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