca6b669fba75f35a9 Mon Sep 17 00:00:00 2001
From: Kristoffer Carlsson <kcarlsson89@gmail.com>
Date: Fri, 24 Apr 2026 21:28:10 +0200
Subject: [PATCH 2/2] add to docs

---
 docs/src/index.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/docs/src/index.md b/docs/src/index.md
index 2bed335..85e7e55 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -36,6 +36,7 @@ Taylor mode:
 ```@docs
 AutoGTPSA
 AutoTaylorDiff
+AutoHyperHessians
 ```
 
 ### Reverse mode