diff --git a/packages/core/src/agent/content-utils.test.ts b/packages/core/src/agent/content-utils.test.ts
index 0ed681c078..6137bde280 100644
--- a/packages/core/src/agent/content-utils.test.ts
+++ b/packages/core/src/agent/content-utils.test.ts
@@ -187,7 +187,7 @@ describe('contentPartsToGeminiParts', () => {
     ]);
   });
 
-  it('serializes unknown ContentPart variants', () => {
+  it('throws on unknown ContentPart variants', () => {
     // Force an unknown variant past the type system
     const content = [
       { type: 'custom_widget', payload: 123 },
diff --git a/packages/core/src/agent/legacy-agent-session.test.ts b/packages/core/src/agent/legacy-agent-session.test.ts
index 525548e292..1be4b023bd 100644
--- a/packages/core/src/agent/legacy-agent-session.test.ts
+++ b/packages/core/src/agent/legacy-agent-session.test.ts
@@ -241,11 +241,10 @@ describe('LegacyAgentSession', () => {
       );
     });
 
-    it('throws for non-message payloads', async () => {
+    it('returns null streamId for non-message payloads', async () => {
       const session = new LegacyAgentSession(deps);
-      await expect(session.send({ update: { title: 'test' } })).rejects.toThrow(
-        'only supports message sends',
-      );
+      const result = await session.send({ update: { title: 'test' } });
+      expect(result.streamId).toBeNull();
     });
 
     it('throws if send is called while a stream is active', async () => {
diff --git a/packages/core/src/agent/legacy-agent-session.ts b/packages/core/src/agent/legacy-agent-session.ts
index e8d5e56ef5..182256323f 100644
--- a/packages/core/src/agent/legacy-agent-session.ts
+++ b/packages/core/src/agent/legacy-agent-session.ts
@@ -105,12 +105,10 @@ export class LegacyAgentProtocol implements AgentProtocol {
     };
   }
 
-  async send(payload: AgentSend): Promise<{ streamId: string }> {
+  async send(payload: AgentSend): Promise<{ streamId: string | null }> {
     const message = 'message' in payload ? payload.message : undefined;
     if (!message) {
-      throw new Error(
-        'LegacyAgentSession.send() only supports message sends for the moment.',
-      );
+      return { streamId: null };
     }
 
     if (this._activeStreamId) {