---
id: architecture/oq-13-cas-failfast
name: Backlog — CAS-before-unpack fail-fast pre-check on push
status: pending
depends_on: []
scope: narrow
risk: low
impact: component
level: implementation
tags: [optimization-backlog]
---

## Description

Optimization backlog item from ADR-013 (§7, §Consequences): a pre-unpack
CAS pre-check on receive-pack — reject commands with obviously-wrong
old-ids before reading the pack body, failing fast instead of ingesting
a pack whose refs would be rejected anyway. The protocol-correct order
(unpack-first, then per-ref checks) is what v1 implements; this is an
optimization, additive, and two-way (internal behavior, not wire shape —
the report shape is unchanged).

## Work

When receive-pack implementation is on the critical path for a
deployment with heavy stale-push traffic (measured, not hypothetical):
add the pre-check to the transport push state machine, after command
parse and before ingestion. Rejected refs report the same `ng <ref>
<reason>`; the pack body is then drained (or the connection closed per
the substrate's error rule).

## Verification

- All existing push tests still pass (real `git push` against the
  transport).
- A stale-old push errors before the pack body is fully read (observable
  via the body-budget counters or a truncated-body test).

## Out of scope

- Changing the report shape or the protocol order (ADR-013 §7 stands).

## Summary

> Filled on completion.