The Dependency Upgrade Optimism Is Now Officially a Procedure
The dependency was upgraded. The tests passed. Then the production behavior that depended on the old version became visible.
Field Guide: Dependency Upgrade Optimism
Subject: Dependency upgrade, production codebase Classification: Developer behavior, release confidence Intended Audience: The engineer currently waiting for CI to go green
Stage 1: The Decision
The dependency is two major versions behind. There is a security advisory. There is a convenient Friday afternoon. The engineer opens the package file.
The upgrade is one line. The tests pass locally. The mood is good.
Stage 2: The Merge
The PR is approved. The CI is green. The merge is clean. The governing rule: any dependency upgrade optimism that restores service once may be reused until someone proves why it worked.
The upgrade has not yet been proven. The tests passing is not the proof. The tests test what was written. They do not test what was assumed.
Stage 3: The First Anomaly
Two days post-deploy, a webhook begins failing at a rate that looks like noise until someone checks the logs. The webhook is calling a serialization method that changed behavior in the new version, silently, documented in the changelog under "behavior change (minor)."
The change was minor. The dependency on the old behavior was not documented anywhere. The dependency existed in production for fourteen months before this upgrade.
Stage 4: The Discovery
The upgrade succeeded. The upgrade is still succeeding, in a way, while three things it quietly relied on are now wrong.
The dependency upgrade optimism produces a local feeling of certainty while leaving the system-level uncertainty untouched. The workaround is applied. The workaround is not explained. The workaround enters the codebase as a correction, not a revert.
Stage 5: Operational Folklore
Eventually the unexplained behavior becomes operational folklore and deleting it requires an incident review. The correction added after the upgrade is now part of how the system works. Nobody remembers why it was added. The comment says "fix webhook serialization after dependency bump." The word "after" is doing significant work.
The next upgrade will encounter this comment and pause.
Status: The system is stable. The stability has a comment attached to it that nobody has opened in seven months.