Append-only record of non-trivial decisions with reasoning. Prevents re-deriving the same question across sessions.
The decision log is the file at logs/decisions.md that captures every non-trivial decision Arkeus makes, along with the reasoning, the rejected alternatives, and the trigger that prompted the question. It exists because sessions do not share memory by default, and without a decision log the same question gets re-answered every time Ryan returns to it.
The format is strict: Decision (what was chosen), Why (the reasoning), Rejected (what was considered and why it was not chosen), Trigger (what prompted the question). This structure forces the decision to be captured in a form that is reusable — a future session can read the log, see the reasoning, and either confirm the decision still holds or surface the specific assumption that has changed.
The rule is: before re-deriving a decision, check the log. If Ryan asks should we X and a prior session already decided X with documented reasoning, surface the reasoning first. Do not start from scratch. Starting from scratch is the failure mode of most assistants: they produce a confident new answer every time, the answers drift over sessions, and the user has no way to tell which answer is load-bearing.
Decision log is append-only. Decisions that turn out to be wrong get marked with a follow-up entry that references the original, explains what changed, and makes the new decision. The history stays — you need the history to understand why the current state is what it is.
The log was introduced in April 2026, specifically to replace the old advisor.md state file that had accumulated cross-session context in a form that was hard to query and easy to let drift. Decisions.md is the canonical cross-session memory surface now. The advisor.md retirement was itself a decision, captured in the log.
The log is the piece of infrastructure that lets Cowork sessions actually build on each other, rather than restarting from zero every time.