diff --git a/ext/TensorOperationsEnzymeExt/TensorOperationsEnzymeExt.jl b/ext/TensorOperationsEnzymeExt/TensorOperationsEnzymeExt.jl
index 4f314d06..8094b908 100644
--- a/ext/TensorOperationsEnzymeExt/TensorOperationsEnzymeExt.jl
+++ b/ext/TensorOperationsEnzymeExt/TensorOperationsEnzymeExt.jl
@@ -16,6 +16,21 @@ using Enzyme.EnzymeCore: EnzymeRules
 @inline EnzymeRules.inactive_type(v::Type{<:Index2Tuple}) = true
 @inline EnzymeRules.inactive_type(v::Type{<:IndexTuple}) = true
 
+"""
+    is_inactive(config, x::Annotation) -> Bool
+
+Check whether the annotated argument `x` should be treated as inactive, i.e. whether no tangent should be read from or accumulated into `x.dval`.
+Under runtime activity, Enzyme may pass an argument that is inactive at run time as a `Duplicated` whose shadow aliases the primal (`x.dval === x.val`);
+writing into that shadow would then corrupt the primal.
+See https://enzymead.github.io/Enzyme.jl/dev/faq/#faq-runtime-activity.
+
+!!! note
+    This is a stopgap until an equivalent helper is available in `EnzymeRules`
+    (see https://github.com/EnzymeAD/Enzyme.jl/pull/3597#discussion_r4014313289).
+"""
+@inline is_inactive(config, ::Const) = true
+@inline is_inactive(config, x::Annotation) = EnzymeRules.runtime_activity(config) && x.dval === x.val
+
 function EnzymeRules.augmented_primal(
         config::EnzymeRules.RevConfigWidth{1},
         func::Const{typeof(TensorOperations.tensoralloc)},
@@ -62,7 +77,7 @@ function EnzymeRules.augmented_primal(
     # form caches if needed
     cache_A = EnzymeRules.overwritten(config)[3] ? copy(A_dA.val) : nothing
     cache_B = EnzymeRules.overwritten(config)[6] ? copy(B_dB.val) : nothing
-    cache_C = !isa(β_dβ, Const) ? copy(C_dC.val) : C_dC.val
+    cache_C = !isa(β_dβ, Const) && !is_inactive(config, C_dC) ? copy(C_dC.val) : C_dC.val
     ba = map(ba_ -> getfield(ba_, :val), ba_dba)
     TensorOperations.tensorcontract!(C_dC.val, A_dA.val, pA_dpA.val, conjA_dconjA.val, B_dB.val, pB_dpB.val, conjB_dconjB.val, pAB_dpAB.val, α_dα.val, β_dβ.val, ba...)
     primal = EnzymeRules.needs_primal(config) ? C_dC.val : nothing
@@ -97,17 +112,17 @@ function EnzymeRules.reverse(
     β = β_dβ.val
     pA, pB, pAB, conjA, conjB = getfield.((pA_dpA, pB_dpB, pAB_dpAB, conjA_dconjA, conjB_dconjB), :val)
 
-    if !isa(A_dA, Const) && !isa(C_dC, Const)
+    if !is_inactive(config, A_dA) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         ΔA = A_dA.dval
         TensorOperations.tensorcontract_pullback_dA!(ΔA, ΔC, Cval, Aval, pA, conjA, Bval, pB, conjB, pAB, α, ba...)
     end
-    if !isa(B_dB, Const) && !isa(C_dC, Const)
+    if !is_inactive(config, B_dB) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         ΔB = B_dB.dval
         TensorOperations.tensorcontract_pullback_dB!(ΔB, ΔC, Cval, Aval, pA, conjA, Bval, pB, conjB, pAB, α, ba...)
     end
-    Δα = if !isa(α_dα, Const) && !isa(C_dC, Const)
+    Δα = if !isa(α_dα, Const) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         TensorOperations.tensorcontract_pullback_dα(ΔC, Cval, Aval, pA, conjA, Bval, pB, conjB, pAB, α, ba...)
     elseif !isa(α_dα, Const)
@@ -115,7 +130,7 @@ function EnzymeRules.reverse(
     else
         nothing
     end
-    Δβ = if !isa(β_dβ, Const) && !isa(C_dC, Const)
+    Δβ = if !isa(β_dβ, Const) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         TensorOperations.pullback_dβ(ΔC, Cval, β)
     elseif !isa(β_dβ, Const)
@@ -123,7 +138,7 @@ function EnzymeRules.reverse(
     else
         nothing
     end
-    !isa(C_dC, Const) && TensorOperations.pullback_dC!(C_dC.dval, β)
+    !is_inactive(config, C_dC) && TensorOperations.pullback_dC!(C_dC.dval, β)
     return nothing, nothing, nothing, nothing, nothing, nothing, nothing, nothing, Δα, Δβ, map(ba_ -> nothing, ba)...
 end
 
@@ -148,7 +163,7 @@ function EnzymeRules.forward(
     β = β_dβ.val
     pA, pB, pAB, conjA, conjB = getfield.((pA_dpA, pB_dpB, pAB_dpAB, conjA_dconjA, conjB_dconjB), :val)
 
-    if !isa(C_dC, Const)
+    if !is_inactive(config, C_dC)
         scale!(C_dC.dval, β)
         if !isa(β_dβ, Const)
             add!(C_dC.dval, C_dC.val, β_dβ.dval)
@@ -156,10 +171,10 @@ function EnzymeRules.forward(
         if !isa(α_dα, Const)
             tensorcontract!(C_dC.dval, A_dA.val, pA, conjA, B_dB.val, pB, conjB, pAB, α_dα.dval, One(), ba...)
         end
-        if !isa(A_dA, Const)
+        if !is_inactive(config, A_dA)
             tensorcontract!(C_dC.dval, A_dA.dval, pA, conjA, B_dB.val, pB, conjB, pAB, α, One(), ba...)
         end
-        if !isa(B_dB, Const)
+        if !is_inactive(config, B_dB)
             tensorcontract!(C_dC.dval, A_dA.val, pA, conjA, B_dB.dval, pB, conjB, pAB, α, One(), ba...)
         end
     end
@@ -189,7 +204,7 @@ function EnzymeRules.augmented_primal(
     ) where {RT, Tα <: Number, Tβ <: Number, TA <: Number, TC <: Number}
     # form caches if needed
     cache_A = EnzymeRules.overwritten(config)[3] ? copy(A_dA.val) : nothing
-    cache_C = !isa(β_dβ, Const) ? copy(C_dC.val) : C_dC.val
+    cache_C = !isa(β_dβ, Const) && !is_inactive(config, C_dC) ? copy(C_dC.val) : C_dC.val
     ba = map(ba_ -> getfield(ba_, :val), ba_dba)
     α = α_dα.val
     β = β_dβ.val
@@ -222,12 +237,12 @@ function EnzymeRules.reverse(
     β = β_dβ.val
     ba = map(ba_ -> getfield(ba_, :val), ba_dba)
 
-    if !isa(A_dA, Const) && !isa(C_dC, Const)
+    if !is_inactive(config, A_dA) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         ΔA = A_dA.dval
         TensorOperations.tensoradd_pullback_dA!(ΔA, ΔC, Cval, Aval, pA, conjA, α, ba...)
     end
-    Δα = if !isa(α_dα, Const) && !isa(C_dC, Const)
+    Δα = if !isa(α_dα, Const) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         TensorOperations.tensoradd_pullback_dα(ΔC, Cval, Aval, pA, conjA, α, ba...)
     elseif !isa(α_dα, Const)
@@ -235,7 +250,7 @@ function EnzymeRules.reverse(
     else
         nothing
     end
-    Δβ = if !isa(β_dβ, Const) && !isa(C_dC, Const)
+    Δβ = if !isa(β_dβ, Const) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         TensorOperations.pullback_dβ(ΔC, Cval, β)
     elseif !isa(β_dβ, Const)
@@ -243,7 +258,7 @@ function EnzymeRules.reverse(
     else
         nothing
     end
-    !isa(C_dC, Const) && TensorOperations.pullback_dC!(C_dC.dval, β)
+    !is_inactive(config, C_dC) && TensorOperations.pullback_dC!(C_dC.dval, β)
     return nothing, nothing, nothing, nothing, Δα, Δβ, map(ba_ -> nothing, ba)...
 end
 
@@ -270,12 +285,12 @@ function EnzymeRules.forward(
     # dD = dα * A + α * dA + β dC + dβ * C
 
     # dC′ = β dC + dβ * C
-    if !isa(C_dC, Const)
+    if !is_inactive(config, C_dC)
         scale!(C_dC.dval, β)
         if !isa(β_dβ, Const)
             add!(C_dC.dval, C_dC.val, β_dβ.dval)
         end
-        if !isa(A_dA, Const)
+        if !is_inactive(config, A_dA)
             TensorOperations.tensoradd!(C_dC.dval, A_dA.dval, pA, conjA, α, One(), ba...)
         end
         if !isa(α_dα, Const)
@@ -309,7 +324,7 @@ function EnzymeRules.augmented_primal(
     ) where {RT, Tα <: Number, Tβ <: Number, TA <: Number, TC <: Number}
     # form caches if needed
     cache_A = EnzymeRules.overwritten(config)[3] ? copy(A_dA.val) : nothing
-    cache_C = !isa(β_dβ, Const) ? copy(C_dC.val) : C_dC.val
+    cache_C = !isa(β_dβ, Const) && !is_inactive(config, C_dC) ? copy(C_dC.val) : C_dC.val
     ba = map(ba_ -> getfield(ba_, :val), ba_dba)
     α = α_dα.val
     β = β_dβ.val
@@ -344,12 +359,12 @@ function EnzymeRules.reverse(
     β = β_dβ.val
     ba = map(ba_ -> getfield(ba_, :val), ba_dba)
 
-    if !isa(A_dA, Const) && !isa(C_dC, Const)
+    if !is_inactive(config, A_dA) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         ΔA = A_dA.dval
         TensorOperations.tensortrace_pullback_dA!(ΔA, ΔC, Cval, Aval, p, q, conjA, α, ba...)
     end
-    Δα = if !isa(α_dα, Const) && !isa(C_dC, Const)
+    Δα = if !isa(α_dα, Const) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         TensorOperations.tensortrace_pullback_dα(ΔC, Cval, Aval, p, q, conjA, α, ba...)
     elseif !isa(α_dα, Const)
@@ -357,7 +372,7 @@ function EnzymeRules.reverse(
     else
         nothing
     end
-    Δβ = if !isa(β_dβ, Const) && !isa(C_dC, Const)
+    Δβ = if !isa(β_dβ, Const) && !is_inactive(config, C_dC)
         ΔC = C_dC.dval
         TensorOperations.pullback_dβ(ΔC, Cval, β)
     elseif !isa(β_dβ, Const)
@@ -365,7 +380,7 @@ function EnzymeRules.reverse(
     else
         nothing
     end
-    !isa(C_dC, Const) && TensorOperations.pullback_dC!(C_dC.dval, β)
+    !is_inactive(config, C_dC) && TensorOperations.pullback_dC!(C_dC.dval, β)
     return nothing, nothing, nothing, nothing, nothing, Δα, Δβ, map(ba_ -> nothing, ba)...
 end
 
@@ -391,7 +406,7 @@ function EnzymeRules.forward(
 
     # dD = dα * tr(A) + α * tr(dA) + dβ * C + β * dC
     # dC1 = dβ * C + β * dC
-    if !isa(C_dC, Const)
+    if !is_inactive(config, C_dC)
         scale!(C_dC.dval, β)
         if !isa(β_dβ, Const)
             add!(C_dC.dval, C_dC.val, β_dβ.dval)
@@ -399,7 +414,7 @@ function EnzymeRules.forward(
         if !isa(α_dα, Const)
             TensorOperations.tensortrace!(C_dC.dval, A_dA.val, p, q, conjA, α_dα.dval, One(), ba...)
         end
-        if !isa(A_dA, Const)
+        if !is_inactive(config, A_dA)
             TensorOperations.tensortrace!(C_dC.dval, A_dA.dval, p, q, conjA, α, One(), ba...)
         end
     end
diff --git a/test/enzyme.jl b/test/enzyme.jl
index aa7245cf..1f6064c6 100644
--- a/test/enzyme.jl
+++ b/test/enzyme.jl
@@ -260,3 +260,28 @@ end
     test_reverse(tensoradd!, Duplicated, (C, Duplicated), (A, Duplicated), (pA, Const), (false, Const), (randn(), Active), (0.0, Active))
     test_reverse(tensoradd!, Duplicated, (C, Duplicated), (A, Duplicated), (pA, Const), (false, Const), (randn(), Active), (randn(), Const))
 end
+
+# under runtime activity, inactive operands can arrive as `Duplicated` with `dval === val`,
+# see https://github.com/EnzymeAD/Enzyme.jl/issues/3623
+const RUNTIME_ACTIVITY_OPS = Dict([1, 2] => randn(4, 4), [2, 3] => randn(4, 4))
+function runtime_activity_f(x)
+    total = 0.0
+    for (_, op) in collect(RUNTIME_ACTIVITY_OPS)
+        C = zeros(length(x))
+        @tensor C[i] += op[i, j] * x[j]
+        D = x * x'
+        @tensor D[j, i] += op[i, j]
+        T = fill(x[1])
+        @tensor T[] += op[i, i]
+        total += sum(C) + sum(D) + T[]
+    end
+    return total
+end
+@testset "runtime activity with inactive operands" begin
+    ops = deepcopy(RUNTIME_ACTIVITY_OPS)
+    x = randn(4)
+    dx = zero(x)
+    Enzyme.autodiff(set_runtime_activity(Reverse), Const(runtime_activity_f), Active, Duplicated(x, dx))
+    @test all(RUNTIME_ACTIVITY_OPS[k] == ops[k] for k in keys(ops))
+    @test dx ≈ sum(vec(sum(op; dims = 1)) .+ 2 * sum(x) .+ [1, 0, 0, 0] for op in values(ops))
+end