ADR-0001 — Append-only graph expansion

• Status: Proposed
• Date: 2026-07-26

Context

An agent runtime must adapt when a step returns something unexpected. Two established options: re-plan from scratch (loses history) or loop back to a prior node (creates cycles and mutates completed state).

Decision

The graph expands by append only. A retry is a new node whose parent is the failure. Terminal nodes are immutable. Cycles are impossible by construction.

Consequences

Positive — complete causal history for free; the graph is the audit log; resumption from the frontier is trivial; persistence is append-only.

Negative — graphs grow without bound in long runs, making frontier policy and budget enforcement mandatory rather than optional; graph storage grows monotonically within a run and needs a retention story.

Alternatives rejected

Mutable node state with retry counters (loses the causal chain); cyclic graphs with checkpoints (LangGraph's model — reconciling checkpoint to trace is exactly the work this design removes).