fix(core): update token count and telemetry on /chat resume history load (#16279)

Co-authored-by: Tommaso Sciortino <sciortino@gmail.com>

packages/core/src/core/client.test.ts

@@ -303,6 +303,17 @@ describe('Gemini Client (client.ts)', () => {
     });
   });
 
+  describe('setHistory', () => {
+    it('should update telemetry token count when history is set', () => {
+      const history: Content[] = [
+        { role: 'user', parts: [{ text: 'some message' }] },
+      ];
+      client.setHistory(history);
+
+      expect(uiTelemetryService.setLastPromptTokenCount).toHaveBeenCalled();
+    });
+  });
+
   describe('resumeChat', () => {
     it('should update telemetry token count when a chat is resumed', async () => {
       const history: Content[] = [

packages/core/src/core/client.ts

@@ -246,6 +246,7 @@ export class GeminiClient {
 
   setHistory(history: Content[]) {
     this.getChat().setHistory(history);
+    this.updateTelemetryTokenCount();
     this.forceFullIdeContext = true;
   }
 

packages/core/src/core/geminiChat.test.ts

@@ -219,6 +219,37 @@ describe('GeminiChat', () => {
     });
   });
 
+  describe('setHistory', () => {
+    it('should recalculate lastPromptTokenCount when history is updated', () => {
+      const initialHistory: Content[] = [
+        { role: 'user', parts: [{ text: 'Hello' }] },
+      ];
+      const chatWithHistory = new GeminiChat(
+        mockConfig,
+        '',
+        [],
+        initialHistory,
+      );
+      const initialCount = chatWithHistory.getLastPromptTokenCount();
+
+      const newHistory: Content[] = [
+        {
+          role: 'user',
+          parts: [
+            {
+              text: 'This is a much longer history item that should result in more tokens than just hello.',
+            },
+          ],
+        },
+      ];
+      chatWithHistory.setHistory(newHistory);
+
+      expect(chatWithHistory.getLastPromptTokenCount()).toBeGreaterThan(
+        initialCount,
+      );
+    });
+  });
+
   describe('sendMessageStream', () => {
     it('should succeed if a tool call is followed by an empty part', async () => {
       // 1. Mock a stream that contains a tool call, then an invalid (empty) part.

packages/core/src/core/geminiChat.ts

@@ -676,6 +676,9 @@ export class GeminiChat {
 
   setHistory(history: Content[]): void {
     this.history = history;
+    this.lastPromptTokenCount = estimateTokenCountSync(
+      this.history.flatMap((c) => c.parts || []),
+    );
   }
 
   stripThoughtsFromHistory(): void {