diff --git a/.gitignore b/.gitignore
index 6bed729c..b6c01326 100644
--- a/.gitignore
+++ b/.gitignore
@@ -19,3 +19,4 @@ lib/*.bundle
 lib/*.so
 log/scout_apm.log
 gems/*.lock
+gems/vendor/
diff --git a/gems/instruments.gemfile b/gems/instruments.gemfile
index fa20eff5..7e0124de 100644
--- a/gems/instruments.gemfile
+++ b/gems/instruments.gemfile
@@ -8,3 +8,7 @@ gem 'actionpack'
 gem 'actionview'
 gem 'httpx'
 gem 'grape'
+# json 3.0 (released 2026-09) dropped the `quirks_mode` keyword that Grape 3's
+# ActiveSupport JSON formatter still passes, so any Grape 3 JSON request raises
+# ArgumentError. Pin to the 2.x line until this matrix moves off Grape 3.
+gem 'json', '< 3'
diff --git a/lib/scout_apm/instruments/grape.rb b/lib/scout_apm/instruments/grape.rb
index efa52695..0fcfcf34 100644
--- a/lib/scout_apm/instruments/grape.rb
+++ b/lib/scout_apm/instruments/grape.rb
@@ -51,6 +51,36 @@ def endpoint_owns_run?
       end
     end
 
+    module GrapeEndpointNaming
+      # Grape >= 4 moved endpoint route metadata off the public `options`
+      # Hash and onto `Endpoint#config` (a Grape::Endpoint::Options Data
+      # object). Support both so the instrument works across Grape 3 and 4.
+      def self.name_for(endpoint)
+        # `config` is a protected reader on Grape::Endpoint, so a public
+        # respond_to? misses it: check with `true` and read via `send`.
+        if endpoint.respond_to?(:config, true) && endpoint.send(:config).respond_to?(:http_methods)
+          config = endpoint.send(:config)
+          method = config.http_methods && config.http_methods.first
+          api = config.api || (endpoint.respond_to?(:api) ? endpoint.api : nil)
+          path = config.path && config.path.first
+        else
+          method = endpoint.options[:method] && endpoint.options[:method].first
+          api = endpoint.options[:for]
+          path = endpoint.options[:path] && endpoint.options[:path].first
+        end
+
+        ["Grape",
+         method,
+         api.to_s,
+         endpoint.namespace.sub(%r{\A/}, ''), # removing leading slashes
+         path,
+        ].compact.map { |n| n.to_s }.join("/")
+      rescue => e
+        ScoutApm::Agent.instance.context.logger.info("Error getting Grape Endpoint Name. Error: #{e.message}. Options: #{endpoint.options.inspect}")
+        "Grape/Unknown"
+      end
+    end
+
     module GrapeEndpointInstruments
       def run_with_scout_instruments(*args)
         request = ::Grape::Request.new(env || args.first)
@@ -64,17 +94,7 @@ def run_with_scout_instruments(*args)
 
         req.set_headers(request.headers)
 
-        begin
-          name = ["Grape",
-                  self.options[:method].first,
-                  self.options[:for].to_s,
-                  self.namespace.sub(%r{\A/}, ''), # removing leading slashes
-                  self.options[:path].first,
-          ].compact.map{ |n| n.to_s }.join("/")
-        rescue => e
-          ScoutApm::Agent.instance.context.logger.info("Error getting Grape Endpoint Name. Error: #{e.message}. Options: #{self.options.inspect}")
-          name = "Grape/Unknown"
-        end
+        name = GrapeEndpointNaming.name_for(self)
 
         req.start_layer( ScoutApm::Layer.new("Controller", name) )
         begin
@@ -101,17 +121,7 @@ def run(*args)
 
         req.set_headers(request.headers)
 
-        begin
-          name = ["Grape",
-                  self.options[:method].first,
-                  self.options[:for].to_s,
-                  self.namespace.sub(%r{\A/}, ''), # removing leading slashes
-                  self.options[:path].first,
-          ].compact.map{ |n| n.to_s }.join("/")
-        rescue => e
-          ScoutApm::Agent.instance.context.logger.info("Error getting Grape Endpoint Name. Error: #{e.message}. Options: #{self.options.inspect}")
-          name = "Grape/Unknown"
-        end
+        name = GrapeEndpointNaming.name_for(self)
 
         req.start_layer( ScoutApm::Layer.new("Controller", name) )
         begin
diff --git a/test/unit/instruments/grape_test.rb b/test/unit/instruments/grape_test.rb
index b7cacb57..272f2f7b 100644
--- a/test/unit/instruments/grape_test.rb
+++ b/test/unit/instruments/grape_test.rb
@@ -53,6 +53,31 @@ def test_endpoint_runs_without_infinite_recursion
       assert_includes body_content, 'hello'
     end
 
+    # Grape >= 4 moved endpoint route metadata (method/path/api) off the
+    # public `options` Hash and onto the protected `Endpoint#config` object.
+    # Reading the old `options[...]` keys under Grape 4 silently returns nil,
+    # so every request gets named "Grape/Unknown" instead of the real
+    # endpoint name.
+    def test_endpoint_name_reflects_the_route
+      captured_names = []
+      original_new = ScoutApm::Layer.method(:new)
+      ScoutApm::Layer.define_singleton_method(:new) do |type, name, *rest|
+        captured_names << name if type == "Controller"
+        original_new.call(type, name, *rest)
+      end
+
+      begin
+        status, _headers, _body = HelloAPI.call(Rack::MockRequest.env_for('/hello'))
+      ensure
+        ScoutApm::Layer.define_singleton_method(:new, original_new)
+      end
+
+      assert_equal 200, status
+      refute_empty captured_names
+      refute_equal "Grape/Unknown", captured_names.first
+      assert_equal "Grape/GET/#{HelloAPI}//hello", captured_names.first
+    end
+
     # Either instrumentation method is valid: alias_method is the default,
     # prepend is used when configured (`use_prepend`/`prepend_instruments`)
     # or when the instrument falls back to it for safety.