diff --git a/CHANGELOG.md b/CHANGELOG.md
index d2d9c2951..89766e950 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -115,6 +115,7 @@
 * [#2905](https://github.com/ruby-grape/grape/pull/2905): Nest any `route_param` requirement under the param name, not just a Regexp, and reject the `requirements` shapes that have no param to attach to where they are written rather than on the first request - [@ericproulx](https://github.com/ericproulx).
 * [#2908](https://github.com/ruby-grape/grape/pull/2908): Stop reassigning method parameters across `lib`, so a parameter keeps the value its caller passed for the whole method; the `oneof` collection in `Grape::Validations::ParamsScope` no longer writes into the Hash it was given - [@ericproulx](https://github.com/ericproulx).
 * [#2910](https://github.com/ruby-grape/grape/pull/2910): Restore `Grape::Middleware::Formatter`'s in-place content-type negotiation as `ensure_content_type!`, which #2908 had turned into a copy of the response headers on every response - [@ericproulx](https://github.com/ericproulx).
+* [#2911](https://github.com/ruby-grape/grape/pull/2911): Copy response headers with `merge!` instead of `merge` in `Grape::API::Instance#call` and `Grape::Middleware::Error`, which allocated a `Grape::Util::Header` only to discard it - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 3.3.5 (2026-07-30)
diff --git a/lib/grape/api/instance.rb b/lib/grape/api/instance.rb
index 8a3e2f1e7..2b43d5a8e 100644
--- a/lib/grape/api/instance.rb
+++ b/lib/grape/api/instance.rb
@@ -118,7 +118,12 @@ def initialize
       def call(env)
         status, headers, response = @router.call(env)
         unless @cascade
-          headers = Grape::Util::Header.new.merge(headers)
+          # +merge!+, not +merge+: the latter is a `dup` plus a `merge!`, so the
+          # Header built on this line would be allocated only to be discarded.
+          # The copy stays because +headers+ can come from a mounted Rack app,
+          # which is free to hand back a frozen or shared Hash that the delete
+          # below must not reach into.
+          headers = Grape::Util::Header.new.merge!(headers)
           headers.delete('X-Cascade')
         end
 
diff --git a/lib/grape/middleware/error.rb b/lib/grape/middleware/error.rb
index 0b43730c0..a2dd7c48a 100644
--- a/lib/grape/middleware/error.rb
+++ b/lib/grape/middleware/error.rb
@@ -68,7 +68,7 @@ def call!(env)
 
       def rack_response(status, headers, message)
         body = html_content_type?(headers[Rack::CONTENT_TYPE]) ? Rack::Utils.escape_html(message) : message
-        Rack::Response.new(Array.wrap(body), Rack::Utils.status_code(status), Grape::Util::Header.new.merge(headers))
+        Rack::Response.new(Array.wrap(body), Rack::Utils.status_code(status), Grape::Util::Header.new.merge!(headers))
       end
 
       # Escaping must key off the media type only, case-insensitively. Comparing
diff --git a/spec/grape/api/instance_spec.rb b/spec/grape/api/instance_spec.rb
index 2af3df688..e7deff512 100644
--- a/spec/grape/api/instance_spec.rb
+++ b/spec/grape/api/instance_spec.rb
@@ -117,4 +117,29 @@ def app
       expect(an_instance.compile!.cascade?).to be(true)
     end
   end
+
+  describe '#call' do
+    context 'when cascade is false' do
+      let(:rack_headers) { { 'content-type' => 'text/plain', 'x-cascade' => 'pass' }.freeze }
+      let(:root_api) do
+        headers = rack_headers
+        Class.new(Grape::API::Instance) do
+          cascade false
+          mount ->(_env) { [200, headers, ['from rack']] } => '/rack'
+        end
+      end
+
+      it 'strips X-Cascade from the response' do
+        get '/rack'
+        expect(last_response.headers).not_to have_key('x-cascade')
+      end
+
+      # The mounted app owns the Hash it returned and is free to hand back a
+      # frozen or shared one, so removing X-Cascade has to happen on a copy.
+      it 'does not write into the headers the mounted app returned' do
+        get '/rack'
+        expect(rack_headers).to eq('content-type' => 'text/plain', 'x-cascade' => 'pass')
+      end
+    end
+  end
 end