diff --git a/packages/core/src/code_assist/converter.test.ts b/packages/core/src/code_assist/converter.test.ts
index 3d3a8ef3..03f388dc 100644
--- a/packages/core/src/code_assist/converter.test.ts
+++ b/packages/core/src/code_assist/converter.test.ts
@@ -24,12 +24,7 @@ describe('converter', () => {
         model: 'gemini-pro',
         contents: [{ role: 'user', parts: [{ text: 'Hello' }] }],
       };
-      const codeAssistReq = toGenerateContentRequest(
-        genaiReq,
-        'my-prompt',
-        'my-project',
-        'my-session',
-      );
+      const codeAssistReq = toGenerateContentRequest(genaiReq, 'my-project');
       expect(codeAssistReq).toEqual({
         model: 'gemini-pro',
         project: 'my-project',
@@ -42,9 +37,8 @@ describe('converter', () => {
           labels: undefined,
           safetySettings: undefined,
           generationConfig: undefined,
-          session_id: 'my-session',
+          session_id: undefined,
         },
-        user_prompt_id: 'my-prompt',
       });
     });
 
@@ -53,12 +47,7 @@ describe('converter', () => {
         model: 'gemini-pro',
         contents: [{ role: 'user', parts: [{ text: 'Hello' }] }],
       };
-      const codeAssistReq = toGenerateContentRequest(
-        genaiReq,
-        'my-prompt',
-        undefined,
-        'my-session',
-      );
+      const codeAssistReq = toGenerateContentRequest(genaiReq);
       expect(codeAssistReq).toEqual({
         model: 'gemini-pro',
         project: undefined,
@@ -71,9 +60,8 @@ describe('converter', () => {
           labels: undefined,
           safetySettings: undefined,
           generationConfig: undefined,
-          session_id: 'my-session',
+          session_id: undefined,
         },
-        user_prompt_id: 'my-prompt',
       });
     });
 
@@ -84,7 +72,6 @@ describe('converter', () => {
       };
       const codeAssistReq = toGenerateContentRequest(
         genaiReq,
-        'my-prompt',
         'my-project',
         'session-123',
       );
@@ -102,7 +89,6 @@ describe('converter', () => {
           generationConfig: undefined,
           session_id: 'session-123',
         },
-        user_prompt_id: 'my-prompt',
       });
     });
 
@@ -111,12 +97,7 @@ describe('converter', () => {
         model: 'gemini-pro',
         contents: 'Hello',
       };
-      const codeAssistReq = toGenerateContentRequest(
-        genaiReq,
-        'my-prompt',
-        'my-project',
-        'my-session',
-      );
+      const codeAssistReq = toGenerateContentRequest(genaiReq);
       expect(codeAssistReq.request.contents).toEqual([
         { role: 'user', parts: [{ text: 'Hello' }] },
       ]);
@@ -127,12 +108,7 @@ describe('converter', () => {
         model: 'gemini-pro',
         contents: [{ text: 'Hello' }, { text: 'World' }],
       };
-      const codeAssistReq = toGenerateContentRequest(
-        genaiReq,
-        'my-prompt',
-        'my-project',
-        'my-session',
-      );
+      const codeAssistReq = toGenerateContentRequest(genaiReq);
       expect(codeAssistReq.request.contents).toEqual([
         { role: 'user', parts: [{ text: 'Hello' }] },
         { role: 'user', parts: [{ text: 'World' }] },
@@ -147,12 +123,7 @@ describe('converter', () => {
           systemInstruction: 'You are a helpful assistant.',
         },
       };
-      const codeAssistReq = toGenerateContentRequest(
-        genaiReq,
-        'my-prompt',
-        'my-project',
-        'my-session',
-      );
+      const codeAssistReq = toGenerateContentRequest(genaiReq);
       expect(codeAssistReq.request.systemInstruction).toEqual({
         role: 'user',
         parts: [{ text: 'You are a helpful assistant.' }],
@@ -168,12 +139,7 @@ describe('converter', () => {
           topK: 40,
         },
       };
-      const codeAssistReq = toGenerateContentRequest(
-        genaiReq,
-        'my-prompt',
-        'my-project',
-        'my-session',
-      );
+      const codeAssistReq = toGenerateContentRequest(genaiReq);
       expect(codeAssistReq.request.generationConfig).toEqual({
         temperature: 0.8,
         topK: 40,
@@ -199,12 +165,7 @@ describe('converter', () => {
           responseMimeType: 'application/json',
         },
       };
-      const codeAssistReq = toGenerateContentRequest(
-        genaiReq,
-        'my-prompt',
-        'my-project',
-        'my-session',
-      );
+      const codeAssistReq = toGenerateContentRequest(genaiReq);
       expect(codeAssistReq.request.generationConfig).toEqual({
         temperature: 0.1,
         topP: 0.2,
diff --git a/packages/core/src/code_assist/converter.ts b/packages/core/src/code_assist/converter.ts
index ffd471da..8340cfc1 100644
--- a/packages/core/src/code_assist/converter.ts
+++ b/packages/core/src/code_assist/converter.ts
@@ -32,7 +32,6 @@ import {
 export interface CAGenerateContentRequest {
   model: string;
   project?: string;
-  user_prompt_id?: string;
   request: VertexGenerateContentRequest;
 }
 
@@ -116,14 +115,12 @@ export function fromCountTokenResponse(
 
 export function toGenerateContentRequest(
   req: GenerateContentParameters,
-  userPromptId: string,
   project?: string,
   sessionId?: string,
 ): CAGenerateContentRequest {
   return {
     model: req.model,
     project,
-    user_prompt_id: userPromptId,
     request: toVertexGenerateContentRequest(req, sessionId),
   };
 }
diff --git a/packages/core/src/code_assist/server.test.ts b/packages/core/src/code_assist/server.test.ts
index 3fc1891f..6246fd4e 100644
--- a/packages/core/src/code_assist/server.test.ts
+++ b/packages/core/src/code_assist/server.test.ts
@@ -14,25 +14,13 @@ vi.mock('google-auth-library');
 describe('CodeAssistServer', () => {
   it('should be able to be constructed', () => {
     const auth = new OAuth2Client();
-    const server = new CodeAssistServer(
-      auth,
-      'test-project',
-      {},
-      'test-session',
-      UserTierId.FREE,
-    );
+    const server = new CodeAssistServer(auth, 'test-project');
     expect(server).toBeInstanceOf(CodeAssistServer);
   });
 
   it('should call the generateContent endpoint', async () => {
     const client = new OAuth2Client();
-    const server = new CodeAssistServer(
-      client,
-      'test-project',
-      {},
-      'test-session',
-      UserTierId.FREE,
-    );
+    const server = new CodeAssistServer(client, 'test-project');
     const mockResponse = {
       response: {
         candidates: [
@@ -50,13 +38,10 @@ describe('CodeAssistServer', () => {
     };
     vi.spyOn(server, 'requestPost').mockResolvedValue(mockResponse);
 
-    const response = await server.generateContent(
-      {
-        model: 'test-model',
-        contents: [{ role: 'user', parts: [{ text: 'request' }] }],
-      },
-      'user-prompt-id',
-    );
+    const response = await server.generateContent({
+      model: 'test-model',
+      contents: [{ role: 'user', parts: [{ text: 'request' }] }],
+    });
 
     expect(server.requestPost).toHaveBeenCalledWith(
       'generateContent',
@@ -70,13 +55,7 @@ describe('CodeAssistServer', () => {
 
   it('should call the generateContentStream endpoint', async () => {
     const client = new OAuth2Client();
-    const server = new CodeAssistServer(
-      client,
-      'test-project',
-      {},
-      'test-session',
-      UserTierId.FREE,
-    );
+    const server = new CodeAssistServer(client, 'test-project');
     const mockResponse = (async function* () {
       yield {
         response: {
@@ -96,13 +75,10 @@ describe('CodeAssistServer', () => {
     })();
     vi.spyOn(server, 'requestStreamingPost').mockResolvedValue(mockResponse);
 
-    const stream = await server.generateContentStream(
-      {
-        model: 'test-model',
-        contents: [{ role: 'user', parts: [{ text: 'request' }] }],
-      },
-      'user-prompt-id',
-    );
+    const stream = await server.generateContentStream({
+      model: 'test-model',
+      contents: [{ role: 'user', parts: [{ text: 'request' }] }],
+    });
 
     for await (const res of stream) {
       expect(server.requestStreamingPost).toHaveBeenCalledWith(
@@ -116,13 +92,7 @@ describe('CodeAssistServer', () => {
 
   it('should call the onboardUser endpoint', async () => {
     const client = new OAuth2Client();
-    const server = new CodeAssistServer(
-      client,
-      'test-project',
-      {},
-      'test-session',
-      UserTierId.FREE,
-    );
+    const server = new CodeAssistServer(client, 'test-project');
     const mockResponse = {
       name: 'operations/123',
       done: true,
@@ -144,13 +114,7 @@ describe('CodeAssistServer', () => {
 
   it('should call the loadCodeAssist endpoint', async () => {
     const client = new OAuth2Client();
-    const server = new CodeAssistServer(
-      client,
-      'test-project',
-      {},
-      'test-session',
-      UserTierId.FREE,
-    );
+    const server = new CodeAssistServer(client, 'test-project');
     const mockResponse = {
       currentTier: {
         id: UserTierId.FREE,
@@ -176,13 +140,7 @@ describe('CodeAssistServer', () => {
 
   it('should return 0 for countTokens', async () => {
     const client = new OAuth2Client();
-    const server = new CodeAssistServer(
-      client,
-      'test-project',
-      {},
-      'test-session',
-      UserTierId.FREE,
-    );
+    const server = new CodeAssistServer(client, 'test-project');
     const mockResponse = {
       totalTokens: 100,
     };
@@ -197,13 +155,7 @@ describe('CodeAssistServer', () => {
 
   it('should throw an error for embedContent', async () => {
     const client = new OAuth2Client();
-    const server = new CodeAssistServer(
-      client,
-      'test-project',
-      {},
-      'test-session',
-      UserTierId.FREE,
-    );
+    const server = new CodeAssistServer(client, 'test-project');
     await expect(
       server.embedContent({
         model: 'test-model',
diff --git a/packages/core/src/code_assist/server.ts b/packages/core/src/code_assist/server.ts
index 08339bdc..7af643f7 100644
--- a/packages/core/src/code_assist/server.ts
+++ b/packages/core/src/code_assist/server.ts
@@ -53,16 +53,10 @@ export class CodeAssistServer implements ContentGenerator {
 
   async generateContentStream(
     req: GenerateContentParameters,
-    userPromptId: string,
   ): Promise<AsyncGenerator<GenerateContentResponse>> {
     const resps = await this.requestStreamingPost<CaGenerateContentResponse>(
       'streamGenerateContent',
-      toGenerateContentRequest(
-        req,
-        userPromptId,
-        this.projectId,
-        this.sessionId,
-      ),
+      toGenerateContentRequest(req, this.projectId, this.sessionId),
       req.config?.abortSignal,
     );
     return (async function* (): AsyncGenerator<GenerateContentResponse> {
@@ -74,16 +68,10 @@ export class CodeAssistServer implements ContentGenerator {
 
   async generateContent(
     req: GenerateContentParameters,
-    userPromptId: string,
   ): Promise<GenerateContentResponse> {
     const resp = await this.requestPost<CaGenerateContentResponse>(
       'generateContent',
-      toGenerateContentRequest(
-        req,
-        userPromptId,
-        this.projectId,
-        this.sessionId,
-      ),
+      toGenerateContentRequest(req, this.projectId, this.sessionId),
       req.config?.abortSignal,
     );
     return fromGenerateContentResponse(resp);
diff --git a/packages/core/src/code_assist/setup.test.ts b/packages/core/src/code_assist/setup.test.ts
index c1260e3f..6db5fd88 100644
--- a/packages/core/src/code_assist/setup.test.ts
+++ b/packages/core/src/code_assist/setup.test.ts
@@ -49,11 +49,8 @@ describe('setupUser', () => {
     });
     await setupUser({} as OAuth2Client);
     expect(CodeAssistServer).toHaveBeenCalledWith(
-      {},
+      expect.any(Object),
       'test-project',
-      {},
-      '',
-      undefined,
     );
   });
 
@@ -65,10 +62,7 @@ describe('setupUser', () => {
     });
     const projectId = await setupUser({} as OAuth2Client);
     expect(CodeAssistServer).toHaveBeenCalledWith(
-      {},
-      undefined,
-      {},
-      '',
+      expect.any(Object),
       undefined,
     );
     expect(projectId).toEqual({
diff --git a/packages/core/src/code_assist/setup.ts b/packages/core/src/code_assist/setup.ts
index 9c7a8043..8831d24b 100644
--- a/packages/core/src/code_assist/setup.ts
+++ b/packages/core/src/code_assist/setup.ts
@@ -34,7 +34,7 @@ export interface UserData {
  */
 export async function setupUser(client: OAuth2Client): Promise<UserData> {
   let projectId = process.env.GOOGLE_CLOUD_PROJECT || undefined;
-  const caServer = new CodeAssistServer(client, projectId, {}, '', undefined);
+  const caServer = new CodeAssistServer(client, projectId);
 
   const clientMetadata: ClientMetadata = {
     ideType: 'IDE_UNSPECIFIED',
diff --git a/packages/core/src/core/client.test.ts b/packages/core/src/core/client.test.ts
index 5101f98b..25ea9bc1 100644
--- a/packages/core/src/core/client.test.ts
+++ b/packages/core/src/core/client.test.ts
@@ -209,9 +209,7 @@ describe('Gemini Client (client.ts)', () => {
 
     // We can instantiate the client here since Config is mocked
     // and the constructor will use the mocked GoogleGenAI
-    client = new GeminiClient(
-      new Config({ sessionId: 'test-session-id' } as never),
-    );
+    client = new GeminiClient(new Config({} as never));
     mockConfigObject.getGeminiClient.mockReturnValue(client);
 
     await client.initialize(contentGeneratorConfig);
@@ -350,19 +348,16 @@ describe('Gemini Client (client.ts)', () => {
 
       await client.generateContent(contents, generationConfig, abortSignal);
 
-      expect(mockGenerateContentFn).toHaveBeenCalledWith(
-        {
-          model: 'test-model',
-          config: {
-            abortSignal,
-            systemInstruction: getCoreSystemPrompt(''),
-            temperature: 0.5,
-            topP: 1,
-          },
-          contents,
+      expect(mockGenerateContentFn).toHaveBeenCalledWith({
+        model: 'test-model',
+        config: {
+          abortSignal,
+          systemInstruction: getCoreSystemPrompt(''),
+          temperature: 0.5,
+          topP: 1,
         },
-        'test-session-id',
-      );
+        contents,
+      });
     });
   });
 
@@ -381,21 +376,18 @@ describe('Gemini Client (client.ts)', () => {
 
       await client.generateJson(contents, schema, abortSignal);
 
-      expect(mockGenerateContentFn).toHaveBeenCalledWith(
-        {
-          model: 'test-model', // Should use current model from config
-          config: {
-            abortSignal,
-            systemInstruction: getCoreSystemPrompt(''),
-            temperature: 0,
-            topP: 1,
-            responseSchema: schema,
-            responseMimeType: 'application/json',
-          },
-          contents,
+      expect(mockGenerateContentFn).toHaveBeenCalledWith({
+        model: 'test-model', // Should use current model from config
+        config: {
+          abortSignal,
+          systemInstruction: getCoreSystemPrompt(''),
+          temperature: 0,
+          topP: 1,
+          responseSchema: schema,
+          responseMimeType: 'application/json',
         },
-        'test-session-id',
-      );
+        contents,
+      });
     });
 
     it('should allow overriding model and config', async () => {
@@ -419,22 +411,19 @@ describe('Gemini Client (client.ts)', () => {
         customConfig,
       );
 
-      expect(mockGenerateContentFn).toHaveBeenCalledWith(
-        {
-          model: customModel,
-          config: {
-            abortSignal,
-            systemInstruction: getCoreSystemPrompt(''),
-            temperature: 0.9,
-            topP: 1, // from default
-            topK: 20,
-            responseSchema: schema,
-            responseMimeType: 'application/json',
-          },
-          contents,
+      expect(mockGenerateContentFn).toHaveBeenCalledWith({
+        model: customModel,
+        config: {
+          abortSignal,
+          systemInstruction: getCoreSystemPrompt(''),
+          temperature: 0.9,
+          topP: 1, // from default
+          topK: 20,
+          responseSchema: schema,
+          responseMimeType: 'application/json',
         },
-        'test-session-id',
-      );
+        contents,
+      });
     });
   });
 
@@ -1017,14 +1006,11 @@ Here are files the user has recently opened, with the most recent at the top:
         config: expect.any(Object),
         contents,
       });
-      expect(mockGenerateContentFn).toHaveBeenCalledWith(
-        {
-          model: currentModel,
-          config: expect.any(Object),
-          contents,
-        },
-        'test-session-id',
-      );
+      expect(mockGenerateContentFn).toHaveBeenCalledWith({
+        model: currentModel,
+        config: expect.any(Object),
+        contents,
+      });
     });
   });
 
diff --git a/packages/core/src/core/client.ts b/packages/core/src/core/client.ts
index 02fbeb38..77683a45 100644
--- a/packages/core/src/core/client.ts
+++ b/packages/core/src/core/client.ts
@@ -106,7 +106,7 @@ export class GeminiClient {
   private readonly COMPRESSION_PRESERVE_THRESHOLD = 0.3;
 
   private readonly loopDetector: LoopDetectionService;
-  private lastPromptId: string;
+  private lastPromptId?: string;
 
   constructor(private config: Config) {
     if (config.getProxy()) {
@@ -115,7 +115,6 @@ export class GeminiClient {
 
     this.embeddingModel = config.getEmbeddingModel();
     this.loopDetector = new LoopDetectionService(config);
-    this.lastPromptId = this.config.getSessionId();
   }
 
   async initialize(contentGeneratorConfig: ContentGeneratorConfig) {
@@ -428,19 +427,16 @@ export class GeminiClient {
       };
 
       const apiCall = () =>
-        this.getContentGenerator().generateContent(
-          {
-            model: modelToUse,
-            config: {
-              ...requestConfig,
-              systemInstruction,
-              responseSchema: schema,
-              responseMimeType: 'application/json',
-            },
-            contents,
+        this.getContentGenerator().generateContent({
+          model: modelToUse,
+          config: {
+            ...requestConfig,
+            systemInstruction,
+            responseSchema: schema,
+            responseMimeType: 'application/json',
           },
-          this.lastPromptId,
-        );
+          contents,
+        });
 
       const result = await retryWithBackoff(apiCall, {
         onPersistent429: async (authType?: string, error?: unknown) =>
@@ -525,14 +521,11 @@ export class GeminiClient {
       };
 
       const apiCall = () =>
-        this.getContentGenerator().generateContent(
-          {
-            model: modelToUse,
-            config: requestConfig,
-            contents,
-          },
-          this.lastPromptId,
-        );
+        this.getContentGenerator().generateContent({
+          model: modelToUse,
+          config: requestConfig,
+          contents,
+        });
 
       const result = await retryWithBackoff(apiCall, {
         onPersistent429: async (authType?: string, error?: unknown) =>
diff --git a/packages/core/src/core/contentGenerator.ts b/packages/core/src/core/contentGenerator.ts
index 797bad73..44ed7beb 100644
--- a/packages/core/src/core/contentGenerator.ts
+++ b/packages/core/src/core/contentGenerator.ts
@@ -25,12 +25,10 @@ import { UserTierId } from '../code_assist/types.js';
 export interface ContentGenerator {
   generateContent(
     request: GenerateContentParameters,
-    userPromptId: string,
   ): Promise<GenerateContentResponse>;
 
   generateContentStream(
     request: GenerateContentParameters,
-    userPromptId: string,
   ): Promise<AsyncGenerator<GenerateContentResponse>>;
 
   countTokens(request: CountTokensParameters): Promise<CountTokensResponse>;
diff --git a/packages/core/src/core/geminiChat.test.ts b/packages/core/src/core/geminiChat.test.ts
index cd5e3841..39dd883e 100644
--- a/packages/core/src/core/geminiChat.test.ts
+++ b/packages/core/src/core/geminiChat.test.ts
@@ -79,14 +79,11 @@ describe('GeminiChat', () => {
 
       await chat.sendMessage({ message: 'hello' }, 'prompt-id-1');
 
-      expect(mockModelsModule.generateContent).toHaveBeenCalledWith(
-        {
-          model: 'gemini-pro',
-          contents: [{ role: 'user', parts: [{ text: 'hello' }] }],
-          config: {},
-        },
-        'prompt-id-1',
-      );
+      expect(mockModelsModule.generateContent).toHaveBeenCalledWith({
+        model: 'gemini-pro',
+        contents: [{ role: 'user', parts: [{ text: 'hello' }] }],
+        config: {},
+      });
     });
   });
 
@@ -114,14 +111,11 @@ describe('GeminiChat', () => {
 
       await chat.sendMessageStream({ message: 'hello' }, 'prompt-id-1');
 
-      expect(mockModelsModule.generateContentStream).toHaveBeenCalledWith(
-        {
-          model: 'gemini-pro',
-          contents: [{ role: 'user', parts: [{ text: 'hello' }] }],
-          config: {},
-        },
-        'prompt-id-1',
-      );
+      expect(mockModelsModule.generateContentStream).toHaveBeenCalledWith({
+        model: 'gemini-pro',
+        contents: [{ role: 'user', parts: [{ text: 'hello' }] }],
+        config: {},
+      });
     });
   });
 
diff --git a/packages/core/src/core/geminiChat.ts b/packages/core/src/core/geminiChat.ts
index 14e8d946..4c3cd4c8 100644
--- a/packages/core/src/core/geminiChat.ts
+++ b/packages/core/src/core/geminiChat.ts
@@ -286,14 +286,11 @@ export class GeminiChat {
           );
         }
 
-        return this.contentGenerator.generateContent(
-          {
-            model: modelToUse,
-            contents: requestContents,
-            config: { ...this.generationConfig, ...params.config },
-          },
-          prompt_id,
-        );
+        return this.contentGenerator.generateContent({
+          model: modelToUse,
+          contents: requestContents,
+          config: { ...this.generationConfig, ...params.config },
+        });
       };
 
       response = await retryWithBackoff(apiCall, {
@@ -396,14 +393,11 @@ export class GeminiChat {
           );
         }
 
-        return this.contentGenerator.generateContentStream(
-          {
-            model: modelToUse,
-            contents: requestContents,
-            config: { ...this.generationConfig, ...params.config },
-          },
-          prompt_id,
-        );
+        return this.contentGenerator.generateContentStream({
+          model: modelToUse,
+          contents: requestContents,
+          config: { ...this.generationConfig, ...params.config },
+        });
       };
 
       // Note: Retrying streams can be complex. If generateContentStream itself doesn't handle retries