diff --git a/app/messages/route_options_message.rb b/app/messages/route_options_message.rb
index f79a2bb6a0..c8b6d82a11 100644
--- a/app/messages/route_options_message.rb
+++ b/app/messages/route_options_message.rb
@@ -3,11 +3,15 @@
 module VCAP::CloudController
   class RouteOptionsMessage < BaseMessage
     # Register all possible keys upfront so attr_accessors are created
-    register_allowed_keys %i[loadbalancing hash_header hash_balance]
+    # RFC-0027 compliant: only string/number/boolean values (no nested objects/arrays)
+    # mtls_allowed_apps, mtls_allowed_spaces, mtls_allowed_orgs are comma-separated GUIDs
+    # mtls_allow_any is a boolean
+    register_allowed_keys %i[loadbalancing hash_header hash_balance mtls_allowed_apps mtls_allowed_spaces mtls_allowed_orgs mtls_allow_any]
 
     def self.valid_route_options
       options = %i[loadbalancing]
       options += %i[hash_header hash_balance] if VCAP::CloudController::FeatureFlag.enabled?(:hash_based_routing)
+      options += %i[mtls_allowed_apps mtls_allowed_spaces mtls_allowed_orgs mtls_allow_any] if VCAP::CloudController::FeatureFlag.enabled?(:app_to_app_mtls_routing)
       options.freeze
     end
 
@@ -21,6 +25,7 @@ def self.valid_loadbalancing_algorithms
     validate :loadbalancing_algorithm_is_valid
     validate :route_options_are_valid
     validate :hash_options_are_valid
+    validate :mtls_allowed_sources_options_are_valid
 
     def loadbalancing_algorithm_is_valid
       return if loadbalancing.blank?
@@ -82,5 +87,95 @@ def validate_hash_options_with_loadbalancing
       errors.add(:base, 'Hash header can only be set when loadbalancing is hash') if hash_header.present? && loadbalancing.present? && loadbalancing != 'hash'
       errors.add(:base, 'Hash balance can only be set when loadbalancing is hash') if hash_balance.present? && loadbalancing.present? && loadbalancing != 'hash'
     end
+
+    def mtls_allowed_sources_options_are_valid
+      # Only validate mtls options when the feature flag is enabled
+      # If disabled, route_options_are_valid will already report them as unknown fields
+      return unless VCAP::CloudController::FeatureFlag.enabled?(:app_to_app_mtls_routing)
+
+      validate_mtls_string_types
+      validate_mtls_allow_any_type
+      validate_mtls_allow_any_exclusivity
+      validate_mtls_guids_exist
+    end
+
+    private
+
+    # Parse comma-separated GUIDs into an array
+    def parse_guid_list(value)
+      return [] if value.blank?
+
+      value.to_s.split(',').map(&:strip).reject(&:empty?)
+    end
+
+    def validate_mtls_string_types
+      # These should be strings (comma-separated GUIDs) per RFC-0027
+      %i[mtls_allowed_apps mtls_allowed_spaces mtls_allowed_orgs].each do |key|
+        value = public_send(key)
+        next if value.blank?
+
+        unless value.is_a?(String)
+          errors.add(key, 'must be a string of comma-separated GUIDs')
+        end
+      end
+    end
+
+    def validate_mtls_allow_any_type
+      return if mtls_allow_any.nil?
+
+      unless [true, false, 'true', 'false'].include?(mtls_allow_any)
+        errors.add(:mtls_allow_any, 'must be a boolean (true or false)')
+      end
+    end
+
+    def validate_mtls_allow_any_exclusivity
+      allow_any = mtls_allow_any == true || mtls_allow_any == 'true'
+      has_specific = [mtls_allowed_apps, mtls_allowed_spaces, mtls_allowed_orgs].any?(&:present?)
+
+      return unless allow_any && has_specific
+
+      errors.add(:mtls_allow_any, 'is mutually exclusive with mtls_allowed_apps, mtls_allowed_spaces, and mtls_allowed_orgs')
+    end
+
+    def validate_mtls_guids_exist
+      return if errors.any? # Skip if already invalid
+
+      validate_app_guids_exist
+      validate_space_guids_exist
+      validate_org_guids_exist
+    end
+
+    def validate_app_guids_exist
+      app_guids = parse_guid_list(mtls_allowed_apps)
+      return if app_guids.empty?
+
+      existing_guids = AppModel.where(guid: app_guids).select_map(:guid)
+      missing_guids = app_guids - existing_guids
+      return if missing_guids.empty?
+
+      errors.add(:mtls_allowed_apps, "contains non-existent app GUIDs: #{missing_guids.join(', ')}")
+    end
+
+    def validate_space_guids_exist
+      space_guids = parse_guid_list(mtls_allowed_spaces)
+      return if space_guids.empty?
+
+      existing_guids = Space.where(guid: space_guids).select_map(:guid)
+      missing_guids = space_guids - existing_guids
+      return if missing_guids.empty?
+
+      errors.add(:mtls_allowed_spaces, "contains non-existent space GUIDs: #{missing_guids.join(', ')}")
+    end
+
+    def validate_org_guids_exist
+      org_guids = parse_guid_list(mtls_allowed_orgs)
+      return if org_guids.empty?
+
+      existing_guids = Organization.where(guid: org_guids).select_map(:guid)
+      missing_guids = org_guids - existing_guids
+      return if missing_guids.empty?
+
+      errors.add(:mtls_allowed_orgs, "contains non-existent organization GUIDs: #{missing_guids.join(', ')}")
+    end
   end
 end
diff --git a/app/models/runtime/feature_flag.rb b/app/models/runtime/feature_flag.rb
index df991d89f7..da4bc026f2 100644
--- a/app/models/runtime/feature_flag.rb
+++ b/app/models/runtime/feature_flag.rb
@@ -24,7 +24,8 @@ class UndefinedFeatureFlagError < StandardError
       hide_marketplace_from_unauthenticated_users: false,
       resource_matching: true,
       route_sharing: false,
-      hash_based_routing: false
+      hash_based_routing: false,
+      app_to_app_mtls_routing: false
     }.freeze
 
     ADMIN_SKIPPABLE = %i[
diff --git a/spec/unit/messages/route_options_message_spec.rb b/spec/unit/messages/route_options_message_spec.rb
index 57646d2195..f081ecc942 100644
--- a/spec/unit/messages/route_options_message_spec.rb
+++ b/spec/unit/messages/route_options_message_spec.rb
@@ -37,6 +37,215 @@ module VCAP::CloudController
       end
     end
 
+    describe 'mTLS allowed sources validations (RFC-0027 compliant flat options)' do
+      context 'when app_to_app_mtls_routing feature flag is disabled' do
+        it 'does not allow mtls_allowed_apps option' do
+          message = RouteOptionsMessage.new({ mtls_allowed_apps: 'app-guid-1' })
+          expect(message).not_to be_valid
+          expect(message.errors_on(:base)).to include("Unknown field(s): 'mtls_allowed_apps'")
+        end
+
+        it 'does not allow mtls_allowed_spaces option' do
+          message = RouteOptionsMessage.new({ mtls_allowed_spaces: 'space-guid-1' })
+          expect(message).not_to be_valid
+          expect(message.errors_on(:base)).to include("Unknown field(s): 'mtls_allowed_spaces'")
+        end
+
+        it 'does not allow mtls_allowed_orgs option' do
+          message = RouteOptionsMessage.new({ mtls_allowed_orgs: 'org-guid-1' })
+          expect(message).not_to be_valid
+          expect(message.errors_on(:base)).to include("Unknown field(s): 'mtls_allowed_orgs'")
+        end
+
+        it 'does not allow mtls_allow_any option' do
+          message = RouteOptionsMessage.new({ mtls_allow_any: true })
+          expect(message).not_to be_valid
+          expect(message.errors_on(:base)).to include("Unknown field(s): 'mtls_allow_any'")
+        end
+      end
+
+      context 'when app_to_app_mtls_routing feature flag is enabled' do
+        before do
+          VCAP::CloudController::FeatureFlag.make(name: 'app_to_app_mtls_routing', enabled: true)
+        end
+
+        describe 'mtls_allowed_apps validation' do
+          it 'allows valid comma-separated app GUIDs' do
+            app1 = AppModel.make
+            app2 = AppModel.make
+            message = RouteOptionsMessage.new({ mtls_allowed_apps: "#{app1.guid},#{app2.guid}" })
+            expect(message).to be_valid
+          end
+
+          it 'allows single app GUID' do
+            app = AppModel.make
+            message = RouteOptionsMessage.new({ mtls_allowed_apps: app.guid })
+            expect(message).to be_valid
+          end
+
+          it 'allows app GUIDs with whitespace around commas' do
+            app1 = AppModel.make
+            app2 = AppModel.make
+            message = RouteOptionsMessage.new({ mtls_allowed_apps: "#{app1.guid} , #{app2.guid}" })
+            expect(message).to be_valid
+          end
+
+          it 'rejects non-existent app GUIDs' do
+            message = RouteOptionsMessage.new({ mtls_allowed_apps: 'non-existent-guid' })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allowed_apps)).to include('contains non-existent app GUIDs: non-existent-guid')
+          end
+
+          it 'reports multiple non-existent app GUIDs' do
+            message = RouteOptionsMessage.new({ mtls_allowed_apps: 'guid-1,guid-2' })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allowed_apps)).to include('contains non-existent app GUIDs: guid-1, guid-2')
+          end
+
+          it 'rejects non-string values' do
+            message = RouteOptionsMessage.new({ mtls_allowed_apps: ['array-not-string'] })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allowed_apps)).to include('must be a string of comma-separated GUIDs')
+          end
+        end
+
+        describe 'mtls_allowed_spaces validation' do
+          it 'allows valid comma-separated space GUIDs' do
+            space1 = Space.make
+            space2 = Space.make
+            message = RouteOptionsMessage.new({ mtls_allowed_spaces: "#{space1.guid},#{space2.guid}" })
+            expect(message).to be_valid
+          end
+
+          it 'allows single space GUID' do
+            space = Space.make
+            message = RouteOptionsMessage.new({ mtls_allowed_spaces: space.guid })
+            expect(message).to be_valid
+          end
+
+          it 'rejects non-existent space GUIDs' do
+            message = RouteOptionsMessage.new({ mtls_allowed_spaces: 'non-existent-space' })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allowed_spaces)).to include('contains non-existent space GUIDs: non-existent-space')
+          end
+
+          it 'rejects non-string values' do
+            message = RouteOptionsMessage.new({ mtls_allowed_spaces: { 'nested' => 'object' } })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allowed_spaces)).to include('must be a string of comma-separated GUIDs')
+          end
+        end
+
+        describe 'mtls_allowed_orgs validation' do
+          it 'allows valid comma-separated org GUIDs' do
+            org1 = Organization.make
+            org2 = Organization.make
+            message = RouteOptionsMessage.new({ mtls_allowed_orgs: "#{org1.guid},#{org2.guid}" })
+            expect(message).to be_valid
+          end
+
+          it 'allows single org GUID' do
+            org = Organization.make
+            message = RouteOptionsMessage.new({ mtls_allowed_orgs: org.guid })
+            expect(message).to be_valid
+          end
+
+          it 'rejects non-existent org GUIDs' do
+            message = RouteOptionsMessage.new({ mtls_allowed_orgs: 'non-existent-org' })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allowed_orgs)).to include('contains non-existent organization GUIDs: non-existent-org')
+          end
+        end
+
+        describe 'mtls_allow_any validation' do
+          it 'allows true value' do
+            message = RouteOptionsMessage.new({ mtls_allow_any: true })
+            expect(message).to be_valid
+          end
+
+          it 'allows false value' do
+            message = RouteOptionsMessage.new({ mtls_allow_any: false })
+            expect(message).to be_valid
+          end
+
+          it 'allows string "true"' do
+            message = RouteOptionsMessage.new({ mtls_allow_any: 'true' })
+            expect(message).to be_valid
+          end
+
+          it 'allows string "false"' do
+            message = RouteOptionsMessage.new({ mtls_allow_any: 'false' })
+            expect(message).to be_valid
+          end
+
+          it 'rejects non-boolean values' do
+            message = RouteOptionsMessage.new({ mtls_allow_any: 'yes' })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allow_any)).to include('must be a boolean (true or false)')
+          end
+        end
+
+        describe 'mtls_allow_any exclusivity validation' do
+          it 'does not allow mtls_allow_any with mtls_allowed_apps' do
+            app = AppModel.make
+            message = RouteOptionsMessage.new({ mtls_allow_any: true, mtls_allowed_apps: app.guid })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allow_any)).to include('is mutually exclusive with mtls_allowed_apps, mtls_allowed_spaces, and mtls_allowed_orgs')
+          end
+
+          it 'does not allow mtls_allow_any with mtls_allowed_spaces' do
+            space = Space.make
+            message = RouteOptionsMessage.new({ mtls_allow_any: true, mtls_allowed_spaces: space.guid })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allow_any)).to include('is mutually exclusive with mtls_allowed_apps, mtls_allowed_spaces, and mtls_allowed_orgs')
+          end
+
+          it 'does not allow mtls_allow_any with mtls_allowed_orgs' do
+            org = Organization.make
+            message = RouteOptionsMessage.new({ mtls_allow_any: true, mtls_allowed_orgs: org.guid })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allow_any)).to include('is mutually exclusive with mtls_allowed_apps, mtls_allowed_spaces, and mtls_allowed_orgs')
+          end
+
+          it 'allows mtls_allow_any: false with specific GUIDs' do
+            app = AppModel.make
+            message = RouteOptionsMessage.new({ mtls_allow_any: false, mtls_allowed_apps: app.guid })
+            expect(message).to be_valid
+          end
+
+          it 'allows string "true" exclusivity check' do
+            app = AppModel.make
+            message = RouteOptionsMessage.new({ mtls_allow_any: 'true', mtls_allowed_apps: app.guid })
+            expect(message).not_to be_valid
+            expect(message.errors_on(:mtls_allow_any)).to include('is mutually exclusive with mtls_allowed_apps, mtls_allowed_spaces, and mtls_allowed_orgs')
+          end
+        end
+
+        describe 'combined options' do
+          it 'allows all mTLS options together (without mtls_allow_any)' do
+            app = AppModel.make
+            space = Space.make
+            org = Organization.make
+            message = RouteOptionsMessage.new({
+              mtls_allowed_apps: app.guid,
+              mtls_allowed_spaces: space.guid,
+              mtls_allowed_orgs: org.guid
+            })
+            expect(message).to be_valid
+          end
+
+          it 'allows mTLS options with loadbalancing' do
+            app = AppModel.make
+            message = RouteOptionsMessage.new({
+              loadbalancing: 'round-robin',
+              mtls_allowed_apps: app.guid
+            })
+            expect(message).to be_valid
+          end
+        end
+      end
+    end
+
     describe 'hash-based routing validations' do
       context 'when hash_based_routing feature flag is disabled' do
         it 'does not allow hash_header option' do