diff --git a/packages/cli/src/gemini.test.tsx b/packages/cli/src/gemini.test.tsx
index f678d9ad71..6795c2a1b0 100644
--- a/packages/cli/src/gemini.test.tsx
+++ b/packages/cli/src/gemini.test.tsx
@@ -1189,6 +1189,39 @@ describe('resolveSessionId', () => {
     expect(sessionId).toBe('new-id');
     expect(resumedSessionData).toBeUndefined();
   });
+
+  it('should exit with FATAL_INPUT_ERROR when explicit resume session is missing', async () => {
+    vi.mocked(SessionSelector).mockImplementation(
+      () =>
+        ({
+          resolveSession: vi
+            .fn()
+            .mockRejectedValue(SessionError.noSessionsFound()),
+        }) as unknown as InstanceType<typeof SessionSelector>,
+    );
+
+    const emitFeedbackSpy = vi.spyOn(coreEvents, 'emitFeedback');
+    const processExitSpy = vi
+      .spyOn(process, 'exit')
+      .mockImplementation((code) => {
+        throw new MockProcessExitError(code);
+      });
+
+    try {
+      await resolveSessionId('explicit-session-id');
+    } catch (e) {
+      if (!(e instanceof MockProcessExitError)) throw e;
+    }
+
+    expect(emitFeedbackSpy).toHaveBeenCalledWith(
+      'error',
+      expect.stringContaining('Error resuming session:'),
+    );
+    expect(processExitSpy).toHaveBeenCalledWith(ExitCodes.FATAL_INPUT_ERROR);
+
+    emitFeedbackSpy.mockRestore();
+    processExitSpy.mockRestore();
+  });
 });
 
 describe('gemini.tsx main function exit codes', () => {
diff --git a/packages/cli/src/gemini.tsx b/packages/cli/src/gemini.tsx
index ab97f7f574..04fc5c2169 100644
--- a/packages/cli/src/gemini.tsx
+++ b/packages/cli/src/gemini.tsx
@@ -85,7 +85,11 @@ import { validateAuthMethod } from './config/auth.js';
 import { runAcpClient } from './acp/acpStdioTransport.js';
 import { validateNonInteractiveAuth } from './validateNonInterActiveAuth.js';
 import { appEvents, AppEvent } from './utils/events.js';
-import { SessionError, SessionSelector } from './utils/sessionUtils.js';
+import {
+  RESUME_LATEST,
+  SessionError,
+  SessionSelector,
+} from './utils/sessionUtils.js';
 
 import { relaunchOnExitCode } from './utils/relaunch.js';
 import { loadSandboxConfig } from './config/sandboxConfig.js';
@@ -309,8 +313,10 @@ export async function resolveSessionId(
     };
   } catch (error) {
     if (error instanceof SessionError && error.code === 'NO_SESSIONS_FOUND') {
-      coreEvents.emitFeedback('warning', error.message);
-      return { sessionId: createSessionId() };
+      if (resumeArg === RESUME_LATEST) {
+        coreEvents.emitFeedback('warning', error.message);
+        return { sessionId: createSessionId() };
+      }
     }
     coreEvents.emitFeedback(
       'error',
diff --git a/packages/cli/src/utils/sessionUtils.test.ts b/packages/cli/src/utils/sessionUtils.test.ts
index 0bc1183e71..cfdadf795f 100644
--- a/packages/cli/src/utils/sessionUtils.test.ts
+++ b/packages/cli/src/utils/sessionUtils.test.ts
@@ -616,6 +616,120 @@ describe('SessionSelector', () => {
     expect(sessions.length).toBe(1);
     expect(sessions[0].id).toBe(mainSessionId);
   });
+
+  it('should list legacy session JSON without timestamps (regression #18593)', async () => {
+    const sessionId = randomUUID();
+
+    const chatsDir = path.join(tmpDir, 'chats');
+    await fs.mkdir(chatsDir, { recursive: true });
+
+    const session = {
+      sessionId,
+      projectHash: 'test-hash',
+      messages: [
+        {
+          type: 'user',
+          content: 'Legacy session message',
+          id: 'msg1',
+          timestamp: '2024-01-01T10:00:00.000Z',
+        },
+      ],
+    };
+
+    const filePath = path.join(
+      chatsDir,
+      `${SESSION_FILE_PREFIX}2024-01-01T10-00-${sessionId.slice(0, 8)}.json`,
+    );
+    await fs.writeFile(filePath, JSON.stringify(session, null, 2));
+    const fallbackTimestamp = new Date('2024-01-01T10:30:00.000Z');
+    await fs.utimes(filePath, fallbackTimestamp, fallbackTimestamp);
+
+    const sessionSelector = new SessionSelector(storage);
+    const sessions = await sessionSelector.listSessions();
+
+    expect(sessions.length).toBe(1);
+    expect(sessions[0].id).toBe(sessionId);
+    expect(sessions[0].startTime).toBe(fallbackTimestamp.toISOString());
+    expect(sessions[0].lastUpdated).toBe(fallbackTimestamp.toISOString());
+  });
+
+  it('should resolve legacy session JSON without timestamps by UUID (regression #18593)', async () => {
+    const sessionId = randomUUID();
+
+    const chatsDir = path.join(tmpDir, 'chats');
+    await fs.mkdir(chatsDir, { recursive: true });
+
+    const session = {
+      sessionId,
+      projectHash: 'test-hash',
+      messages: [
+        {
+          type: 'user',
+          content: 'Legacy session message',
+          id: 'msg1',
+          timestamp: '2024-01-01T10:00:00.000Z',
+        },
+      ],
+    };
+
+    const filePath = path.join(
+      chatsDir,
+      `${SESSION_FILE_PREFIX}2024-01-01T10-00-${sessionId.slice(0, 8)}.json`,
+    );
+    await fs.writeFile(filePath, JSON.stringify(session, null, 2));
+    const fallbackTimestamp = new Date('2024-01-01T10:30:00.000Z');
+    await fs.utimes(filePath, fallbackTimestamp, fallbackTimestamp);
+
+    const sessionSelector = new SessionSelector(storage);
+    const result = await sessionSelector.resolveSession(sessionId);
+
+    expect(result.sessionData.sessionId).toBe(sessionId);
+    expect(result.sessionData.startTime).toBe(fallbackTimestamp.toISOString());
+    expect(result.sessionData.lastUpdated).toBe(
+      fallbackTimestamp.toISOString(),
+    );
+  });
+
+  it('should throw INVALID_SESSION_IDENTIFIER for a UUID that does not exist on disk at all', async () => {
+    const existingSessionId = randomUUID();
+    const nonExistentId = randomUUID();
+
+    const chatsDir = path.join(tmpDir, 'chats');
+    await fs.mkdir(chatsDir, { recursive: true });
+
+    const session = {
+      sessionId: existingSessionId,
+      projectHash: 'test-hash',
+      startTime: '2024-01-01T10:00:00.000Z',
+      lastUpdated: '2024-01-01T10:30:00.000Z',
+      messages: [
+        {
+          type: 'user',
+          content: 'Hello',
+          id: 'msg1',
+          timestamp: '2024-01-01T10:00:00.000Z',
+        },
+      ],
+    };
+
+    await fs.writeFile(
+      path.join(
+        chatsDir,
+        `${SESSION_FILE_PREFIX}2024-01-01T10-00-${existingSessionId.slice(0, 8)}.json`,
+      ),
+      JSON.stringify(session, null, 2),
+    );
+
+    const sessionSelector = new SessionSelector(storage);
+
+    await expect(sessionSelector.findSession(nonExistentId)).rejects.toSatisfy(
+      (error) => {
+        expect(error).toBeInstanceOf(SessionError);
+        expect((error as SessionError).code).toBe('INVALID_SESSION_IDENTIFIER');
+        return true;
+      },
+    );
+  });
 });
 
 describe('extractFirstUserMessage', () => {
diff --git a/packages/cli/src/utils/sessionUtils.ts b/packages/cli/src/utils/sessionUtils.ts
index 437e32c465..a2918eae3e 100644
--- a/packages/cli/src/utils/sessionUtils.ts
+++ b/packages/cli/src/utils/sessionUtils.ts
@@ -270,15 +270,23 @@ export const getAllSessionFiles = async (
           }
 
           // Validate required fields
-          if (
-            !content.sessionId ||
-            !content.startTime ||
-            !content.lastUpdated
-          ) {
+          if (!content.sessionId) {
             // Missing required fields - treat as corrupted
             return { fileName: file, sessionInfo: null };
           }
 
+          const fileTimestamp =
+            !content.startTime || !content.lastUpdated
+              ? (
+                  await fs.stat(filePath).catch(() => undefined)
+                )?.mtime.toISOString()
+              : undefined;
+          const fallbackTimestamp = fileTimestamp ?? new Date().toISOString();
+          const startTime =
+            content.startTime || content.lastUpdated || fallbackTimestamp;
+          const lastUpdated =
+            content.lastUpdated || content.startTime || fallbackTimestamp;
+
           // Skip sessions that only contain system messages (info, error, warning)
           if (!content.hasUserOrAssistantMessage) {
             return { fileName: file, sessionInfo: null };
@@ -319,8 +327,8 @@ export const getAllSessionFiles = async (
             id: content.sessionId,
             file: file.replace(/\.jsonl?$/, ''),
             fileName: file,
-            startTime: content.startTime,
-            lastUpdated: content.lastUpdated,
+            startTime,
+            lastUpdated,
             messageCount: content.messageCount ?? content.messages.length,
             displayName: content.summary
               ? stripUnsafeCharacters(content.summary)
@@ -546,12 +554,17 @@ export class SessionSelector {
       if (!sessionData) {
         throw new Error('Failed to load session data');
       }
+      const normalizedSessionData = {
+        ...sessionData,
+        startTime: sessionData.startTime || sessionInfo.startTime,
+        lastUpdated: sessionData.lastUpdated || sessionInfo.lastUpdated,
+      };
 
       const displayInfo = `Session ${sessionInfo.index}: ${sessionInfo.firstUserMessage} (${sessionInfo.messageCount} messages, ${formatRelativeTime(sessionInfo.lastUpdated)})`;
 
       return {
         sessionPath,
-        sessionData,
+        sessionData: normalizedSessionData,
         displayInfo,
       };
     } catch (error) {