Six Bugs That Taught Me How to Build an AI Agent That Doesn't Fall Over

Kiita — Mon, 14 Sep 2026 08:49:51 +0000

For the past several months I've been running a voice-driven AI assistant of my own design, continuously, unattended — 240+ tools, real API integrations, real background jobs, no human babysitting it around the clock.

Building the demo version took a weekend. Keeping it alive, unsupervised, for months took a lot longer — and almost none of what I had to fix ever shows up in a framework's quickstart guide. It only shows up after enough real hours logged, with a real user (me) depending on it not breaking silently at 2am.

Here are three of the failures that taught me the most, and what I ended up building because of them.

1. The OAuth helper that hung for hours

One integration used a standard OAuth flow: open a browser, wait for the user to click "allow." That's fine when a human is sitting there. It is not fine when the same code path gets triggered from a scheduled background task with an expired token and nobody anywhere near a screen.

It just... waited. For hours. No error, no timeout, no log line saying anything was wrong — just a thread quietly blocked forever, because nothing in the code had ever considered "what if no human answers."

The fix sounds obvious in hindsight: never let anything block indefinitely when it might run unattended. In practice, doing this correctly from any thread, on both Windows and POSIX, without being able to forcibly kill a stuck call, took real care — Python has no cross-platform signal-based way to interrupt an arbitrary blocking call from a non-main thread.

2. Two duplicate tool calls, two live sessions

The model occasionally emits the same tool call twice in a single turn — once on a first pass, once on what looks like a retry. Most of the time this is harmless. Once, it wasn't: two near-simultaneous calls to a "launch session" tool spun up two concurrent audio sessions on the same physical device. Two processes fighting over one microphone is exactly as unpleasant as it sounds.

The fix is a short time-windowed dedupe check — tool name plus arguments, hashed, checked against what ran in the last few seconds. Simple once you know you need it. I didn't know I needed it until it happened.

3. One shared API key, two features starving each other

Two unrelated features shared a single provider API key. When one had a burst of usage, it silently ate into the quota the other feature needed — mid-conversation, with no warning, no isolation, no way to tell which feature was actually responsible for the 429 that showed up somewhere else entirely.

The fix: every paid call now sits behind a budget ceiling tracked per feature, not per account. A runaway loop in one capability can no longer starve every other capability sharing the same key.

What I did with all of this

Every pattern above — plus three more (code-enforced confirmation gates for irreversible actions, a structured audit trail, and a watchdog that restarts what dies) — is now a small, tested, zero-dependency Python library called kevlar-agent.

pip install kevlar-agent

from kevlar_agent import dedupe, silent, budget_guard

@dedupe(window_seconds=5)
@budget_guard("image_gen", monthly_limit_usd=3.0, cost_usd=0.04)
def generate_clip(prompt: str) -> str:
    ...

token = silent(refresh_oauth_token, timeout_seconds=15)

24 tests, MIT licensed, no framework lock-in — it's plain Python decorators and functions, so it drops into whatever you're already building (LangChain, CrewAI, MCP-based agents, or nothing at all) instead of asking you to adopt a new architecture.

Code: https://github.com/KiitaInternet/kevlar
Full write-up with all six patterns: https://kiitainternet.github.io/kevlar/

If you're running an agent unattended and have hit any of these — or a failure mode I haven't — I'd genuinely like to hear about it.

DEV Community: Kiita

Six Bugs That Taught Me How to Build an AI Agent That Doesn't Fall Over

1. The OAuth helper that hung for hours

2. Two duplicate tool calls, two live sessions

3. One shared API key, two features starving each other

What I did with all of this