diff --git a/CHANGELOG.md b/CHANGELOG.md
index fcef2bd..6514493 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Fixed
+- Make `testRunStartedId` optional ([#4](https://github.com/cucumber/javascript-core/pull/4))
 
 ## [0.2.0] - 2025-07-24
 ### Added
diff --git a/cucumber-core.api.md b/cucumber-core.api.md
index 1f477b2..41bd75c 100644
--- a/cucumber-core.api.md
+++ b/cucumber-core.api.md
@@ -170,7 +170,7 @@ export interface TestPlanIngredients {
     gherkinDocument: GherkinDocument;
     pickles: ReadonlyArray<Pickle>;
     supportCodeLibrary: SupportCodeLibrary;
-    testRunStartedId: string;
+    testRunStartedId?: string;
 }
 
 // @public
diff --git a/src/types.ts b/src/types.ts
index 977a35d..55f1929 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -298,7 +298,7 @@ export interface TestPlanIngredients {
   /**
    * Identifier for the test run within which this plan will be executed
    */
-  testRunStartedId: string
+  testRunStartedId?: string
   /**
    * The Gherkin document that has been processed
    */