Build AI Apps with Python: Tool Error Handling — Make Tools Bulletproof | Episode 11

Tools break. Files go missing, users divide by zero, paths are invalid. Without error handling, one bad call crashes your entire agent. In this episode, we make every tool bulletproof with try/except, input validation, and error dictionaries. Three scenarios prove it works: reading a missing file (graceful error message), dividing by zero then recovering with a valid division (error + recovery), and a successful write+read (normal flow unaffected). Claude reads error results, explains them, and tries a different approach. Phase 2 complete! Code: https://github.com/GoCelesteAI/build-ai-apps-python/tree/main/episode11 Every keystroke is shown on screen with 3-second pauses so you can follow along at your own pace. What You'll Learn: • try/except wrapping every tool function • Input validation — empty paths, directory traversal, zero division • Error dictionaries instead of exceptions • Security check: ".." path traversal prevention • Claude reads error results and explains them to users • Error recovery — Claude tries a different approach after failure • Multi-block tool handling (multiple tool calls in one response) • System prompt for error-aware behavior • Running Python scripts with :!python % Timestamps: 0:00 - Introduction 0:12 - Why Error Handling? (Preview) 0:44 - Creating error_handling.py 1:00 - Imports and setup 2:00 - Workspace directory 2:28 - Tool 1: read_file with try/except 3:15 - Directory traversal security check 4:22 - except catches everything 5:05 - Tool 2: write_file with validation 5:28 - Empty path check 6:28 - Write logic with error catching 7:28 - Tool 3: divide with zero check 8:35 - Cannot divide by zero — clear error 9:15 - Save progress 9:28 - Dispatch and tool definitions 11:40 - Save definitions 11:55 - Agent function with error-aware system prompt 13:12 - While loop from Episode 10 14:32 - Multi-block tool handling (for loop) 16:45 - Append results and break 17:40 - Three error scenarios 18:02 - Save and run 18:14 - Error: file not found (graceful) 18:24 - Error + recovery: divide by zero, then success 18:34 - Success: write + read (normal flow) 19:17 - Code review 19:37 - Recap: 3 Key Takeaways 20:09 - End Screen