diff --git a/docs/design/INDEX.md b/docs/design/INDEX.md
index 4379ff8..446a5ca 100644
--- a/docs/design/INDEX.md
+++ b/docs/design/INDEX.md
@@ -748,7 +748,14 @@ lib API is retained: `rank_by`/`optimize` keep live consumers (`optimize` backs
 walk-forward's in-sample step, `rank_by` backs `runs family … rank`); `append`/
 `load` (the flat-store half) remain public API with **no in-tree caller** after
 this retire — tested, available to external consumers, a latent dead-code surface
-a later sweep may revisit.
+a later sweep may revisit. **Unknown-id contract (ratified, Runway fieldtest
+2026-06).** `aura runs family <id>` treats an unknown-but-well-formed id as an
+*empty family* (prints nothing, exit 0) — the same treat-as-empty discipline as
+`Registry::load` reading a missing store as `Ok(empty)`, and deliberately
+distinct from the retired `list`/`rank` exit-2, which is argv-shape rejection
+*before* any store access, not a found-nothing lookup. Tightening to a non-zero
+`no such family <id>` exit (typo-safety) is an available future UX choice, not a
+current contract.
 
 ### C19 — Bootstrap: blueprint → instance (recursive)
 **Guarantee.** Construction is a distinct phase, recursive at every level. Each