6 Ways Your AI Agent Fails Silently (With Code to Catch Each One)

Your AI agent says "Done! Order placed successfully." But it ordered the wrong product. Or it ignored a tool error and hallucinated the rest. Or someone changed the system prompt mid-session and the agent quietly shifted its behavior. The agent didn't crash. It didn't raise an exception. It just... did the wrong thing and reported success. I've been building agents in production and I keep seeing the same failure patterns. Here are the 6 most common ones, with concrete code examples showing how each one happens -- and how to detect it. 1. Hallucinated Tool Output What happens: A tool returns an error, but the agent ignores it and proceeds as if the tool succeeded. # The tool returns an error search_result = search_api ( " Galaxy S25 Ultra " ) # -> {"error": "Product not found"} # But the agent's next decision says: # "Based on the search results, the Galaxy S25 Ultra costs $470..." # # What search results?! The tool returned an error! Why it's dangerous: The agent builds its entire dec