Recompile the router when a route is declared after compilation

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).
+* [#2848](https://github.com/ruby-grape/grape/pull/2848): Recompile the router when a route is declared after the API has been compiled, so it is served instead of listed by `routes` and answering 404 - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 3.3.5 (2026-07-30)

lib/grape/dsl/routing.rb

@@ -206,6 +206,11 @@ def route(methods, paths = ['/'], requirements: nil, anchor: true, **route_optio
         )
         endpoints << new_endpoint unless endpoints.include?(new_endpoint)
 
+        # The router is built once, when the API is first compiled, so an
+        # endpoint added after that would be listed by #routes and still 404.
+        # `helpers` and `mount` already invalidate for the same reason.
+        change!
+
         inheritable_setting.route_end
         reset_validations!
       end

spec/grape/api_spec.rb

@@ -4120,6 +4120,48 @@ def my_method
     end
   end
 
+  # The router is built once, when the API is first compiled. A route declared
+  # after that was listed by .routes and still answered 404, so the API's own
+  # metadata disagreed with what it served.
+  describe 'a route declared after the API has been compiled' do
+    before do
+      subject.format :json
+      subject.get('/first') { 'first' }
+    end
+
+    it 'is served after a request has already been made' do
+      get '/first'
+      subject.get('/second') { 'second' }
+
+      get '/second'
+      expect(last_response.status).to eq(200)
+      expect(last_response.body).to eq('second'.to_json)
+    end
+
+    it 'is served after recognize_path has compiled the router' do
+      subject.recognize_path('/first')
+      subject.get('/second') { 'second' }
+
+      get '/second'
+      expect(last_response.status).to eq(200)
+    end
+
+    it 'is recognised by recognize_path' do
+      get '/first'
+      subject.get('/second') { 'second' }
+
+      expect(subject.recognize_path('/second')).not_to be_nil
+    end
+
+    it 'leaves the routes declared before it serving' do
+      get '/first'
+      subject.get('/second') { 'second' }
+
+      get '/first'
+      expect(last_response.body).to eq('first'.to_json)
+    end
+  end
+
   describe '.endpoint' do
     before do
       subject.format :json

spec/grape/dsl/routing_spec.rb

@@ -12,6 +12,10 @@
       class << self
         attr_reader :instance, :base
         attr_accessor :configuration
+
+        # Part of the contract Grape::DSL::Routing expects of its host, the
+        # same way #mount already relies on it.
+        def change!; end
       end
     end
   end