- Looked like
- Code regression
- Actually
- Outdated toolchain version
- Layer
- Agent runtime / Toolchain
Field note
An agent workflow stopped working during ordinary use. Because the team had been actively committing, attention went first to recent diffs — but reverting changes did not restore the behavior. Widening the search past the repository to the runtime and toolchain layer surfaced the real problem: the installed version no longer matched what the workflow expected.
- Symptom
- The workflow failed in normal usage after previously working.
- Misleading hypothesis
- The first assumption was that a recent code change had broken the behavior.
- Boundary expansion
- The investigation moved beyond Git history into the runtime and toolchain layer.
- Root cause
- The installed agent/toolchain version was older than the behavior expected by the workflow.
- Signals
-
- Recent code changes did not line up with the failure
- The same workflow depended on external runtime behavior
- The failure pattern matched a compatibility mismatch more than a local code regression
- Checks
-
- Verified installed toolchain version
- Compared expected behavior with the local runtime
- Confirmed the issue by aligning the runtime/toolchain version
Evidence trail
Recent code changes did not line up with the failure
Runtime behavior differed from the expected workflow behavior
Installed toolchain version was below the expected baseline
Aligning the toolchain version confirmed the cause
Reusable lesson
For agent systems, check runtime and toolchain compatibility before assuming the application code changed.
Dealing with a failure that looks like one layer but might be another?