PatchSession classifies errors by parsing err.message

[flyingrobots]

Migrated from Method backlog

GitHub Issues are now the live work tracker. Repository docs remain Method evidence.

Field	Value
Source backlog	docs/method/backlog/bad-code/BND_patch-session-message-parsing.md
Archived source	docs/archive/backlog/github-issue-migration-2026-06-01/docs/method/backlog/bad-code/BND_patch-session-message-parsing.md
Original lane	bad-code
Original id	BND_patch-session-message-parsing
Original legend	BND
Original feature	observer-admission-runtime
Original release home	v19.0.0

Original backlog card

PatchSession classifies errors by parsing err.message

Effort: S

What's Wrong

PatchSession._classifyCommitError() uses errMsg.includes('Concurrent commit detected') and errMsg.includes('has advanced') to classify errors. This is the "raccoon in a dumpster" anti-pattern from SSJS doctrine — behaviorally significant branching driven by string matching on human-readable text. If anyone rewords the message, the classification silently breaks.

Error type should be primary, not message content.

Suggested Fix

Introduce ConcurrentCommitError and RefAdvancedError classes extending the appropriate domain error base. Throw those from the origin sites. Replace _classifyCommitError() string matching with instanceof dispatch (P7).