From dc3c7994d10ed1c39566489c0f006ddc6a8191df Mon Sep 17 00:00:00 2001
From: Adam Weidman <adamfweidman@google.com>
Date: Thu, 7 May 2026 14:09:21 -0400
Subject: [PATCH] refactor: address review comments for local protocol

---
 packages/core/src/agent/content-utils.test.ts    | 16 ++++------------
 packages/core/src/agent/content-utils.ts         | 13 +++++++------
 .../core/src/agent/legacy-agent-session.test.ts  |  7 +++----
 packages/core/src/agent/legacy-agent-session.ts  |  6 ++----
 4 files changed, 16 insertions(+), 26 deletions(-)

diff --git a/packages/core/src/agent/content-utils.test.ts b/packages/core/src/agent/content-utils.test.ts
index acf8a4a329..346b0e2a56 100644
--- a/packages/core/src/agent/content-utils.test.ts
+++ b/packages/core/src/agent/content-utils.test.ts
@@ -187,22 +187,14 @@ 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 },
     ] as unknown as ContentPart[];
-
-    const warnSpy = vi.spyOn(debugLogger, 'warn');
-    const result = contentPartsToGeminiParts(content);
-
-    expect(warnSpy).toHaveBeenCalled();
-    expect(result).toHaveLength(1);
-    expect(result[0]).toEqual({
-      text: JSON.stringify({ type: 'custom_widget', payload: 123 }),
-    });
-
-    warnSpy.mockRestore();
+    expect(() => contentPartsToGeminiParts(content)).toThrow(
+      'Unhandled ContentPart type: {"type":"custom_widget","payload":123}',
+    );
   });
 });
 
diff --git a/packages/core/src/agent/content-utils.ts b/packages/core/src/agent/content-utils.ts
index 42b0b7fec7..1c716ffd11 100644
--- a/packages/core/src/agent/content-utils.ts
+++ b/packages/core/src/agent/content-utils.ts
@@ -93,13 +93,14 @@ export function contentPartsToGeminiParts(content: ContentPart[]): Part[] {
         // References are converted to text for the model
         result.push({ text: part.text });
         break;
-      default:
-        debugLogger.warn(
-          `Unhandled ContentPart type: ${JSON.stringify(part)} fallback to serialization`,
-        );
-        // Serialize unknown ContentPart variants instead of dropping them
-        result.push({ text: JSON.stringify(part) });
+      default: {
+        ((x: never) => {
+          throw new Error(`Unhandled ContentPart type: ${JSON.stringify(x)}`);
+        })(part);
         break;
+      }
+        break;
+      }
     }
   }
   return result;
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) {