diff --git a/libs/ruby/warp_engine/app/controllers/warp_engine/api_controller.rb b/libs/ruby/warp_engine/app/controllers/warp_engine/api_controller.rb
index 97577fb..78a93af 100644
--- a/libs/ruby/warp_engine/app/controllers/warp_engine/api_controller.rb
+++ b/libs/ruby/warp_engine/app/controllers/warp_engine/api_controller.rb
@@ -31,7 +31,13 @@ module WarpEngine
     private
 
     def set_version_header
-      response.headers[WarpEngine::VERSION_HEADER] = WarpEngine::VERSION
+      # The name is spelled out here rather than taken from WarpEngine::VERSION_HEADER on
+      # purpose. A deployed process can end up with these controllers and an older
+      # `lib/` — it happened on the first deploy of this feature — and a controller that
+      # needs a constant from the newer half answers 500 to every request instead of
+      # serving the catalog. A response header is not worth that fragility. The constant
+      # is still the documented name, and a spec holds the two together.
+      response.headers["WarpEngine-Version"] = WarpEngine::VERSION
     end
 
     def resolve_mime(path)
diff --git a/libs/ruby/warp_engine/spec/requests/version_header_spec.rb b/libs/ruby/warp_engine/spec/requests/version_header_spec.rb
index a2818d5..f74304c 100644
--- a/libs/ruby/warp_engine/spec/requests/version_header_spec.rb
+++ b/libs/ruby/warp_engine/spec/requests/version_header_spec.rb
@@ -3,6 +3,12 @@ require "rails_helper"
 # Every response the engine serves carries the version that served it, so a client can
 # branch on the engine's age without asking a separate endpoint for it.
 RSpec.describe "the WarpEngine-Version header", type: :request do
+  # The controller writes the name as a literal so that it cannot depend on a constant a
+  # half-updated deploy might not have. This is what keeps the two in step.
+  it "is the name the constant documents" do
+    expect(WarpEngine::VERSION_HEADER).to eq("WarpEngine-Version")
+  end
+
   it "is on a normal response" do
     create(:software)