fix(core): serialize workflow request context safely (#16061)

Author: mbenhamd

.changeset/rare-kiwis-throw.md

@@ -0,0 +1,5 @@
+---
+'@mastra/core': patch
+---
+
+Fixed workflow request context serialization to skip values that cannot be safely stored as JSON. Fixes #16043.

packages/core/src/workflows/default.test.ts

@@ -608,6 +608,23 @@ describe('DefaultExecutionEngine.fmtReturnValue stepExecutionPath and payload de
 });
 
 describe('DefaultExecutionEngine.deserializeRequestContext', () => {
+  it('should produce JSON-safe serialized request context values', () => {
+    const engine = new TestableExecutionEngine({ mastra: undefined });
+    const requestContext = new RequestContext();
+    const circular: any = { name: 'service' };
+    circular.self = circular;
+
+    requestContext.set('userId', 'user-123');
+    requestContext.set('progressEmitter', () => undefined);
+    requestContext.set('service', circular);
+
+    const serialized = engine.serializeRequestContext(requestContext);
+
+    expect(() => JSON.stringify(requestContext.toJSON())).not.toThrow();
+    expect(() => JSON.stringify(serialized)).not.toThrow();
+    expect(serialized).toEqual({ userId: 'user-123' });
+  });
+
   it('should return a RequestContext instance with all entries from the plain object', () => {
     const engine = new TestableExecutionEngine({ mastra: undefined });
     const plainObj = { userId: 'user-123', tenantId: 'tenant-456', nested: { flag: true } };

packages/core/src/workflows/default.ts

@@ -615,11 +615,7 @@ export class DefaultExecutionEngine extends ExecutionEngine {
    * Used by durable execution engines to persist context across step replays.
    */
   serializeRequestContext(requestContext: RequestContext): Record<string, any> {
-    const obj: Record<string, any> = {};
-    requestContext.forEach((value, key) => {
-      obj[key] = value;
-    });
-    return obj;
+    return requestContext.toJSON();
   }
 
   /**