diff --git a/ext/QuantumControlFiniteDifferencesExt.jl b/ext/QuantumControlFiniteDifferencesExt.jl
index 3c97e698..e6163f20 100644
--- a/ext/QuantumControlFiniteDifferencesExt.jl
+++ b/ext/QuantumControlFiniteDifferencesExt.jl
@@ -4,15 +4,10 @@ using LinearAlgebra
 
 import FiniteDifferences
 import QuantumControl.Functionals:
-    _default_chi_via, make_gate_chi, make_automatic_chi, make_automatic_grad_J_a
+    make_gate_chi, make_automatic_chi, make_automatic_grad_J_a
 
 
-function make_automatic_chi(
-    J_T,
-    trajectories,
-    ::Val{:FiniteDifferences};
-    via=_default_chi_via(trajectories)
-)
+function make_automatic_chi(J_T, trajectories, ::Val{:FiniteDifferences}; via=:states)
 
     # TODO: Benchmark if χ should be closure, see QuantumControlZygoteExt.jl
 
diff --git a/ext/QuantumControlZygoteExt.jl b/ext/QuantumControlZygoteExt.jl
index 858d28af..3e4bf22a 100644
--- a/ext/QuantumControlZygoteExt.jl
+++ b/ext/QuantumControlZygoteExt.jl
@@ -4,15 +4,10 @@ using LinearAlgebra
 
 import Zygote
 import QuantumControl.Functionals:
-    _default_chi_via, make_gate_chi, make_automatic_chi, make_automatic_grad_J_a
+    make_gate_chi, make_automatic_chi, make_automatic_grad_J_a
 
 
-function make_automatic_chi(
-    J_T,
-    trajectories,
-    ::Val{:Zygote};
-    via=_default_chi_via(trajectories)
-)
+function make_automatic_chi(J_T, trajectories, ::Val{:Zygote}; via=:states)
 
     # TODO: At some point, for a large system, we could benchmark if there is
     # any benefit to making χ a closure and using LinearAlgebra.axpby! to
@@ -26,7 +21,14 @@ function make_automatic_chi(
         χ = Vector{eltype(Ψ)}(undef, length(Ψ))
         ∇J = Zygote.gradient(_J_T, Ψ...)
         for (k, ∇Jₖ) ∈ enumerate(∇J)
-            χ[k] = 0.5 * ∇Jₖ  # ½ corrects for gradient vs Wirtinger deriv
+            if isnothing(∇Jₖ)
+                # Functional does not depend on Ψₖ. That probably means a buggy
+                # J_T, but who knows: maybe there are situations where that
+                # makes sense. It would be extremely noisy to warn here.
+                χ[k] = zero(χ[k])
+            else
+                χ[k] = 0.5 * ∇Jₖ  # ½ corrects for gradient vs Wirtinger deriv
+            end
             # axpby!(0.5, ∇Jₖ, false, χ[k])
         end
         return χ
@@ -43,7 +45,12 @@ function make_automatic_chi(
         χ = Vector{eltype(Ψ)}(undef, length(Ψ))
         ∇J = Zygote.gradient(_J_T, τ...)
         for (k, traj) ∈ enumerate(trajectories)
-            ∂J╱∂τ̄ₖ = 0.5 * ∇J[k]  # ½ corrects for gradient vs Wirtinger deriv
+            if isnothing(∇J[k])
+                # Functional does not depend on τₖ
+                ∂J╱∂τ̄ₖ = zero(ComplexF64)
+            else
+                ∂J╱∂τ̄ₖ = 0.5 * ∇J[k]  # ½ corrects for gradient vs Wirtinger deriv
+            end
             χ[k] = ∂J╱∂τ̄ₖ * traj.target_state
             # axpby!(∂J╱∂τ̄ₖ, traj.target_state, false, χ[k])
         end
diff --git a/src/functionals.jl b/src/functionals.jl
index d3c9c00c..c262027f 100644
--- a/src/functionals.jl
+++ b/src/functionals.jl
@@ -9,12 +9,21 @@ export make_grad_J_a, make_chi
 using LinearAlgebra: axpy!, dot
 
 
-# default for `via` argument of `make_chi`
-function _default_chi_via(trajectories)
-    if any(isnothing(traj.target_state) for traj in trajectories)
-        return :states
-    else
-        return :tau
+function _check_chi(chi; states, trajectories, tau, via)
+    try
+        if via == :tau
+            chi_states = chi(states, trajectories; tau)
+        else
+            chi_states = chi(states, trajectories)
+        end
+        if typeof(chi_states) ≠ typeof(states)
+            msg = "`chi` must return a vector of states"
+            error(msg)
+        end
+    catch exception
+        msg = "The chi generated by `make_chi` does not have the required interface"
+        @error msg exception
+        error("Cannot make chi")
     end
 end
 
@@ -86,15 +95,25 @@ chi = make_chi(
     trajectories;
     mode=:any,
     automatic=:default,
-    via=(any(isnothing(t.target_state) for t in trajectories) ? :states : :tau),
+    via=:automatic,  # one of :automatic, :tau, :states
 )
 ```
 
-creates a function `chi(Ψ, trajectories; τ)` that returns
-a vector of states `χ` with ``|χ_k⟩ = -∂J_T/∂⟨Ψ_k|``, where ``|Ψ_k⟩`` is the
-k'th element of `Ψ`. These are the states used as the boundary condition for
-the backward propagation propagation in Krotov's method and GRAPE. Each
-``|χₖ⟩`` is defined as a matrix calculus
+creates a function `chi(Ψ, trajectories)` or `chi(Ψ, trajectories; tau)` that
+returns a vector of states `χ` with ``|χ_k⟩ = -∂J_T/∂⟨Ψ_k|``, where ``|Ψ_k⟩``
+is the k'th element of `Ψ`. These are the states used as the boundary condition
+for the backward propagation propagation in Krotov's method and GRAPE.
+
+The resulting `chi` function takes the keyword argument `tau`
+if and only if `via=:tau` or `via=:automatic` (default) if the following
+conditions are met:
+
+* All `trajectories` have a defined `target_state` component (not `nothing`)
+* `J_T` takes `tau` as a keyword argument (determined via introspection)
+
+Both of these conditions are _requirements_ for `via=:tau`.
+
+Each ``|χₖ⟩`` is defined as a matrix calculus
 [Wirtinger derivative](https://www.ekinakyurek.me/complex-derivatives-wirtinger/),
 
 ```math
@@ -193,25 +212,53 @@ and the definition of the Zygote gradient with respect to a complex scalar,
     gradients). Always test automatic derivatives against finite differences
     and/or other automatic differentiation frameworks.
 """
-function make_chi(
-    J_T,
-    trajectories;
-    mode=:any,
-    automatic=:default,
-    via=_default_chi_via(trajectories),
-)
+function make_chi(J_T, trajectories; mode=:any, automatic=:default, via=:automatic,)
+    states = [traj.initial_state for traj in trajectories]
+    tau = [zero(ComplexF64) for _ in states]
+    J_T_takes_tau = hasmethod(J_T, Tuple{typeof(states),typeof(trajectories)}, (:tau,))
+    has_target_states = all((traj.target_state ≢ nothing) for traj in trajectories)
+    if (via == :tau) && !J_T_takes_tau
+        msg = "Called `make_chi` with `via=:tau`, but given J_T does not take `tau` keyword argument"
+        error(msg)
+    end
+    if (via == :tau) && !has_target_states
+        msg = "Called `make_chi` with `via=:tau`, but not all `trajectories` define a `target_state`"
+        error(msg)
+    end
+    if via == :automatic
+        via = :states
+        if J_T_takes_tau && has_target_states
+            via = :tau
+        end
+    end
+    chi = nothing
+    try
+        if via == :tau
+            J_T_val = J_T(states, trajectories; tau)
+        else
+            J_T_val = J_T(states, trajectories)
+        end
+        if !(J_T_val isa Float64)
+            msg = "J_T passed to `make_chi` must return a Float64, not $(typeof(J_T_val))"
+            error(msg)
+        end
+    catch exception
+        msg = "The J_T passed to `make_chi` does not have the required interface"
+        @error msg exception
+        error("Cannot make chi")
+    end
     if mode == :any
         try
             chi = make_analytic_chi(J_T, trajectories)
             @debug "make_chi for J_T=$(J_T) -> analytic"
-            # TODO: call chi to compile it and ensure required properties
+            _check_chi(chi; states, trajectories, tau, via)
             return chi
         catch exception
             if exception isa MethodError
                 @info "make_chi for J_T=$(J_T): fallback to mode=:automatic"
                 try
                     chi = make_automatic_chi(J_T, trajectories, automatic; via)
-                    # TODO: call chi to compile it and ensure required properties
+                    _check_chi(chi; states, trajectories, tau, via)
                     return chi
                 catch exception
                     if exception isa MethodError
@@ -228,7 +275,7 @@ function make_chi(
     elseif mode == :analytic
         try
             chi = make_analytic_chi(J_T, trajectories)
-            # TODO: call chi to compile it and ensure required properties
+            _check_chi(chi; states, trajectories, tau, via)
             return chi
         catch exception
             if exception isa MethodError
@@ -241,7 +288,7 @@ function make_chi(
     elseif mode == :automatic
         try
             chi = make_automatic_chi(J_T, trajectories, automatic; via)
-            # TODO: call chi to compile it and ensure required properties
+            _check_chi(chi; states, trajectories, tau, via)
             return chi
         catch exception
             if exception isa MethodError
diff --git a/test/test_functionals.jl b/test/test_functionals.jl
index 8675bed9..0e501af2 100644
--- a/test/test_functionals.jl
+++ b/test/test_functionals.jl
@@ -299,13 +299,13 @@ end
         throw(DomainError("XXX"))
     end
 
-    @test_throws DomainError begin
+    @test_throws Exception begin
         IOCapture.capture() do
             make_chi(J_T_xxx, trajectories)
         end
     end
 
-    @test_throws DomainError begin
+    @test_throws Exception begin
         IOCapture.capture() do
             make_chi(J_T_xxx, trajectories; mode=:automatic)
         end