diff --git a/ext/ParameterSetYAMLExt.jl b/ext/ParameterSetYAMLExt.jl
index f7b50f69d..9aa8c02f2 100644
--- a/ext/ParameterSetYAMLExt.jl
+++ b/ext/ParameterSetYAMLExt.jl
@@ -9,7 +9,12 @@ import Unitful
     _parse_units!(data::Dict{String, Any})
 
 Recursively walk a parsed YAML dict. String values that parse into a
-`Unitful.Quantity` (e.g. `"150μm"`) are converted in place.
+`Unitful.Quantity` (e.g. `"150μm"`) are converted in place via
+[`DeviceLayout.uparse`](@ref), so length-dimensioned values share the
+package's promotion context (`ContextUnits`) instead of carrying bare
+`FreeUnits` (which would later trip the mixed-context promotion bug
+fixed in https://github.com/JuliaPhysics/Unitful.jl/pull/845 — surfaced
+in DeviceLayout via `layer_z` arithmetic).
 
 Bare unit names like `"s"`, `"m"`, `"cm"` parse into `Unitful.Units`, not
 `Quantity` - those are left as strings so that ordinary text values (e.g.
@@ -21,7 +26,7 @@ function _parse_units!(data::Dict{String, Any})
             _parse_units!(v)
         elseif v isa AbstractString
             try
-                parsed = Unitful.uparse(v)
+                parsed = DeviceLayout.uparse(v)
                 parsed isa Unitful.Quantity && (data[k] = parsed)
             catch
                 # not a valid unit expression, keep as-is
diff --git a/src/units.jl b/src/units.jl
index 729fc7801..fde9468c0 100644
--- a/src/units.jl
+++ b/src/units.jl
@@ -90,6 +90,32 @@ Mixed unit operations with these imports will be converted based on the unit
 preference set by [`DeviceLayout.set_unit_preference!`](@ref) (default `nm`).
 """ PreferredUnits
 
+"""
+    uparse(str)
+
+Parse `str` as a `Unitful.Quantity`, resolving length symbols (`fm`, `pm`, `nm`,
+`μm`, `mm`, `cm`, `dm`, `m`) against [`PreferredUnits`](@ref) so the parsed value
+carries the package's promotion context (`Unitful.ContextUnits`) instead of bare
+`Unitful.FreeUnits`. Non-length symbols (e.g. `s`, `Hz`, `fH`) fall back to
+`Unitful` and behave exactly as `Unitful.uparse`.
+
+Use this in place of `Unitful.uparse` when ingesting unit strings from external
+sources (YAML, JSON, user input) — bare `FreeUnits` lengths cause the mixed-context
+promotion bug in https://github.com/JuliaPhysics/Unitful.jl/pull/845 when later
+combined with package-internal `ContextUnits` values (e.g. inside `layer_z`).
+"""
+function uparse(str::AbstractString)
+    # Try PreferredUnits first; fall back to Unitful for any non-length symbol
+    # that PreferredUnits does not redefine. This avoids `Unitful.uparse`'s
+    # "found in multiple registered unit modules" warning for shared length
+    # symbols (μm, nm, ...) that exist in both modules with different values.
+    try
+        return Unitful.uparse(str; unit_context=PreferredUnits)
+    catch
+        return Unitful.uparse(str)
+    end
+end
+
 onemicron(v::T) where {T <: Unitful.Length} =
     one(T) * Unitful.ContextUnits(Unitful.μm, Unitful.unit(Unitful.upreferred(v)))
 onemicron(T::Type{<:Unitful.Length}) =
diff --git a/test/test_parameter_set.jl b/test/test_parameter_set.jl
index b18b504bf..ae477ee0e 100644
--- a/test/test_parameter_set.jl
+++ b/test/test_parameter_set.jl
@@ -690,6 +690,45 @@ end
         @test ps2.components.transmon.island.cap_width == 24μm
         @test ps2.components.transmon.junction.w_jj == 1μm
     end
+
+    @testset "Parsed lengths share DeviceLayout's promotion context" begin
+        # `Unitful.uparse` returns FreeUnits quantities (promotion target `m`),
+        # which mismatch DeviceLayout's ContextUnits (target `nm` or `μm`).
+        # Mixing them through `+` (e.g. inside `layer_z`) used to fail with
+        # `MethodError: no method matching (::FreeUnits{(m,), 𝐋, nothing})()` -
+        # see https://github.com/JuliaPhysics/Unitful.jl/pull/845. Lengths
+        # parsed from YAML must therefore enter the system already wrapped in
+        # the package's preferred context via `DeviceLayout.uparse`.
+        yaml_str = """
+        components:
+          cap:
+            finger_length: 150μm
+            t_chip: 525μm
+            inv_length: 1/μm
+        """
+        ps = ParameterSet(IOBuffer(yaml_str))
+        v = ps.components.cap.finger_length
+
+        @test v isa Unitful.Quantity
+        @test Unitful.unit(v) isa Unitful.ContextUnits
+        # The parsed length must add cleanly to a value carrying the package's
+        # preferred context - this was the operation that previously threw.
+        @test (v + 1 * DeviceLayout.PreferredUnits.UPREFERRED) isa Unitful.Quantity
+
+        # Compound expressions involving length symbols must also resolve
+        # against PreferredUnits so the embedded length carries ContextUnits.
+        @test ps.components.cap.inv_length isa Unitful.Quantity
+
+        # Reproduce the original `level_z` failure path: mixing YAML-loaded
+        # chip thicknesses (parsed as μm) with package-internal nm-targeted
+        # ContextUnits inside the same arithmetic chain. Pre-fix this raised
+        # the FreeUnits MethodError on the second `+`/`-`.
+        t_chips = [v, v]
+        t_gap = ps.components.cap.t_chip
+        nm_value = 1 * DeviceLayout.nm
+        z = sum(t_chips) + t_gap - nm_value
+        @test (z + nm_value) isa Unitful.Quantity
+    end
 end
 
 @testitem "Composite ParameterSet flow" setup = [CommonTestSetup] begin