diff --git a/app/controllers/api/lockfiles_controller.rb b/app/controllers/api/lockfiles_controller.rb
index 0674a32..670eaaa 100644
--- a/app/controllers/api/lockfiles_controller.rb
+++ b/app/controllers/api/lockfiles_controller.rb
@@ -51,8 +51,16 @@ def poll_after_seconds(lockfile)
         estimate.clamp(MIN_POLL_SECONDS, MAX_POLL_SECONDS)
       end
 
+      # Some requests arrive with `lockfile` as a bare scalar instead of the
+      # nested `lockfile[content]` hash. Accept both and let Inspection
+      # reject anything that is not a Gemfile.lock.
       def lockfile_content
-        params.require(:lockfile).fetch(:content, "").to_s.strip
+        lockfile = params[:lockfile]
+
+        case lockfile
+        when ActionController::Parameters then lockfile[:content]
+        when String                       then lockfile
+        end.to_s.strip
       end
   end
 end
diff --git a/app/controllers/lockfiles_controller.rb b/app/controllers/lockfiles_controller.rb
index d80a540..78852dd 100644
--- a/app/controllers/lockfiles_controller.rb
+++ b/app/controllers/lockfiles_controller.rb
@@ -12,11 +12,15 @@ def create
   end
 
   def show
-    if FeatureFlags.new_check_flow?
-      @lockfile = Lockfile.includes(lockfile_checks: [:rails_release, :gem_checks]).find_by!(slug: params[:id])
-      render :show_new
-    else
-      @lockfile = Lockfile.find_by!(slug: params[:id])
+    respond_to do |format|
+      format.html do
+        if FeatureFlags.new_check_flow?
+          @lockfile = Lockfile.includes(lockfile_checks: [:rails_release, :gem_checks]).find_by!(slug: params[:id])
+          render :show_new
+        else
+          @lockfile = Lockfile.find_by!(slug: params[:id])
+        end
+      end
     end
   end
 
@@ -49,6 +53,6 @@ def create_legacy_flow
     end
 
     def lockfile_params
-      params.require(:lockfile).permit(:content)
+      params.expect(lockfile: [:content])
     end
 end
diff --git a/spec/controllers/api/lockfiles_controller_spec.rb b/spec/controllers/api/lockfiles_controller_spec.rb
index 672ea87..170b666 100644
--- a/spec/controllers/api/lockfiles_controller_spec.rb
+++ b/spec/controllers/api/lockfiles_controller_spec.rb
@@ -76,6 +76,26 @@
       end
     end
 
+    context "when lockfile is sent as a scalar instead of a hash" do
+      it "returns 422 with reason invalid_content instead of raising" do
+        expect {
+          post :create, params: { lockfile: "not a hash" }, as: :json
+        }.not_to raise_error
+
+        expect(response).to have_http_status(:unprocessable_content)
+        expect(JSON.parse(response.body)["reason"]).to eq("invalid_content")
+      end
+    end
+
+    context "when the lockfile param is missing entirely" do
+      it "returns 422 with reason invalid_content" do
+        post :create, params: {}, as: :json
+
+        expect(response).to have_http_status(:unprocessable_content)
+        expect(JSON.parse(response.body)["reason"]).to eq("invalid_content")
+      end
+    end
+
     context "when the lockfile is already on the latest known Rails" do
       it "returns 200 with reason up_to_date and errors, and does not persist the lockfile" do
         FactoryBot.create(:rails_release, version: "7.1")
diff --git a/spec/controllers/lockfiles_controller_spec.rb b/spec/controllers/lockfiles_controller_spec.rb
index 01807b8..ae39a16 100644
--- a/spec/controllers/lockfiles_controller_spec.rb
+++ b/spec/controllers/lockfiles_controller_spec.rb
@@ -178,5 +178,21 @@
       get :show, params: { id: lockfile.to_param }
       expect(response).to be_successful
     end
+
+    # UnknownFormat is mapped to 406 by Rails and excluded from Sentry by
+    # default, unlike the MissingTemplate 500 this replaces.
+    it "raises UnknownFormat (406) for non-HTML formats instead of MissingTemplate" do
+      expect {
+        get :show, params: { id: lockfile.to_param }, format: :json
+      }.to raise_error(ActionController::UnknownFormat)
+    end
+  end
+
+  describe "POST #create with a scalar lockfile param" do
+    it "raises ParameterMissing (400) instead of NoMethodError" do
+      expect {
+        post :create, params: { lockfile: "not a hash" }
+      }.to raise_error(ActionController::ParameterMissing)
+    end
   end
-end
\ No newline at end of file
+end