Keep a :version path capture when the API declares no version

CHANGELOG.md

@@ -55,6 +55,7 @@
 * [#2827](https://github.com/ruby-grape/grape/pull/2827): Make the `cascade` DSL getter return the configured value (`cascade false` read back as `true`) - [@ericproulx](https://github.com/ericproulx).
 * [#2829](https://github.com/ruby-grape/grape/pull/2829): Fix a cascading route handing over only to the last route registered for the path, making a middle version (3+ mounted versions with a catch-all) answer 406 - [@ericproulx](https://github.com/ericproulx).
 * [#2826](https://github.com/ruby-grape/grape/pull/2826): Fix `api.version` not being set for the root route of a path-versioned API (`GET /v1`) - [@ericproulx](https://github.com/ericproulx).
+* [#2846](https://github.com/ruby-grape/grape/pull/2846): Keep a `:version` path capture in `params` when the API declares no version, instead of always dropping it as Grape's own - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 3.3.5 (2026-07-30)

lib/grape/request.rb

@@ -168,15 +168,39 @@ def cookies?
 
     def make_params
       params = @params_builder.call(rack_params)
-      routing_args = env[Grape::Env::GRAPE_ROUTING_ARGS]
-      filtered = routing_args&.except(:version, :route_info)
+      filtered = routing_args_as_params(env[Grape::Env::GRAPE_ROUTING_ARGS])
       return params if filtered.blank?
 
       params.deep_merge!(filtered)
     rescue *Grape::RACK_ERRORS
       raise Grape::Exceptions::RequestError
     end
 
+    # The routing args carry two things that are not request params:
+    # +:route_info+, which is always Grape's own, and +:version+, which is only
+    # Grape's own when the API declared a version — that is captured as a path
+    # segment and exposed through +env['api.version']+ instead.
+    #
+    # An API that declares no version can legitimately name a param +:version+
+    # (`route_param :version`, `get '/:version'`), and that capture belongs to
+    # the application. Dropping it unconditionally left `params[:version]` nil
+    # on a route that had matched, losing the segment silently.
+    def routing_args_as_params(routing_args)
+      return if routing_args.nil?
+      return routing_args.except(:version, :route_info) if grape_owns_version?(routing_args)
+
+      routing_args.except(:route_info)
+    end
+
+    # A route reports a +version+ only when the API declared one, which is the
+    # case where the captured segment is Grape's rather than the application's.
+    def grape_owns_version?(routing_args)
+      return false unless routing_args.key?(:version)
+
+      route = routing_args[:route_info]
+      route.respond_to?(:version) && !route.version.nil?
+    end
+
     # Uses a plain `each_header` block instead of `each_header.with_object`:
     # `with_object` can only pass the block one value plus the memo, so the
     # `k, v` pair would be boxed into a throwaway Array on every header. A

spec/grape/request_spec.rb

@@ -50,13 +50,29 @@
       let(:routing_args) do
         {
           version: '123',
-          route_info: '456',
+          route_info: instance_double(Grape::Router::Route, version: route_version),
           c: 'ccc'
         }
       end
 
-      it 'cuts version and route_info' do
-        expect(request.params).to eq(ActiveSupport::HashWithIndifferentAccess.new(a: '123', b: 'xyz', c: 'ccc'))
+      context 'when the route carries a version of its own' do
+        let(:route_version) { 'v1' }
+
+        it 'cuts version and route_info' do
+          expect(request.params).to eq(ActiveSupport::HashWithIndifferentAccess.new(a: '123', b: 'xyz', c: 'ccc'))
+        end
+      end
+
+      # Without a declared version the captured segment is the application's:
+      # `route_param :version` on an unversioned API has to reach the endpoint.
+      context 'when the route carries no version' do
+        let(:route_version) { nil }
+
+        it 'cuts only route_info' do
+          expect(request.params).to eq(
+            ActiveSupport::HashWithIndifferentAccess.new(a: '123', b: 'xyz', c: 'ccc', version: '123')
+          )
+        end
       end
     end