diff --git a/.toys/ci.rb b/.toys/ci.rb
index cb5650a2fc2a..a1cd344821c7 100644
--- a/.toys/ci.rb
+++ b/.toys/ci.rb
@@ -237,8 +237,8 @@ def interpret_github_event
       logger.info "Using local commits"
       [base_commit, head_commit]
     end
-  base_ref = nil if base_ref&.empty?
-  head_ref = nil if head_ref&.empty?
+  base_ref = nil if base_ref && base_ref.empty?
+  head_ref = nil if head_ref && head_ref.empty?
   [base_ref, head_ref]
 end
 
diff --git a/google-cloud-error_reporting/AUTHENTICATION.md b/google-cloud-error_reporting/AUTHENTICATION.md
index d3e9a71b86c5..6adea5259616 100644
--- a/google-cloud-error_reporting/AUTHENTICATION.md
+++ b/google-cloud-error_reporting/AUTHENTICATION.md
@@ -96,20 +96,24 @@ client = Google::Cloud::ErrorReporting.new
 
 ### Configuration
 
-The **Project ID** and the path to the **Credentials JSON** file can be configured
+The **Project ID**, **Quota Project**, and the path to the **Credentials JSON** file can be configured
 instead of placing them in environment variables or providing them as arguments.
 
 ```ruby
 require "google/cloud/error_reporting"
 
 Google::Cloud::ErrorReporting.configure do |config|
-  config.project_id  = "my-project-id"
-  config.credentials = "path/to/keyfile.json"
+  config.project_id    = "my-project-id"        # The project where errors are reported
+  config.quota_project = "my-billing-project"   # The project billed for quota/billing (optional)
+  config.credentials   = "path/to/keyfile.json"
 end
 
 client = Google::Cloud::ErrorReporting.new
 ```
 
+> [!NOTE]
+> **Project ID** (where errors are sent) and **Quota Project** (which project is billed for the API call) are distinct. By default, the library bills the project associated with the credentials. Use `config.quota_project` if you need to bill a different project.
+
 ### Cloud SDK
 
 This option allows for an easy way to authenticate during development. If
diff --git a/google-cloud-error_reporting/README.md b/google-cloud-error_reporting/README.md
index efaa355a5451..2ebf27bad7d5 100644
--- a/google-cloud-error_reporting/README.md
+++ b/google-cloud-error_reporting/README.md
@@ -168,9 +168,12 @@ Google::Cloud.configure do |config|
   # Shared parameters
   config.project_id = "your-project-id"
   config.keyfile = "/path/to/key.json"
+  config.quota_project = "your-billing-project"
+
   # Or Error Reporting specific parameters
   config.error_reporting.project_id = "your-project-id"
   config.error_reporting.keyfile = "/path/to/key.json"
+  config.error_reporting.quota_project = "your-billing-project"
 end
 ```
 
diff --git a/google-cloud-error_reporting/lib/google/cloud/error_reporting.rb b/google-cloud-error_reporting/lib/google/cloud/error_reporting.rb
index 28aa231c4ef4..51844b2b9cff 100644
--- a/google-cloud-error_reporting/lib/google/cloud/error_reporting.rb
+++ b/google-cloud-error_reporting/lib/google/cloud/error_reporting.rb
@@ -93,7 +93,10 @@ def self.new project_id: nil,
         credentials = resolve_credentials credentials, scope
         project_id = resolve_project_id project_id, credentials
 
-        service = ErrorReporting::Service.new project_id, credentials, host: endpoint, timeout: timeout
+        quota_project = configure.quota_project
+        service = ErrorReporting::Service.new project_id, credentials,
+                                              host: endpoint, timeout: timeout,
+                                              quota_project: quota_project
         ErrorReporting::Project.new service
       end
 
diff --git a/google-cloud-error_reporting/lib/google/cloud/error_reporting/project.rb b/google-cloud-error_reporting/lib/google/cloud/error_reporting/project.rb
index cf65e373c50d..551328404eb8 100644
--- a/google-cloud-error_reporting/lib/google/cloud/error_reporting/project.rb
+++ b/google-cloud-error_reporting/lib/google/cloud/error_reporting/project.rb
@@ -122,8 +122,8 @@ def project_id
         #   error_event = error_reporting.error_event "Error with Backtrace"
         #   error_reporting.report error_event
         #
-        def report *args, &block
-          service.report(*args, &block)
+        def report(*args, &)
+          service.report(*args, &)
         end
 
         ##
diff --git a/google-cloud-error_reporting/lib/google/cloud/error_reporting/service.rb b/google-cloud-error_reporting/lib/google/cloud/error_reporting/service.rb
index ef43277e5aa8..467cb654a8ff 100644
--- a/google-cloud-error_reporting/lib/google/cloud/error_reporting/service.rb
+++ b/google-cloud-error_reporting/lib/google/cloud/error_reporting/service.rb
@@ -29,14 +29,17 @@ class Service
         attr_accessor :credentials
         attr_accessor :timeout
         attr_accessor :host
+        attr_accessor :quota_project
 
         ##
         # Creates a new Service instance.
-        def initialize project, credentials, timeout: nil, host: nil
+        def initialize project, credentials, timeout: nil, host: nil, quota_project: nil
           @project = project
           @credentials = credentials
           @timeout = timeout
           @host = host
+          @quota_project = quota_project
+          @quota_project ||= credentials.quota_project_id if credentials.respond_to? :quota_project_id
         end
 
         def error_reporting
@@ -46,6 +49,7 @@ def error_reporting
               config.credentials = credentials if credentials
               config.timeout = timeout if timeout
               config.endpoint = host if host
+              config.quota_project = quota_project if quota_project
               config.lib_name = "gccl"
               config.lib_version = Google::Cloud::ErrorReporting::VERSION
             end
diff --git a/google-cloud-error_reporting/support/doctest_helper.rb b/google-cloud-error_reporting/support/doctest_helper.rb
index b179fbd0de4b..38392f631381 100644
--- a/google-cloud-error_reporting/support/doctest_helper.rb
+++ b/google-cloud-error_reporting/support/doctest_helper.rb
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 require "minitest/focus"
+require "ostruct"
 
 require "google/cloud/error_reporting"
 
@@ -73,13 +74,13 @@ def mock_error_reporting
 
   doctest.before "Google::Cloud#error_reporting" do
     mock_error_reporting do |mock|
-      mock.expect :report_error_event, nil, [Hash]
+      mock.expect :report_error_event, nil, [], project_name: String, event: Google::Cloud::ErrorReporting::V1beta1::ReportedErrorEvent
     end
   end
 
   doctest.before "Google::Cloud.error_reporting" do
     mock_error_reporting do |mock|
-      mock.expect :report_error_event, nil, [Hash]
+      mock.expect :report_error_event, nil, [], project_name: String, event: Google::Cloud::ErrorReporting::V1beta1::ReportedErrorEvent
     end
   end
 
@@ -89,13 +90,13 @@ def mock_error_reporting
 
   doctest.before "Google::Cloud::ErrorReporting::ErrorEvent" do
     mock_error_reporting do |mock|
-      mock.expect :report_error_event, nil, [Hash]
+      mock.expect :report_error_event, nil, [], project_name: String, event: Google::Cloud::ErrorReporting::V1beta1::ReportedErrorEvent
     end
   end
 
   doctest.before "Google::Cloud::ErrorReporting::Project" do
     mock_error_reporting do |mock|
-      mock.expect :report_error_event, nil, [Hash]
+      mock.expect :report_error_event, nil, [], project_name: String, event: Google::Cloud::ErrorReporting::V1beta1::ReportedErrorEvent
     end
   end
 
@@ -103,7 +104,7 @@ def mock_error_reporting
 
   doctest.before "Google::Cloud::ErrorReporting::Service" do
     mock_error_reporting do |mock|
-      mock.expect :report_error_event, nil, [Hash]
+      mock.expect :report_error_event, nil, [], project_name: String, event: Google::Cloud::ErrorReporting::V1beta1::ReportedErrorEvent
     end
   end
 end
diff --git a/google-cloud-error_reporting/test/google/cloud/error_reporting/service_test.rb b/google-cloud-error_reporting/test/google/cloud/error_reporting/service_test.rb
new file mode 100644
index 000000000000..29176cb63a02
--- /dev/null
+++ b/google-cloud-error_reporting/test/google/cloud/error_reporting/service_test.rb
@@ -0,0 +1,67 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+require "helper"
+require "signet/oauth_2/client"
+
+describe Google::Cloud::ErrorReporting::Service do # rubocop:disable Metrics/BlockLength
+  let(:project) { "test-project" }
+  let :credentials do
+    creds = Signet::OAuth2::Client.new
+    def creds.quota_project_id
+      "credentials-quota-project"
+    end
+
+    def creds.disable_universe_domain_check
+      true
+    end
+    creds
+  end
+
+  describe ".new" do
+    it "sets project and credentials" do
+      service = Google::Cloud::ErrorReporting::Service.new project, credentials
+      _(service.project).must_equal project
+      _(service.credentials).must_equal credentials
+    end
+
+    it "accepts quota_project" do
+      quota_project = "test-quota-project"
+      service = Google::Cloud::ErrorReporting::Service.new project, credentials, quota_project: quota_project
+      _(service.quota_project).must_equal quota_project
+    end
+
+    it "falls back to credentials quota_project_id if not explicitly passed" do
+      service = Google::Cloud::ErrorReporting::Service.new project, credentials
+      _(service.quota_project).must_equal "credentials-quota-project"
+    end
+  end
+
+  describe "#error_reporting" do
+    it "configures the gRPC client with quota_project" do
+      quota_project = "test-quota-project"
+      service = Google::Cloud::ErrorReporting::Service.new project, credentials, quota_project: quota_project
+
+      client = service.error_reporting
+      _(client.configure.quota_project).must_equal quota_project
+    end
+
+    it "configures the gRPC client with credentials quota_project" do
+      service = Google::Cloud::ErrorReporting::Service.new project, credentials
+
+      client = service.error_reporting
+      _(client.configure.quota_project).must_equal "credentials-quota-project"
+    end
+  end
+end
diff --git a/google-cloud-error_reporting/test/google/cloud/error_reporting_test.rb b/google-cloud-error_reporting/test/google/cloud/error_reporting_test.rb
index f01c3c01eb56..7d5c96a28e18 100644
--- a/google-cloud-error_reporting/test/google/cloud/error_reporting_test.rb
+++ b/google-cloud-error_reporting/test/google/cloud/error_reporting_test.rb
@@ -66,11 +66,12 @@ def creds.is_a? target
 
     it "uses provided endpoint" do
       endpoint = "errorreporting-endpoint2.example.com"
-      stubbed_service = ->(project, credentials, timeout: nil, host: nil) {
+      stubbed_service = ->(project, credentials, timeout: nil, host: nil, quota_project: nil) {
         _(project).must_equal "project-id"
         _(credentials).must_equal default_credentials
         _(timeout).must_be :nil?
         _(host).must_equal endpoint
+        _(quota_project).must_be :nil?
         OpenStruct.new project: project
       }
       ENV.stub :[], nil do
@@ -85,6 +86,7 @@ def creds.is_a? target
       end
     end
 
+
     it "uses provided project (alias), keyfile (alias), service, and version" do
       stubbed_credentials = ->(keyfile, scope: nil) {
         _(keyfile).must_equal "/path/to/a/keyfile"
@@ -315,11 +317,12 @@ def creds.is_a? target
         _(scope).must_equal default_scopes
         "error_reporting-credentials"
       }
-      stubbed_service = ->(project, credentials, timeout: nil, host: nil) {
+      stubbed_service = ->(project, credentials, timeout: nil, host: nil, quota_project: nil) {
         _(project).must_equal "project-id"
         _(credentials).must_equal "error_reporting-credentials"
         _(timeout).must_be :nil?
         _(host).must_equal default_endpoint
+        _(quota_project).must_be :nil?
         OpenStruct.new project: project
       }
 
@@ -352,11 +355,12 @@ def creds.is_a? target
         _(scope).must_equal default_scopes
         "error_reporting-credentials"
       }
-      stubbed_service = ->(project, credentials, timeout: nil, host: nil) {
+      stubbed_service = ->(project, credentials, timeout: nil, host: nil, quota_project: nil) {
         _(project).must_equal "project-id"
         _(credentials).must_equal "error_reporting-credentials"
         _(timeout).must_be :nil?
         _(host).must_equal default_endpoint
+        _(quota_project).must_be :nil?
         OpenStruct.new project: project
       }
 
@@ -389,11 +393,12 @@ def creds.is_a? target
         _(scope).must_equal default_scopes
         "error_reporting-credentials"
       }
-      stubbed_service = ->(project, credentials, timeout: nil, host: nil) {
+      stubbed_service = ->(project, credentials, timeout: nil, host: nil, quota_project: nil) {
         _(project).must_equal "project-id"
         _(credentials).must_equal "error_reporting-credentials"
         _(timeout).must_equal 42
         _(host).must_equal default_endpoint
+        _(quota_project).must_be :nil?
         OpenStruct.new project: project
       }
 
@@ -423,11 +428,12 @@ def creds.is_a? target
 
     it "uses error_reporting config for endpoint" do
       endpoint = "errorreporting-endpoint2.example.com"
-      stubbed_service = ->(project, credentials, timeout: nil, host: nil) {
+      stubbed_service = ->(project, credentials, timeout: nil, host: nil, quota_project: nil) {
         _(project).must_equal "project-id"
         _(credentials).must_equal default_credentials
         _(timeout).must_be :nil?
         _(host).must_equal endpoint
+        _(quota_project).must_be :nil?
         OpenStruct.new project: project
       }
 
@@ -454,11 +460,12 @@ def creds.is_a? target
         _(scope).must_equal default_scopes
         "error_reporting-credentials"
       }
-      stubbed_service = ->(project, credentials, timeout: nil, host: nil) {
+      stubbed_service = ->(project, credentials, timeout: nil, host: nil, quota_project: nil) {
         _(project).must_equal "project-id"
         _(credentials).must_equal "error_reporting-credentials"
         _(timeout).must_equal 42
         _(host).must_equal default_endpoint
+        _(quota_project).must_be :nil?
         OpenStruct.new project: project
       }
 
@@ -485,5 +492,29 @@ def creds.is_a? target
         end
       end
     end
+
+    it "uses error_reporting config for quota_project" do
+      stubbed_service = lambda do |project, credentials, timeout: nil, host: nil, quota_project: nil|
+        _(project).must_equal "project-id"
+        _(credentials).must_equal default_credentials
+        _(timeout).must_be :nil?
+        _(host).must_equal default_endpoint
+        _(quota_project).must_equal "configure-quota-project"
+        OpenStruct.new project: project
+      end
+
+      ENV.stub :[], nil do
+        Google::Cloud::ErrorReporting.configure do |config|
+          config.project_id = "project-id"
+          config.quota_project = "configure-quota-project"
+        end
+
+        Google::Cloud::ErrorReporting::Service.stub :new, stubbed_service do
+          error_reporting = Google::Cloud::ErrorReporting.new credentials: default_credentials
+          _(error_reporting).must_be_kind_of Google::Cloud::ErrorReporting::Project
+          _(error_reporting.project).must_equal "project-id"
+        end
+      end
+    end
   end
 end