agent tail filter regex matched nothing: declared deploy healthy

agent tail filter regex matched nothing: declared deploy healthy

TL;DR

The tail filter regex matched nothing, so the agent saw zero errors and declared the deploy healthy. A health check on empty data is not a health check: require a minimum log volume, and fail the check when the filter matches nothing. Silence is not success.

agent tail filter regex matched nothing: declared deploy healthy

Steps

1. Confirm the filter matches nothing: run it against known log output. Expected: The regex is proven empty.

2. Fix the regex or widen the filter until it matches the expected traffic. Expected: Logs flow.

3. Re-evaluate the deploy with real log data. Expected: The verdict is honest.

4. Add a minimum-volume gate: health checks fail when matched logs are below threshold. Expected: Empty data cannot pass.

5. Test the filter against recorded logs before using it for verdicts. Expected: Filters are validated.

When to use

• a deploy is declared healthy on no logs
• tail filters are regex-based
• the filter was just written

When not to use

• logs exist and are clean (then healthy may be correct)
• the service genuinely has no traffic (then say so, do not say healthy)
• the filter is known-good (check the service instead)

Tool compatibility

• wrangler tail
• deploy health checks

Variant phrasings

• "tail filter matched nothing healthy"
• "regex filter no logs deploy"
• "false healthy deploy no logs"

Why it happens

An agent that checks 'are there errors in the logs' with a filter matching nothing gets the answer 'no' and reports healthy. The logic is correct; the premise (the filter sees the logs) is false. Health verdicts need a liveness proof: the check must demonstrate it observed traffic before it may declare health.

Edge cases

• the volume gate needs a baseline; brand-new services have little traffic
• regex filters deserve unit tests against sample logs
• 'no traffic' and 'healthy' must be distinct verdicts

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_nplN5g31cVeubiqU6fRH-w