diff --git a/Project.toml b/Project.toml
index 3fe422fc..29a6dcc6 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "TensorAlgebra"
 uuid = "68bd88dc-f39d-4e12-b2ca-f046b68fcc6a"
-version = "0.21.0"
+version = "0.21.1"
 authors = ["ITensor developers <support@itensor.org> and contributors"]
 
 [workspace]
diff --git a/ext/TensorAlgebraTensorKitExt.jl b/ext/TensorAlgebraTensorKitExt.jl
index a17957e1..608f8d4c 100644
--- a/ext/TensorAlgebraTensorKitExt.jl
+++ b/ext/TensorAlgebraTensorKitExt.jl
@@ -23,6 +23,7 @@ TensorAlgebra.size(t::AbstractTensorMap, i::Int) = dim(space(t, i))
 TensorAlgebra.size(t::AbstractTensorMap) = ntuple(i -> dim(space(t, i)), numind(t))
 # A `TensorMap` stores its codomain/domain split, so its codomain rank is `numout`.
 TensorAlgebra.ndims_codomain(t::AbstractTensorMap) = numout(t)
+TensorAlgebra.has_bipartition(::AbstractTensorMap) = true
 
 # `t[]` on a rank-0 `TensorMap` requires a trivial sector type; `TensorKit.scalar` is the
 # general spelling.
diff --git a/src/TensorAlgebra.jl b/src/TensorAlgebra.jl
index 63968afb..92887326 100644
--- a/src/TensorAlgebra.jl
+++ b/src/TensorAlgebra.jl
@@ -5,7 +5,7 @@ export contract, contract!, contractalign, dual, isdual, MatrixAlgebra
 if VERSION >= v"1.11.0-DEV.469"
     eval(
         Meta.parse(
-            "public AbstractAlgorithm, add!, AddBroadcasted, addends, allocate_output, allocate_project, arguments, axes, bipartition, bipartition_axes, biperm, bipermutedims, bipermutedims!, bipermutedimsopadd!, cat_axis, cat_similar, check_input, concatenate, concatenate!, ConjBroadcasted, contractadd!, ContractAlgorithm, contractopadd!, contractperm, contractperm!, contractpermadd!, contractpermalign, contractpermopadd!, data, datatype, default_algorithm, dims2cat, directsum, eig_full, eig_trunc, eig_vals, eigh_full, eigh_trunc, eigh_vals, fill_map, flattenlinear, infer_aux_space, invsqrth_safe, is_output_view, is_projected, isidentitybiperm, label_type, left_null, left_orth, left_polar, LinearBroadcasted, linearbroadcasted, lq_compact, lq_full, matricize, MatricizeContract, matricizeop, matricizeop!, matricizeopcopy, matricizeopview, MatricizeStyle, MATRIX_FUNCTIONS, ndims, ndims_codomain, ndims_domain, one, ones_map, operation, output_axes, PermutedDims, permuteddims, permutedims, permutedims!, permutedimsadd!, permutedimsop, permutedimsopadd!, project, project!, project_aux, project_hermitian, projectto!, qr_compact, qr_full, rand_map, randn_map, right_null, right_orth, right_polar, scalar, scale!, ScaledBroadcasted, select_algorithm, similar_map, size, sqrth_invsqrth_safe, sqrth_safe, sum, svd_compact, svd_full, svd_trunc, svd_vals, TensorOperationsContract, to_range, tr, trivialrange, tryflattenlinear, tryproject, tryproject_aux, unchecked_project, unchecked_project_aux, ungrade, unmatricize, unmatricize!, unmatricize_factors, unproject, unscaled, zero!, zeros_map"
+            "public AbstractAlgorithm, add!, AddBroadcasted, addends, allocate_output, allocate_project, arguments, axes, bipartition, bipartition_axes, biperm, bipermutedims, bipermutedims!, bipermutedimsopadd!, cat_axis, cat_similar, check_input, concatenate, concatenate!, ConjBroadcasted, contractadd!, ContractAlgorithm, contractopadd!, contractperm, contractperm!, contractpermadd!, contractpermalign, contractpermopadd!, data, datatype, default_algorithm, dims2cat, directsum, eig_full, eig_trunc, eig_vals, eigh_full, eigh_trunc, eigh_vals, fill_map, flattenlinear, has_bipartition, infer_aux_space, invsqrth_safe, is_output_view, is_projected, isidentitybiperm, label_type, left_null, left_orth, left_polar, LinearBroadcasted, linearbroadcasted, lq_compact, lq_full, matricize, MatricizeContract, matricizeop, matricizeop!, matricizeopcopy, matricizeopview, MatricizeStyle, MATRIX_FUNCTIONS, ndims, ndims_codomain, ndims_domain, one, ones_map, operation, output_axes, PermutedDims, permuteddims, permutedims, permutedims!, permutedimsadd!, permutedimsop, permutedimsopadd!, project, project!, project_aux, project_hermitian, projectto!, qr_compact, qr_full, rand_map, randn_map, right_null, right_orth, right_polar, scalar, scale!, ScaledBroadcasted, select_algorithm, similar_map, size, sqrth_invsqrth_safe, sqrth_safe, sum, svd_compact, svd_full, svd_trunc, svd_vals, TensorOperationsContract, to_range, tr, trivialrange, tryflattenlinear, tryproject, tryproject_aux, unchecked_project, unchecked_project_aux, ungrade, unmatricize, unmatricize!, unmatricize_factors, unproject, unscaled, zero!, zeros_map"
         )
     )
 end
diff --git a/src/projectto.jl b/src/projectto.jl
index 59c53862..048a5159 100644
--- a/src/projectto.jl
+++ b/src/projectto.jl
@@ -80,13 +80,28 @@ The codomain and domain ranks of `a`'s intrinsic split, for a type that stores o
 no intrinsic split, so it defaults to all codomain and an empty domain, and a type that does store
 one overloads `ndims_codomain` (a `TensorMap` returns `numout`).
 
-Only `ndims_codomain` needs overloading: `ndims_domain` is whatever rank is left over, so the two
-agree by construction.
+`ndims_domain` never needs overloading: it is whatever rank is left over, so the two agree by
+construction. A type that does overload `ndims_codomain` should also overload
+[`has_bipartition`](@ref).
 """
 ndims_codomain(a) = ndims(a)
 @doc (@doc ndims_codomain)
 ndims_domain(a) = ndims(a) - ndims_codomain(a)
 
+"""
+    TensorAlgebra.has_bipartition(a) -> Bool
+
+Whether `a`'s type stores an intrinsic codomain/domain split, so that
+[`ndims_codomain`](@ref) reports a split `a` genuinely carries rather than the all-codomain
+fallback. A `TensorMap` returns `true`, an array `false`; a type that overloads
+`ndims_codomain` should also overload this.
+
+Lets a consumer tell a genuinely all-codomain `a` apart from one with no notion of a split,
+which is what validating a caller-supplied split against `a` needs: there is nothing to
+validate it against unless `a` stores one.
+"""
+has_bipartition(a) = false
+
 """
     is_projected(dest, src, ndims_codomain::Val; kwargs...) -> Bool
     is_projected(dest, src; kwargs...) -> Bool
diff --git a/test/test_exports.jl b/test/test_exports.jl
index 90a3c2f9..36e952e1 100644
--- a/test/test_exports.jl
+++ b/test/test_exports.jl
@@ -28,7 +28,8 @@ using Test: @test, @testset
                 :datatype,
                 :default_algorithm, :dims2cat, :directsum, :eig_full, :eig_trunc,
                 :eig_vals, :eigh_full, :eigh_trunc, :eigh_vals, :fill_map,
-                :flattenlinear, :infer_aux_space, :invsqrth_safe, :is_output_view,
+                :flattenlinear, :has_bipartition, :infer_aux_space, :invsqrth_safe,
+                :is_output_view,
                 :is_projected, :isidentitybiperm, :label_type, :left_null, :left_orth,
                 :left_polar, :LinearBroadcasted, :linearbroadcasted, :lq_compact,
                 :lq_full, :matricize, :MatricizeContract, :matricizeop, :matricizeop!,
diff --git a/test/test_projectto.jl b/test/test_projectto.jl
index c83acb43..942c5168 100644
--- a/test/test_projectto.jl
+++ b/test/test_projectto.jl
@@ -1,6 +1,6 @@
-using TensorAlgebra: TensorAlgebra, is_projected, project, project!, project_aux,
-    projectto!, tryproject, tryproject_aux, unchecked_project, unchecked_project_aux,
-    unproject
+using TensorAlgebra: TensorAlgebra, has_bipartition, is_projected, ndims_codomain,
+    ndims_domain, project, project!, project_aux, projectto!, tryproject, tryproject_aux,
+    unchecked_project, unchecked_project_aux, unproject
 using Test: @test, @test_throws, @testset
 
 const elts = (Float32, Float64, ComplexF32, ComplexF64)
@@ -83,6 +83,14 @@ end
     @test tryproject(raw, (Base.OneTo(2),), (Base.OneTo(3),)) == raw
 end
 
+@testset "the codomain/domain accessors on an array" begin
+    a = randn(2, 3, 4)
+    @test ndims_codomain(a) == 3
+    @test ndims_domain(a) == 0
+    # An array is all-codomain by fallback, not because it stores that split.
+    @test !has_bipartition(a)
+end
+
 @testset "unproject (dense default) ($T)" for T in elts
     raw = randn(T, 2, 3, 2, 3)
     cod = (Base.OneTo(2), Base.OneTo(3))
diff --git a/test/test_tensorkitext.jl b/test/test_tensorkitext.jl
index e4446fdc..6125a37c 100644
--- a/test/test_tensorkitext.jl
+++ b/test/test_tensorkitext.jl
@@ -2,10 +2,10 @@ using Base.Broadcast: broadcasted
 using LinearAlgebra: LinearAlgebra, norm
 using StableRNGs: StableRNG
 using TensorAlgebra: TensorAlgebra, contract, contractalign, eig_full, eig_vals, eigh_full,
-    eigh_vals, left_null, left_orth, left_polar, lq_compact, lq_full, matricize, project,
-    project_aux, projectto!, qr_compact, qr_full, rand_map, randn_map, right_null,
-    right_orth, right_polar, similar_map, svd_compact, svd_full, svd_vals, tryflattenlinear,
-    tryproject, unchecked_project, unmatricize, zeros_map
+    eigh_vals, has_bipartition, left_null, left_orth, left_polar, lq_compact, lq_full,
+    matricize, project, project_aux, projectto!, qr_compact, qr_full, rand_map, randn_map,
+    right_null, right_orth, right_polar, similar_map, svd_compact, svd_full, svd_vals,
+    tryflattenlinear, tryproject, unchecked_project, unmatricize, zeros_map
 using TensorKit: TensorKit, @tensor, AbstractTensorMap, DiagonalTensorMap, Irrep, Rep, SU₂,
     TensorMap, U₁, dim, dual, fuse, isomorphism, randn, reduceddim, space, storagetype, ←, ⊗
 using Test: @test, @test_throws, @testset
@@ -493,3 +493,11 @@ end
     s = Irrep[U₁](1)
     @test TensorAlgebra.dual(s) == dual(s)
 end
+
+@testset "the codomain/domain accessors on a TensorMap" begin
+    W = Rep[U₁](0 => 1, 1 => 1)
+    t = zeros_map(Float64, (W, W), (W,))
+    @test has_bipartition(t)
+    @test TensorAlgebra.ndims_codomain(t) == 2
+    @test TensorAlgebra.ndims_domain(t) == 1
+end