diff --git a/src/Tearing/Runner/Control.jl b/src/Tearing/Runner/Control.jl
index 17128a3d6..1a12ff7e6 100644
--- a/src/Tearing/Runner/Control.jl
+++ b/src/Tearing/Runner/Control.jl
@@ -29,7 +29,9 @@ constructor.
 
 # Physics knobs
 
-  - `bt`       -- toroidal field [T]. `nothing` → use `equil.config.b0exp`
+  - `bt`       -- toroidal field `[T]`. `nothing` (default) resolves the physical
+    `B_T = F(ψ)/(2π·R₀)` per surface from the equilibrium's F-spline; a scalar or a
+    callable of `psi` overrides it
   - `mu_i`     -- ion mass in proton-mass units (default 2.0 for D)
   - `zeff`     -- effective charge
   - `chi_perp`, `chi_tor` -- fallback perpendicular / toroidal heat
diff --git a/src/Tearing/Runner/run_slayer.jl b/src/Tearing/Runner/run_slayer.jl
index b42980465..2a29c63a8 100644
--- a/src/Tearing/Runner/run_slayer.jl
+++ b/src/Tearing/Runner/run_slayer.jl
@@ -360,6 +360,9 @@ read from `control.profile_file` (relative to `dir_path`) through the shared
 profiles they set χ⊥(ψ)/χ_φ(ψ), otherwise the scalar `control.chi_perp`/
 `chi_tor` fallbacks are used.
 
+The toroidal field comes from `control.bt`; leaving it unset (the default) makes
+`build_slayer_inputs` evaluate the physical `B_T = F(ψ)/(2π·R₀)` per surface.
+
 Returns an `enabled=false` `SLAYERResult` when `control.enabled` is
 false.
 """
@@ -394,7 +397,12 @@ function run_slayer(equil, surfaces::AbstractVector, delta_prime_matrix::Abstrac
             resistivity_model=_build_resistivity_model(control.resistivity_model),
             lnLambda_form=control.lnLambda_form)
     else
-        bt = control.bt === nothing ? equil.config.b0exp : control.bt
+        # `equil.config.b0exp` is a NORMALIZATION (commonly exactly 1.0), not the toroidal
+        # field, so substituting it here silently ran the layer physics at B_T = 1 T. Pass the
+        # control value through instead: `nothing` makes build_slayer_inputs compute the
+        # physical B_T = F(psi)/(2*pi*R_0) per surface from the equilibrium's F-spline, which is
+        # what its docstring already prescribes.
+        bt = control.bt
         # χ⊥/χ_φ from the kinetic file when present, else the scalar fallbacks.
         chi_perp = loaded.chi_perp === nothing ? control.chi_perp : loaded.chi_perp
         chi_tor = loaded.chi_tor === nothing ? control.chi_tor : loaded.chi_tor
diff --git a/test/runtests_slayer_inputs.jl b/test/runtests_slayer_inputs.jl
index c0b7d2d7d..55d6124e9 100644
--- a/test/runtests_slayer_inputs.jl
+++ b/test/runtests_slayer_inputs.jl
@@ -106,6 +106,34 @@
         @test sl[1].Q_i == -sl[1].tauk * profiles.omega_i(0.3)
     end
 
+    @testset "build_slayer_inputs: bt defaults to the physical B_T, not a normalization" begin
+        # `b0exp` is a normalization, not a field; passing it as `bt` ran the layer physics at the
+        # wrong toroidal field. Asserted behaviourally rather than by grepping the source, so a
+        # refactor cannot silently break the check and a reintroduction anywhere in the chain still
+        # fails: tau_h = R0*sqrt(mu0*rho)/(n*sval_r*bt), so lu is linear in the field actually used
+        # and the resolved bt is recoverable from the returned parameters.
+        #
+        # Note this fixture cannot exhibit the original bug: Solovev is normalized so that
+        # b0exp == 1.0 and F(psi)/(2*pi*R0) == 1.0 to roundoff, which is exactly why the defect
+        # survived. It shows up on a deck whose normalization differs from its field -- the
+        # DIII-D-like EFIT deck has b0exp = 1.0 against a physical ~1.95 T. What is pinned here is
+        # therefore the resolution rule itself, which is deck-independent.
+        sings = [_mk_sing(psi=0.3, q=2.0, q1=1.5, m=2, n=1)]
+        bt_phys = Float64(equil.profiles.F_spline(0.3)) / (2π * equil.ro)
+
+        sl_default = build_slayer_inputs(equil, sings, profiles; dr_val=0.0, compute_omega_star=false)
+        sl_at_phys = build_slayer_inputs(equil, sings, profiles; bt=bt_phys, dr_val=0.0, compute_omega_star=false)
+        sl_double = build_slayer_inputs(equil, sings, profiles; bt=2 * bt_phys, dr_val=0.0, compute_omega_star=false)
+
+        # Leaving bt unset must be identical to passing the physical field explicitly.
+        @test sl_default[1].lu ≈ sl_at_phys[1].lu rtol = 1e-12
+
+        # An explicit bt overrides, and lu tracks it linearly -- so the field that was actually
+        # used is what the parameters encode, not merely some field.
+        @test sl_double[1].lu / sl_default[1].lu ≈ 2.0 rtol = 1e-10
+        @test sl_double[1].lu != sl_default[1].lu
+    end
+
     @testset "build_slayer_inputs: chi_perp/chi_tor as scalars and callables" begin
         sings = [_mk_sing(psi=0.5, q=2.4, q1=1.2, m=2, n=1)]
 
diff --git a/test/runtests_slayer_runner.jl b/test/runtests_slayer_runner.jl
index fb4b923dd..0eacc18ed 100644
--- a/test/runtests_slayer_runner.jl
+++ b/test/runtests_slayer_runner.jl
@@ -306,4 +306,5 @@
             end
         end
     end
+
 end