diff --git a/decisions/0016-warn-when-a-tier-resolves-to-no-model.md b/decisions/0016-warn-when-a-tier-resolves-to-no-model.md
new file mode 100644
index 0000000..56c8127
--- /dev/null
+++ b/decisions/0016-warn-when-a-tier-resolves-to-no-model.md
@@ -0,0 +1,44 @@
+# 0016 — Warn when a tier resolves to no model
+
+Date: 2026-10-07
+Status: accepted
+
+## Context
+
+An agent's `tier` resolves per tool through the built-in defaults in
+`engine/lib/common.sh` and any `models.<tool>.<tier>` override in
+`intelligence.yaml`. A tier that neither source defines — a misspelled `haevy`,
+as a rule — rendered an empty `model` for every tool while sync exited 0 and
+printed nothing, so the mistake surfaced only when a tool refused the agent or
+quietly fell back to its own default (issue #39). Custom tier names are
+legitimate: a project may define `models.claude.review-deep` for one tool and
+no other.
+
+## Decision
+
+1. A tier that resolves to no model still renders, with an empty `model`, and
+   sync prints one `WARNING:` per tool and tier — naming both and the two ways
+   out: a standard tier, or a `models.<tool>.<tier>` override. The warning is
+   deduplicated across every agent carrying the tier.
+2. It is a `WARNING:` line, not an indented note, so `sync --compact` and an
+   unchanged compact run keep it.
+3. A tier the manifest defines for a tool resolves quietly for that tool.
+
+## Consequences
+
+- A typo is visible on the first sync without blocking anyone's render.
+- A project using a custom tier for one tool sees a warning for each other tool
+  until it defines the tier there too; that is the information it needs.
+- Nothing refuses the render, so a project that ignores warnings can still ship
+  an agent without a model.
+
+## Rejected
+
+- **Refusing the sync.** A project that deliberately defines a custom tier for
+  one tool would fail on every other enabled tool until it added an override
+  for each.
+- **Falling back to `heavy`.** It hides the typo and silently substitutes a
+  model the author did not choose — costlier than a misspelled `light` or
+  `standard` meant, weaker than a misspelled `frontier` — and is right only
+  when the misspelled tier was `heavy` itself.
+- **Leaving it silent.** The issue this record closes.
diff --git a/engine/lib/common.sh b/engine/lib/common.sh
index 116df5a..7b89cb4 100644
--- a/engine/lib/common.sh
+++ b/engine/lib/common.sh
@@ -1132,7 +1132,7 @@ load_model_tiers() {
 # the tool has no default for and the manifest overrides nowhere — a typo, as a
 # rule — still renders an empty model, but never silently: the run warns once
 # per tool and tier, unindented on purpose because `sync --compact` keeps only
-# `WARNING:` lines.
+# `WARNING:` lines (decision 0016).
 # shellcheck disable=SC2034  # IS_MODEL is the return channel read by adapters
 resolve_model_var() {
     case "$1" in