# I Was Filming a Demo of My Monitoring Tool. The Monitor Wasn't Monitoring.

> Source: <https://dev.to/dannwaneri/i-was-filming-a-demo-of-my-monitoring-tool-the-monitor-wasnt-monitoring-1p7d>
> Published: 2026-07-27 08:40:19+00:00

*This is a submission for DEV's Summer Bug Smash: Smash Stories powered by Sentry.*

`workers-monitor`

is a Cloudflare Worker I run to watch a small fleet of

my own Workers. Hourly cron, deterministic threshold gate, Claude Haiku

judgement only if the gate trips, Telegram alert if Haiku confirms

something's actually wrong. A quiet hour makes zero LLM calls.

[github.com/dannwaneri/workers-monitor](https://github.com/dannwaneri/workers-monitor)

It pages me on Telegram when something's wrong with the fleet it

watches. What it didn't have, until recently, was an equivalent safety

net for itself — if `workers-monitor`

broke, nobody got told. So I added

Sentry: automatic error capture, and `Sentry.withMonitor()`

around the

hourly run so a dead cron trigger would be caught immediately instead of

silently waiting up to 24 hours for the next daily heartbeat to go quiet.

I deployed it. A cron tick ran clean. I moved on, fairly pleased with

myself.

A few days later I sat down to record a demo video of the Sentry

integration — screen capture, narration, the whole thing, for a separate

part of this challenge. Simple plan: open the Sentry dashboard, show the

captured errors, show the cron monitor's check-in, done.

I opened the Monitors page to get the screenshot.

There was nothing there. Just Sentry's own auto-created generic "Error

Monitor" — no `workers-monitor-hourly-poll`

, no Cron-type entry

whatsoever, despite the code having run successfully, every hour, for

days. I'd been carrying around a completely false belief: that deploying

`Sentry.withMonitor()`

and watching it execute without error meant it was

monitoring. It wasn't. It never had been.

``` js
Sentry.withMonitor("workers-monitor-hourly-poll", () => run(event, env))
```

Nothing about this throws. It compiles. It deploys. The wrapped function

runs exactly as intended, every hour, on schedule. And it still wasn't

doing the one thing it existed to do.

By accident, honestly. Not through review, not through testing — I found

it because I was trying to film proof that something worked, and the

proof wasn't there. If I hadn't been making a video, I might not have

looked at that specific dashboard page for weeks.

`withMonitor`

's check-in has nowhere to attach without a schedule —

Sentry needs to know what "on time" even means for this monitor before it

will create a Cron Monitor entity to check in against. Without that

config, the call silently has no monitor to report to.

``` js
Sentry.withMonitor(
  "workers-monitor-hourly-poll",
  () => run(event, env),
  {
    schedule: { type: "crontab", value: "0 * * * *" }, // matches the real cron trigger
    checkinMargin: 5,
    maxRuntime: 5,
    timezone: "UTC",
  },
)
```

One config object. I deployed it, then made myself actually wait for a

real hourly tick — not a local test, the genuine production cron —

before I let myself believe it was fixed. `workers-monitor-hourly-poll`

showed up as a real Cron monitor afterward, "Every hour" schedule and

all.

Because it's the same lie twice, if I'm honest about it. I built a

monitoring tool because I don't trust systems to tell me the truth about

their own state unprompted. Then I shipped a piece of that exact tool

without applying the same skepticism to itself. "It ran without throwing"

is not evidence of "it's doing its job" — I know that, I'd have said it

confidently to anyone who asked — and I still fell for the gap between

those two claims on my own code, in the one project whose entire purpose

is not falling for that gap.

Real Sentry dashboard, the actual moment the cron monitor showed up after the fix not a re-enactment.

Treat "I deployed it and it didn't error" as a hypothesis, not a

conclusion — for every feature, not just the ones I already suspect are

fragile. The KV read bug I fixed earlier in this project (a separate

entry, if you're comparing notes) came from a structured spec review.

This one came from dumb luck — I happened to need a screenshot. I'd

rather it come from the habit than the accident next time.

[github.com/dannwaneri/workers-monitor/pull/3](https://github.com/dannwaneri/workers-monitor/pull/3)

One file changed, 14 insertions, 1 deletion — the isolated fix, nothing

else riding along with it.