diff --git a/packages/cli/src/acp/acpResume.test.ts b/packages/cli/src/acp/acpResume.test.ts
index c38b23d7151..c59a73205ba 100644
--- a/packages/cli/src/acp/acpResume.test.ts
+++ b/packages/cli/src/acp/acpResume.test.ts
@@ -310,4 +310,67 @@ describe('GeminiAgent Session Resume', () => {
       );
     });
   });
+
+  it('should resolve session before initializing config or fresh chat recording', async () => {
+    const callOrder: string[] = [];
+    const resolveSessionMock = vi.fn().mockImplementation(async () => {
+      callOrder.push('resolveSession');
+      return {
+        sessionData: {
+          sessionId: 'same-minute-session-id',
+          messages: [{ type: 'user', content: [{ text: 'Hello' }] }],
+        },
+        sessionPath: '/path/to/session.jsonl',
+      };
+    });
+
+    (SessionSelector as unknown as Mock).mockImplementation(() => ({
+      resolveSession: resolveSessionMock,
+    }));
+    mockConfig.initialize.mockImplementation(async () => {
+      callOrder.push('config.initialize');
+    });
+    vi.mocked(mockConfig.getGeminiClient().resumeChat).mockImplementation(
+      async () => {
+        callOrder.push('geminiClient.resumeChat');
+      },
+    );
+    (convertSessionToClientHistory as unknown as Mock).mockReturnValue([
+      { role: 'user', parts: [{ text: 'Hello' }] },
+    ]);
+
+    await agent.loadSession({
+      sessionId: 'same-minute-session-id',
+      cwd: '/tmp',
+      mcpServers: [],
+    });
+
+    expect(callOrder).toEqual([
+      'resolveSession',
+      'config.initialize',
+      'geminiClient.resumeChat',
+    ]);
+    expect(mockConfig.getGeminiClient().initialize).not.toHaveBeenCalled();
+  });
+
+  it('should not initialize config if session resolution fails', async () => {
+    (SessionSelector as unknown as Mock).mockImplementation(() => ({
+      resolveSession: vi
+        .fn()
+        .mockRejectedValue(
+          new Error('No previous sessions found for this project.'),
+        ),
+    }));
+
+    await expect(
+      agent.loadSession({
+        sessionId: 'missing-session-id',
+        cwd: '/tmp',
+        mcpServers: [],
+      }),
+    ).rejects.toThrow('No previous sessions found for this project.');
+
+    expect(mockConfig.initialize).not.toHaveBeenCalled();
+    expect(mockConfig.getGeminiClient().resumeChat).not.toHaveBeenCalled();
+  });
 });
diff --git a/packages/cli/src/acp/acpSessionManager.ts b/packages/cli/src/acp/acpSessionManager.ts
index cfa7037a24b..f1a76a01dba 100644
--- a/packages/cli/src/acp/acpSessionManager.ts
+++ b/packages/cli/src/acp/acpSessionManager.ts
@@ -126,7 +126,9 @@ export class AcpSessionManager {
 
     const geminiClient = config.getGeminiClient();
 
-    const chat = await geminiClient.startChat();
+    const chat = geminiClient.isInitialized?.()
+      ? geminiClient.getChat()
+      : await geminiClient.startChat();
 
     const session = new Session(
       sessionId,
@@ -165,22 +167,26 @@ export class AcpSessionManager {
     { sessionId, cwd, mcpServers }: acp.LoadSessionRequest,
     authDetails: AuthDetails,
   ): Promise<acp.LoadSessionResponse> {
-    const config = await this.initializeSessionConfig(
+    const config = await this.prepareSessionConfig(
       sessionId,
       cwd,
       mcpServers,
       authDetails,
     );
 
+    await config.storage?.initialize?.();
     const sessionSelector = new SessionSelector(config.storage);
 
     const { sessionData, sessionPath } =
       await sessionSelector.resolveSession(sessionId);
 
+    await config.initialize();
+    startupProfiler.flush(config);
+    startAutoMemoryIfEnabled(config);
+
     const clientHistory = convertSessionToClientHistory(sessionData.messages);
 
     const geminiClient = config.getGeminiClient();
-    await geminiClient.initialize();
     await geminiClient.resumeChat(clientHistory, {
       conversation: sessionData,
       filePath: sessionPath,
@@ -228,7 +234,7 @@ export class AcpSessionManager {
     return response;
   }
 
-  private async initializeSessionConfig(
+  private async prepareSessionConfig(
     sessionId: string,
     cwd: string,
     mcpServers: acp.McpServer[],
@@ -274,12 +280,6 @@ export class AcpSessionManager {
       config.setFileSystemService(acpFileSystemService);
     }
 
-    // 4. Now that we are authenticated, it is safe to initialize the config
-    // which starts the MCP servers and other heavy resources.
-    await config.initialize();
-    startupProfiler.flush(config);
-    startAutoMemoryIfEnabled(config);
-
     return config;
   }
 
diff --git a/packages/core/src/core/client.ts b/packages/core/src/core/client.ts
index 6dc4b60fda3..c39c9b815c3 100644
--- a/packages/core/src/core/client.ts
+++ b/packages/core/src/core/client.ts
@@ -340,6 +340,20 @@ export class GeminiClient {
     history: ReadonlyArray<Content | HistoryTurn>,
     resumedSessionData?: ResumedSessionData,
   ): Promise<void> {
+    if (resumedSessionData?.filePath) {
+      const previousRecordingService = this.chat?.getChatRecordingService?.();
+      if (
+        previousRecordingService &&
+        previousRecordingService.getConversationFilePath?.() !==
+          resumedSessionData.filePath
+      ) {
+        try {
+          await previousRecordingService.deleteCurrentSessionIfNotResumableAsync?.();
+        } catch {
+          // Best-effort cleanup of abandoned startup-only session file
+        }
+      }
+    }
     this.chat = await this.startChat(history, resumedSessionData);
     this.updateTelemetryTokenCount();
   }
diff --git a/packages/core/src/services/chatRecordingService.test.ts b/packages/core/src/services/chatRecordingService.test.ts
index 8a63e3e541e..6a9209919a0 100644
--- a/packages/core/src/services/chatRecordingService.test.ts
+++ b/packages/core/src/services/chatRecordingService.test.ts
@@ -213,6 +213,49 @@ describe('ChatRecordingService', () => {
       expect(files[0]).toMatch(/^session-.*-test-ses\.jsonl$/);
     });
 
+    it('should not append to or poison an existing session file in the same UTC minute', async () => {
+      await chatRecordingService.initialize();
+      chatRecordingService.recordMessage({
+        type: 'user',
+        content: 'Reply with exactly: alpha',
+        model: 'gemini-pro',
+      });
+      chatRecordingService.recordMessage({
+        type: 'gemini',
+        content: 'alpha',
+        model: 'gemini-pro',
+      });
+
+      const originalFilePath = chatRecordingService.getConversationFilePath()!;
+      expect(fs.existsSync(originalFilePath)).toBe(true);
+
+      // A second fresh initialization in the same UTC minute for the same sessionId
+      // (e.g. during eager config initialization before resumeChat) must not append a
+      // context-only checkpoint onto the existing session file.
+      const secondRecordingService = new ChatRecordingService(mockConfig);
+      await secondRecordingService.initialize();
+      secondRecordingService.updateMessagesFromHistory([
+        {
+          id: 'ctx-1',
+          content: {
+            role: 'user',
+            parts: [{ text: '<session_context>env</session_context>' }],
+          },
+        } as HistoryTurn,
+      ]);
+
+      const secondFilePath = secondRecordingService.getConversationFilePath()!;
+      expect(secondFilePath).not.toBe(originalFilePath);
+      expect(path.basename(secondFilePath)).toMatch(
+        /^session-.*-1-test-ses\.jsonl$/,
+      );
+
+      const reloadedOriginal = await loadConversationRecord(originalFilePath);
+      expect(reloadedOriginal).not.toBeNull();
+      expect(reloadedOriginal?.hasResumableContent).toBe(true);
+      expect(reloadedOriginal?.messages).toHaveLength(2);
+    });
+
     it('should include the conversation kind when specified', async () => {
       await chatRecordingService.initialize(undefined, 'subagent');
       chatRecordingService.recordMessage({
diff --git a/packages/core/src/services/chatRecordingService.ts b/packages/core/src/services/chatRecordingService.ts
index 186282eb1d0..7bd6156b06a 100644
--- a/packages/core/src/services/chatRecordingService.ts
+++ b/packages/core/src/services/chatRecordingService.ts
@@ -511,10 +511,12 @@ export class ChatRecordingService {
         if (this.kind === 'subagent') {
           filename = `${safeSessionId}.jsonl`;
         } else {
-          filename = `${SESSION_FILE_PREFIX}${timestamp}-${safeSessionId.slice(
-            0,
-            8,
-          )}.jsonl`;
+          const shortId = safeSessionId.slice(0, 8);
+          filename = `${SESSION_FILE_PREFIX}${timestamp}-${shortId}.jsonl`;
+          let collisionIndex = 1;
+          while (fs.existsSync(path.join(chatsDir, filename))) {
+            filename = `${SESSION_FILE_PREFIX}${timestamp}-${collisionIndex++}-${shortId}.jsonl`;
+          }
         }
         this.conversationFile = path.join(chatsDir, filename);