chore: improve error messages for --resume (#15360)

packages/cli/src/utils/sessionUtils.test.ts

@@ -10,6 +10,7 @@ import {
   extractFirstUserMessage,
   formatRelativeTime,
   hasUserOrAssistantMessage,
+  SessionError,
 } from './sessionUtils.js';
 import type { Config, MessageRecord } from '@google/gemini-cli-core';
 import { SESSION_FILE_PREFIX } from '@google/gemini-cli-core';
@@ -333,10 +334,10 @@ describe('SessionSelector', () => {
 
     await expect(
       sessionSelector.resolveSession('invalid-uuid'),
-    ).rejects.toThrow('Invalid session identifier "invalid-uuid"');
+    ).rejects.toThrow(SessionError);
 
     await expect(sessionSelector.resolveSession('999')).rejects.toThrow(
-      'Invalid session identifier "999"',
+      SessionError,
     );
   });
 

packages/cli/src/utils/sessionUtils.ts

@@ -23,6 +23,47 @@ import { stripUnsafeCharacters } from '../ui/utils/textUtils.js';
  */
 export const RESUME_LATEST = 'latest';
 
+/**
+ * Error codes for session-related errors.
+ */
+export type SessionErrorCode =
+  | 'NO_SESSIONS_FOUND'
+  | 'INVALID_SESSION_IDENTIFIER';
+
+/**
+ * Error thrown for session-related failures.
+ * Uses a code field to differentiate between error types.
+ */
+export class SessionError extends Error {
+  constructor(
+    readonly code: SessionErrorCode,
+    message: string,
+  ) {
+    super(message);
+    this.name = 'SessionError';
+  }
+
+  /**
+   * Creates an error for when no sessions exist for the current project.
+   */
+  static noSessionsFound(): SessionError {
+    return new SessionError(
+      'NO_SESSIONS_FOUND',
+      'No previous sessions found for this project.',
+    );
+  }
+
+  /**
+   * Creates an error for when a session identifier is invalid.
+   */
+  static invalidSessionIdentifier(identifier: string): SessionError {
+    return new SessionError(
+      'INVALID_SESSION_IDENTIFIER',
+      `Invalid session identifier "${identifier}".\n  Use --list-sessions to see available sessions, then use --resume {number}, --resume {uuid}, or --resume latest.`,
+    );
+  }
+}
+
 /**
  * Represents a text match found during search with surrounding context.
  */
@@ -370,7 +411,7 @@ export class SessionSelector {
     const sessions = await this.listSessions();
 
     if (sessions.length === 0) {
-      throw new Error('No previous sessions found for this project.');
+      throw SessionError.noSessionsFound();
     }
 
     // Sort by startTime (oldest first, so newest sessions get highest numbers)
@@ -398,9 +439,7 @@ export class SessionSelector {
       return sortedSessions[index - 1];
     }
 
-    throw new Error(
-      `Invalid session identifier "${identifier}". Use --list-sessions to see available sessions.`,
-    );
+    throw SessionError.invalidSessionIdentifier(identifier);
   }
 
   /**
@@ -430,9 +469,13 @@ export class SessionSelector {
       try {
         selectedSession = await this.findSession(resumeArg);
       } catch (error) {
-        // Re-throw with more detailed message for resume command
+        // SessionError already has detailed messages - just rethrow
+        if (error instanceof SessionError) {
+          throw error;
+        }
+        // Wrap unexpected errors with context
         throw new Error(
-          `Invalid session identifier "${resumeArg}". Use --list-sessions to see available sessions, then use --resume {number}, --resume {uuid}, or --resume latest.  Error: ${error}`,
+          `Failed to find session "${resumeArg}": ${error instanceof Error ? error.message : String(error)}`,
         );
       }
     }